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.
Highlights
Oyster beds play important ecological and economic roles in coastal ecosystems [1,2,3]
Study areas and collection of green crabs and oysters In 2014, ten estuaries distributed across Prince Edward Island (PEI) were chosen for sampling following two criteria: the presence of mid-sized scattered oyster beds that had been subject to wild oyster fishing, and historical records documenting the distribution of green crabs [45, 49]
Frequency histograms of oyster size classes indicated that small oysters (
Summary
Oyster beds play important ecological and economic roles in coastal ecosystems [1,2,3]. The European green crab (Carcinus maenas) has well documented impacts on shellfish and other invertebrates throughout its native and invaded ranges [28,29,30,31]. This species is a significant predator of small sized cockles (Katelysia scalarina), softshell clams (Mya arenaria), blue mussels (Mytilus edulis), ribbed mussels (Aulacomya atra), and American oysters (Crassostrea virginica) [32,33,34,35,36,37]. The overall impact of these non-indigenous species will be not fully understood until their interactions with native prey are studied in a range of sites or locations and experimental approaches have been applied to quantify their effects [42]
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