Exploring the decline of oyster beds in Atlantic Canada shorelines: potential effects of crab predation on American oysters (Crassostrea virginica)

Abstract

Atlantic Canada's American oyster (Crassostrea virginica) beds, while economically and ecologically important, have been in decline over the past few decades. Predation by crabs, in particular by the European green crab (Carcinus maenas), has been proposed as one of the potential causes of such decline. Hence, this study examined oyster mortality levels in multiple beds across Prince Edward Island (PEI) and then experimentally assessed the contribution of green crab predation to oyster mortality. Results from surveys conducted in 10 estuaries across PEI in 2014 indicate that the probability of mortality for small oysters was significantly higher when green crabs were present then in areas without green crabs. This probability of mortality was significantly less when there was the presence of alternative prey like natural mussel beds (Mytilus edulis). The odds of oyster mortality were also higher when beds had rock crabs (Cancer irroratus) compared to beds with no rock crabs. Given the potential importance of green crab predation, its influence was assessed in 2015 using two field experiments with tethered oysters. Our results indicate that odds of small oyster mortality occurring were much higher in green crab inclusion cages than in the open environment and the exclusion cages. These results reaffirm that oysters up to ~40 mm SL are vulnerable to predation, and at least some of the mortality affecting these oysters can be causally attributed to green crab predation. Green crab predation rates upon small oysters are relevant given the economic benefits and ecosystem services provided by these bivalves. They highlight the need for the industry to consider mitigation measures and potentially adapt their oyster growing strategies.

Revisiting recent history: records of occurrence and expansion of the European green crab across Prince Edward Island, Atlantic Canada

“…However, we remain confident on the patterns described here, as alternative sources of information, including sightings and surveys conducted for other purposes in selected areas and times (e.g. Pickering and Quijón 2011;Tummon Flynn et al 2015;Poirier et al 2017) are all consistent with the records presented here.…”

“…Trap deployments were performed from a boat or directly from the shore in the shallow subtidal depending on location and accessibility. In general, traps were deployed at least 10-15 m apart from each other, attempting to cover as much area as possible particularly in bottoms rich in vegetation or known to have abundant shellfish (see Pickering et al 2017;Poirier et al 2017). The total number of traps deployed and the number of sites surveyed was slightly higher along the north shore.…”

“…Green crabs are well known disruptors of eelgrass habitats (Malyshev and Quijón 2011;Garbary et al 2014;Neckles 2015;Matheson et al 2016) and predators of shellfish species in the region (e.g. Floyd and Williams 2004;Pickering and Quijón 2011;Pickering et al 2017;Poirier et al 2017) and elsewhere (e.g. Grosholz et al 2000;Palacios and Ferraro 2003;Murray et al 2007).…”

Effects of habitat complexity on cannibalism rates in European green crabs (Carcinus maenas Linnaeus, 1758)

“…The European green crab (Carcinus maenas Linnaeus, 1758) is an interesting model species given its foraging abilities (Cunningham & Hughes, 1984) and its use of a broad range of coastal habitats (Grosholz et al, 2000). In their invaded range, green crabs have been associated with declines of several commercially important bivalve species (Grosholz et al, 2000;Poirier et al, 2017) and negative interactions with native crustaceans (Gehrels et al, 2016;Gregory & Quijón, 2011;Rossong et al, 2011;Rossong, Williams, Comeau, Mitchell, & Apaloo, 2006). However, despite the common occurrence of cannibalism in green crabs, studies of green crab cannibalism in relation to habitat properties have lagged behind.…”

Site fidelity and population structure of blue land crabs (Cardisoma guanhumi Latreille, 1825) in a restricted-access mangrove area, analyzed using PIT tags

“…Carnivorous crabs (Carcinus maenas) may affect the structure of local oyster beds through philopatric, site-specific predation [9]. Ocypodid ghost crabs [10,11], lobsters [12] and tropical land crabs [13] have the ability to return to the same sites after foraging, thus showing a strong philopatric behavior.…”