Linking “Sink or Swim” Indicators to Delayed Mortality in Red Snapper by Using a Condition Index

Sandra L. Diamond,Matthew D. Campbell

doi:10.1577/c08-043.1

Abstract

AbstractThe 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.

Highlights

Abstract.—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
We looked at correlations between environmental variables to reduce the number of parameters in the logistic models by means of analysis of variance (ANOVA) and Tukey’s honestly significant difference (HSD) tests (SPSS version 15)
Our experiment shows that delayed mortality, which often occurs in the first 24–48 h after fish are caught, is much higher than immediate mortality, making it likely that surface observations of a fish’s ability to submerge are underestimates of delayed discard mortality

Summary

Introduction

Abstract.—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. 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). All of these measures result in discarding of red snapper because of red snapper bycatch outside of the legal limits or seasons. 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

Methods

Results

Conclusion