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Fleet selectivity, sorting behaviour, catchability and partial fishing mortality studies

Fleet based catchabilities and partial fishing mortality has been calculated through analyses of whole fishery selectivity, fishing patterns, and fleet catchability dynamics in international Baltic Sea cod fisheries - from observed spatio-temporal patterns in resource availability and fleet specific selection, relative fishing power, and fisherman sorting behaviour. This work and the results of these analyses area reported in the scientific paper listed below.

Fishery selectivity was analysed for variation when estimated as relative resource outtake by international fishing fleet on a spatio-temporal dis-aggregated scale. Coupled estimates of fleet specific relative effort allocation, gear selection pattern, relative sorting (discard) behaviour, standardized fishing power, and relative resource availability were used. To avoid bias in fleet catchability estimates (and resulting management advice) it has shown necessary to consider significant variation in those factors between fleets, areas, and periods, and to categorize accordingly. Vessels catching Baltic cod were categorized into fleets from GIS-analysis of effort and catch rate stratification also considering impor¬tance of landings resulting in spatio-temporal relative effort distribution by fleet. Modelling and ANOVA of fleet-gear selection estimated directly from comparison of research survey catch rates, reflecting resource availability, with fleet specific catch rates including discards from international observer sampling programmes revealed significant spatio-temporal diffe¬rences for important fleets. Modelling and ANOVA of fleet specific sorting behaviour estimated by comparing landings with total catch including discards by vessel and trip showed typically steep sorting ogives. Mainly fish under the minimum legal landing size was discarded. Fleet specific fishing power was estimated from GLM-ANOVA of cod-log-CPUE to obtain relative effici¬en¬cy between fleets and standardized aggregated effort. It was significantly different between fleets with highest effici¬en¬¬cy among large and medium sized otterboard-trawlers. Fisheries management and socio-economic management evaluation should consider those factors and their varia¬tion influen¬cing catchability and overall fishing pattern when evaluating effects of spatio-temporal switching behaviour of fleets in response to regulations. This is becoming especially important when calculating, predicting, and distributing fleet specific partial fishing mortality or effort in effort regulation systems.

Nielsen, J.R., Bastardie, F., Nielsen, J.N., and Pedersen, E.M.F. (2008, In revision). Whole fishery selectivity, fishing patterns, and fleet catchability dynamics in international Baltic Sea cod fisheries – from observed spatio-temporal patterns in resource availability and fleet specific selection, relative fishing power, and fisherman sorting behaviour. (In revision) ICES J. Mar. Sci.

See also Bastardie et al. 2008 (submitted).

 
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