Use of Ultrasonic Telemetry to Estimate Natural and Fishing Mortality of Red Snapper

Abstract

An accurate estimate of natural mortality is critical for the management of any fishery but is typically difficult to directly measure. Mortality rates for Red Snapper Lutjanus campechanus (n = 87) were estimated by telemetry from December 2005 to June 2009 in the northeastern Gulf of Mexico. At five separate sites an array of five receivers was deployed with one receiver at the center (reef) and four receivers placed 1,100 m (or 420 m) north, south, east, and west of center. These arrays enabled the direct estimation of fishing mortality, natural mortality, and emigration of acoustically tagged Red Snapper. Out of the 70 fish that remained after the 6-d recovery period, 19 were caught, 10 died naturally, and 27 emigrated from the 2-km-radius study sites. The Kaplan–Meier staggered-entry method was used to estimate survival from different mortality events, and survival was converted to instantaneous mortality rates. For all years combined, total mortality (95% confidence interval) was 0.39 (0.19–0.64), fishing mortality (F) was 0.27 (0.11–0.54), and natural mortality (M) was 0.11 (0.06–0.20). Each year estimated annual M increased,

M = 0 in 2006, M = 0.19 (0.07–0.48) in 2007, and M = 0.21 (0.09–0.44) in 2008, while annual F rates decreased, F = 0.61 (0.16–1.67) in 2006, F = 0.22 (0.09–0.51) in 2007, and F = 0.14 (0.05–0.35) in 2008. Thus, in more recent years (2007–2008) M estimates were higher, while F estimates were lower than past estimates used in stock assessments

(M = 0.1; F = 0.35). These decreasing rates of F over 2006–2008 suggests that in recent years restrictive management efforts have succeeded in reducing fishing mortality, and although M has increased it may have leveled off near M = 0.2 such that populations will continue to increase.

Received September 2, 2012; accepted March 25, 2013

ACKNOWLEDGMENTS

We thank S. Beyer, D. Miller, P. Mudrak, C. Simmons, and T. Syc for field assistance. We thank L. Swann and Y. Brady for reviewing earlier versions of this manuscript. This project was funded by the National Oceanic and Atmospheric Administration, National Fisheries Service MARFIN program award number NA06NMF4330054 and Federal Aid in Sport Fish Restoration 5-101-12 to 15, Alabama Marine Resources Division. This study is a contribution of the Alabama Agricultural Experiment Station and Department of Fisheries and Allied Aquacultures, Auburn University.