Date of Award

Spring 2025

Project Type

Dissertation

Program or Major

Biological Sciences

Degree Name

Doctor of Philosophy

First Advisor

Bonnie Brown

Second Advisor

Stephen Jones

Third Advisor

Brittany Jellison

Abstract

Invasive species may alter how an ecosystem functions, such as the Great Bay Estuary (GBE) inNew Hampshire. In GBE, an invasive species of particular interest is the green cab (Carcinus maenas) because historically (and currently) there are no mitigation or eradication plans that have been implemented. It is not well-known how green crabs are interacting with oysters in GBE, whether that be wild, restored, or aquacultured. The aim of this dissertation was to document NH oyster growers’ perspectives on the issue of green crabs and oyster predation, understand their abundance, distribution and diet among natural/restored oyster areas and oyster farms as well as understand what size green crabs can consume what size aquacultured oysters. The NH oyster growers are the people who view oyster biology and ecology daily and their interactions with green crabs constitute crucial information to help understand the potential effects of this now well-established species. Integrating social aspects (i.e., perspectives) from the industry helps to identify challenges farmers face and what research may be beneficial to mitigate these challenges. This survey enhanced our understanding of how NH oyster growers view green crabs, what type of research is beneficial to them, and other issues that are of concern to them. From this survey, it was learned that trapping green crabs and studying their diet is beneficial to the oyster growers, they are concerned green crabs are threat to oysters, and other issues such as tunicates and drills may be even more troublesome. Trapping is a data gathering mechanism and a potential means of green crab mitigation. It helps provide data to improve management programs, inform oyster growers, and inform the overall GBE watershed community. Another study in this dissertation focused on trapping green xi crabs, in spring, summer, and autumn of 2021 and 2022 at two natural oyster reef/restoration areas and two oyster aquaculture areas. Crabs were weighed for mass (g), sexed, and measured for carapace width (mm) each week. Overall, the oyster farm area Cedar Point (most downstream site) had the highest estimated population of green crabs and Moody Point (most upstream site) had the least. The captures were biased heavily towards male and large crabs. Factors such as salinity, temperature, and surrounding habitat most likely affect the distribution and abundance of green crabs at these four sites. These data have enhanced our understanding of the estimated population, size, and sex of green crabs near various oyster populations in GBE. Great Bay is the estuarine home to diverse organisms including submerged aquatic vegetation, fishes, crustaceans, mollusks, tunicates, and more. Several native species in this area have undergone significant declines including eelgrass, Zostera marina, and Eastern oyster, Crassostrea virginica. Many species are expanding their ranges due to climate change or are introduced. Among those species that are not native, but that now are naturalized, are invasive species such as a European green crab, Carcinus maenas. These crabs have been shown to alter habitats and reduce biodiversity in this estuary. It is not well-known, however, what green crabs of GBE consume. Green crabs were collected from GBE, Cape Cod (Nauset Marsh), and Maine (Wells). Metagenomic analysis was performed on gut contents collected from adult green crabs to identify their prey. Sequences of gut content DNAs were analyzed to identify taxa using a pipeline at the UNH Hubbard Center for Genome Studies. Data were analyzed using a Two-way PERMANOVA based on site type/sex as well as month/year. A NMDS was used to help visualize similarities or dissimilarities among gut samples. This study focused on specifically the top eukaryotic orders found among green crab gut samples. Prey species that were of importance and xii interest to the NH oyster growers were oysters, oyster drills, and tunicates. Analyses showed green crabs in this region have an omnivorous diet with some differences spatially and temporally. As the northeast Atlantic coast waters warm, populations of some well-established invasive species, e.g., green crabs Carcinus maenas, are increasing. Concurrently, populations of oysters, Crassostrea virginica, have been decreasing coastwide due to non-climate related pressures such as disease and overharvesting. The possibility that green crabs could be a significant factor in the decline of oysters in GBE, was considered. Green crabs of different sizes were inserted into oyster cultivation bags to investigate whether green crabs are effective oyster predators and if so, what size of green crabs can pierce through what size of oyster shell. All oysters and bags were examined for signs of predation, mortality, and presence/absence of other predators. Findings showed that large green crabs (~7.6 cm, carapace width) could break open and consume medium and small oysters (~2.4cm-5.1cm), and predominantly smaller oysters (£5 cm, shell length) were most vulnerable. Videos showed green crabs were interested in oysters but often conceded defeat when attempting to break into them. Videos also showed crabs picking and eating biofouling organisms off bags, and fish swimming into and out of bags. These results can inform best management practices on farms about how to protect their oysters from green crab predation, especially due to the lack of permitting of floating gear in GBE and can aid restoration planning by illustrating that larger oysters (>7.5 cm) may be best for reef supplementation. This information can aid oyster restoration, oyster aquaculture, as well as give us a better understanding of how green crabs impact them and a need for more management on these invasive species in this watershed.

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