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Tethering assays, or the physical restraint of test organisms, has been used in the past to measure selected organisms' response to stressors while removing the observer from the experimental setting. Although informative for monitoring and hypothesis testing, these assays often used microfilaments that have been found to be too invasive or prone to biases given their effects on test organisms' behavior. Here, we describe a new variation of tethering using American oysters ( Crassostrea virginica ) and illustrate its use in the study of their mortality rates as a result of two stressors: siltation and predation by a non-indigenous species. Our protocol identified a resistant (non-toxic) glue that could be used to attach oysters to stone slabs, thus partially mimicking the natural cementation of the shell to natural substrates. This variation of tethering was harmless and maintained oysters' body position and natural ability to filter feed. Using tethered oysters in separate two-week field cage experiments, we also show how siltation and predation by a non-indigenous species (the European green crab, Carcinus maenas ), caused a gradual, easily measurable increase in oyster mortality rates. We argue that this variation of tethering is a cost-effective and advantageous way to monitor or test the effects of these and other stressors on oysters and similar species. [ABSTRACT FROM AUTHOR]

Non-indigenous green crabs ( Carcinus maenas ) are emerging as important predators of autogenic engineers like American oysters ( Crassostrea virginica ) throughout the eastern seaboard of Canada and the United States. To document the spreading distribution of green crabs, we carried out surveys in seven sites of Prince Edward Island during three fall seasons. To assess the potential impact of green crabs on oyster mortality in relation to predator and prey size, we conducted multiple predator-prey manipulations in the field and laboratory. The surveys confirmed an ongoing green crab spread into new productive oyster habitats while rapidly increasing in numbers in areas where crabs had established already. The experiments measured mortality rates on four sizes of oysters exposed to three sizes of crab, and lasted 3–5 days. The outcomes of experiments conducted in Vexar ® bags, laboratory tanks and field cages were consistent and were heavily dependent on both crab size and oyster size: while little predation occurred on large oysters, large and medium green crabs preyed heavily on small sizes. Oysters reached a refuge within the 35–55 mm shell length range; below that range, oysters suffered high mortality due to green crab predation and thus require management measures to enhance their survival. These results are most directly applicable to aquaculture operations and restoration initiatives but have implications for oyster sustainability. [ABSTRACT FROM AUTHOR]

• Gear choice is critical to new harvestings, including those to control invasive species • Fukui traps and fyke nets were assessed for green crab catch efficiency and bycatch • Fukui traps were less efficient but caught considerably less bycatch than fyke nets • Fukui traps modifications and fyke net bycatch reduction devices should be explored The European green crab (Carcinus maenas) continues to expand in Atlantic Canada and other coastal regions worldwide. Since most control measures for this species have failed, there is a need for innovative and ecologically responsible mitigation measures. One such measure is a green crab control harvesting wherein affected fishers could recoup some of their losses through value-added products. As with any new control initiative, gear choice must be carefully considered. This study assessed catch efficiency and bycatch of two common gear: Fukui foldable traps and fyke nets. Both types of gear were systematically compared in two soft-bottom areas with abundant green crab populations over the course of a field season (early May–late September). In this study, fyke nets were more efficient, catching roughly three times the number of green crabs caught by individual Fukui foldable traps. However, fyke nets also caught nearly ten times more bycatch and doubled the number of native species caught. Fyke net bycatch also included proportionally higher numbers of endangered and commercially important species, such as American eel (Anguilla rostrata) and winter flounder (Pleuronectes americanus). Although in principle Fukui foldable traps seem better suited for green crab control because of lesser bycatch impacts, further studies should find ways to increase Fukui foldable trap efficiency or test fyke nets with bycatch reduction devices. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]