Discard Mortality in the Summer Flounder Trawl Fishery: A New Approach to Evaluation

Fish bycatch discard mortality is one of the most significant issues influencing marine fisheries management in the US and worldwide.  For summer flounder (Paralichthys dentatus) an 80% discard mortality is assumed but not verified.  Discard mortality studies to date have often relied on on-deck evaluation of mortality or reflex impairment, and evaluation of delayed mortality through holding captured fish for varying lengths of time to determine survival.  Because of the inability of laboratory or holding experiments to replicate the natural environment, the mortality rates derived from these experiments may be questionable.  We suggest that telemetry of ultrasonically tagged fish provides an improved technique for evaluating fish bycatch discard mortality, and especially latent mortality, under natural conditions in the sea. In order to determine the mortality of these discarded fish, both live and dead summer flounder from commercial fishery length tows were tagged and released in a fixed hydrophone array on 15 September 2009 off Brigantine, New Jersey.  We were able to re-detect both live and dead fish within the array during mobile tracking for approximately 24 hours before a northeast storm event.  Fish of poor initial health that were re-detected after the storm were found in a concentrated area inshore of the release site (and later presumed dead), while live fish exited the array as they moved offshore as is typical in the fall migration. The final discard mortality estimate, combining on deck mortality (32.7%) and latent mortality (50.0%), is 82.7%, a rate similar to current estimates.  Together, these findings suggest that telemetry can provide a new approach to evaluating discard mortality.