Exploring the expansion, impact and a control option for European green crab populations in Prince Edward Island, Atlantic Canada

One of the world’s most successful marine invaders, the European green crab (Carcinus maenas), continues to spread and invade the coastlines of Atlantic Canada. In Prince Edward Island in particular, populations have continued to expand since the late 1990s. In areas like these, there is a distinct need to document this species’ expansion into new habitats, to monitor its impacts on important native species, and to develop innovative mitigation strategies to control some of its effects. This thesis addressed the large information gap that still exists on this species’ effect in the Atlantic Canadian region by focusing on four main areas: First, I explored over a decade of green crab population expansion records along Prince Edward Island’s main shorelines. Second, I surveyed and examined the effects of green crabs on vulnerable size classes of an important shellfish species, the American oyster (Crassostrea virginica). Third, I performed a preliminary study into the occurrence and synchrony of molting in green crabs, identifying suitable physical features of pre-molting crabs as a first step for developing a soft-shell crab industry. And fourth, I developed and tested a novel bycatch reduction device to be used with fyke nets to facilitate the implementation of the aforementioned directed fishery for this species. Samples collected during the years 2000-2013 indicated that green crab expansion rates on Prince Edward Island vary spatially and temporarily. Expansion rates were higher along the south shore than the north shore of the island, and it was hypothesized that this dissimilarity was possibly related to the higher availability of suitable habitat along the south shore. The speed of the spread was such that it seemed more likely related to the benthic movement of juvenile and adult crabs than associated to larval spread or new anthropogenic vectors. With regards to green crab impacts, multiple oyster bed surveys conducted in 2014 measured mortality levels of vulnerable size classes of oysters. These surveys revealed that the probability of mortality of small oysters was higher in areas where green crabs were present. This probability decreased in the presence of other food sources (i.e. mussel beds), a result likely linked to green crab prey preferences. Field experiments using inclusion/exclusion cages indicated that the odds of oyster mortality was higher in inclusion cages than in the open environment and exclusion cages. These results suggested that at least some of the mortality of oysters could be attributed to green crab predation. With the recent implementation of a fishery for green crab as a potential mitigation strategy, there is a need for a marketable product to prompt fishermen to harvest green crabs. A soft-shell green crab product modelled after the Venetian “moleche” is expected to provide a lucrative incentive beyond the existing hard shell bait market. The examination of the timing and characteristics of molting revealed that synchronized “molting windows” or times of increased molting occur in the early summer for male green crabs. Recorded molting rates in the laboratory and the field were as high as 75% and 60%, respectively. External physical characteristics such as the presence of a “halo” on the episternites of the carapace, were deemed a reliable indicator that a crab would molt within 3 weeks. Although these results were preliminary in nature, they were promising and suggested that further studies be performed on the potential scaling up of these results. Fyke nets are a natural choice for a directed fishery of green crabs, as this type of gear is readily available and used locally, primarily for American eel (Anguilla rostrata). These nets are also a variant of the fyke net used in Italy to fish for a similar species of crab (Carcinus aestuarii) during its molting season. However, before fyke nets can be used in a directed fishery for green crab in Atlantic Canada, bycatch must be reduced. A novel Bycatch Reduction Device (BRD) for fyke nets was therefore developed and evaluated. Bycatch numbers and diversity, including three key commercial species, were significantly lower in the BRD equipped nets. Overall, the results of this thesis advance green crab research in the region, especially with regards to the quantification of impacts and development of mitigation measures. Further research questions as a result of my thesis work are identified and briefly discussed