Underwater video as a tool to examine American lobster (Homarus americanus) trapping behaviour and interactions with invasive European green crab (Carcinus maenas)

Zargarpour, Nicola (2018) Underwater video as a tool to examine American lobster (Homarus americanus) trapping behaviour and interactions with invasive European green crab (Carcinus maenas). Masters thesis, Memorial University of Newfoundland.

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Abstract

Species invasions pose a global threat to marine biodiversity, but their impacts can be complex and nonlinear. The European green crab (Carcinus maenas) is a marine invasive species present in nearshore waters of insular Newfoundland (NL). While the negative impact of green crab on eelgrass (Zostera marina) – a habitat-forming species – is well-understood, their direct impact on local fisheries is unclear. The collapse of American lobster (Homarus americanus) catch rates in green crab-invaded systems in Placentia Bay, NL led industry to suspect that the invasive species may be interfering with the ability of lobster traps to catch their target species. In this thesis, I examine whether green crab in and around lobster traps affect catch and retention rates of lobster using underwater video to observe traps as they fished. Our results from 2015 showed little co-occurrence between lobster and green crab in both catch and video data. However, this research provided the information necessary to design future quantitative studies involving the impact of invasive green crabs on the catchability of lobster, which we examined during a second field season in 2016. During this field study we conducted an in situ experiment in which we tethered green crabs in lobster traps, and assessed impacts on gear performance using scuba divers, underwater cameras, and by comparing catch rates of traps. Our primary finding was that crabs tethered in traps reduced lobster entry rates into the gear, but that the effect occurred both with invasive and native crabs.

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