Reproductive biology of farmed purple-hinged rock scallop (Crassadoma gigantea)

Abstract

Native species are increasingly chosen for aquaculture expansion to avoid negative environmental impacts associated with the introduction of foreign species. However, the culture of native species is not without risks, as interbreeding with wild populations and can impact naturally occurring genetic diversity and population structure. The risk of adverse impacts depends, among other factors, on the frequency of interbreeding and genetic diversity of hatchery produced animals. Thus, cultured stocks that reach sexual maturity prior to harvest and have reduced genetic diversity because of small effective broodstock size, Nb, are particularly risky. Here, we evaluated both factors, first by establishing age and size at maturity and second, by determining the distribution of reproductive success and Nb in a voluntary mass spawn of purple hinged rock scallop, Crassadoma gigantea.Age and size at first maturation were determined by repeat sampling of a known age cohort grown in Dabob Bay WA for 25 months. To establish the effects of local environment on first maturation additional scallops were deployed in Totten Inlet, Dabob Bay, and Neah Bay for ten months and sampled once at the end of growout. Size and age at first maturation was 55.2 mm shell height and 25.1 months of age, well before the projected harvest size and age for cultured C. gigantea. The additive effect of age and size produced the most accurate prediction of first maturation in C. gigantea, and there was a significant effect of environment on first maturation suggesting that under optimal conditions first maturation could be even earlier than our size and age prediction. To determine the distribution of reproductive success in a mass spawn of cultured C. gigantea, scallops were deployed at the three Washington State sites were sequenced using restriction site-associated DNA sequencing to identify SNPs for parentage assignment in COLONY. Three methods were then used to calculate the Nb from sibship assignments (COLONY), parentage assignment without parents (PwoP) and linkage disequilibrium (LDNe). A total of 408 loci were identified for parentage analysis and offspring were assigned to 74.5% of candidate parents at one or more sites. Survival was family specific, as Nb was significantly different across sites and Nb/N ratios were significantly lower than 1.00 at all sites (Totten Inlet=0.33, Dabob Bay=0.26, Neah Bay=0.15). Low Nb/N ratios suggest a loss of genetic diversity in the hatchery which is worsened by mortality on during growout and are an important consideration when determining the risk of genetic diversity loss in wild populations in the event of interbreeding. Unless wild population Ne/N ratios are orders of magnitude lower than their cultured conspecifics than escapes will reduce the genetic diversity of wild populations. Our study determined that interbreeding between cultured and wild C. gigantea is possible, but the magnitude of this risk is reduced by the likely protandry and skewed sex ratio of young C. gigantea. In addition, increasing the number of broodstock used in each spawn, equalizing parental contributions prior to fertilization and using partial or full factorial spawn designs will help increase the Nb of cultured C. gigantea.