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\nmaenas), continues to spread and invade the coastlines of Atlantic Canada. In Prince Edward\nIsland in particular, populations have continued to expand since the late 1990s. In areas like these,\nthere is a distinct need to document this species’ expansion into new habitats, to monitor its\nimpacts on important native species, and to develop innovative mitigation strategies to control\nsome of its effects. This thesis addressed the large information gap that still exists on this species’\neffect in the Atlantic Canadian region by focusing on four main areas: First, I explored over a\ndecade of green crab population expansion records along Prince Edward Island’s main shorelines.\nSecond, I surveyed and examined the effects of green crabs on vulnerable size classes of an\nimportant shellfish species, the American oyster (Crassostrea virginica). Third, I performed a\npreliminary study into the occurrence and synchrony of molting in green crabs, identifying\nsuitable physical features of pre-molting crabs as a first step for developing a soft-shell crab\nindustry. And fourth, I developed and tested a novel bycatch reduction device to be used with\nfyke nets to facilitate the implementation of the aforementioned directed fishery for this species.\nSamples collected during the years 2000-2013 indicated that green crab expansion rates\non Prince Edward Island vary spatially and temporarily. Expansion rates were higher along the\nsouth shore than the north shore of the island, and it was hypothesized that this dissimilarity was\npossibly related to the higher availability of suitable habitat along the south shore. The speed of\nthe spread was such that it seemed more likely related to the benthic movement of juvenile and\nadult crabs than associated to larval spread or new anthropogenic vectors. With regards to green\ncrab impacts, multiple oyster bed surveys conducted in 2014 measured mortality levels of\nvulnerable size classes of oysters. These surveys revealed that the probability of mortality of\nsmall oysters was higher in areas where green crabs were present. This probability decreased in\nthe 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\nmortality was higher in inclusion cages than in the open environment and exclusion cages. These\nresults suggested that at least some of the mortality of oysters could be attributed to green crab\npredation.\nWith the recent implementation of a fishery for green crab as a potential mitigation\nstrategy, there is a need for a marketable product to prompt fishermen to harvest green crabs. A\nsoft-shell green crab product modelled after the Venetian “moleche” is expected to provide a\nlucrative incentive beyond the existing hard shell bait market. The examination of the timing and\ncharacteristics of molting revealed that synchronized “molting windows” or times of increased\nmolting occur in the early summer for male green crabs. Recorded molting rates in the laboratory\nand the field were as high as 75% and 60%, respectively. External physical characteristics such as\nthe presence of a “halo” on the episternites of the carapace, were deemed a reliable indicator that\na crab would molt within 3 weeks. Although these results were preliminary in nature, they were\npromising and suggested that further studies be performed on the potential scaling up of these\nresults. Fyke nets are a natural choice for a directed fishery of green crabs, as this type of gear is\nreadily available and used locally, primarily for American eel (Anguilla rostrata). These nets are\nalso a variant of the fyke net used in Italy to fish for a similar species of crab (Carcinus aestuarii)\nduring its molting season. However, before fyke nets can be used in a directed fishery for green\ncrab in Atlantic Canada, bycatch must be reduced. A novel Bycatch Reduction Device (BRD) for\nfyke nets was therefore developed and evaluated. Bycatch numbers and diversity, including three\nkey commercial species, were significantly lower in the BRD equipped nets.\nOverall, the results of this thesis advance green crab research in the region, especially\nwith regards to the quantification of impacts and development of mitigation measures. Further\nresearch questions as a result of my thesis work are identified and briefly discussed.

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