Cockles are generally associated with the intertidal
but can be found in the subtidal. Most studies on
the effects of dredging for cockles (Cerastoderma
edule) have examined the use of mechanical dredges
which use a plough-like blade to remove sediment
from which the cockles are subsequently sorted.
Tractor harvesting - Studies have shown that tractor-towed
harvesters leave vehicle tracks as well as dredging
furrows which remain visible for varying amounts
of time depending on the conditions at the site5.
In an area of stable sediment (poorly sorted fine
sand) dredge tracks may be visible for long periods
(more than 6 months have been recorded) whereas
in more mobile sediments there may be no alteration
in sediment parameters6. On areas of
cohesive sediment the tracks appeared to act as
lines from which erosion of the surface layer spread
out. This appeared to accelerate the erosion phase
of a natural cycle of cohesion of the surface sediment
by worm tube mats62. Dredged areas often
had a lot more dead shell scattered on the surface,
an effect which can persist for several months.
In undisturbed beds, most dead shell is normally
under the surface which can create a shell layer
limiting the depth to which small drainage channels
can normally erode into a cockle flat62.
The effect on infauna also depends on the exposure
of the site6,18,36. Research to date
suggests that in an area of stable sediments, as
well as large reductions in the target species,
mechanical dredging can result in a significant
decline in numbers of the lower spire shell (Hydrobia
ulvae) and decreased numbers of Pygospio
elegans, a segmented worm whose tubes may be
removed by the dredge6,18. These effects
may still be apparent 6 months later6.
The sand mason worm (Lanice conchilega),
on the other hand, has more robust tubes and can
retract below the depth disturbed by the dredge18,
62 and although the distribution of white
ragworm (Nephtys hombergii) was affected
by dredging, populations have been shown to recover
within six months6.
In Scotland there is a general prohibition on dredging
for cockles from or by means of any vehicle, eg.
tractor dredging. This was specifically adopted
following concerns as to the direct effects of large
scale dredging operations on cockle stocks and the
indirect effects to the intertidal habitat15,36,93.
In England and Wales some Sea Fisheries Committees
have in place byelaws which specify the design or
type of equipment that can be used to target cockles.
In this way, tractor dredging has been prohibited
in many areas.
Suction dredging - Suction dredgers or hydraulic
continuous lift dredgers - to be more accurate -
are deployed from specially adapted or specially
built shallow draft vessels and are used to harvest
cockles in the Wash and Thames in particular. Depending
on the stability of the sediment surface at the
time and the prevailing tide or wind conditions,
evidence of the tracks left by the dredge head,
can persist for several months62. Where
dredging was carried out in a sheltered area with
eel grass (Zostera) beds, (Auchencairn Bay,
Solway Firth), breaking the sward allowed erosion
that produced clearly visible grooves down the shore62.
The immediate effect of hydraulic dredging on the
infauna can be significant. Studies have shown up
to 30% reductions in the number of species and 50%
reduction in number of individuals. Comparison between
dredged and undredged areas have shown recovery
times varying from 14-56 days 93.
In general the overall decrease in biomass of target
species and non-target species is likely to be more
pronounced in areas with stable environmental conditions
and diverse communities. In sites with moderately
mobile sediments it is possible for natural disturbances
to have a greater effect than dredging6, 77.
Sites with more tube dwelling and sedentary species
appear to take longer to recover to pre-fishing
levels than areas with more mobile fauna.
The time of year of exploitation will also influence
recovery36. Avoiding dredging during
periods of larval settlement or spawning for example,
can reduce time required for the restoration of
infaunal communities. The sediment may change, at
least in the short term, but how long this remains
the case also depends on the exposure and stability
of the site.
Effects on birds are varied. In some cases short-term
increases of gulls and waders in the harvesting
area, followed by a long term significant reduction
in feeding opportunities for these birds=
has been noted5. In contrast, research
linked to the Solway fishery concluded that because
natural changes are very large the fishery may not
have a significant effect on bird numbers unless
a high proportion of the cockles are harvested62.
A simulation model tested on the Exe estuary has
been developed to explore the consequences of changes
in fishing activities and bird numbers on commercial
shellfish stocks and on the birds themselves63.
Key predictions include that where a number of conditions
apply it is possible to exploit shellfish stocks
without increasing the winter mortality of shorebirds,
that the effects of a given intensity of shellfishing
depend crucially on local conditions of the climate
and general abundance of food and that as fishing
effort increases, shorebird mortality may be hardly
affected initially but then may suddenly increase
dramatically once a threshold level of fishing effort
has been reached63.
Hand gathering - Hand gathering for cockles is
the only permitted form of cockle fishery in some
areas. No information was found on the effects of
large scale hand gathering for cockles. However,
disturbance to feeding and roosting birds, which
is a concern in relation to bait digging on intertidal
flats could also be an issue for cockle gathering
from intertidal areas. This issue is addressed in
more detail in the related report on collecting
bait and other shoreline animals (Fowler, S. 1999).