An experimental assessment of the impact of gear saturation on an abundance index for an exploited rock lobster resource

Increased fishing effort may downwardly bias indices of abundance, such as catch-per-unit-effort (CPUE), due to gear saturation. A field experiment was developed to assess gear-saturation effects on an index of relative abundance used for stock-assessments of rock lobster, Palinurus gilchristi, off South Africa. A progressive decrease in the value of the index, which is derived from commercial trap catch rates after accounting for the influences of vessel characteristics, trap soak-times, depth, area and month, has led to cuts in the total allowable catch (TAC) over the past 10 years. Concerns have been raised, however, that recent steep increases in fishing effort might have led to gear saturation effects, so that the index may now underestimate abundance relative to periods of lower effort, and hence the resource may not have declined to the extent that the index suggests. In the experiment, the fishing grounds were subdivided into the Agulhas Bank and Eastern fishing grounds, and both areas were subjected to three 2-month treatments of either unrestricted fishing effort or reduced fishing effort. In both areas, fishing in the first 2 months was unrestricted. In the second 2-month period, the number of trap-days was reduced by 25–30%, whereas in the third 2-month period this number was further reduced on the Agulhas Bank (by 65%), but increased by 25% on the Eastern fishing grounds. In terms of the saturation hypothesis, CPUE was predicted to rise during periods when effort was reduced. Changes in CPUE were assessed by expressing CPUE during periods of reduced effort as a proportion of CPUE during periods of unrestricted effort. These proportions were compared to proportions for equivalent time periods in previous years (1991–1998) when fishing effort was always unrestricted. The null hypothesis that CPUE is unaffected by effort (i.e. no gear-saturation effect) was rejected in only one of four treatments. In the other three treatments, the experiment failed to separate responses to effort-manipulations from those due to seasonal and areal fluctuations in catchability. However, a model that integrates the results of all four treatments suggests that, if GLM-standardized effort is used as the index of potential saturation, there is only a 2.5% probability that saturation could account for as much as 0.004 of the drop in CPUE from 0.15 to 0.09 kg tails/trap between 1991/1992 and 1997/1998. If trap-days instead are used as the saturation index, this 0.004 figure increases, but only to 0.03. This thus suggests with very high probability that saturation effects cannot alone explain the drop in CPUE in the fishery during the 1990s.