| Shrimp trawler discards account for 1.9 million mt of the 6.8 million mt of annual global fisheries discards. This study examined the effects of dead discards from the North Carolina estuarine shrimp trawl fishery on scavenger populations and examined the trophic changes resulting from fishery practices. Bycatch and discards onboard estuarine shrimp trawlers were quantified, identified, and monitored for short-term survival. The major taxa in the bycatch both by weight and number were juvenile finfish and portunid crabs. Crabs showed high survival while 78% of fish died, generating a substantial amount of carrion in the estuaries, most of which sinks. Scavenging for discards occurred mainly by blue crabs and pinfish in the lower meter of the water column. In laboratory experiments blue crabs fed on discards six times more than on a natural prey, the juvenile hard clam, and this was replicated in mesocosm experiments lasting three days, where predation on juvenile hard clams was 33-60% less in treatments where discards were added. Thus, the shrimp, blue crab, and bivalve fisheries in estuarine waters of North Carolina are interconnected. Early and late summer quantitative ecosystem mass-balance models were constructed for two years of varying intensity of disturbance from hypoxia with four fishing treatments within each of the four time periods modeled: no fishing or discarding, fishing extraction alone, discarding alone, and combined fishing extraction and discarding of carrion. The effects of fishing on trophic structure, carbon flow and system network properties as calculated by Ecopath software were minimal compared to differences due to season and disturbance by hypoxia. However, fishing and discarding changed the trophic level at which many consumers fed, decreased transfer efficiency of energy through and out of the system, and altered trophic interactions. Pathways for energy transfer increased when discards were present, increasing measures of system stability. More system production was retained as detritus when discards were included in models. Management must acknowledge the ecosystem impacts of fisheries, and ecosystem-based management will be more successful if eutrophication is minimized and if the suite of interactions between fisheries and fished species are considered in management plans. |
