Use of telemetry methods to estimate natural and fishing mortality of striped bass in Lake Gaston, North Carolina

Natural mortality can substantially affect fish population dynamics, but th e rate is difficult to estimate because natural deaths are rarely observed and it is difficult to separate the effects of natural and fishing mortalit y on abundance. We developed a new telemetry approach for estimating natura l and fishing mortality rates and applied it to the population of striped b ass Morone saxatilis in Lake Gaston, North Carolina and Virginia. Our analy ses were based on a sample size of 51 telemetered striped bass that were kn own to be alive and in Lake Gaston at least 1 month after capture and surge ry. Relocations of live fish and fish that died of natural causes were used to estimate natural and fishing mortality rates and the probability of rel ocating telemetered fish. Fishing mortality rates varied seasonally, but fe w natural deaths were observed, so the best model incorporated a constant a nnual instantaneous natural mortality rate (M; +/- SE) of 0.14 +/- 0.02. Wi th the uncertainty in model selection accounted for, the average annual M w as 0.16 +/- 0.04 For 1997 and 0.12 +/- 0.04 for 1998. Estimated annual fish ing mortality rates (F) were 0.74 +/- 0.13 for 1997 and 0.34 +/- 0.18 for 1 998. This telemetry approach for estimating mortality rates does not rely o n angler reporting of tagged fish. The relative standard errors for;M (24-3 3%) were comparable to those obtained from traditional ragging methods with large sample sizes. This approach is most applicable in closed systems, wh ere fishing mortality estimates are not biased by emigration. A high reloca tion probability is critical to reliably establishing seasonal changes in m ortality.