This thesis examines the impact of the shore crab, Carcinus maenas, on
commercial mussel beds. The seasonal abundance of C. maenas was determined
in the Menai Strait, where densities varied from around 0.2 crabs m¯² in winter to
1.5 crabs m¯² in summer. Day length showed a strong seasonal correlation with
crab abundance and may be the cue which initiates migration offshore by crabs.
Laboratory experiments showed that the number of mussels consumed by C.
maenas decreased exponentially as the size of mussels presented was increased.
The number of mussels eaten was greatest at 13 °C, decreasing at higher and lower temperatures. Catch per unit effort by the commercial shore crab fishery in the Menai Strait did not accurately reflect abundance but co-varied with temperature due to the temperature-dependence of feeding rates, which were the main influence on catches. Thus, baited trap catches are likely to be a relatively poor estimator of crab abundance.
Shore crabs foraged more efficiently in the presence of conspecifics, by foraging
optimally sized mussels, and were able to identify the most profitable patches of
mussels in which to forage. This behaviour presumably influences the distribution
of crabs in the wild. Mixing larger mussels, above the size most often selected by
crabs, with smaller, more vulnerable, mussels may help to reduce losses by
increasing foraging time.
The biodiversity of mussel assemblages was examined in the commercial mussel
beds in the Menai Strait and in natural and cultivated mussels in Maine, USA.
Rope-grown mussels possessed a significantly greater biomass of associated
macrofauna than cultivated subtidal mussels or natural intertidal mussels. There
were significantly fewer individuals associated with subtidal mussels than with
intertidal or rope-grown mussels in Maine. Twenty-seven taxa were found among
cultivated intertidal mussels in the Menai Strait, while 15 taxa were associated
with naturally occurring intertidal mussels in Maine.
A predation model is presented that is used to estimate mussels lost to crab
predation during the cultivation process. This model can also potentially be
applied to other areas of mussel cultivation subject to shore crab predation. The
model may also be more generally applicable to other bivalve species subject to
crab predation, although further work will be necessary to determine appropriate
coefficients for other species.
It is concluded that Carcinus maenas is a major predator of mussels during the
three year cultivation process, consuming approximately 10% of the mussels relaid to the Menai Strait. The main factor influencing the numbers of mussels lost to crab predation is the growth of mussels; thus most effort to reduce the impact
of crabs should be expended on protecting mussels during the first year of
cultivation.