COS 128-1
Current rates of boat injuries to diamondback terrapins imperil long-term population persistence
Diamondback terrapins (Malaclemys terrapin) are a keystone species in estuarine ecosystems, many of which are economically important recreational boating areas. Competition for space between boats and terrapins leads to propeller injury and death. We developed a size-structured exponential population model in Vensim 6.2 to simulate impacts of decreased adult survivorship of boat-injured terrapins. We used the following assumptions based on a mark-recapture study of terrapins in Barnegat Bay, NJ: (1) all adult female terrapins nest yearly, (2) sex ratio was 0.66 female, (3) adult females lay 1.2 clutches of 12 eggs each year, (4) hatch success was 0.5, (5) uninjured adult survival was 0.7 for both sexes, (6) hatchlings become juveniles in one year, and (7) juveniles become adults in 4 years. We simulated population growth for 100 years at an initial size of 5000 adult individuals, establishing baseline equilibrium to between 4500 and 5500 individuals. We assigned variables with the following values: (1) female injury rate was 21%, (2) female injured survivorship was 0.5, (3) male injury rate was 15%, and (4) male injured survivorship was 0.1. We compared scenarios of 75, 50, 25 and 0% management reduction of current injury rates to baseline after 100 years.
Results/Conclusions
Hatchling and juvenile survival rates at equilibrium population growth are negatively correlated, ranging from 0.26 to 0.56 and 0.7 to 0.58, respectively. Limits of these ranges assumed increased survivorship with size class. For all boat injury management scenarios, hatchling and juvenile survival rates were 0.31 and 0.67, respectively; these values established baseline equilibrium population without boat injury at 4881 individuals after 100 years. The modeled boat injury reduction scenarios of 75, 50, 25% and current rates (0%) predicted after 100 years this terrapin population decreases in size by 41.1% (2873 individuals), 64.9% (1713), 78.8% (1034), and 87.1% (632), respectively. Of note is that even with 75% reduction of boat injury rates, the population decreases by more than 40% and if we continue at current rates, this population is in danger of extirpation largely due to sensitivity to adult reproductive potential. Since current management practices are unsustainable, we strongly suggest partial or complete closures of high terrapin density areas (mating, nesting or foraging grounds) to boats, regulations on speed limit and propeller size, increased enforcement, and inclusion of an ecological component to boater safety certification courses.