Abstract
In response to declining Chesapeake Bay blue crab populations, an effort to better understand the life cycle of the blue crab has been launched in hopes of using the information to create an effective hatchery system to restore the bay's populations. Cell size significantly affects the growth of numerous crustaceans, suggesting that a similar effect could be found with blue crabs. This study examines the effect of both cell area and depth upon the growth of juvenile blue crabs.
Literature Review
The blue crab (Callinectes sapidus Rathbun). Photo courtesy of Center of Marine Biotechnology, University of Maryland Biotechnology Institute
Only during the past century has the blue crab, Callinectes sapidus Rathbun, been the subject of experimentation and study. In earlier centuries the blue crab population was abundant and healthy, faithfully providing for a huge, profitable industry. Giving no cause for special study, the biology and life cycle of the blue crab remained largely mysterious. However, in this past century the blue crab population has become less abundant, giving alarm to those that rely on this economically and ecologically important crustacean and thus sparking experimentation and study. Several projects have confirmed that the population has been greatly reduced from what it was only 200 years ago by utilizing modern dredge surveys in comparison with past crabbing figures (Lipcius and van Engel, 1990; Rugolo, et al., 1998). As a result, further studies of the blue crab were finally put into motion to understand what factors affect the blue crab and what could possibly have caused this recent decline. A common subject of study has been the relationship between blue crabs and the declining number of seagrass refuges (Heck and Thoman, 1984; Ryer, 1987; Ryer et al., 1990), as well as with shallow water refuges (Dittel, et al., 1995; Hines and Ruiz, 1995). Tidal-stream transport and migration, once mysterious aspects of the blue crab life cycle, have also been studied, and it has been found that blue crabs do in fact utilize the tides to migrate throughout estuaries (Cargo, 1958; Olmi, 1994; Tankersley et al., 1998). Blue crab prey selection and response to stimuli, a complex formula that factors in relative size, distance, contrast to the surroundings, movement, and energy usage (Hughes and Seed, 1995; Hughes and Seed, 1997; Seed and Hughes, 1997; Taylor and Eggleston, 2000; Cote et al., 2001), is yet another important, studied aspect of the blue crab life cycle. As for the quality of the water itself and its impact on blue crabs, the adverse effects of hypoxia and anoxia on blue crabs (Tankersley and Wieber, 2000; Taylor and Eggleston, 2000) have been the most common subjects of study. Beyond these few examples, there are still many more new studies that cover a broad range of items related to the blue crab, such as early development, nutrition, and metabolism.
|
Juvenile blue crabs. Photo courtesy of Center of Marine Biotechnology, University of Maryland Biotechnology Institute.
None of these projects, however, have fully "unveiled the poorly understood, yet complex, basic biology and life cycle" of the blue crab (Zohar, 2002). Therefore, because of this uncertainty and the need for blue crab hatcheries that could restore the bay's populations, a joint, multi-state effort between Maryland, Virginia, North Carolina, and Mississippi was created in 2001 to better understand the blue crab population and to use the newly gained information to create an effective hatchery system (Zohar, 2002). Within its first year, this effort greatly advanced the current understanding of the blue crab life cycle and created a hatchery system that demonstrated the capability to produce tens of thousands of crabs (Zohar, 2002). Currently, this research is continuing so as to perfect a blue crab hatchery system, including the study of optimal conditions for juvenile blue crabs. The goal of these "optimal condition" experiments is to fine-tune a hatchery system to make it more efficient and worthwhile—in other words, to attain maximum growth per crab and a maximum yield of crabs. Ultimately, the goal is to perfect a blue crab hatchery system that is capable of restocking the Chesapeake Bay. Specifically, a separate-cell hatchery system will be used to produce adults that can be used for study or for spawning. The adult females capable of spawning could then be released into protected areas unthreatened by crabbers and spawn in the Chesapeake Bay itself. Since females can produce as many as 8 million eggs over a period of several sponges (Chesapeake Bay Program), a relatively low release count of adult females could greatly boost the crab populations in the bay.
It is common knowledge that blue crabs are cannibalistic once they reach the juvenile stages where they grow claws. This means that a single population tank for a crab hatchery would be inefficient, since the larger-sized half of the population will cannibalize the smaller-sized half of the population. The solution to this problem is to use a separate cell system, where each individual crab is in its own solitary cell, or cage. One of the single most important questions that must be addressed when developing this type of hatchery system is what impact, if any, cell size has on the growth of juvenile blue crabs. If cell size does affect the growth of the crabs, then cell size would correspond to crab size, which would dictate how large the hatchery system needs to be, and ultimately whether or not the blue crab hatchery system is a viable option for blue crab production for the restocking of the declining populations within the Chesapeake Bay. With this question in mind, the purpose of this project was to study the impact of cell size on the growth of juvenile blue crabs, Callinectes sapidus Rathbun. The impact of cell size, both area and depth, on juvenile blue crab growth were analyzed for statistical significance.
|
