Resource degradation: a subtle effect of bottom fishing

Populations of hermit crabs are critically limitedby the availability of suitable gastropod shells thatthey utilise to reduce their risk of predation and environmentalstress. Common whelks are the main sourceof shells for large hermit crabs in the northern Atlanticbut are vulnerable to direct and indirect effects of fishingactivity. This study examined the potential consequencesof degrading shell resources for common hermit crabs.Laboratory trials demonstrated that hermit crabs avoidlow-quality damaged shells throughout their life history.This laboratory preference was corroborated by directfield observations of shells preferentially occupied byhermit crabs, compared with shells available for occupation.In the field, 8 times as many empty shells hadholes compared to shells occupied by hermit crabs. Inthe North Sea, the abundance and biomass of livewhelks and hermit crabs collected at sites where they cooccurredwere significantly related. However, whelksoccurred at far fewer sites overall and were morepatchily distributed at high abundance than hermitcrabs, which were more widespread. At a subset of sites,whelks of the same body-mass range occurred in theIrish and North Sea. However, at these sites, hermitcrabs sampled from the North Sea had a significantlylower biomass. This suggests that the shells available foroccupation at the North Sea sites would not supportcrabs of a body mass comparable to that found in theIrish Sea. Using published data, we calculated that insome of the intensively fished areas of the North Sea,24% of the available shell resource will be damaged eachyear. The reduction in shell quality in the North Sea mayimpose a physical constraint on the upper size limitcurrently attainable by hermit crabs and hence may haveimplications for population viability