Vol. 55: 141-147, 1989 MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. Published July 27 Effects of soak-time and spatial heterogeneity on sampling populations of spanner crabs Ranina ranina Steven J. Kennelly Fisheries Research Institute, NSW Agriculture & Fisheries. PO Box 21, Cronulla. NSW 2230, Australia 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. Cost- benefit 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 stud~es in designing surveys of such species' populations. INTRODUCTION Fisheries research involving the sampling of popula- tions often employs catch per unit of effort data (CPUE) to estimate relative abundances and age/size struc- tures of exploited populations. This requires accurate estimates of both catch and effort (Collie & Sissenwine 1983; for review see Sissenwine 1984). Although data on catch are relatively simple to obtain as counts and measurements of caught animals, the effort involved in obtaining given catches is often more difficult to quan- tify and standardize. Ideally, methods should be uniform, unbiased, optimal with respect to the quantity of catch obtained, and replicated in space and time so as to allow reliable estimates of relative abundances and size structures of target species. The focus of many previous studies has been the reliability and accuracy of methods that sample abundances of commercially- important marine species (Larkins 1963, 1964, Jester 1973, Hamley 1975, Kjelson & Colby 1977, Fogarty & Borden 1980; see renew by Sissenwine et al. 1983). 3 Inter-Research/Printed In F. R. Germany There have also been several papers which have assessed baited trapping techniques as a means of estimating relative abundances of exploited popula- tions of large decapods (Thomas 1953, Sinoda & Kobay- asi 1969, Miller 1978, 1979, 1981, 1983). These papers point out several factors that may influence such CPUE data apart from the absolute abundances of crabs and lobsters. These factors include the shape of traps, the net used in traps, the bait used, competition between traps, the soak-time of traps (length of time traps are available to animals), and the position of fishing effort in space and time. An earlier paper (Kennelly & Craig 1989) considered the first 4 of these factors in develop- ing a sampling unit which could be used in subsequent sampling of the relative abundances of populations of spanner crabs Ranina ranina (Linnaeus) off the east coast of Australia. In the present paper I consider the last 2 of these factors by describing experiments which determine the best deployment of the sampling tool in large-scale, long-term surveys of R. ranina distributions and abundances. These experiments involved assess-