Kingsley-Smith, P. R. | Search bioRxiv

bioRxiv2023

Evidence for a Male Bias in Atlantic Blue Crab Pot-Based Sampling

The Atlantic Blue Crab Callinectes sapidus is an ecologically- and economically-important species that supports one of the highest valued commercial fisheries in coastal South Carolina, USA. Researchers at the South Carolina Department of Natural Resources conduct multiple surveys to monitor the status of the Atlantic Blue Crab using a variety of gear configurations. Demographic characterizations, however, can often be influenced by sampling gear-related biases. We compared blue crab sex ratios between passive, pot-based, sampling and active, trawl-based, sampling across three estuaries in the fall and for year-round sampling in a single estuary. For fall sampling, the percent of males in pots was 20.1% higher than values observed for trawl-based sampling, while an overall male bias of 22.7% was observed for year-round sampling; however, this bias was only significant in certain months. Our findings suggest that while different sampling gears may offer their own advantages, such as greater suitability to effectively sample specific habitats, the resulting characterizations of population demographics can differ among gear types. Recognizing gear related biases is important for ensuring that field surveys are representative of populations, particularly when sex-specific modeling approaches are used as part of stock assessments to determine population status.

ecology↗

bioRxiv2022

The use of alternative spawning habitats by the American horseshoe crab, Limulus polyphemus

For animals that develop externally, habitats where environmental conditions are optimal for embryonic development are sometimes assumed to represent the highest recruitment potential and thus support the majority of reproductive output for a species. However, organisms may spawn in areas considered sub-optimal for embryonic development. Thus, understanding spawning habitat selection decisions and their potential impacts on recruitment and ecological interactions is necessary for predicting population status and identifying critical habitats to inform sustainable conservation decisions and effective management approaches. The American horseshoe crab, Limulus polyphemus, is ecologically, economically, and biomedically important. Females come ashore to spawn in the sediment where eggs develop for 2 - 4 weeks. Horseshoe crabs have been thought to primarily use sandy beach habitat for spawning in part because this habitat has been shown to be optimal for embryonic development. Horseshoe crab eggs on sandy beaches are an essential part of the diet of many organisms, including shorebirds such as the rufa red knot which requires the eggs to fuel their migration to arctic spawning grounds. While horseshoe crabs have been observed spawning in alternative habitats such as salt marshes and peat beds, this behavior has been assumed to be rare and non-adaptive. In this study, we compare the use of beach and alternative habitats by horseshoe crabs for spawning. To do so, we conducted adult horseshoe crab spawning surveys and horseshoe crab egg surveys in beach and Spartina-dominated salt marsh alternative habitats in South Carolina, Connecticut, and New Hampshire, U.S.A. While spawning horseshoe crabs were more likely to be observed on beach habitats than in alternative habitats, potentially due to logistical constraints surveying alternative habitats, we found similar densities of spawning horseshoe crabs in both habitat types. We also tended to find more eggs in alternative habitats than on beaches. Taken together, these results suggest that alternative habitats likely represent a significant source of horseshoe crab spawning activity and recruitment that had not previously been quantified. We recommend this information be incorporated into horseshoe crab population assessments, habitat protections, and more directed research at understanding variability in habitat-specific horseshoe crab spawning and its relationship to migratory shorebirds.

ecology↗