Capture efficiency and size selectivity of hydraulic clam dredges used in fishing for ocean quahogs ( Arctica islandica ): simultaneous estimation in the SELECT model
AbstractThorarinsdóttir, G. G., Jacobson, L., Ragnarsson, S. Á., Garcia, E. G., and Gunnarsson, K. 2010. Capture efficiency and size selectivity of hydraulic clam dredges used in fishing for ocean quahogs ( Arctica islandica ): simultaneous estimation in the SELECT model. – ICES Journal of Marine Science, 67: 345–354. Estimates of capture efficiency and size selectivity for commercial dredges are important in estimating stock biomass and setting catch limits for the ocean quahog off Iceland and the United States. Ocean quahogs are long-lived, slow-growing, and sensitive to overharvest resulting from poor estimates of capture efficiency and stock biomass. Capture efficiency and size selectivity were estimated simultaneously in mixed-effect SELECT models using diver and commercial dredge experiment data from the shallow-water habitats off Iceland. Estimated capture efficiency for the commercial dredge E = 92% ( CV = 8%) was high for large (107.5 mm shell length (SL)) ocean quahogs. Size selectivity followed an ascending logistic curve, with L 50 = 70.5 mm SL ( CV 4%), a selectivity range of 17.6 mm SL, and substantial variability among experimental dredge tows. Estimated capture efficiency was higher than that for ocean quahogs in US waters, possibly because of the deep habitats off the United States or uncertainty about dredge position during US depletion experiments. Scaling maximum selectivity from the SELECT model to one reduces correlations between size-selectivity and capture-efficiency parameters and may enhance the utility of selectivity estimates in stock assessment modelling. Our experimental and modelling approach may be useful for studies with other non-mobile benthic species.