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

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 1,100m (or 420m) 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