Evaluating discard mortality of summer flounder (Paralichthys dentatus) in the commercial trawl fishery: Developing acoustic telemetry techniques

Fish bycatch discard mortality is one of the most significant issues influencing marine fisheries management worldwide. Discard mortality studies to date rely on potentially biased on-deck evaluation of immediate mortality or reflex impairment, and evaluation of delayed mortality through holding captured fish for varying lengths of time to determine survival. Telemetry of ultrasonically tagged fish provides a technique for evaluating fish bycatch discard mortality, and especially latent mortality, under natural conditions in the sea. For summer flounder (Paralichthys dentatus), along the eastern United States, an 80% discard mortality is assumed but not verified. To determine the mortality of discarded fish, both live (n = 41; excellent condition = 4, good condition = 16, poor condition = 21) and dead (n = 16) summer flounder from commercial fishery-length tows were tagged and released in a fixed hydrophone array (mean depth of 8.8 m) on 15 September 2009 off Brigantine, NJ. We were able to re-detect both live and dead fish within the array and during mobile tracking for approximately 24 h before a storm. Fish of poor initial health and known dead fish were redetected after the storm in a concentrated area inshore of the release site and were presumed dead. Live fish exited the array offshore, as is typical in the fall migration. The final discard mortality estimate, combining on-deck mortality (32.7%) and latent mortality (49.0%), was 81.7% similar to current estimates. Latent mortality contributed at least as much to total discard mortality as on-deck mortality, confirming assumptions of earlier assessments. Several new telemetry metrics can lead to a better understanding of these important latent effects.

► Commercial trawl summer flounder discards were telemetered.
► Known dead specimens served as controls.
► Live fish behaved differently from severely injured and dead fish.
► In situ latent mortality contributed substantially to total discard mortality.
► An assumed 80% total discard mortality for summer flounder is supported.