Impacts of newly established non-indigenous green crab (Carcinus maenas) on native fauna in Placentia Bay, Newfoundland

The recent invasion of the European green crab (Carcinus maenas) populations in Placentia Bay, Newfoundland and Labrador (NL) raises great concern about potential impacts on local fisheries and native biodiversity. Green crab are highly adaptable and in both native and invaded areas, green crab are well established predators that can outcompete other similarly sized decapods. The main objectives of this thesis were to: 1) identify the native species that green crab compete with for resources; 2) determine the depths and substrate types in which these interactions likely occur; 3) assess the indirect effects of green crab on native crustaceans and their changes in behavior; 4) assess the impacts of green crab on benthic community structure; 5) compare the NL population with other Atlantic Canadian populations in terms of competitive abilities; and 6) compare morphological features of the NL population with other Atlantic Canadian populations. I found that green crab overlap in space and diet with both rock crab (Cancer irroratus) and American lobster (Homarus americanus), potentially leading to a shift in habitat. Laboratory studies on naïve juvenile lobster also suggested shifts in behavior related to green crab, in that lobster decreased foraging activity and increased shelter use in the presence of green crab. Benthic community analyses showed fewer species in mud, sand, and eelgrass sites heavily populated by green crab compared to sites without green crab, although results depended on the taxa involved and I could not eliminate environmental differences through a short term caging study. Foraging ability of green crab varied in intraspecific competition experiments, with populations from NL and Prince Edward Island dominating longer-established populations from Nova Scotia and New Brunswick. Additional studies excluded claw size as a factor driving these results and behavioral differences likely reflected differences in invasion time and population genetics. Overall, green crab in Placentia Bay appear to be altering community structure of benthic invertebrates through predation and they also appear to indirectly impact native crustaceans through competition.

INVERTEBRATE CONTRIBUTION TO LEAF BREAKDOWN IN PYNN’S BROOK, NL.

Leaf bags of fine and coarse mesh were placed at two locations, one with an open tree canopy, the other with a closed tree canopy, in Pynn’s Brook on June 30th 2015. Bags were collected after 2, 30, 37 and 44 days. After collection, invertebrates were counted and leaf material remaining was determined to measure leaf breakdown rate. There was no significant difference in leaf mass remaining (R) between the two sites. Comparisons between mesh types found a difference in leaf breakdown at two collection days. The difference at 2 days was small (2.7%) and may not be biologically meaningful. At 37 days, the difference was larger (8.41%) and may be related to a larger proportion of shredder taxa, seen in coarse mesh bags, or higher absolute numbers of invertebrates. The invertebrate community was dominated by Diptera spp. across all collection days and mesh types, but after 37 days, communities in coarse mesh bags had a higher proportion of shredder orders than did fine mesh bags.