Overcompensation and the European green crab, Carcinus maenas

Background/Question/Methods

Overcompensation occurs when an increase in adult mortality results in an increase in the number of individuals in a given population. In at least one instance overcompensation has occurred as the result of efforts to control invasive species. Management of nonindigenous nuisance species includes efforts to remove them from their recipient ranges, but potential for overcompensation in response to removal of adults is rarely assessed. Species that exhibit strong negative conspecific interactions are more likely to overcompensate. To examine the overcompensation potential of the European green crab, Carcinus maenas, experiments were performed in Bodega Harbor, CA, where adult C. maenas have been removed. These experiments examined adult impacts on juvenile survivorship, foraging, and growth. Cannibalism rates by C. maenas with and without alternative prey (the native crab Hemigrapsus oregonensis), and survivorship of tethered juvenile C. maenas compared to H. oregonensis at varying densities of adult C. maenas. Effects of adult presence were assessed by measuring foraging rates of juveniles over a 24 hour period and the growth rate of juveniles over one month with and without adult conspecifics. Five years of catch data from Bodega Harbor and nearby C. maenas populations where removal efforts did not occur were compared to look for signs of overcompensation.

Results/Conclusions

Adult C. maenas decreased foraging by juvenile conspecifics, but adult presence did not significantly decrease juvenile growth or survivorship. Adults consumed on average 0.5 juvenile per feeding trail without alternative prey. When alternative prey was available, adults consumed even fewer conspecifics but significantly more prey overall. A non-significant positive correlation was found between adult presence and survival of tethered juveniles, while tethered H. oregonensis survival declined with increasing abundance of adult C. maenas. In the short term, the presence of adult conspecifics significantly reduced foraging rates of juvenile C. maenas. However, adult presence had no affect on juvenile growth rate over a month-long period. These findings suggest that while adults can impact juveniles in negative ways, adult removal via harvest would not significantly increase juvenile survival or growth rates. Comparisons of  past catch data across locations showed no evidence of overcompensation occurring in Bodega Harbor, with no obvious trend of increasing juvenile or total C. maenas abundance with ongoing removal.  Therefore, C. maenas is unlikely to overcompensate in response to removal efforts. We expect that harvesting adults will not cause overcompensation in species in which adults only minimally affect juvenile growth and survival rates.