The Marine Life Information Network

Substratum loss

Benchmark. All of the substratum occupied by the species or biotope under consideration is removed. A single event is assumed for sensitivity assessment. Once the activity or event has stopped (or between regular events) suitable substratum remains or is deposited. Species or community recovery assumes that the substratum within the habitat preferences of the original species or community is present. Further details

Evidence

As Dipturus batis is highly motile, it would move away from an area that lost a suitable substratum and return when the area was back to normal. A certain amount of stress maybe caused by loss of food items with the substratum and because of the need to find new foraging/spawning areas. Therefore an intolerance of low has been recorded. Recoverability is likely to be high, resulting in a sensitivity assessment of low. Substrate removal is likely to destroy egg cases, but the location of important breeding and nursery grounds is poorly understood.

Smothering

Benchmark. All of the population of a species or an area of a biotope is smothered by sediment to a depth of 5 cm above the substratum for one month. Impermeable materials, such as concrete, oil, or tar, are likely to have a greater effect. Further details.

Evidence

Dipturus batis would move away from an area that was being smothered but with some stress due to loss of food and energetic costs of migrating to new foraging areas. Therefore an intolerance of low has been recorded. Recoverability is likely to be high, resulting in a sensitivity assessment of low. Egg cases on the sea bed are likely to be more sensitive.

Increase in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

It is not known whether an increase in suspended sediment will have an effect on Dipturus batis. Not relevant has been recorded because the skate is mobile enough to avoid local adverse effects.

Decrease in suspended sediment

Benchmark. An arbitrary short-term, acute change in background suspended sediment concentration e.g., a change of 100 mg/l for one month. The resultant light attenuation effects are addressed under turbidity, and the effects of rapid settling out of suspended sediment are addressed under smothering. Further details

Evidence

It is not known whether an decrease in suspended sediment will have an effect on Dipturus batis. Not relevant has been recorded because the skate is mobile enough to avoid local adverse effects.

Desiccation

1. A normally subtidal, demersal or pelagic species including intertidal migratory or under-boulder species is continuously exposed to air and sunshine for one hour.
2. A normally intertidal species or community is exposed to a change in desiccation equivalent to a change in position of one vertical biological zone on the shore, e.g., from upper eulittoral to the mid eulittoral or from sublittoral fringe to lower eulittoral for a period of one year. Further details.

Evidence

Dipturus batis is a sublittoral species unlikely to be subject to exposure to air. Therefore desiccation is not relevant.

Increase in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Dipturus batis is a sublittoral species unlikely to be subject to exposure to air. Therefore an increase in emergence is not relevant.

Decrease in emergence regime

Benchmark. A one hour change in the time covered or not covered by the sea for a period of one year. Further details

Evidence

Dipturus batis is a sublittoral species unlikely to be subject to exposure to air. Therefore a decrease in emergence is not relevant.

Increase in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Dipturus batis has been recorded from sites around the UK with varying hydrodynamic conditions (see adult distribution) and therefore is unlikely to be affected by changes in flow rate (JNCC, 1999).

Decrease in water flow rate

A change of two categories in water flow rate (view glossary) for 1 year, for example, from moderately strong (1-3 knots) to very weak (negligible). Further details

Evidence

Dipturus batis has been recorded from sites around the UK with varying hydrodynamic conditions (see adult distribution) and therefore is unlikely to be affected by changes in flow rate (JNCC, 1999).

Increase in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Sudden changes in temperature are unlikely to affect adults because they can move away but developing young may be affected. A study on a related species, Raja eglanteria, found that the embryos of this species do not develop at temperatures over 24 °C (Whitehead et al., 1984). As such intolerance is assessed as intermediate. Chronic changes in temperature would also have little effect as the adults experience large temperature changes when moving between deep and shallow water. In addition, the world distribution of Dipturus batis is from the coasts of north-western Africa to the North coast of Norway so it is unlikely to experience temperatures outside of its tolerance range in British and Irish waters. Therefore recoverability of adults is likely to be high, however due to a lack of information on the effects on developing young, recoverability is assessed as moderate, resulting in a moderate sensitivity rating.

Decrease in temperature

1. A short-term, acute change in temperature; e.g., a 5°C change in the temperature range for three consecutive days. This definition includes ‘short-term’ thermal discharges.
2. A long-term, chronic change in temperature; e.g. a 2°C change in the temperature range for a year. This definition includes ‘long term’ thermal discharges.

For intertidal species or communities, the range of temperatures includes the air temperature regime for that species or community. Further details

Evidence

Sudden changes in temperature are unlikely to affect adults because they can move away but developing young may be affected. As such intolerance is assessed as intermediate. Chronic changes in temperature would have little effect as the adults experience large temperature changes when moving between deep and shallow water. In addition, the world distribution of Dipturus batis is from the coasts of north-western Africa to the North coast of Norway so it is unlikely to experience temperatures outside of its tolerance range in British and Irish waters. Therefore recoverability of adults is likely to be high, however due to a deficit of information on the effects on developing young, recoverability is assessed as moderate, resulting in a moderate sensitivity rating.

Increase in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

An increase in turbidity could potentially interfere with foraging by inhibiting visual location of prey. However, Dipturus batis is a bottom feeder that is probably adapted to murky, silty water and utilize smell and electromagnetic cues to locate prey. Therefore the species is considered tolerant, and not sensitive has been recorded.

Decrease in turbidity

1. A short-term, acute change; e.g., two categories of the water clarity scale (see glossary) for one month, such as from medium to extreme turbidity.
2. A long-term, chronic change; e.g., one category of the water clarity scale (see glossary) for one year, such as from low to medium turbidity. Further details

Evidence

A decrease in turbidity may aid predators. However, since man is the main threat to Dipturus batis, a decrease in turbidity is unlikely to increase the predation rate on this species. A decrease in turbidity may however, influence foraging success, either because prey gain an earlier warning of the skates' approach or because the skates' visual acquisition of prey is improved. Therefore the species is considered tolerant, and not sensitive has been recorded.

Increase in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Dipturus batis has been recorded at sites all over the UK which vary from very sheltered to very exposed (see adult distribution) and anyway can swim to deeper areas if wave action increases to the extent that oscillatory movements on the seabed become excessive. As a result Dipturus batis is unlikely to be affected by an increase or decrease in wave exposure, so is considered tolerant, and not sensitive has been recorded.

Decrease in wave exposure

A change of two ranks on the wave exposure scale (view glossary) e.g., from Exposed to Extremely exposed for a period of one year. Further details

Evidence

Dipturus batis has been recorded at sites all over the UK which vary from very sheltered to very exposed (see adult distribution). Therefore the species is unlikely to be affected by an increase or decrease in wave exposure, so is considered tolerant, and not sensitive has been recorded.

Noise

1. Underwater noise levels e.g., the regular passing of a 30-metre trawler at 100 metres or a working cutter-suction transfer dredge at 100 metres for one month during important feeding or breeding periods.
2. Atmospheric noise levels e.g., the regular passing of a Boeing 737 passenger jet 300 metres overhead for one month during important feeding or breeding periods. Further details

Evidence

Fish with swimbladders have been reported to be the most sensitive to noise (Vella et al., 2001). Dipturus batis is an elasmobranch and therefore does not have a swimbladder so is among the fish that are less sensitive to noise. However, sudden loud noises of low frequency have been shown to elicit an avoidance response in most fish (Vella et al. 2001). Noise rarely has a physiological affect on fish so Dipturus batis has been deemed tolerant, and therefore not sensitive to noise.

Visual presence

Benchmark. The continuous presence for one month of moving objects not naturally found in the marine environment (e.g., boats, machinery, and humans) within the visual envelope of the species or community under consideration. Further details

Evidence

Adult Dipturus batis can be found at depths from the shallow sublittoral down to 600 m so are unlikely to be disturbed by boats or divers, although divers might disturb young skate in shallow water. Recoverability is likely to be immediate, however, since the skate can swim away from the disturbance and return when it has gone. Therefore an intolerance of low has been recorded, and the species is deemed not sensitive.

Abrasion & physical disturbance

Benchmark. Force equivalent to a standard scallop dredge landing on or being dragged across the organism. A single event is assumed for assessment. This factor includes mechanical interference, crushing, physical blows against, or rubbing and erosion of the organism or habitat of interest. Where trampling is relevant, the evidence and trampling intensity will be reported in the rationale. Further details.

Evidence

Dipturus batis has a high resilience when trawled or caught by rod and line and then released again (Little, 1995). Therefore adults are probably tolerant of abrasion and physical disturbance at the benchmark level. Because of the shape of rays, they cannot escape trawl nets once they have been captured. A newborn skate is about 22 cm long and almost as wide, therefore is unable to pass through the mesh of fishing nets. Because of their small size there is a greater chance a juvenile skate will be damaged in a net than an adult skate. This could lead to high mortality/stress in the juveniles and affect the processes maintaining the population. Therefore an intolerance of intermediate has been recorded. Recoverability is probably moderate (see information below) hence sensitivity is assessed as moderate.

Displacement

Benchmark. Removal of the organism from the substratum and displacement from its original position onto a suitable substratum. A single event is assumed for assessment. Further details

Evidence

Dipturus batis has a high resilience when trawled or caught by rod and line and then released again (Little, 1995). In addition, this species is found on a variety of substrata all around the UK and therefore is probably quite tolerant of displacement. Recoverability is likely to be high, resulting in a low sensitivity recording.

Synthetic compound contamination

Sensitivity is assessed against the available evidence for the effects of contaminants on the species (or closely related species at low confidence) or community of interest. For example:

• evidence of mass mortality of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as high sensitivity;
• evidence of reduced abundance, or extent of a population of the species or community of interest (either short or long term) in response to a contaminant will be ranked as intermediate sensitivity;
• evidence of sub-lethal effects or reduced reproductive potential of a population of the species or community of interest will be assessed as low sensitivity.

The evidence used is stated in the rationale. Where the assessment can be based on a known activity then this is stated. The tolerance to contaminants of species of interest will be included in the rationale when available; together with relevant supporting material. Further details.

Evidence

No information found.

Heavy metal contamination

Evidence

General information on the tolerance of fish to metal contamination reveals that part per billion concentrations are not lethal but may reduce gill activity, growth and hatching success of eggs. Copper was reported to be the most toxic of metals and suppressed egg hatching at concentrations of 10 parts per billion in certain teleost fish (Bryan, 1984). The leathery egg case of rays may make them less susceptible to metal contamination. However, in the absence of evidence on the effects in Dipturus batis no assessment can be made.

Hydrocarbon contamination

Evidence

No information found.

Radionuclide contamination

Evidence

No information found.

Changes in nutrient levels

Evidence

No information was found on the effect of nutrient enrichment or algal blooms was found.

Increase in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

As with many of the other factors, the high motility of this species allows it to escape adverse changes in salinity. Therefore intolerance is recorded as low and recoverability is likely to be high, resulting in a sensitivity assessment of low.

Decrease in salinity

1. A short-term, acute change; e.g., a change of two categories from the MNCR salinity scale for one week (view glossary) such as from full to reduced.
2. A long-term, chronic change; e.g., a change of one category from the MNCR salinity scale for one year (view glossary) such as from reduced to low. Further details.

Evidence

As with many of the other factors, the high motility of this species allows it to escape adverse changes in salinity. Therefore intolerance is recorded as low and recoverability is likely to be high, resulting in a sensitivity assessment of low.

Changes in oxygenation

Benchmark.  Exposure to a dissolved oxygen concentration of 2 mg/l for one week. Further details.

Evidence

No information was found on the effects of hypoxia on Dipturus batis.

Introduction of microbial pathogens/parasites

Benchmark. Sensitivity can only be assessed relative to a known, named disease, likely to cause partial loss of a species population or community. Further details.

Evidence

No information found.

Introduction of non-native species

Sensitivity assessed against the likely effect of the introduction of alien or non-native species in Britain or Ireland. Further details.

Evidence

No non-native species are known to compete with the common skate.

Extraction of this species

Benchmark. Extraction removes 50% of the species or community from the area under consideration. Sensitivity will be assessed as 'intermediate'. The habitat remains intact or recovers rapidly. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

The slow growth rate, late maturity and low fecundity make Dipturus batis vulnerable to overfishing and it has disappeared from much of its former range due to fishing pressure (Brander; 1981; Walker & Hislop, 1998; Jennings et al. 1999; Rogers & Ellis, 2000). Only about 40 eggs are laid every other year and each generation takes 11 years to reach maturity, therefore populations cannot recover quickly from large mortalities. It has been estimated that a mortality of greater than 38% per year will lead to continual decline in the population and recovery is unlikely to occur until mortality is relaxed (Walker & Hislop, 1998). Numbers of common skate caught in trawls began to decline in the 1920s and again in the 1950s after a recovery period during the second world war and disappeared from the North Sea between the mid 1950s and early 1980s (Walker & Hislop, 1998). Dipturus batis was recorded as 'not uncommon in trawls' in the Marine fauna of the Isle of Man (Bruce et al., 1963) and was regarded as a common species by Hureau & Monod (1979). However the common skate had become all but extinct by the late 1970s due to overfishing (Brander, 1981). Similarly, between 1901 and 1907, the common skate made up 4% of all elasmobranchs caught in trawls in southwest England but between 1989 and 1997 none were caught (Rogers & Ellis, 2000). Therefore an intolerance of high and a recoverability of low has been recorded. The species is highly sensitive to this factor.

Extraction of other species

Benchmark. A species that is a required host or prey for the species under consideration (and assuming that no alternative host exists) or a keystone species in a biotope is removed. Any effects of the extraction process on the habitat itself are addressed under other factors, e.g. displacement, abrasion and physical disturbance, and substratum loss. Further details.

Evidence

Dipturus batis feeds on flatfish and some of the larger individuals take cod, haddock and herring (Dipper, 2001), and fishing pressure on these species may affect the skates food supply. Therefore an intolerance of low has been recorded. Recovery is dependant on the recovery of prey stocks, for which insufficient information has been found to make an assessment. Hence a moderate sensitivity has been recorded.