Marine and Coastal Fisheries

—The ability of fish to submerge after discarding is often used as a proxy for survival, but this practice underestimates total discard mortality because delayed mortality is overlooked. Fishery managers need a way to link ‘‘sink or swim’’ indicators, or variables observed during capture and release, with delayed mortality rates. We conducted a cage study of red snapper Lutjanus campechanus off the coast of Texas to estimate delayed mortality rates and to find factors that could link immediate and delayed mortality. Immediate mortality (17%) was predicted by the interaction of depth and the difference in temperature between surface and bottom waters. Lactate levels were also significant predictors of immediate mortality in fish whose blood was tested. Delayed mortality (64%) was predicted primarily by a condition index consisting of the presence or absence of injuries, symptoms of barotrauma, and fish behaviors immediately after capture. Specific categories included bleeding, protruding intestines, everted stomach, exopthalmia, the presence or absence of flapping and gilling behaviors, and problems with cage submergence. The majority of fatalities occurred within 24 h after fish were placed in the cages. Our mortality estimates indicate that red snapper discard mortality was significantly underestimated in the 2005 stock assessment for red snapper. The use of indices that relate the condition of an individual fish at capture to its probability of delayed mortality is an excellent method for linking immediate and delayed mortality and will likely be applicable to many species that are subject to catch-and-release fishing. Fish are discarded in almost every commercial and recreational fishery. Fish may be discarded because the species or size of fish is not desired for economic or personal reasons. Fishermen may discard less-desirable fish when more desirable fish are obtained. Fish may also be discarded due to regulations such as minimum size limits, bag limits, or closed seasons that are intended to limit catches or to protect some segment of the population. The assumptions behind these regulations are that very few fish are discarded and that most fish survive the catch-and-discard process without detrimental effects. Accurate estimates of discard mortality are critical not only to gauge the effectiveness of regulations but also for setting unbiased catch quotas. In fact, the underestimating of discard mortality has been cited as one cause of the collapse of northern Atlantic cod Gadus morhua stocks off the coast of eastern Canada in the early 1990s (Myers et al. 1997). Because discarded fish are often juveniles or young adults, discard mortality mimics recruitment failure in terms of its population effects, since young fish are killed before they can recruit into the adult population (Diamond et al. 1999). Simulations show that stock assessments that exclude discard mortality overestimate spawning stock biomass and yield compared to assessments that include discard mortality (Breen and Cook 2002). Underestimating discard mortality can therefore lead to overestimates of the total allowable catch, which can result in depletion and, in some cases, collapse. Survival rates of discarded fish are generally estimated over three time scales: (1) over minutes to hours after capture (immediate survival) by surface observation studies that assess the proportion of fish that can submerge immediately after release; (2) over hours to weeks (short-term or delayed survival) by holding fish in cages after capture or by means of telemetry to monitor fish after release; or (3) over days to years (long-term survival) by marking and recapturing released fish (Pollock and Pine 2007). However, using submergence data in place of survival data can be misleading, as the ability to swim immediately after release is not a direct measure of survival. Fish that submerge immediately after release may still die afterwards, while fish that are unable to submerge may survive (Patterson et al. 2002). Fishery managers therefore need some way to link ‘‘sink or swim’’ indicators observed during capture and release with Subject editor: Richard Brill, Virginia Institute of Marine Science, USA * Corresponding author: [email protected] 1 Present address: School of Natural Sciences, University of Western Sydney, Locked Bag 1797, Penrith South DC NSW 1797, Australia. 2 Present address: Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, Louisiana 70808, USA. Received November 18, 2008; accepted March 10, 2009 Published online May 28, 2009 107 Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science 1:107–120, 2009 Copyright by the American Fisheries Society 2009 DOI: 10.1577/C08-043.1 [Article] D ow nl oa de d by [ U ni ve rs ity o f W es te rn S yd ne y W ar d] a t 1 8: 42 3 1 A ug us t 2 01 1 longer-term survival rates that can be used in stock assessments. An example of the need for such links is the directed fisheries for red snapper Lutjanus campechanus in the Gulf of Mexico (hereafter, the Gulf). The red snapper is the most economically important reef fish species in the Gulf. Red snapper have been classified as overfished since 1984 (Goodyear and Phares 1990), and fishery managers have implemented a complex series of measures to regulate the catch and rebuild the stock of red snapper, including size limits, bag limits, trip limits, fleet quotas, and seasonal closures (see Hood et al. 2007 for historical review). Unfortunately, all of these measures result in discarding of red snapper because of red snapper bycatch outside of the legal limits or seasons. In the 2005 stock assessment, release mortality in the recreational fisheries was estimated to be 40% for the western Gulf (Louisiana and Texas) and 15% for the eastern Gulf (Mississippi, Alabama, and Florida) based on literature estimates and the depth distribution of the recreational fisheries (GMFMC 2005). One major criticism of these estimates was that surface observation studies were treated as direct estimates of discard mortality and given equal weight to cage studies where short-term mortality was actually measured. There are concerns that the discard mortality rates used in the assessment are underestimates, introducing a large source of uncertainty into the assessment and biasing the resulting management actions. Our objectives in this study were to estimate immediate and delayed mortality of red snapper caught in the recreational fisheries and to investigate sink or swim indicators observed during capture and release that could be used to predict delayed mortality. To accomplish our objectives, we caught fish off the coast of Texas at three depths and seasons and held them in cages for 1–7 d. We collected data on fishing parameters and fish condition, including physical injuries, fish behavior, and physiological status (symptoms of barotrauma and blood cortisol, lactate, and osmolality levels). Metrics on fish condition were combined into a condition index, which was tested as a predictor of immediate and delayed mortality. Our hypotheses were that delayed mortality would be higher than immediate mortality and that we could find predictors of delayed mortality based on capture and release conditions.