Modeling the impacts of the European green crab on commercial shellfisheries

Abstract

Coastal resource managers are often tasked with managing coastal ecosystems that are stressed by overexploitation, climate change, contaminants, and habitat loss, as well as biological invasions. Therefore, managers increasingly need better economic data to help them prioritize their management strategies and distribute their increasingly limited resources to those strategies. Despite frequent pronouncements about the substantial ecological and economic impacts of invasive species, there have been few if any rigorous analyses of the economic impacts of invasive species in coastal systems. Here we present a bioeconomic analysis of the impacts of the European green crab, Carcinus maenas, on commercial shellfisheries along the West Coast of the United States. Green crabs are among the most comprehensively studied and widely distributed invasive species in coastal systems, with established populations on every continent except Antarctica. Their impacts on commercial bivalve fisheries have been alleged or substantiated to varying degrees, but no formal analysis of the economic impacts of the green crab has been conducted. We assess economic impacts using a combination of ecological and economic models. The ecological models incorporate green crab dispersal and description of estuarine habitat and the relationship between green crab abundance and predation on prey populations. The economic analysis focuses on the green crab impacts on commercial shellfisheries, including both historical and present impacts of green crabs on several important shellfisheries, including soft-shell clams, blue mussels, scallops, hard-shell clams, and Manila clams. We conclude that the past and present economic impacts on the West Coast shellfisheries are minor, although losses could increase significantly if densities increase or with northward range expansion into Alaska.

The role of shore crabs and mussel density in mussel losses at a commercial intertidal mussel plot after seeding

“…These results are in line with the Plass-Johnson et al (2010), who attributed 35 % of the mussel losses to predation and with estimations of Grosholz et al (2011) who extrapolated results from Beal and Kraus (2002) and from (Beal 2006) where predation losses varied between 13 and 55 %. Murray et al (2007) estimated that losses as a result of shore crab predation are 9.5 % over 12 months in the Menai Street in Wales, which is substantially lower than in the present study.…”

“…The feeding rate of shore crabs depends on the predator-prey size ratio, where the predation rate shows a rapid decrease with increasing mussel size (Crothers 1968;Mascaró and Seed 2001;Murray et al 2007;Kamermans et al 2009). The impact of crab predation over the entire culture cycle has been estimated to range from 9.5 % (Murray et al 2007) to 52 % mussel loss (Grosholz et al 2011). These data are extrapolations of laboratory results on feeding rates to estimated field abundances.…”

Production efficiency of mussel bottom culture

“…These results are in line with the Plass-Johnson et al (2010), who attributed 35% of the mussel losses to predation and with estimations of Grosholz et al (2011) who extrapolated results from Beal and Kraus (2002) and from (Beal 2006) where predation losses varied between 13% and 55%. Murray et al (2007b) estimated that losses as a result of shore crab predation are 9.5% over 12 months in the Menai Street in Wales, which is substantially lower than in the present study.…”

“…Our results show that shore crabs take a substantial share in the seeding losses and suggest that the effect of shore crab predation on mussel biomass production is higher than previous studies would suggest (Murray et al 2007b, Grosholz et al 2011. Effect sizes, such as the 10% suggested by Murray et al (2007b), may apply to already established mussel populations on culture plots, or on natural mussel beds.…”

“…Murray et al (2007b) estimated a 10 percent loss due to shore crab predation over the entire culture cycle from seeding to harvest in the Menai strait in Wales. In a study on the economic impact of the invasion of green crabs in North America, Grosholz et al (2011) estimated a 52 percent loss of shellfish including blue mussels. Crabs can have a large impact on culture yields, but results are not always consistent, which is not surprising, because shore crabs…”