Analysis of bycatch in the fishery for orange roughy, Hoplostethus atlanticus, on the South Tasman Rise

Government fisheries observers made detailed records of the catch weights of all species caught on 545 trawls between October 1997 and August 2000 in the South Tasman Rise orange roughy (Hoplostethus atlanticus) fishery. Bycatch ratios, the ratio of bycatch weight to tow duration, were derived from these data and used to make estimates of total annual bycatch for several species groups. Bycatch ratios based on tow duration were chosen over ratios based on orange roughy catch weights after comparing the coefficients of variation (c.v.) of sets of trial data. Bycatch ratios and total bycatch were estimated for three species of oreos (Oreosomatidae), corals and all other bycatch species combined, for the fishing years from 1997-1998 to 2000-2001. Total oreo bycatch dropped from about 7400 t to less than 350 t during this time. These estimates agreed well with recorded oreo landings data for three of the four years. There was a considerable bycatch of corals, with both the bycatch ratio and the total bycatch reducing during the period examined, the latter from about 1750 t to 100 t per year. The coral bycatch comprised a large number of species, but was dominated by the reef-forming stony coral Solenosmilia variabilis. Annual bycatch of all other species combined, mainly rattails (Macrouridae) and dogfishes (Squalidae), was low (13-120 t). Bycatch of this group dropped sharply in each year as the result of a combination of decreasing bycatch ratio and decreasing fishing effort.