Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Mary C. Fabrisio
The blue crab is an iconic species in Chesapeake Bay, supporting important commercial and recreational fisheries and functioning as a critical link in the food web. Structurally complex habitats are often cited as nurseries for the blue crab, and other commercially important fish and crustacean species, by providing enhanced growth and survival for juveniles. I quantified the value of shallow habitats as nurseries for blue crabs through field studies and a demographic model. In Chapter 2, I utilized a two-year juvenile survey in vegetated habitats of the lower Bay to examine the effect of habitat complexity on the density of juvenile blue crabs. The functional relationship between seagrass cover and juvenile density was exponential, such that there were proportionally more crabs per unit increase in cover of vegetated habitat at high percent cover than at low percent cover. The relationship varied spatially, with higher densities on the eastern shore, and between the two years. The high spatial and annual variability led to questions about how habitat utilization varied throughout the recruitment season. I addressed the timing of recruitment and migration between habitats in Chapter 3 through the development of a survey of shallow habitats in the York River with high temporal and spatial resolution. The study provided evidence for a carrying capacity of juvenile blue crabs in vegetated habitats at 10-15 crabs m2. I found substantially higher densities of small juveniles in shallow unvegetated habitats than previously documented, which suggested that the current paradigm for blue crab recruitment requires modification to include the importance of shallow unvegetated habitats for small juveniles. In Chapter 4, I examined the effect of habitat utilization patterns as a function of age or ontogeny on the blue crab stock assessment by comparing juvenile density and abundance estimates from shallow vegetated and unvegetated habitats to estimates from deep habitats sampled by the primary survey for the stock assessment. Juvenile abundance was very high in both shallow habitats despite the relatively smaller area, thus suggesting that the winter dredge survey substantially underestimated the abundance of juvenile crabs. If this bias is inconsistent inter-annually, potentially as a function of temperature, then stock assessments may be producing biased reference points. Finally, I developed an exploratory habitat-specific demographic model to quantify the effects of habitat on population fitness in Chapter 5. Under all fishing mortality rates, including a complete fishing moratorium, the population growth rate was less than 1 when only unvegetated habitat was present; the increased survival of age-0 crabs provided by vegetated habitats led to increases in the population growth rates. The vegetated habitats provided a buffer from fishing mortality; that is, as the survival of juveniles increased in vegetated habitats, the population could sustain higher fishing mortality rates while still remaining stable or even increasing. Shallow vegetated habitats substantially influence juvenile blue crabs and the overall population growth rate. It is essential that these habitats be considered in future explorations of the dynamics of blue crabs, as well as other species that exhibit ontogenetic shifts in habitat utilization.
© The Author
Ralph, Gina M., "Quantification of Nursery Habitats for Blue Crabs in Chesapeake Bay" (2014). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539616819.