SS1.06 The Ecological Impacts of Pelagic Longline Fisheries
Date: Monday, June 10, 2002
Time: 3:45:00 PM
Location: Carson A
CoxS, Cneter for Limnology/University of Wisconsin, Madison, USA,
Trophic effects of longline fishing in the central north Pacific (1952-1998)
Commercial yield of tunas in the central Pacific increased three-fold between 1952 and 1998, mainly as a result of increasing fishing effort. Large-scale surface fisheries generally target small tunas (Thunnus spp; Katsumonus pelamis), whereas deepwater longline fisheries target piscivorous adult tunas and take billfish (Xiphias gladius,Makaira spp., Tetrapturus spp., Istiophorus platypterus) and shark species (Prionace glauca,Alopias superciliosus, Isurus oxyrinchus ,Carcharinus longimanus, Galeocerdo cuvieri) as bycatch. We developed single- and multi-species (Ecosim) age-structured production models that incorporate information from multiple fisheries to estimate population trends and potential changes in ecosystem structure owing to large-scale fishing. Results suggest that adult stocks of tunas and billfishes have been depleted by up to 50% (tunas) to 85% (billfishes) since the 1950s. Estimated biomasses of juvenile tunas increased since the mid-1970s and current biomass of albacore is approaching historical highs. Multi-species analyses suggested that target and bycatch exploitation of large piscivorous species has reduced predation mortality on small tunas. However, reduced predation mortality was largely overwhelmed by increases in fishing mortality on these groups. High fishing mortality on small yellowfin initiated trophic changes that lead to increased growth rates and egg production by large yellowfin and caused small yellowfin biomass to increase to the same degree as suggested by abundance index data and biomass estimates. Our results suggest that within-species interactions have combined with fishing effects to drive tuna dynamics in the central Pacific.