Characterization of the stress response of red snapper: connecting individual responses to population dynamics
Campbell, Matthew Denis
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In the Gulf of Mexico, red snapper (Lutjanus campechanus) are considered to be the most important species for both commercial and recreational fishing. Red snapper were first classified as an overfished species in 1984 and are still considered as such (Schirripa and Legault 1999). Various studies have shown a high degree of uncertainty with the assumption of post-release survival in the fishery. The intent of this dissertation was to evaluate responses of red snapper to stress in the laboratory and the field, and then model those effects at the population level of organization. Both lab and field data demonstrated that with increasing depth and water temperature, the physiological health of red snapper diminished. This decrease in physiological health resulted in elevated immediate mortality rates due to their inability to cope with elevated stress and increased predation. A triage procedure called BtR score was developed and is a synergistic metric that accounts for external features of barotrauma and loss of reflex response capability. The BtR score showed a significant logistic functional relationship to observed mortality in the field and represents a tool that will allow prediction of immediate mortality. Mean BtR score from field data generated 4 different estimates of release mortality (20%, 30%, 33%, and 39%). These release mortality estimates were used to calculate concomitant increases in fishing mortality for this population, and were then applied in a matrix projection model. Results of the model indicate that release mortality rates 35% and greater generate declining population trends. Application of a slot limit to the fishery or reduction of age 1 red snapper bycatch, were both capable of returning the worst case scenario model back to equilibrium. The overall picture is that release mortality is a significant issue for the red snapper fishery. Continued investigation is recommended as more information about density dependent effects for this population become available. Reductions in total catch, release mortality, and bycatch are recommended to achieve sustainability for this population.