The Marine Life Information Network

Temperature increase (local)

Benchmark. A 5°C increase in temperature for one month, or 2°C for one year. Further detail

Evidence

In the eastern Atlantic, a tagged shark experienced a temperature range of 5.8 to 21°C but demonstrated an apparent preference for temperatures of 15-17.5°C with 72% of temperature recordings falling within this range (Skomal et al., 2009).  Basking sharks in the western Atlantic were reported to make transequatorial migrations, entering the warm waters of the tropics and occupying mesopelagic depths (Skomal et al., 2009).  Skomal et al. (2004) cited prior studies by Owen in which basking sharks were sighted at sea surface temperatures between 11 to 24°C in the Gulf of Maine, although peak densities occurred at 22-24°C (Owens, 1984 cited in Skomal et al., 2004). Skomal et al. (2004) also suggested that basking shark departed the waters of the Gulf of Maine in early October as mean daily waters temperatures dropped from 15.8°C to 12.7°C.  Witt et al. (2016) noted that tagged basking sharks in the Sea of Hebrides also began to move south in October when the daily mean water temperatures were ca 13-14°C.  The tagged sharks experienced a range of temperatures of ca 9-15°C throughout the year during the duration of the tags.  

Sims (2008) suggested that the basking shark was tolerant of a wide range of temperatures ranging from 5.8 to 21°C.  The species can tolerate rapid changes in temperature associated with depth, on dives through the thermocline in stratified summer waters (Sims et al., 2003).  A shark in this study experienced temperature gradients of up to 15°C in dives from 20 m depths to 100 m depths, reaching a maximum depth of 180 m in water with a temperature of 1°C (Sims et al., 2003).  However, one individual was found beached and moribund in waters of 24°C, suggesting the species usually avoids warmer waters (Sims, 2008).  

Sensitivity assessment.  Basking sharks are found in temperate and tropical waters and are exposed to rapid temperature fluctuations (vertical temperature gradients of up to 15°C over ca 100 m) associated with swimming in surface and deep waters (up to 1264 m) (Sims et al., 2003; Sims, 2008; Gore et al., 2008). Although they may avoid warm waters (ca 24°C) their mobility would allow them to avoid localised warming at the level of the benchmark. Therefore, resistance is assessed as 'High'. Hence, resilience is also High (by default) and sensitivity is assessed as 'Not sensitive' at the benchmark level.

Temperature decrease (local)

Benchmark. A 5°C decrease in temperature for one month, or 2°C for one year. Further detail

Evidence

In the eastern Atlantic, a tagged shark experienced a temperature range of 5.8 to 21°C but demonstrated an apparent preference for temperatures of 15-17.5°C with 72% of temperature recordings falling within this range (Skomal et al., 2009).  Basking sharks in the western Atlantic were reported to make transequatorial migrations, entering the warm waters of the tropics and occupying mesopelagic depths (Skomal et al., 2009).  Skomal et al. (2004) cited prior studies by Owen in which basking sharks were sighted at sea surface temperatures between 11 to 24°C in the Gulf of Maine, although peak densities occurred at 22-24°C (Owens, 1984 cited in Skomal et al., 2004). Skomal et al. (2004) also suggested that basking shark departed the waters of the Gulf of Maine in early October as mean daily waters temperatures dropped from 15.8°C to 12.7°C.  Witt et al. (2016) noted that tagged basking sharks in the Sea of Hebrides also began to move south in October when the daily mean water temperatures were ca 13-14°C.  The tagged sharks experienced a range of temperatures of ca 9-15°C throughout the year during the duration of the tags.  

Sims (2008) suggested that the basking shark was tolerant of a wide range of temperatures ranging from 5.8 to 21°C.  The species can tolerate rapid changes in temperature associated with depth, on dives through the thermocline in stratified summer waters (Sims et al., 2003).  A shark in this study experienced temperature gradients of up to 15°C in dives from 20 m depths to 100 m depths, reaching a maximum depth of 180 m in water with a temperature of 1°C (Sims et al., 2003).  However, one individual was found beached and moribund in waters of 24°C, suggesting the species usually avoids warmer waters (Sims, 2008).  

Sensitivity assessment.  Basking sharks are found in temperate and tropical waters and are exposed to rapid temperature fluctuations (vertical temperature gradients of up to 15°C over ca 100 m) associated with swimming in surface and deep waters (up to 1264 m). Their mobility would allow them to avoid localised cooling at the level of the benchmark. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as 'Not sensitive' at the benchmark level.

Salinity increase (local)

Benchmark. A increase in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

Sensitivity assessment. Cetorhinus maximus is a fully marine species. It has a broad geographic range suggesting that the basking shark is able to cope with varying salinity levels. Supporting evidence is provided by studies that correlated environmental variables with basking shark distribution and found that their distribution could not be predicted by salinity levels alone (Soldo et al., 2008; Lucifora et al., 2015). In the case of hypersaline conditions, it is likely that the highly mobile Cetorhinus maximus would move to an area of normal salinity. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.

Salinity decrease (local)

Benchmark. A decrease in one MNCR salinity category above the usual range of the biotope or habitat. Further detail

Evidence

The basking shark is a fully marine species.  It has a broad geographic range suggesting that it can cope with varying salinity levels.  Soldo et al. (2008) and Lucifora et al. (2015) demonstrated that basking shark distribution could not be predicted by salinity levels alone.  In New Zealand, the species has been reported to enter the brackish Lake Ellesmere (Ryan, 1974; Dodgshun, 1980; Francis & Duffy, 2002).  A single five metre basking shark was recorded in the lake (Ryan, 1974) and in September 1979, numerous basking sharks were found in the same lake, with a maximum of 21 sharks observed in one day (Dodgshun, 1980).  Lake Ellesmere is known to exhibit variable salinity, both spatially and temporally.  In 1979, at the time of the basking shark encounters, the salinity at the entrance of the lake was thought to be 18 ppt.  The basking sharks were presumably attracted by the high concentrations of zooplankton within the lake (Francis & Duffy, 2002). 

Sensitivity assessment. The salinity change at the benchmark level is a decrease in one MNCR salinity category. Cetorhinus maximus is normally exposed to full salinity (30-40 ppt) and a reduction to variable salinity (18-40) did not have a negative impact on the health of the basking shark, in Lake Ellesmere. Also, as a highly mobile species, Cetorhinus maximus would be able to move away from any localised changes in salinity if they were to reach intolerable levels. Therefore, resistance is assessed as 'High'. Hence, resilience is also High (by default) and sensitivity is assessed as 'Not sensitive'.

Water flow (tidal current) changes (local)

Benchmark. A change in peak mean spring bed flow velocity of between 0.1 m/s to 0.2 m/s for more than one year. Further detail

Evidence

Cetorhinus maximus is highly mobile so can move to areas with favourable feeding conditions, and is unlikely to be affected by local changes in water flow. A study by Witt et al. (2014) indicated that basking sharks spent most time in areas of low to moderate tidal speeds (mean 0.3 m/s) but the standard deviation of this value was from 0.06 - 1.0 m/s, which suggested that they can cope with varying tidal speeds. Therefore, a localised change of 0.1-0.2 m/s is unlikely to be significant, and resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is recorded as Not sensitive.

Emergence regime changes

Benchmark.  1) A change in the time covered or not covered by the sea for a period of ≥1 year or 2) an increase in relative sea level or decrease in high water level for ≥1 year. Further detail

Evidence

Wave exposure changes (local)

Benchmark. A change in near shore significant wave height of >3% but <5% for more than one year. Further detail

Evidence

As a mobile pelagic shark, with the ability to dive to depths of up to 1264 m (Gore et al., 2008), it is unlikely that the basking shark will be impacted by small-scale changes in near shore wave height. However, wave exposure caused by stormy weather may have an effect.  There are very few records of basking shark sightings during stormy weather, partly because of the logistical difficulties involved, but also because the increased mixing of the water causes a breakdown of the coastal fronts, so zooplankton is more widely distributed, and not aggregated near the surface. Although there may be small energy losses resulting from reduced efficiency of feeding, it is likely that basking sharks can dive to greater depths to continue feeding. Sims et al. (2003) showed how basking sharks continue to forage in the winter when prey are concentrated at depth rather than at the surface.  A similar behavioural change may occur in stormy weather.

During calm weather in the summer, the water column becomes stratified and dense aggregations of zooplankton form along coastal fronts. This may be beneficial to Cetorhinus maximus due to increased feeding efficiency on the highly concentrated plankton. Therefore, a decrease in wave action may be of benefit.

Sensitivity assessment.  Cetorhinus maximus is likely to avoid storms by diving to a greater depth to feed. In addition, at the benchmark level, a change of 3-5% of significant wave height is only a small change and is unlikely to affect the basking shark, especially in the open ocean. Therefore, resistance is assessed as High and resilience is High (by default). Hence, sensitivity is assessed as Not sensitive. 

Transition elements & organo-metal contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

No specific accounts of transition element or organo-metal contamination in Cetorhinus maximus have been found. However, Cadmium and Lead were detected in the tissue of six different shark species in the eastern Mediterranean, whilst a component of antifouling paints, Tributyltin (TBT), was detected in blue shark kidneys (Watts et al., 2001). Though little is known about the impacts of these chemicals on the health of sharks, Watts et al. (2001) stated ‘they are likely to cause severe damage to basic biological functions’.

As a filter-feeder, Cetorhinus maximus ­is also vulnerable to the indirect consumption of toxic substances via contaminated prey (zooplankton) however; there are currently no accounts of this in the scientific literature.

Hydrocarbon & PAH contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available.

There is little information available in the scientific literature about the impacts of Hydrocarbon and PAH contamination on the basking shark/ However PCBs along with MEHP (plasticizer) and DDTs (toxic chemicals that adsorb onto the surface of plastics) were found incorporated into incidentally caught basking shark tissue in the Channel of Sicily, in the south Mediterranean (Fossi et al., 2014b). This study also found MEHP in Euphausia kronii (krill), samples; a prey species for the basking shark, which indicated that some component of the chemical ingestion was indirect.

Synthetic compound contamination

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed but evidence is presented where available. Little is known about the impact of synthetic compounds on Cetorhinus maximus. However, PCB was detected in basking shark tissue (Zitko et al., 1972; Fossi et al., 2014b). (See ‘Litter’ for more evidence of PCBs).

Radionuclide contamination

Benchmark. An increase in 10µGy/h above background levels. Further detail

Evidence

Introduction of other substances

Benchmark. Exposure of marine species or habitat to one or more relevant contaminants via uncontrolled releases or incidental spills. Further detail

Evidence

This pressure is Not assessed.

De-oxygenation

Benchmark. Exposure to dissolved oxygen concentration of less than or equal to 2 mg/l for one week (a change from WFD poor status to bad status). Further detail

Evidence

No information could be found on Cetorhinus maximus ability to tolerate hypoxia, but as the species is large and pelagic, it is unlikely to be able to tolerate low levels of oxygen. However, as a highly mobile species, Cetorhinus maximus would be able to move to an area with preferable oxygen levels. Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

Nutrient enrichment

Benchmark. Compliance with WFD criteria for good status. Further detail

Evidence

No information on the effect of nutrient enrichment or algal blooms was found. However, basking sharks are known to utilise oceanographic fronts that host an abundant food source (zooplankton) supported by the diffusion of nutrients from cold mixed water to warmer water and the subsequent growth of phytoplankton (Sims, 2008). As Cetorhinus maximus feeds on zooplankton, an increase in phytoplankton may increase the available food supply, not only in fronts but in other areas of enhanced nutrients. However, hypoxia caused by eutrophication may cause the basking shark to move to a more desirable area if the nutrient load rapidly increases.

Sensitivity assessment. Cetorhinus maximus is unlikely to be negatively impacted by nutrient enrichment at the benchmark level as it will lead to an increase in the food source. However, if nutrient levels lead to toxic blooms or hypoxia the, the highly mobile basking shark is likely to move to a more desirable area. Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

Organic enrichment

Benchmark. A deposit of 100 gC/m²/yr. Further detail

Evidence

No information was found on the specific effect of organic enrichment on Cetorhinus maximus. However, as a filter feeder, an increase in organic enrichment would likely affect the basking shark indirectly by influencing primary productivity and, therefore, prey abundance. Additionally, the potential for gill-raker clogging associated with increased suspended solids is low due to the method of filter-feeding used (cross-step filtration), which is thought to concentrate particles away from the gills using vortical flow to resuspend the particles that might otherwise clog the gill-rakers (Sanderson et al., 2016). Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

Physical loss (to land or freshwater habitat)

Benchmark. A permanent loss of existing saline habitat within the site. Further detail

Evidence

Basking shark abundance is related to plankton abundance within shelf-sea and headland fronts (Sims & Quayle, 1998; Sims, 2008; Speedie et al., 2009).  Important areas for basking sharks include migratory pathways, such as the Irish Sea and the Firth of Clyde (Sims et al., 2003; Solandt & Chassin, 2013), and locations associated with feeding activity such as oceanic fronts e.g. tidal fronts in the English Channel and the Ushant Front, Brittany (Sims et al., 2003) and other hotspots in the south and west of Cornwall and west coast of Scotland (Speedie & Johnson, 2008; Speedie et al., 2009; Solandt & Chassin, 2013). Sims (2008) suggested that food rich areas where the sharks aggregated provided the opportunity to initiate courtship and were potential important areas for the sharks to find mates as well as to feed.  Mating has not been observed and probably occurs in deep water (Mathews, 1950; Sims, 2008).  Courtship-like behaviour includes close-following (one shark following another closely), nose-to-tail swimming, parallel swimming, echelon swimming (sharks stationed behind and to the side of another in front of them), stacking (swimming below or slightly below and behind another), close-swimming (swimming within a body length of each other), and breaching (the shark leaps completely or partly out of the water) (Sims, 2008; Gore et al., 2019).  These behaviours have been reported from feeding aggregations of basking sharks in the Western English Channel (Sims et al., 2000), west Cornwall (Speedie & Johnson, 2008); west coast of Scotland and the Inner Hebrides (Speedie et al., 2009; Gore et al., 2019) and the coast of Nova Scotia (Harvey-Clark, et al., 1999).  However, Gore et al. (2019) found no relationship between the sex or size of a shark and close-following and suggested that 'following behaviours' were not related to gender.  Abrasions typical of male behaviour in other shark species were found on both sexes of basking shark while the abrasion of pectoral fins, typical of mating behaviour, were mainly on females.  Also, there was no clear evidence that breaching was related to mating.  They concluded that close-swimming behaviours were probably related to hydrodynamic advantage for feeding.  Nevertheless, they stated that mature sharks possibly use feeding aggregations to initiate pre-courtship behaviour (Gore et al., 2019).  

Doherty et al. (2017a) reported that basking sharks undertook post-summer migrations along the western coast of the British Isles from the vicinity of the Faeroes south to North Africa (perhaps further) via continental shelf and oceanic waters (up to ca 1000 km) at depths of 50-200 m. Post-summer densities were greatest in the Celtic and Irish Seas, the west coast of Scotland, and continental shelf of the west coast of Ireland (Doherty et al., 2017a).  Doherty et al. (2017a) also reported that some individuals returned to the summer hotspots where they were tagged off the west coast of Scotland and the Isle of Man.  Similarly, Doherty et al. (2017b), noted that three (or 36) tagged individuals shown inter-annual fidelity, returning to their tagged locations off the west coast of Scotland within a year of tagging.  It is theoretically possible that an obstruction due to an offshore wind farm, wave or tidal device arrays, mariculture infrastructure could prevent access to fronts in the vicinity of headlands, currently used by this species.  It is also theoretically possible that major engineering projects (e.g. barrages) in coastal seas could change the local hydrography significantly so that the fronts do not persist or do not form.  However, no direct evidence of either situation was found to support this supposition. 

Sensitivity assessment. The basking shark has a broad geographic range and is capable of ocean migrations (Sims, 2008; Gore et al., 2008; Doherty et al., 2017a) so that any loss of food supply is likely to be temporary as the animals find other frontal systems to frequent or areas to feed (see ‘reduction in prey’ below).  For example, the decline in the Achill Island shark fishery (west Ireland) between 1925 and 1975, correlated with a similar decline in copepod abundance (Sims & Reid, 2002; Sims, 2008; Speedie et al., 2009).  Also, the Norwegian fishery saw an increase in basking shark numbers after 1958, which suggested that the shark distribution shifted north in the mid-1950s in search of prey (Sims, 2008).  Therefore, while local ‘hotspots’ or aggregations may be lost, the animal itself may experience some energy-loss at most and is capable of relocating to other areas in search of food.   Therefore, resistance is assessed as 'High' and resilience as 'High' (by default). Hence, sensitivity is assessed as Not sensitive. 

Physical change (to another seabed type)

Benchmark. Permanent change from sedimentary or soft rock substrata to hard rock or artificial substrata or vice-versa. Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Physical change (to another sediment type)

Benchmark. Permanent change in one Folk class (based on UK SeaMap simplified classification). Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Habitat structure changes - removal of substratum (extraction)

Benchmark. The extraction of substratum to 30 cm (where substratum includes sediments and soft rock but excludes hard bedrock). Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Abrasion / disturbance of the surface of the substratum or seabed

Benchmark. Damage to surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Penetration or disturbance of the substratum subsurface

Benchmark. Damage to sub-surface features (e.g. species and physical structures within the habitat). Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Changes in suspended solids (water clarity)

Benchmark. A change in one rank on the WFD (Water Framework Directive) scale e.g. from clear to intermediate for one year. Further detail

Evidence

An increase in suspended solids may affect the basking shark in numerous ways.  Firstly, turbid waters attenuate light more rapidly than clear waters, which may result in a reduction in zooplankton prey (see ‘Reduction in prey’).  Basking sharks have been recorded in turbid regions.  An individual was tracked in the vicinity of the Amazon river mouth for approximately one month (Skomal et al., 2009).  In addition, the basking shark has been known to penetrate estuaries in some cases (Knickle et al., 2017).  Turbid waters might be thought to pose a risk of gill-raker clogging.  However, there were no reports in the literature of basking sharks suffering from this problem.  In addition, Sanderson et al. (2016) presented a model that showed how the basking shark might avoid gill-raker clogging by a particular filter-feeding method (vortical cross- step filtration). 

Sensitivity assessment. The turbidity change at the benchmark level is a change in one rank on the WFD scale for one year. As a highly mobile species, Cetorhinus maximus would be able to move away from any localised changes in turbidity if they were to reach intolerable levels. Energy losses may occur if the increase in turbidity occurs over a broad geographic range, as Cetorhinus maximus would be required to travel further to find food. Therefore, resistance is assessed as 'High'. Hence, resilience is also 'High' (by default) and sensitivity is assessed as 'Not sensitive'.

Smothering and siltation rate changes (light)

Benchmark. ‘Light’ deposition of up to 5 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus which is restricted to open water.

Smothering and siltation rate changes (heavy)

Benchmark. ‘Heavy’ deposition of up to 30 cm of fine material added to the seabed in a single discrete event. Further detail

Evidence

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus which is restricted to open water.

Litter

Benchmark. The introduction of man-made objects able to cause physical harm (surface, water column, seafloor or strandline). Further detail

Evidence

A high abundance of marine plastics and microplastics has been reported in areas of basking shark activity.  These include the north Atlantic, the North Sea and the Pelagos Sanctuary in the Mediterranean (a specific feeding area for the basking shark) where plastics are increasing (Thompson et al., 2004; Fossi et al., 2012; Panti et al., 2015).  In the Mediterranean, Fossi et al. (2014a) calculated that the basking shark (with a swimming speed of 0.85 m/s and mouth gape of 0.4 m²) could theoretically ingest 13,110 microplastic items per day, which suggested a vulnerability to contamination by both microplastics and their associated contaminants.  Fossi et al. (2014b) reported microplastic chemical consumption by basking sharks.  They found MEHP (a plasticizer) along with PCBs and DDTs (adsorbed on the surface of plastics) incorporated into incidentally caught basking shark tissue in the Channel of Sicily, south Mediterranean.  They also found MEHP in Euphausia kronii (krill) samples; a prey species for the basking shark, which indicated that some component of the plastic contaminant ingestion was indirect.  However, no evidence of the effect of microplastic consumption or the contaminant burden on health or viability was found.  Macroplastic litter such as discarded (ghost) fishing gear could be a major threat to the basking shark.  The United Nations Environment Programme (UNEP) and the Food and Agriculture Organization of the United Nations (FAO) have estimated that at least 640,000 tonnes of fishing gear are left in our oceans each year (World Animal Protection, 2014).  Stelfox et al. (2016) concluded that ‘ghost fishing’ (by discarded or lost fishing gear) could be a significant source of mortality in elasmobranchs but that there is a lack of data on direct effects. 

Sensitivity assessment.  Basking sharks are routinely observed with injuries and scars consistent with entanglement in ropes or nets (Darling & Keogh, 1994; Bloomfield & Solandt, 2006; Speedie et al., 2009; Solandt & Chassin, 2013; Gore et al., 2016) indicating that they can survive, and elasmobranchs are thought to be naturally resilient to some types of injury (Riley et al., 2009; Chin et al., 2015).  However, while evidence of entanglement by set fishing gear exists (see ‘Removal of non-target species’) no direct evidence of the effect of ‘ghost fishing’ on basking sharks was found.  Similarly, while the evidence suggests that basking shark ingest microplastics and adsorbed contaminants, any adverse effect of the contaminant burden on the individual (if any) was not reported.  Therefore, there is not enough evidence on which to base an assessment.  

Electromagnetic changes

Benchmark. A local electric field of 1 V/m or a local magnetic field of 10 µT. Further detail

Evidence

Electromagnetic detection is well recorded in elasmobranch species and is thought to be a method of both prey detection and navigation (Kalmijn, 1971, 1982; Meyer et al., 2005; Hart & Colin, 2015).  The basking shark is thought to forage for zooplankton using passive electroreception, performed by the electrosensory pores focussed on its snout (Kempster & Collin, 2011).  Zooplankton produce weak electric fields (up to 0.1V/m, Kempster & Collin, 2011).  Sharks can detect voltage gradients of ca 5 nV/m and the biopotentials of prey (1-500 mV) at distances of up to 0.5 m (Hart & Collin, 2015).  Therefore, if the basking shark is able to detect these signals, it is probable that it will also detect electric fields at the benchmark level of 1 V/m.  However, little is known about the direct impact of changing electric fields on basking sharks.  Gill & Kimber (2005) stated that electric fields may cause an attraction or avoidance response in some shark species.  Kalmijn (1982) suggested that elasmobranchs (including Mustelus canis and Prionace glauca) were generally attracted to electric fields in the range 0.005 to 1 mV/cm and avoid those around 10 mV/cm, due to the field being perceived as prey or a threat depending on its intensity.  In shark repellent tests, the effects of electromagnetic repellents were often species-specific (Hart & Collin, 2015). 

Less is known about the detection of magnetic fields by the basking shark.  Other sharks (Carcharhinus plumbeus and Sphyrna lewini) are attracted to magnetic fields in the range of 25-100 mT over <7 m (Meyer et al., 2005).  However, the magnetic field used in that study was far more intense than the benchmark level (10µT).  Hart & Collin (2015) reported mixed results with the use of magnets on long-line and hook-line fisheries.  For example, shark catch rate was not reduced by strong magnets (14,800 G; 1.4 T) but was significantly less than controls with weaker magnets (3,850 G; 0.38 T), and the repellent effect was species-specific.  Hart & Collin (2015) concluded that further research was required in the use of electromagnetic fields as shark repellents.  

Sensitivity assessment.  There is little direct evidence of the impact of electromagnetic fields on the basking shark.  However, if the behaviour of this species reflects that of other sharks (see above) it may be attracted or repelled by fields at different strengths.  The basking shark can probably detect electric fields at the benchmark level (1V /m) and, if it reflects the behaviour of Mustelus canis and Prionace glauca, a field of this strength may elicit an avoidance response.  There is no evidence to show the direct impact of a magnetic field of 10 µT on basking sharks.  But magnetic fields have been shown to attract sharks and might, therefore, affect the behaviour of this species too.  However, individuals may avoid or move away from localised areas of strong electric and magnetic fields and any temporary attractive or avoidance responses caused by fields at the benchmark level are likely to result in little more than small-scale energy loss.  Therefore, resistance is assessed as 'High'. Hence, resilience is also 'High' (by default) and sensitivity is assessed as 'Not sensitive' at the benchmark level.

Underwater noise changes

Benchmark. MSFD indicator levels (SEL or peak SPL) exceeded for 20% of days in a calendar year. Further detail

Evidence

Elasmobranchs have been shown to react to noise (Myrberg, 2001; Casper et al., 2012; Hart & Collin, 2015). Casper et al. (2012) and Hart & Collin (2015) noted that elasmobranchs have a relatively narrow auditory range and poor sensitivity when compared to many teleosts, although they also noted that studies were limited to only a few of the hundred species of elasmobranches.  Nelson & Gruber (1963) found that some sharks (including Carcharhinus leucas, Sphyrna sp., Negaprion brevirostris and Galeocerdo cuvieri) could be strongly attracted to rapidly and irregularly pulsed sounds (mirroring the vibrations caused by struggling prey) at below 60 Hz.  Sudden loud noises of low frequency have been shown to elicit an avoidance response in most fish (Vella et al., 2001).  Similar findings by Myrberg et al. (1978) show avoidance behaviour in some sharks in response to rapidly changing sounds and sudden onset of transmission of an intense sound (impulsive sounds).  Casper et al. (2012) noted that sharks were startled by sudden onset loud noise (20-30 dB above ambient) but habituated to the sound after a few trials.  Casper et al. (2012) also suggested that noise from offshore wind farm operation and boats (shipping) were unlikely to cause hearing loss or damage in sharks, but that the noise of pile driving (that can reach ca 237 dB re 1 µPa at 100-1000 Hz) could cause a short-term decrease in hearing sensitivity.  Barotrauma (due to the impulse energy caused by the hammer hitting the pile) was shown to damage the internal organs of teleost fish and suggested that the resultant vibration through the substratum might be a particular concern for demersal sharks and rays in contact with the substratum (Casper et al., 2012).  Hart & Collin (2015) reported that broad-band, low frequency biased, ‘pink noise’ was effective at repelling sharks, especially if suddenly or rapidly increased in loudness, and that a personal protection device claimed to repel sharks using pulsed sound in the range of 30-500 Hz or 200-1500 Hz.  But they also noted that sharks rapidly habituate to both attractive and repulsive sounds (Hart & Collin, 2015).  

Little information on sound detection in the basking shark was found.  Basking sharks have been reported to dive and move away from the area if disturbed by boats (Bloomfield & Solandt, 2008) but have also been noted to be relatively unaware of surface vessels (Speedie & Johnson, 2008).  Basking sharks killed by the prior harpoon fishery were shot at very close range and they generally show little reaction to being tagged.  Speedie & Johnson (2008) note that slow-moving vessels elicit hardly any response when groups of basking shark are feeding.  Wilson (2000; cited in Speedie & Johnson, 2008) noted that engine noise and angle of approach had a limited effect.  However, in the Isle of Man, courtship-like behaviour appeared to be disturbed by an approaching motorised craft, at a range of 1 km (Bloomfield & Solandt, 2006).  On the other hand, at Gwennap Head, Seawatch Southwest wildlife observers in 2007 began to see a change in behaviour when vessels came within 10 m of individual basking sharks.  Observations from Gwennap Head by Seawatch Southwest wildlife observers in 2007 reported that the sharks only showed altered behaviour when vessels approached very close to them (within 10 m) (Bloomfield & Solandt, 2006).  Darling & Keogh (1994) also suggested that basking sharks were attracted to vessel propellers. 

Sensitivity assessment.  There is no direct evidence of sound causing basking shark mortality or stress, however, the behaviour of other sharks can be altered by sound in the short-term.  The response of basking shark to boats may be due to either their noise or visual disturbance (see ‘Visual disturbance’).  Hence, if sound at the benchmark level, elicited an attractive or avoidance response in the basking shark, it would be likely to experience some energy-loss at most due to short-term interruption in feeding.  Therefore, Cetorhinus maximus is probably resistant to noise at the benchmark level so resistance is assessed as 'High'.  Hence, resilience is also 'High' (by default) and sensitivity is assessed as 'Not sensitive' at the benchmark level.  However, the applicability of the behaviour seen in other shark species to Cetorhinus maximus needs further study, particularly considering its feeding strategy as a filter-feeder (no need to detect struggling prey), and the confidence in the assessment is ‘Low’. 

Introduction of light or shading

Benchmark. A change in incident light via anthropogenic means. Further detail

Evidence

There are no reports of Cetorhinus maximus being disturbed by light pollution. In addition, they have very small eyes in proportion to their body (SharkTrust, 2010). Moreover, if temporarily disturbed by high light levels, Cetorhinus maximus is highly mobile and able to move towards more preferable conditions. Therefore, resistance is assessed as 'High'. Hence, resilience is also 'High' (by default) and sensitivity is recorded as 'Not sensitive'.

Barrier to species movement

Benchmark. A permanent or temporary barrier to species movement over ≥50% of water body width or a 10% change in tidal excursion. Further detail

Evidence

Basking shark (Cetorhinus maximus) abundance has been shown to be related to plankton abundance within shelf-sea and headland fronts (Sims & Quayle, 1998; Sims, 2008; Speedie et al., 2009) where they feed on the seasonal abundance of plankton, especially copepods (Sims, 2008).  Important areas for basking sharks include migratory pathways, such as the Irish Sea and the Firth of Clyde (Sims et al., 2003; Solandt & Chassin, 2013), and locations associated with feeding activity such as oceanic fronts e.g. tidal fronts in the English Channel and the Ushant Front, Brittany (Sims et al., 2003) and other hotspots in the south and west of Cornwall and west coast of Scotland (Speedie & Johnson, 2008; Speedie et al., 2009; Solandt & Chassin, 2013).  Doherty et al. (2017a) reported that basking sharks undertook post-summer migrations along the western coast of the British Isles from the vicinity of the Faeroes south to North Africa (perhaps further) via continental shelf and oceanic waters (up to ca 1000 km) at depths of 50-200 m. Post-summer densities were greatest in the Celtic and Irish Seas, the west coast of Scotland, and continental shelf of the west coast of Ireland (Doherty et al., 2017a).  Doherty et al. (2017a) also reported that some individuals returned to the summer hotspots where they were tagged off the west coast of Scotland and the Isle of Man.  Similarly, Doherty et al. (2017b), noted that three (of 36) tagged individuals showed inter-annual fidelity, returning to their tagged locations off the west coast of Scotland within a year of tagging.  It is theoretically possible that obstruction due to an offshore wind farm, wave or tidal device arrays, or mariculture infrastructure could reduce or prevent access to fronts in the vicinity of headlands, currently used by this species.  It is also theoretically possible that major engineering projects (e.g. barrages) in coastal seas could change the local hydrography significantly so that the fronts do not persist or do not form.  However, no direct evidence of either situation was found to support this supposition. 

Sensitivity assessment. Basking sharks have a broad geographic range and are capable of ocean migrations (Sims, 2008; Gore et al., 2008; Doherty et al., 2017a) so that obstruction or loss of access to current aggregation sites is likely to be temporary as the animals find other frontal systems to frequent or areas to feed.  It is also likely that an individual would be able to swim around obstructions and continue its migration along another route, resulting in little more than small-scale energy loss.  Although local ‘hotspots’ or aggregations may be lost, or move, the animal itself may experience some energy-loss at most.  Therefore, resistance is assessed as 'High', resilience as 'High', and sensitvity is assesed as 'Not sensitive'. 

Death or injury by collision

Benchmark. Injury or mortality from collisions of biota with both static or moving structures due to 0.1% of tidal volume on an average tide, passing through an artificial structure. Further detail

Evidence

As a mobile and broadly distributed species, the basking shark could encounter anthropogenic objects (such as vessels and marine infrastructure) that may result in collisions.  The basking shark is at risk from a collision with boat traffic because of their habit of feeding very close to the surface and at slow speeds (Sims, 2000; Speedie, 2017).  There have been numerous accounts of basking shark collisions, particularly ship-strikes (Kelly et al., 2004; Speedie & Johnson, 2008).  The Marine Conservation Society (MCS) reported 63 basking sharks suffering from ship strike or entanglement in fishing gear between 1992 and 2013 (Solandt & Chassin, 2013).  Despite having tough skin covered in dermal denticles, there is evidence of ship-strike causing scarring or injury and basking shark surveys routinely record evidence of injuries consistent with ship-strikes (Darling & Keogh, 1994; Bloomfield & Solandt, 2006; Speedie et al., 2009).  Speedie (2017) suggested that fatalities from boat collisions in both basking shark and humans had occurred but did not provide evidence to confirm the observation.  Speedie (2017) also noted that breaching basking sharks were reported to land on and accidentally damage fishing vessels, although one individual was reported to have deliberately rammed and damaged a trawler. 

Elasmobranchs are thought to be naturally resilient to some types of injury (Riley et al., 2009; Chin et al., 2015).  For example, wounds several centimetres long were undetectable within weeks or months in blacktip reef sharks (Carcharhinus melanopterus) and fresh bite wounds healed in 3-5 weeks; while a deep bite wound 20 cm wide had closed within three days and almost completely recovered in 40 days (Chin et al., 2015).  Longer healing times were reported in grey nurse sharks (Carcharias taruus) where necrosis from hook injuries took over six months to heal.  Healing rates were probably slower in cooler waters.  Similarly, minor abrasions in white sharks (Carcharodon carcharias) in the cooler waters (ca 18-20°C) of the Guadalupe Islands were visible for several months but a large bite wound healed in about nine months (Domeier & Nasby-Lucas, 2007; Chin et al., 2015).  Riley et al. (2009) reported that a whale shark (Rhincodon typus) survived harpooning.  It was observed with a wooden harpoon through its body and its poor condition suggested internal injuries.  However, it was observed 331 days later having lost the harpoon and with signs of healing.  Another whale shark was observed with a decapitated dorsal fin over four years, although the long-term effects on feeding and reproduction were unknown (Riley et al., 2009).  However, in the Canadian long-line fishery, hooking mortality varies between 10 and 31% in blue sharks (Prionace glauca), the shortfin mako (Isurus oxyrhinus) and porbeagle (Lamna nasus) but about half of hooked porbeagles and makos died during or after fishing, mostly with two days after release (Campana et al., 2016).  Capture by fishing is probably more traumatic than injury alone.  But they also noted that their study could not detect delayed mortality due to altered behaviour and feeding or altered reproductive success (Campana et al., 2016).  

No evidence on healing rates in basking sharks was found.  However, photo-identification and observational studies of basking sharks regularly record injuries, scars (including lamprey scars), notches in fins, propeller injuries, ship-strikes, and marks from nets or ropes (Speedie & Johnson, 2008; Speedie et al., 2009; Solandt & Chassin, 2013; Gore et al., 2016).  These observations suggest that the basking shark can heal and recover from a range of injuries.  

Sensitivity assessment.  It is difficult to quantify the impact of collisions on the basking shark.  However, with 63 reported collisions over 21 years (Solandt & Chassin, 2013) the occurrence appears to be relatively low, although Speedie et al. (2009) suggested incidents were increasing.  Sharks can heal a wide variety of injuries quickly, particularly in the larger species.  The evidence from nurse angel sharks, white sharks, and whale sharks suggests that minor injuries in basking sharks might heal within a few months while even significant (but not fatal) injuries might heal within a year.  However, there is no evidence of delayed or long-term effects on feeding and reproductive success.  Mortalities may go un-noticed if the affected individual sank to the seabed but no documented evidence of mortalities was found.  Therefore, resistance is assessed as 'Medium' as a precaution to represent the potential for some mortality but with 'Low' confidence due to the lack of direct evidence. Hence, resilience is assessed as 'Medium' (2-10 yrs), and sensitivity as 'Medium'. 

Visual disturbance

Benchmark. The daily duration of transient visual cues exceeds 10% of the period of site occupancy by the feature. Further detail

Evidence

The basking shark is thought to be relatively tolerant of visual presence (Compagno, 1984; Speedie & Johnson, 2008).  They have very small eyes in proportion to their body (Shark Trust, 2010), suggesting that vision is not a key sensory mechanism in this species.  However, if disturbed by boats, individuals have been reported to dive and move away from the area (Bloomfield & Solandt, 2006).  They have also been reported to be relatively unaware of surface vessels (Speedie & Johnson, 2008).  Basking sharks killed by the prior harpoon fishery were shot at very close range and they generally show little reaction to being tagged.  Speedie & Johnson (2008) noted that slow-moving vessels elicit hardly any response when groups of basking shark are feeding.  The Marine Conservation Society (MCS) received accounts of behavioural changes in the basking shark, from experienced wildlife observers in the Isle of Man.  In the accounts, courtship-like behaviour appeared to be disturbed by an approaching motorised craft, at a range of 1 km (Bloomfield & Solandt, 2006).  On the other hand, at Gwennap Head, Seawatch Southwest wildlife observers (in 2007) began to see a change in behaviour when vessels came within 10 m of individuals (Bloomfield & Solandt, 2006).  Observations from Gwennap Head by Seawatch Southwest wildlife observers in 2007 reported that the sharks only showed altered behaviour when vessels approached very close to them (within 10 m) (see ‘Noise’ above).  The impact of visual disturbance may be magnified in tourist areas where disturbance by boat traffic and marine tourism activities is more frequent (Speedie & Johnson, 2008). 

Sensitivity assessment.  Visual disturbance appears to elicit a variety of reactions in the basking shark.  Different outcomes documented include diving and moving away from the disturbance, disruption of courtship-like behaviour (Bloomfield & Solandt, 2008) or staying relatively undisturbed (Compagno, 1984; Speedie et al., 2009).  However, the response of basking shark to boats may be due to either their noise or visual disturbance (see ‘Noise’). If visual disturbance or noise from passing vessels altered behaviour it would likely experience some energy-loss at most. Therefore, resistance has been assessed as 'High', resilience as 'High' (by default), and sensitivity is assessed as 'Not sensitive'.

Genetic modification & translocation of indigenous species

Benchmark. Translocation of indigenous species or the introduction of genetically modified or genetically different populations of indigenous species that may result in changes in the genetic structure of local populations, hybridization, or change in community structure. Further detail

Evidence

Introduction or spread of invasive non-indigenous species

Benchmark. The introduction of one or more invasive non-indigenous species (INIS). Further detail

Evidence

There were no reports of direct impacts of invasive species on the success of Cetorhinus maximus.  However, there is the potential for indirect threats caused by invasive species lower down in the food chain. As a zooplanktivore, any invasive species impacting on the zooplankton assemblage is likely to indirectly impact the basking shark by altering food availability. An example is Mnemiopsis leidyi, an invasive ctenophore native to North and South America feeds on zooplankton (Colin et al., 2010). This species has already been found in the Mediterranean, Baltic and North Seas, however little is known about its future impacts.  

Sensitivity assessment. There is currently no evidence to suggest that the basking shark is affected by the introduction of invasive species. However, this may require re-evaluation as more information becomes available. 

Introduction of microbial pathogens

Benchmark. The introduction of relevant microbial pathogens or metazoan disease vectors to an area where they are currently not present (e.g. Martelia refringens and Bonamia, Avian influenza virus, viral Haemorrhagic Septicaemia virus). Further detail

Evidence

The first account of disease in the basking shark was reported by Dagleish et al. (2010) based on the post-mortem of a juvenile male found on Musselburgh beach, East Lothian, UK, in October 2007.  Pyogranulomatous meningoencephalitis was found alongside multifocal, myocarditis (damage and inflammation of heart muscle) with myocyte necrosis (muscle cell death), oedema (fluid build-up in body cavities) and haemorrhage.  The exact cause of the disease was not found.  However, the evidence suggested an infectious origin (possibly caused by bacteria) (Dagleish et al., 2010).  The study could not conclude whether the meningoencephalitis was the cause of repeated live-stranding and subsequent death of the shark.  The specimen was found in south-east Scotland, where it is uncommon, which was likely due to the impact of the disease on navigation (Dagleish et al., 2010).  In addition, 27 basking sharks were reported to the UK Cetacean Stranding Investigation Programme (CSIP)  between 2005 and 2010 (Deaville & Jepson, 2010); 14 were found stranded in England, 12 in Scotland and one in Wales.  Of the 27 reported basking shark strandings, three were investigated at post mortem (two in Scotland and one in England. Of these, one was found to have died as a consequence of live-stranding, one from a generalised bacterial infection and one from a meningoencephalitis (Deaville & Jepson, 2010).  

There have been multiple accounts of basking shark associated parasites.  Matthews & Parker (1950) reported the presence of three types of parasitic copepod Dinematura producta (now Dinemoura producta), Caligus rapax and Nemesis lamna on Cetorhinus maximus.  These were found on the surface of the skin and the gills.  The effects of the copepods attachment varied from minor skin erosion (Dinemoura producta) to extensive gill damage (Nemesis lamna).  Lampreys are also commonly found attached to basking sharks with little apparent damage (Matthews & Parker, 1950).  Further, there have been accounts of blood flukes (Hyperandrotrema cetorhini) or endoparasitic flat worms, found in the heart of Cetorhinus maximus (Orélis-Ribeiro et al., 2013).  Despite little information about their impacts on basking sharks, they have been known to cause inflammation and a decrease in the physiological and mechanical efficiency of the infected organs in other fishes (Bullard & Overstreet, 2002). 

Sensitivity assessment.  Most parasites found on the basking shark are seemingly benign, except for blood flukes which are suspected to cause inflammation in the infected organ (Bullard & Overstreet, 2002).  Individuals probably live with a number of parasites throughout their life with limited effect on their viability, which suggests a ‘Low’ sensitivity.  But, the natural mortality of basking sharks is unknown (Sims, 2008).  The reports by Dagleish et al. (2010) and Deaville & Jepson (2010) suggest that once infected, an individual basking shark may experience deteriorating health and death.  Therefore, resistance is assessed as 'Medium' to represent the potential for 'some mortality' due to disease in basking shark, but with 'Low' confidence due to the limited evidence.  Hence, resilience is assessed as 'Medium' and sensitivity as 'Medium'. 

Removal of target species

Benchmark. Removal of species targeted by fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

Historically, the basking shark has been fished throughout its range, most commonly by harpoon (for reviews of fishing methods see: Kunzlik, 1988; Fairfax, 1998).  Exploitation by fisheries (for its valuable liver oil and fins) in the North East Atlantic began in the 1700s in Norwegian, Scottish and Irish waters, and ended in the mid-1800s after a decline in basking shark abundance.  In this period, landings were as high as 1000 individuals per year in Irish waters (ICES, 2016).  The Norwegian fishery restarted in 1920, later to be joined by the Scottish (the 1940s) and the Irish (1947).  The Norwegians dominated the market by taking between 1266 and 4266 basking sharks per year (from the years 1959-1980), compared with lower numbers in Scottish (total estimate of 970 individuals, from the years 1946-1953) and Irish waters (average of 1475 individuals per year, from the years 1951-1955).  Although the extent and scale of these fisheries are not well recorded, in the 51 years between 1946 and 1997, at least 105,730 sharks (mainly females) were likely to have been captured in the North East Atlantic (Sims, 2008) with peak landings (5266 metric tonnes) observed in 1979 (ICES, 2016).  Most basking shark fisheries reported declines in landings before they were terminated (Sims et al., 2015). 

North East Atlantic fisheries experienced a large decline in basking shark total catch with a total of 3680 t in 1977, compared with only 119 t in 2000 (ICES, 2016).  Sims et al. (2015) stated that the overall result of fishery efforts was thought to have reduced the basking shark population to less than half of its original size over the previous three generation spans (>100 years).  In 2001, Norway reduced its basking shark landing quota to zero tonnes (Sims et al., 2015), and in 2006 ICES advised a zero total allowable catch (TAC) placing the basking shark on the Prohibited Species List.  It was also recommended that by-catch should be minimized (ICES, 2016).  In addition, EU legislation prohibits Union fishing vessels from fishing basking sharks in all waters under Article 13 of the Council Regulation 2016/72 (ICES, 2016).  This ban continues, however, dead or dying incidentally caught basking sharks can be landed, but must be reported.  In the UK, the basking shark has been protected since 1998 (ICES, 2016). 

It is not known whether the basking shark population has recovered since protective measures were initiated (Sims, 2008), however, there is some evidence for improvement.  Public sighting schemes in some locations (e.g. Irish waters) have seen an increase in total abundance, but whether this reflects the basking shark population or an increase in public sightings efforts is unknown (Sims et al., 2015).  In addition, though many countries have banned the targeted removal of this species, the increased demand for shark fins due to human consumption likely puts basking sharks at risk in less regulated areas (Sims, 2008) especially since its fins are amongst the most lucrative on the international market (Fowler, 2009). 

Sensitivity assessment. Although direct fishing of basking shark is illegal in UK waters, fisheries is a potential threat in other areas. Sims et al. (2015) estimated that the overall result of past fishery efforts reduced the basking shark population to less than half of its original size over less than 100 years. Despite some signs of improvement (e.g. increases in public sightings in Irish waters), with a generation time of ca 34 years, the basking shark is unlikely to have fully recovered from this loss. Therefore, resistance is assessed as 'Low', resilience as 'Low' and sensitivity is assessed as 'High'. 

Removal of non-target species

Benchmark. Removal of features or incidental non-targeted catch (by-catch) through targeted fishery, shellfishery or harvesting at a commercial or recreational scale. Further detail

Evidence

The basking shark was reported to be a victim of entanglement in fishing gear (e.g. trawls, longlines, prawn and cod traps) in the Pacific Canadian waters (DFO, 2009; McFarlane et al., 2009).  In the North East Atlantic, there are anecdotal reports of the basking shark being incidentally caught in gillnet and trawl fishing gear (ICES, 2016).  In 1993, 28 records of basking sharks entangled in fishing gear were reported in the Irish Sea (Berrow, 1994; Berrow & Heardman, 1994) and at least 22% of the sharks died as a result of the entanglement.  Furthermore, the Marine Conservation Society (MCS) reported 63 sharks suffering from ship strike or entanglement in fishing gear between 1992 and 2013 (Solandt & Chassin, 2013).  Entanglement in ropes and nets was reported from Scotland and south-west England (Bloomfield & Solandt, 2006).  Basking sharks are also accidentally caught by towed gear (Francis & Duffy, 2002).  Small numbers (130 individuals over 21 years) of incidentally caught basking sharks continue to be reported in the UK (Witt et al., 2012).  As a result of the zero total allowable catch (TAC) and the requirement of the EU fishing industry to discard all incidentally caught basking sharks, there is little recorded information about these incidents.  It is also difficult to quantify the impacts (ICES, 2016).  Although the impact of accidental removal by fisheries and entanglement on populations is not quantified, fishing gear poses a threat to individuals of this species and the population as a whole. Finally, the high value of shark fins to the Asian market may result in basking sharks that are found alive being killed instead of released (Bloomfield & Solandt, 2006),  although there are currently no records of the practice taking place in the UK (Bloomfield & Solandt, 2008).

Sensitivity assessment. Although the impact of accidental removal by fisheries and discarded (ghost) fishing gear on Cetorhinus maximus populations cannot be quantified, fishing gear poses a threat to this species.  A threat that is presumably still present, with small numbers (130 individuals over 21 years) of incidentally caught basking reported in the UK and at least 22% mortality due to entanglement in the Irish Sea (Berrow & Heardman, 1994; Witt et al., 2012). Therefore, resistance is assessed as 'Medium', resilience as 'Medium', and sensitivity is assessed as 'Medium'.