Impacts on other species

Impacts on habitat

Impacts on other shore users

Only about 10% of the crab population is moulting at any one time, and therefore potentially vulnerable to collection (depending on whether the shelters on the shores are the main locations for moulting crab, or whether there are alternative shelters on sandbanks or in the sublittoral). The whole crab population will, however, pass through many moulting stages during its life cycle, making each individual vulnerable to collection several times. Additionally, mature females mate during the moult, so collection removes a reproductively active proportion of the population. The effect of their removal on recruitment of young to the population is unknown.

The Tamar Estuaries Working Group (1998) gives an estimate of an annual yield of 110,000 crabs within this complex of estuaries. Of these, commercial collection is thought to yield 90,000 crabs, and recreational anglers 20,000. The former figure is much higher because the commercial collectors are active most regularly, better at finding the hidden crabs, and maintain the shelters more effectively. Anecdotal evidence from anglers and collectors in the south-west suggests that recent increases in numbers and densities of shelters have not actually increased the yield of crabs from each estuary – it merely requires more shelters to be searched to provide the same number. There is, however, no information on the effect of crab collection inside estuaries using shelters on the local crab population, although shellfish farmers in the Teign Estuary report no change in the numbers of crabs on shellfish beds since large scale collection from crab shelters commenced (Philip Gibbons pers. comm.). It would be useful to undertake such studies in an estuary that is only just beginning to be exploited in this way.

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.

The shore crab is also a very common and important component of the shore and shallow sublittoral community. Not only does it take a wide range of prey, but it also acts as an important food source for many larger species, mainly teleost and elasmobranch fishes. The effect on prey and predators of removal of tens of thousands of adult crabs by collectors and possibly an overall decline in recruitment to the population is unknown.

Finally, the presence of large numbers of crab shelters on muddy shores means that collectors are now seen regularly in areas of soft sediment that were previously only used by feeding birds, with consequent problems of disturbance as described above as a result of bait digging in estuaries. Huggett (1995b) reports disturbance to feeding wildfowl and waders as a result of this activity.

Section 3.7.3 describes how the presence of crab shelters on the shore changes the habitat by introducing hard substrata for colonisation by rocky shore species, and changes sediment characteristics by affecting water and oxygen exchange and sedimentation rates. 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. Firm sediment beaches are used by a larger number of people for recreational purposes. Shelters (tiles and drains) are laid flat on the sediment in these situations, where they are not visually obtrusive and less likely to cause problems for other beach users.

Where car tyres are used as shelters, these are 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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