Estimating and Mitigating Discard Mortality of Reef Fishes.

RUNDE, BRENDAN JAMES. Estimating and Mitigating Discard Mortality of Reef Fishes. (Under the direction of Dr. Jeffrey A. Buckel). Discard mortality (also called bycatch or release mortality) is one of the most pervasive threats to fisheries sustainability. For many reef fishes in the US southeast, discards now exceed harvests as a result of increasingly-strict regulations. As a result, the mortality of discarded fish has become a crucial component of assessing stocks. Also of recent interest are techniques for mitigating or alleviating the stresses that result in increased risk of mortality. Our study objectives were to 1) robustly estimate discard mortality of gray triggerfish (Balistes capriscus) and 2) investigate the efficacy of forced recompression on improving survival of three species of deepwater groupers. For gray triggerfish (Balistes capriscus), we estimated condition-specific survival rates using a tag-return approach and extrapolated these values to the fishery to estimate an overall discard mortality rate. Estimates of survival were calculated using a Cox proportional hazards regression model. Importantly, capture gear had a significant effect on survival: fish caught with hook-and-line had lower survival than fish caught with traps. For best condition surface-released fish caught with hook-and-line, survival was estimated to be 0.21 (95% confidence interval 0.08, 0.58). For fish exhibiting visible trauma, estimated survival was 0.16 (0.03, 0.96). Fish that were unable to submerge had survival of zero. For surface-released fish caught with traps, the survival estimates were 0.81 (0.51, 1.28) for fish in best condition, 0.36 (0.16, 0.81) for fish with trauma, and zero for floating fish. We estimated the overall adjusted survival rate for triggerfish in 36-40 m to be 0.20 and a maximal estimate across all depths to be approximately 0.52. These results have implications for future gray triggerfish management since our estimates are substantially lower than the value (0.875) used for the most recent stock assessment. Other reef fish are of similar importance in the US southeast. Discard survival of deepwater (>60 m) groupers (Serranidae; Epinephelinae) is often assumed to be 0% given the severity of their barotrauma and associated inability to submerge. We used acoustic telemetry to study the activity of 19 deepwater groupers after a recompressed release with a descender device, achieved by rapidly returning fish to a depth where expanded gases can contract. The species tested were Scamp (Mycteroperca phenax, n=8), Snowy Grouper (Epinephelus niveatus, n=7), and Speckled Hind (E. drummondhayi, n=4). Individuals of all three species showed postrecompression variation in water depth and acceleration indicative of survival, while information from other tags indicated discard mortality. Nonparametric Kaplan-Meier survivorship procedures yielded a 14 d survival estimate of 0.50 (95% CI: 0.10, 0.91); although low, this is higher than currently assumed 0% survival. Additionally, our estimate of discard survival is likely biased low because we assumed no individuals shed their tag, which is unlikely for our external attachment method. A technique to increase discard survival of deepwater groupers may lead to better-constructed regulations for reef fishes in the US southeast and elsewhere where these species are caught and released. Management of species in the snapper-grouper complex is often complicated, given the overlapping ranges and cohabitation of many species in the management unit. Robust data for these species are critical for better-constructed management. The findings of both chapters of this thesis will hopefully contribute to better-informed stock assessments and increased sustainability for reef fishes in the US southeast. Estimating and Mitigating Discard Mortality of Reef Fishes by Brendan James Runde A thesis submitted to the Graduate Faculty of North Carolina State University in partial fulfillment of the requirements for the degree of Master of Science Fisheries, Wildlife, and Conservation Biology Raleigh, North Carolina 2017