UW Aquatic & Fishery Sciences Quantitative Seminar
University of British Columbia
Deriving species specific abundance information form aggregated commercial catch and effort statistics
The Hawaiian bottomfish fishery is a multi-species fishery targeting 13 species associated with deep reef structures and caught using handlines. This fishery exemplifies many multi-species fisheries, where fishing gear simultaneously captures multiple species. Multi-species fisheries dependent data generally consist of species specific catch and total effort, which are used develop species specific indices of abundance. These indices are then used in single species assessment models to determine the status for each species. Using single-species assessment models with effort-aggregated data may be problematic because these data assume that the capture probabilities and targeting of each species are constant over time and therefore species-specific catches per unit effort (CPUE) indices are proportional to abundance. This assumption is likely to be violated due to changes in targeting and changes in species composition.
Total effort does not represent the effort put forth to capture each individual species. To account for this and changes in species composition effects in multi-species fisheries, I use Holling’s disc equation to partition total effort into components of handling time and time spent actively fishing. This explicitly allows for differences in catchability among target and non-target species. Stochastic simulation-estimation experiments were done, where the dynamics of individual fish populations that make up a multi-species fishery and typical fisheries dependent data (i.e., species specific catch data and aggregate effort data) were simulated. Simulation results demonstrate handling time leads to a hyperstable index of abundance which would lead to overestimates of abundance in the classic single-species assessments. Ignoring handling time effects for fisheries where a large component of effort could be taken up by gear set-up and retrieval and removal of catch from the gear, results in biased abundance estimates. Collecting information about handling time directly from the fishery could lead to improved estimates of species specific abundance.