Large-scale juvenile production of the blue crab Callinectes sapidus

Abstract

Rearing experiments and mass production of blue crab Callinectes sapidus juveniles were carried out as the first step in a feasibility study of blue crab stock enhancement in the Chesapeake Bay. During February through September 2002, four culture cycles were conducted with C. sapidus seed obtained from photoperiod-manipulated broodstock. A feeding protocol for early life stages, as well as for juvenile crabs, was established based on microalgae, rotifers, Artemia nauplii, and off-the-shelf diets. Developmental patterns of the different life stages were studied and growth and development kinetic curves were established. The captive rearing process was divided into two phases: (1) zoea 1–zoea 8/megalopa and (2) zoea 8/megalopa to 15–30 mm crab juveniles. Each of the phases was accomplished in both open system and recirculated system (RS). Different larval (zoea 1) stocking densities were tested. Within the examined range (40–110 individuals/l), no negative correlation was found for high density. Maximum survival to the zoea 8/megalopa stage was 74% at 95 larvae/l and the average was 30%. Cannibalism was found to be the main cause for blue crab mortality during the second rearing phase. In an effort to reduce the prevalence of cannibalism, experiments providing different shelter substrates, performing size grading, and decreasing stocking density were conducted. During this phase (zoea 8/megalopa until C₂–C₄), 57% survival was obtained in mass production tanks, using a shelter substrate of snow fence nets and stocking tanks with 2.5 individuals/l. Increasing stocking density by 16-fold (39 individuals/l) and doubling the shelter density resulted in production of about 3000 juvenile crabs/m³, although the survival rate dropped to 7.5% (or 7.6-fold). Additional studies are suggested to determine the tradeoff between survival and crab output, which in turn will establish the economic feasibility of the rearing operations. Cumulatively in the four culturing cycles, 40,000 juveniles were produced, of which 25,000 were individually tagged and experimentally released to the wild. This study is the first to mass-produce both postlarvae and juvenile blue crabs in captivity.

Introduction

Global crab fisheries have continuously declined over the last decade (FAO, 2002). As a result, crab aquaculture gained momentum, and production technologies for different species of crabs have recently been reported (Hamasaki, 1996, Keenan and Blackshaw, 1999, Aileen et al., 2000). In addition, efforts to replenish dwindling crab populations led to the establishment of major crab hatchery programs, most notably for the blue swimming crab (Portunus pelagicus), a species for which federal and prefectural hatcheries in Japan produce in excess of 50 million juveniles per year (Takeuchi, 2000).

The blue crab, Callinectes sapidus, represents the most valuable fishery in the Chesapeake Bay and the mid-Atlantic states of Maryland, Virginia, and North Carolina, with a tristate 2001 fishery value of US$150 million. However, during the last decade, Chesapeake harvests of blue crab have declined steadily, with about a 55% drop from the 1993 record-high catches to historically low levels of 22,362 MT (49.3 million pounds) and 23,586 MT (52 million pounds) during 2001 and 2002, respectively (Blue Crab Technical Work Group Report, 2003). More alarming than the decline of the harvests is the sharp drop in the Chesapeake Bay's spawning stocks: 81% in abundance and 84% in biomass (Lipcius and Stockhausen, 2002). Consequently, larval abundance and postlarval recruitment have been reduced by an order of magnitude (Lipcius and Stockhausen, 2002). Similarly, in North Carolina's Pamlico Sound, a precipitous decline in adult abundance (down 74%), spawning stock (down 75%), young-of-the-year (down 63%), and postlarval stages (down 71%) of the blue crab has been recently observed (Etherington and Eggleston, 2000, Etherington et al., 2003). Clearly, the combination of fishing pressure and destruction of coastal nursery habitats has driven the Chesapeake and Carolina crab populations to a crisis situation.

Most strategies aimed at reversing the declining trend of blue crab populations in the Chesapeake Bay involved the regulation of fisheries and the development of sanctuaries to protect the spawning stocks (Secor et al., 2002). The above cited declines in spawning stocks and larval abundance and recruitment suggest that the blue crab population in Chesapeake Bay is severely exploited and recruitment-limited, which has driven the Chesapeake Bay to be largely below its optimal carrying capacity for C. sapidus (Lipcius and Stockhausen, 2002). This situation makes the bay's blue crab an excellent candidate for stock enhancement (Blankenship and Leber, 1995, Munro and Bell, 1997) and has led us to test the feasibility of such an approach to replenish the blue crab's severely reduced breeding stocks (Davis et al., 2004a, Davis et al., 2004c).

As a first step in studying the feasibility of blue crab stock replenishment, and also in view of developing blue crab aquaculture, we set out to establish intensive hatchery and nursery technologies for massive production of blue crab juveniles. Although such technologies were previously developed for other portunid species, in particular the blue swimming crabs, P. pelagicus and Portunus trituberculatus (Hamasaki, 2000), hatchery production of the Chesapeake blue crab has never been accomplished. This is mainly due to the complex early development process of C. sapidus. While most cultured crabs go through only five (e.g., mud crab Scylla serrata) or four (e.g., P. pelagicus) larval zoeal stages (Takeuchi, 2000, Suprayudi et al., 2002, Hamasaki, 1998), the blue crab molts through eight zoeal stages that are significantly shorter than those of the other species (Costlow and Bookhout, 1959). Multiple early life stages require more elaborate feeding and larval rearing protocols. At the end of their zoeal stage, larval crabs metamorphose into megalopae, the stage at which they develop functional claws and become highly cannibalistic (Moksnes et al., 1997). The megalopa then transforms into the early crab instar (C₁ stage), which is the first postlarval stage having the body organization of the adult crab. Historically, cannibalism at the megalopa and juvenile crab stages has prevented any success in developing intensive nursery technologies for any species of crabs.

The present study describes the first successful year-round mass production of larvae and juvenile blue crab, C. sapidus, in intensive tank culture conditions.