Use of Telemetry Methods to Estimate Natural and Fishing Mortality of Striped Bass in Lake Gaston, North Carolina

Abstract

Natural mortality can substantially affect fish population dynamics, but the rate is difficult to estimate because natural deaths are rarely observed and it is difficult to separate the effects of natural and fishing mortality on abundance. We developed a new telemetry approach for estimating natural and fishing mortality rates and applied it to the population of striped bass Morone saxatilis in Lake Gaston, North Carolina and Virginia. Our analyses were based on a sample size of 51 telemetered striped bass that were known to be alive and in Lake Gaston at least 1 month after capture and surgery. Relocations of live fish and fish that died of natural causes were used to estimate natural and fishing mortality rates and the probability of relocating telemetered fish. Fishing mortality rates varied seasonally, but few natural deaths were observed, so the best model incorporated a constant annual instantaneous natural mortality rate (M; ±SE) of 0.14 ± 0.02. With the uncertainty in model selection accounted for, the average annual M was 0.16 ± 0.04 for 1997 and 0.12 ± 0.04 for 1998. Estimated annual fishing mortality rates (F) were 0.74 ± 0.13 for 1997 and 0.34 ± 0.18 for 1998. This telemetry approach for estimating mortality rates does not rely on angler reporting of tagged fish. The relative standard errors for M (24–33%) were comparable to those obtained from traditional tagging methods with large sample sizes. This approach is most applicable in closed systems, where fishing mortality estimates are not biased by emigration. A high relocation probability is critical to reliably establishing seasonal changes in mortality.