Applications of Population Genetics to Assess Survivorship
of Hatchery-Reared Crabs
The Japanese Sea Ranching Program has also completed successful large scale rearing of other portunid crab species, including the Scylla tranquebarica (mangrove crab), which, like Portunus trituberculatus, is primarily fished from August to October (Figure 11). A recent population genetics study of S. tranquebarica released in Urado Bay, a small semi-enclosed bay in southern Shikoku (Figure 1), provides a test of whether hatchery-reared portunid crabs may survive and grow to contribute to the reproductive stock of a sub-population (Kochi Fisheries Cooperative Association, Yokohama Branch). The study used mitochondrial restriction length polymorphism (mtDNA) haplotypes as an inherent, maternally inherited DNA marker that can be followed in progeny. A stock enhancement program was initiated in 1986 and reached a peak stocking level of nearly 200,000 C2 instar crabs in 1997. In 1995 and 1996, mtDNA haplotypes in the natural population and hatchery-reared crabs were determined. Nine major haplotypes were found in the natural population. In 1997, one haplotype (Type 2) was selected for propagation and release into Urado Bay. From analysis of a statistically appropriate sample size of 500 mature crabs (Kishino et al. 1994), the Type 2 haplotype accounted for 29% of the mtDNA genotypes. Based upon baseline Type 2 haplotype frequencies observed for 1995 and 1996, the authors estimated that 17% of the stocked individuals survived to join the reproductive stock. This estimate assumed that the Type 2 haplotype frequency in the natural population remained invariant between 1995-1997. Karakawa attempted to use mtDNA haplotypes to distinguish hatchery reared P. trituberculatus from wild stocks in Okayama Prefecture. Fourteen distinct P. trituberculatus haplotypes were found in the wild population (ranging from 44.2% to 1.1% in frequency). They chose haplotype Type 5 (4.2% frequency) as their founder stock for release. Upon subsequent recapture, they saw no change in haplotype Type 5 frequency in caught crabs, which they interpreted as being due to the low frequency of the founder stock haplotype in the wild population (should be > 20%). Although this study was not definitive, it does emphasize that the approach is widely applicable.
– Allen Place
Figure 11. Catch and hatchery release information for mangrove crab in Urado Bay (Figure 1, Shikoku Island, Japan). MtDNA markers indicate that released crabs have made contributions to the overall gene pool of the Urado Bay population. See text for details.