Environmental
Damage |
Crab
Shelters
As noted in the previous section, peeler and soft shell crabs take
shelter during vulnerable moulting stages. In areas where there are
no or few natural shelters for these crabs, particularly on sediment
shores and in estuaries, anglers and commercial collectors place artificial
shelters on the shore to attract moulting crabs. These shelters may
consist of roofing tiles, field drains, or car tyres placed onto the
shore. They are either laid on top of firm sediment, or embedded at
an angle into softer muddy sediments, so that the crabs can burrow
underneath
Setting crab shelters appears to have started in the south-western
estuaries, where the mild climate provides the longest season for
collection of moulting crabs, but is now spreading all over the country.
Very few studies have been carried out of this activity, but Godden
(1995) suggested that numbers had grown from none to 8,750 traps at
Plymouth, and increased 10-fold in the Exe and Teign estuaries. A
few years later, the Tamar Estuaries Bait Collection Working Group
(1998) gives an estimate of some 20,000 crab ‘traps’ within
the Tamar Estuaries (Tamar, Plym, Lynher and Tavy). Of these, some
8,000 are used on a commercial basis with the 70% of the crab collected
being sold elsewhere in the UK.
Emplacement of crab shelters provides artificial hard substrata on
shores that are predominantly sediment. This enables the settlement
of species characteristic of rocky shores, artificially increasing
the overall biodiversity of the area. This effect is easy to monitor.
Less well understood or studied is the potential effect of placing
large numbers of tiles on the natural sediment habitat and its associated
species. For example, the presence of many structures may change patterns
of water movement over the shore and hence sediment characteristics.
Water and oxygen exchange may be reduced, fine sediments and organic
material accumulate, the surface oxygenated zone become shallower,
and infaunal species composition alter. Additionally, the presence
of many collectors on the shore, particularly in muddy areas, means
that previously undisturbed soft sediments are now regularly trampled
by collectors, and disturbed to a depth of 20-30 cm or more.
The presence of crab shelters in very muddy sediments will only directly
affect a small number of beach users: mainly individuals with moorings
or shellfish beds. However, they may be seriously inconvenienced or
endangered by large numbers of tiles and drains protruding 10-30 cm
from the sediment. Shelters in these muddy areas are also very obvious
visually, and change the appearance of the landscape considerably.
Where car tyres are used as shelters, these are even more likely to
cause problems. They are a potential obstruction to anchoring vessels,
inconvenience swimmers, walkers, and other shore users, are more visually
obtrusive than tiles, and may float away if not well anchored.
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