Alison Rowe - Estuarine Fish & Invertebrates
University of the Pacific, Stockton, CA
Analysis of Stomach Contents of the Blue Crab (Callinectes sapidus): a comparison of diet and prey preference of juvenile hatchery-reared and wild blue crabs
The spawning stock of the blue crab (Callinectes sapidus) in the Chesapeake Bay has declined over 80% since the early 1990s. Fewer mature females produce fewer eggs resulting in fewer larvae and even fewer juvenile blue crabs. The decrease in juvenile blue crabs has caused recruitment limitation; there are not enough juvenile blue crabs in the system and therefore many available habitats of the bay are underutilized. Because there are too few juvenile recruits to fill potential habitats, the blue crab population in the Chesapeake Bay is below carrying capacity. More specifically, most of the upper bay coves well suited for juveniles are far below their carrying capacity, while the coves of the lower bay may be at or only slightly below their carrying capacity.
The reduction of juvenile blue crabs in the upper bay is largely a result of the blue crab’s complex life cycle which begins with larval development on the continental shelf. Only later in the life cycle as juveniles do blue crabs move up the bay on currents looking for habitats in which to settle, feed and grow. Blue crabs exhibit density dependent behavior in habitat choice: they settle first in habitats of the lower bay, which are nearer to the continental shelf and, as the coves of the lower bay reach a threshold density, the unsettled crabs continue to travel up the bay. Small juvenile populations, a consequence of the decreasing spawning stock, result in diminished migration of juvenile blue crabs to the upper bay.
Stock enhancement is one way to address the declining populations of the blue crab in the Chesapeake Bay. Hatchery-reared crabs may be released into the bay, thus augmenting the natural population. Hatched from wild caught gravid females, these crabs are raised in hatcheries until they reach a size of about 20mm. Once blue crabs complete their larval development they are tagged with magnetic microwire and/or elastomer paint so that after release they may be distinguished from wild crabs. Hatchery-reared juvenile crabs are released into pre-selected coves and sampled to assess growth, survival, and mortality.
Growth and survival rates of juvenile crabs are significantly affected by the food resources available to them in the release coves. Data on food resources—in combination with environmental conditions and predator abundances—are crucial to determining optimal coves. Ideal coves will have an abundance of accessible, preferred prey items. This project addressed several factors related to the food resources blue crabs exploit in Quiet Waters, a South River cove on the western shore of Maryland’s upper Chesapeake Bay. Our first objectives were to determine the diet of the blue crabs living in the South River cove, and to compare the diet of wild crabs to the diet of hatchery-reared crabs. We also evaluated ontogenetic shifts in the diets of hatchery-released crabs by comparing the stomach contents of these crabs in two size classes: small (40mm-80mm carapace width) and medium (81mm-110mm carapace width) sized. The prey preferences of hatchery and wild crabs were studied by comparing the stomach contents of these crabs to prey available in the cove’s benthic communities. Our final goal was to determine whether quantities of prey available to blue crabs could be adequately surveyed in the field.
Three methods were used to collect blue crabs from Quiet Waters: seining, neuston netting, and towing with a benthic sled. After capture, crabs were put on ice to stop digestion and later sorted by size and source (hatchery or wild). The crabs were dissected for their stomach contents. A wet weight of the stomach was taken and the percent fullness of the stomach was estimated. The percent of each prey item in the stomach of each crab was recorded. Available prey items were determined by identifying and counting invertebrates in benthic samples.
The diet of blue crabs in Quiet Waters consisted of clams, plants, fish, mud crabs, blue crabs, amphipods, isopods, shrimp, worms, barnacles, insects, snails, and bryozoans. Hatchery-reared crabs ate significantly more clams than wild crabs, whereas wild crabs ate significantly more blue crabs and fish than their hatchery-reared counterparts. There was no difference between the stomach fullness of hatchery-reared and wild blue crabs. However, the stomachs of hatchery-reared crabs weighed significantly more than those of the wild crabs, which may be related to the observed differences in prey preferences between hatchery and wild crabs. Previous research has shown that blue crabs exhibit ontogenetic shifts in diet over their lifespan. In the present study, however, blue crabs from Quiet Waters evinced no ontogenetic dietary shift between small and medium sized crabs.
Our analysis of stomach contents revealed that 70% of the blue crab diet is composed of macrofauna (clams, fish, mud crabs, and blue crabs). It therefore appears that the food resources of a cove can be adequately assessed by the use of a field assessment tool. Such a methodology allows for more rapid assessment of available prey than does the customary strategy of removing benthic samples to the laboratory for processing.
Our study suggests that hatchery crabs are acquiring necessary food resources as efficiently as the wild crabs of Quiet Waters. Although hatchery-raised and wild crabs had similar diets overall, there were notable differences. Wild crabs consumed more fish and conspecifics than did hatchery-reared crabs which mostly consumed sedentary clams. The high incidence of a cannibalistic diet in wild crabs suggests that the ideal release-coves for hatchery-reared crabs are those with low densities of wild crabs.
Quote: The extended scientific community I encountered while at SERC, as well as the opportunity to contribute to the research of the Fish and Invertebrate Ecology Lab, has strengthened my commitment to enroll in a Ph.D. program in marine or conservation biology
Funding provided by the National Science Foundation – Research Experience for Undergraduates (REU)