Razor shells

Razor shell fisheries (Ensis siliqua, Ensis ensis, Ensis arcuatus) are still in their developmental stage, particularly in England and Wales. Information on the effect of this method of fishing is limited. Razor shells occur in intertidal and subtidal habitats. Owing to their relatively deep burrowing ability, adapted hydraulic cockle dredges, which allow for deeper penetration into the substrate, are required to harvest these species.

Studies have indicated that the fishing operation initially causes substantial physical disturbance to the substrate with trenches and holes throughout the fished area (0.5 - 3.5 m wide and 0.25 - 0.6 m deep)27. The length of time these features remain depends on the sites exposure. In one study, undertaken in a relatively exposed area, fished tracks were no longer visible after a 40 day period. This included a period of stormy conditions which may have caused exceptional sediment disturbance27.

The same study showed that this fishing method can reduce the abundance of a significant proportion of the species in a fished area immediately after the fishing operation. Recovery to pre-fishing levels of non-target species was shown after 40 days. The effect on long lived bivalve species, which includes the target species, could be more serious - E.siliqua is estimated as living to 25 years27.

A comparative study of the effects on E.arcuatus showed that suction dredging directly affected the size-class structure of the population and that shells from the dredged site showed signs of damage. Animals subsequently returned to the seabed were slow to re-bury and were considered to be highly vulnerable to attack from predatory crabs79.

Migration and passive translocation may help sites return to pre-fishing levels. Local population reductions may only persist if the population or the sediments in which they live are immobile or the affected area is large relative to the remainder of the habitat so a dilution effect cannot occur. It has been suggested that neither of these conditions are likely to hold because the current technology limits the use of hydraulic dredging for razor shells to shallow water (around 7 m). This would tend to be in areas which are strongly influenced by wind and tide-induced currents and therefore with mobile sediments27. In calmer seasons the effects may persist for longer. Because of limited knowledge of the relative importance of various processes which contribute to animal movement, any cascading effect caused by the removal of razor shells on the structure of benthic communities, is unknown.

Experimental studies of the use of water jet dredgers concluded that there was little difference between the effects of this gear when compared to suction dredgers. In a sandy area swept by strong tidal flow where the gear was tested, trenches were created, there was fluidisation of sediments and although an immediate reduction in species abundance and biomass was apparent the biological effects were only considered to be short-term75.

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