DescriptionSummer flounder, Paralichthys dentatus, is a commercially and recreationally important
flatfish species along the east coast of the United States. There are some clear and
important data gaps in our understanding of summer flounder and their ecology. In this
study we address two of those data gaps using acoustic telemetry; discard mortality in the
commercial trawl fishery and the potential for pelagic behavior. Discard mortality
studies to date rely on potentially biased on-deck evaluation of immediate mortality, and
evaluation of delayed mortality through holding captured fish 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 along the eastern United States, an 80% discard mortality is assumed
but not verified. There is also a growing body of literature on flatfish species exhibiting
pelagic behaviors for various essential functions. In captivity, summer flounder use
“stroke and glide” behavior in the water column and they can feed actively there. To
determine the discard mortality, and explore their potential vertical movement, adult summer flounder were collected from a commercial trawl vessel and tagged and released
in a fixed hydrophone array on 15 September 2009 off Brigantine, New Jersey. In 2010,
an additional set of summer flounder were collected in the Great Bay – Mullica River
estuary, tagged, and released into an array nearby the 2009 site for further examination of
pelagic behaviors. Fish were re-detected both alive and dead within the array and during
mobile tracking. Pressure sensing tags recorded depth at a resolution of 0.68 meters.
Signal values indicating depths two meters or greater above an individual’s greatest depth
were considered to be pelagic in nature. Fish of poor initial health and dead individuals
were redetected after the storm in a concentrated area inshore of the release site and were
presumed dead. The final discard mortality estimate from the commercial trawl,
combining on-deck mortality (32.7%) and latent mortality (49.0%), was 81.7%; similar to
prior estimates. Latent mortality contributed at least as much to total discard mortality as
on-deck mortality. The individual depth profiles show clear patterns of active pelagic
behavior in 6 out of 14 live fish in 2009 and 6 out of 11 live fish in 2010. The mean percent of time above a 2-meter floor was 16.8% with a standard deviation of 24.4% in 2009, and 1.2%, with a standard deviation of 3.0% in 2010. This pelagic behavior occurred more frequently (86.0%) during nighttime, but there were no other obvious environmental correlates.