Modeling the Effects of Commercial Harvest on Population Growth of River Turtles

Commercial turtle harvest is considered one of the major contributing factors to declines in turtle populations. Few long-term studies have evaluated turtle population response to harvest and little is known about demographic rates for many turtle species. We gathered demographic rates from the literature for snapping turtles (Chelydra serpentina), smooth softshells (Apalone mutica), and spiny softshells (Apalone spinifera), which are harvested in Missouri, and developed deterministic, density-independent, stage-based matrix models to assess turtle population response to plausible harvest rates we estimated from field sampling. Further, we used population modeling to determine annual harvest rates that would result in lambda = 1 for each demographic scenario. We developed one model for snapping turtles and another for both softshell species combined due to the lack of available species-specific demographic data for either softshell species. Using mean demographic rates for survival and fecundity, snapping turtle populations had a growth rate (lambda) of 1.023; at minimum demographic rates lambda = 0.891 and at maximum demographic rates lambda = 1.199. When we applied plausible, field-estimated annual harvest rates under mean demographic rates, populations decreased in all instances except when harvesting only juveniles at the minimum harvest rate. At mean demographic rates, annual harvest of both adults and juveniles should be <= 2.3% to maintain a stationary population (lambda = 1). For softshell turtles, lambda was 0.952 at mean demographic rates, 0.838 at minimum demographic rates, and 1.163 at maximum demographic rates. Under mean and minimum demographic rates, no field-estimated harvest could be sustained, as any annual harvest rate resulted in lambda < 1. For both snapping turtles and softshells, harvest was sustainable when demographic rates were at the maximum values, which are highly improbable to occur frequently: annual harvest of 16.3% of both adult and juvenile softshells and 18.6% of adult and juvenile snapping turtles resulted in lambda >= 1. In both species, elasticity analyses demonstrated that adults, which are the most vulnerable to commercial harvest, were the most important segment of the population demographically. These results corroborate the findings of other studies which indicate that even low annual harvest rates may have detrimental effects on the long-term sustainability of turtle populations at localized scales.