Differences between Hatchery-Raised and Wild Blue Crabs: Implications for Stock Enhancement Potential

Abstract

Stock enhancement of severely exploited, recruitment‐limited fisheries has been controversial for several reasons, one of which is the lack of information about competency, competitiveness, and survivorship of hatchery‐reared individuals released into the field. Because enhancement efforts have focused on finfish, even less information is available with which to assess enhancement potential of crustaceans. The Chesapeake Bay stock of blue crabs Callinectes sapidus has declined by more than 80% over the past 12 years and has exhibited recruitment limitation, leading to recent efforts to study the potential of enhancing populations with hatchery‐reared juveniles. To assess how hatchery‐raised juvenile blue crabs may fare after release into the Chesapeake Bay, we compared several aspects of hatchery and wild crabs. Hatchery crabs readily fed on natural prey, moved in the field similarly to wild crabs, and grew at rates similar to those of wild crabs; however, the two crab groups differed in other factors important to field survival. Prerelease and laboratory‐held hatchery crabs had different carapace morphology (smaller spines) than wild crabs, though spine lengths increased to normal proportions by several weeks after release. Hatchery crabs did not initially bury in sediment as often as wild crabs, suggesting inexperience with an important predator escape response. Hatchery crabs were also preyed upon at higher rates in the field than wild crabs. Conditioning experiments suggest that inexperience with sediment and low burial rates were not the main cause of higher predation. By identifying areas in which hatchery individuals may be relatively weak and deficits that can potentially be mitigated, studies such as this can lead to improving the success of hatchery‐raised individuals in the field. On a broader scale, such studies also contribute to determining whether stock enhancement is possible in the case of the Chesapeake blue crab.

Do Hatchery-Reared Blue Crabs Differ from Wild Crabs, and Does it Matter?

“…Juvenile topshell (Trochus niloticus) possess spines that are thought to provide effective protection from predation by allowing them to physically lock into crevices (Purcell, 2002). We found that crabs maintained in the laboratory (both cultured and wild individuals kept for 30 days) had shorter spines than wild crabs (Davis et al, 2004(Davis et al, , 2005b (Figure 1). After time in the field, however, hatchery-reared crabs had spines of similar length to wild crabs, resulting from either individual spine increases (suggesting spine length is a plastic trait) or differential mortality of short-spined crabs (Davis et al, 2005b).…”

“…Under typical hatchery conditions (blue fiberglass tanks), cultured crabs were significantly bluer in hue, brighter, and less saturated in color than their wild counterparts (Davis et al, 2004). These differences, however, faded after cultured crabs were exposed to sediment for 1-2 days in the laboratory and 5 days in the field (Davis et al, 2005b).…”

“…Similarly, juvenile hatchery-reared topshell lack the spines on the lateral whorl of the shell that are present in wild individuals of similar size (Purcell, 2002). For both blue crabs and conch, the hatchery environment may impact shell/carapace shape by failing to provide the necessary cues or stimuli for proper development of predator defenses (Delgado et al, 2002;Davis et al, 2004). Inducible defenses, triggered by the presence of a predator, have long been noted to affect the morphology of prey species, most famously in freshwater Daphnia spp., where presence of a predator stimulates the lengthening of dorsal spines (Grant and Bayly, 1981).…”

“…Both the proportion of an individual's life spent in the hatchery and the number of generations broodstock have been held in captivity can be important in determining how divergent cultured individuals are from their wild counterparts (Fleming et al, 1994), and therefore how much conditioning is required. Identifying how hatchery-reared animals differ both morphologically and behaviorally from wild counterparts (Davis et al, 2004), and whether or not these differences contribute to decreased performance of cultured relative to wild individuals after release, is a critical step in assessing the potential of stock enhancement for any species. Laboratory and field experiments designed to identify and mitigate differences through conditioning may improve survivorship and increase the likelihood of successful releases.…”

Exposure of silver carp (Hypophthalmichthys molitrix) to environmentally relevant levels of cadmium: hematology, muscle physiology, and implications for stock enhancement in the Xiangjiang River (Hunan, China)

“…For decades, stock enhancement has been used to manage severely exploited recruitment-limited fisheries around the world [1,2]. However, for a number of reasons, wild populations have not always shown signs of recovery [3].…”