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,
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.