Effects of soak-time and spatial heterogeneity on sampling populations of spanner crabs Ranina ranina

Abstract

A series of field experiments was done in spanner crab Ranina ranina (Linnaeus) fishing grounds to determine the effects on catch per unit effort (CPUE) of crabs due to soak-time and the species' spatial heterogeneity. Standard and optimally-designed baited tangle-traps were used in a nested experimental design to catch crabs on gear set in place for various periods of time. To determine the degree of spatial and short-term temporal variability in CPUE, replicate sets of traps were set at different locations, depths and times of day. Cost-benefit analyses of this experiment yielded optimal numbers of traps and sets of traps to be used at each location and depth in subsequent sampling. The minimum time one should leave traps in the fishing grounds to achieve maximal CPUE was determined as 60 min. Different locations and depths yielded quite different CPUE's indicating that future surveys should encompass several locations and depths. Time of day had no significant effects on CPUE. Costbenefit analyses showed that 5 traps on each of 3 sets should be used at each location and depth to optimize CPUE given the limited time availabe to survey a given place. The consequences of this replication on the sizes of standard errors in future sampling were estimated. A uniform and optimal methodology is developed from the results dscussed in this and a previous paper whlch wdl be used in subsequent censussing of this species' distnbutions and abundances. Finally I discuss the worth of manipulative experimentation to test specific hypotheses about species which are sampled using baited traps and note the value of cost-benefit analyses of pilot s tud~es in designing surveys of such species' populations.