This research was published in the INSPIRED: A Publication of the New Hampshire Agricultural Experiment Station (Fall 2024)
Researchers: A. Strickland, K. Meyer-Rust, E. Williams, G. Bradt and B. L. Brown
The Gulf of Maine is projected to continue enduring numerous ecological changes due to climate change. Increasing water temperatures can shift marine species' habitats, and studies have shown that benthic organisms will expand northward as temperatures increase. Great Bay Estuary in New Hampshire, within the Gulf of Maine region, is experiencing notable ecological changes due to warming waters, including an invasion of non-native species that are successfully establishing sustained populations. An example of a marine organism shifting its range northward is the Atlantic blue crab (Callinectes sapidus). Not only can these crabs affect native species, but they also can affect the flourishing oyster aquaculture industry Great Bay supports.
Fig. 2. Map of Great Bay Estuary, New Hampshire showing location of crab traps at oyster reefs (blue diamonds) and oyster farms (orange diamonds). Sites include Cedar Point (CP), Fox Point (FP), Moody Point (MP) and Nannie Island (NI).
Mated blue crabs were first documented in Great Bay Estuary, New Hampshire, in September and again in October 2022—evidence of a continued habitat range expansion. The presence of blue crabs may threaten local species, such as eelgrass and oysters, risking declines that could contribute to further ecological imbalances. Strategic and continued monitoring of blue crabs in Great Bay and proactive development of regional adaptive management plans are key to reducing adverse impacts of the blue crab range expansion.
Background and Key Concepts
The New Hampshire oyster aquaculture industry has seen substantial growth in recent years, going from two businesses in 2010 to 32 in 2023. Oyster farms contribute significantly to the local economy and provide essential ecosystem services such as water filtration, excess nutrient mitigation, shoreline stabilization, and habitat for other organisms. However, oysters and other bivalves can be preyed upon by blue crabs. While New Hampshire has not been a traditional ecosystem for blue crabs, climate change is expanding their habitat range. Monitoring the presence and movement of these predators will help mitigate impacts on both the native and farmed oysters and the associated industry.
Key Findings
- Mated blue crabs were first documented in Great Bay Estuary, New Hampshire, in September and again in October 2022—evidence of a continued habitat range expansion.
- The presence of blue crabs may threaten local species, such as eelgrass and oysters, risking declines that could contribute to further ecological imbalances.
- Strategic and continued monitoring of blue crabs in Great Bay and proactive development of regional adaptive management plans are key to reducing adverse impacts of the blue crab range expansion.
About the Co-author
Bonnie Brown, Professor of Biological Sciences
Contact information: Bonnie.Brown@unh.edu, FindScholars profile
Fig. 1B. A Chesapeake cube style blue crab trap from Ketcham Supply, New Bedford, MA.
Methodology
Researchers deployed traps (Fig. 1A and 1B) at four sites in Great Bay Estuary (Nannie Island, Moody Point [outside the mouth of Lamprey River], Fox Point, and Cedar Point) (Fig. 2) to collect crabs from April through November 2022. In July 2022, additional traps specifically designed for blue crabs were introduced.
All traps were baited with preserved herring or frozen mackerel and were monitored weekly. All captured blue crabs were bagged and frozen for later analysis of size, sex, and diet. All crabs were wet weighed (in grams), measured (carapace width in millimeters), and molted females were dissected to determine the presence of sperm plugs.
Discussion of Findings
Fig. 1A. Trapezoid green crab traps from Brooks Trap Mill, Thomaston, ME.
On September 2, 2022, the first documented mated pair of blue crabs (Callinectes sapidus) in Great Bay Estuary (GBE) were captured at Nannie Island. A second pair was captured on October 7, 2022, at Fox Point. This confirmed the presence of an emerging blue crab population in GBE. The male caught in September weighed 159 g (wet) and its carapace width was 134 mm. The female, from the same sampling date, weighed 145 g (wet) with a carapace width of 155 mm. The male caught in October weighed 194 g (wet) with a carapace width of 141 mm, and the female, from the same sampling date, weighed 146 g (wet) with a carapace width of 147 mm.
Fig. 3. Blue crabs (Callinectes sapidus) caught in Great Bay Estuary, NH. (A) Male (top) and female (bottom). (B) Distended seminal receptables with sperm plugs (arrows).
The discovery of mated blue crabs in GBE marks the first scientific documentation of such an event. Blue crabs could have significant implications for the Estuary’s ecosystem, particularly for the already vulnerable oyster populations, as captures happened at an oyster reef and at an oyster farm. Both captured females (Fig. 3) showed clear signs of recent insemination, with turgid seminal receptacles and pink sperm plugs. This finding extends previous observations of blue crabs in the Gulf of Maine, suggesting that the species is establishing a breeding population in GBE.
A further study of the diet of these and additional blue crabs captured in 2023 is underway. If blue crabs are verified to be consuming oysters, they may be a factor affecting oyster populations due to predation. The result could lead to a trophic cascade, increasing primary production and disrupting the ecological balance of GBE.
Strategic Recommendations and Conclusion
To mitigate the potential impact of blue crabs in GBE, it is crucial to implement targeted monitoring and management strategies. Further observations are needed to confirm the establishment of a sustained blue crab population, including the detection of gravid females and early-stage larvae. Collaboration with local end users, enabling activities such as placing traps near oyster farms and restoration areas, can provide valuable data, enhance monitoring efforts, and provide protection by trapping and removing crabs. Additionally, exploring methods to control the blue crab populations, such as selective trapping, can help protect native species. Policymakers should consider adaptive management plans that address the dynamic nature of invasive species and climate change.
The documentation of mating blue crabs in GBE and continued captures afterward highlights the increasing urgency to address the ecological implications of range-expanding marine species. This discovery underscores the importance of continued research and monitoring to understand and manage the impacts on local ecosystems. Community involvement and proactive management strategies are essential to mitigate the potential threats posed by blue crabs and to ensure the resilience of GBE’s native species. Current monitoring of blue crabs by trapping throughout GBE is underway through collaboration of GBNERR, New Hampshire Sea Grant, and Wells Research Reserve.
This material is based on work supported by the NH Agricultural Experiment Station through joint funding from the USDA National Institute of Food and Agriculture (under Hatch award number 1026105) and the state of New Hampshire.
