An assessment approach to estimate biomass of fish communities from bycatch data in a tropical shrimp-trawl fishery

The biomass of the finfish as bycatch of the shrimp-trawl fishery in the southeastern Gulf of California was estimated by use of a heuristic approach for fish biomass, by the use of the central-tendency hypotheses on the proportion of fish landed, the proportion of fish in the bycatch estimated in our work, and from data available from previous studies on bycatch for the area. Data from 1950 experimental trawls made at depths of 8–76 m from 1992 to 2004 during the closed shrimp season were analyzed. The central tendencies were calculated using three methods: the arithmetic mean and the estimators of both Pennington and Kappenman. A total of 250 species were caught during these surveys. From these, 17 species are 50% of the total biomass of the species caught from shallow waters to 76 m. The most abundant species were the grunt Orthopristis chalceus, the pacific moon fish Selene peruviana, the lizardfish Synodus scituliceps and the panamian grunt Pomadasys panamensis. From our results we noted that the Kappenman estimator is more precise than the other two estimators, therefore, for the period 1993–2004 the available biomass of the total fish community was (90 ± 45) × 103 tonnes, the mean proportion shrimp:bycatch ranged from 6 to 31 kg and the mean value of the fish bycatch landed was 1.44 ± 0.54 kg/kg of shrimp.