Influence of reproductive and post-settlement processes on recruitment in scallops

Knowledge about the key factors influencing recruitment of exploited benthic marine invertebrates is important, as it provides information necessary for sustainable fisheries management and population recovery. This information is especially relevant for stocks that have been severely depleted, such as the populations of Pecten fumatus, Equichlamys bifrons and Mimachlamys asperrimus in the D‚ÄövÑvºEntrecasteaux Channel, Tasmania, Australia. Stock declines related to a combination of fishing pressure and recruitment failure have raised concerns about density related mechanisms that could influence recruitment. Correlations between numbers of recruits and numbers of adults have led to hypotheses that distribution patterns of adults might be explained by habitat-related characteristics. Therefore this study examined reproductive and post-settlement processes likely to contribute to patterns of recruitment in scallops, with a particular focus on P. fumatus to identify strategies for management and conservation of scallop populations in south-eastern Tasmania. This aim was addressed using a combination of field experiments, underwater observations and laboratory studies. Spatial patterns of distribution and abundance for the three species of scallops were explained by sediment type, habitat structural components, and/or presence of predators. However, the nature of the relationships between these factors and the distribution patterns differed markedly among species. While Pecten fumatus was strongly associated with finer sediments and Equichlamys bifrons with coarse grain sediments, Mymachlamys asperrima had a less selective association, possibly related to its ability to attach on a wide range of substrates. Other habitat characteristics explaining the abundance of P. fumatus were depth, Asterias amurensis abundance, shell and macroalgae cover. Equichlamys bifrons was strongly associated with macroalgae and seagrass cover, whereas M. asperrima abundance was greatly explained by sponge cover. These relationships are likely mediated by predation pressure as well as the specific behavioural characteristics of each species. The findings highlighted the specific habitat characteristics relevant for spatial management and habitat restoration plans. The role of predation during early post-settlement stages was explored by assessing the survival of recently settled Pecten fumatus using a range of field experiments and sampling. The role of the macroalgae Hincksia sordida as a settlement substrate and as a refuge from predation for this species was examined. Predation on spat and juveniles was a major factor affecting local population sizes; mortality rates up to 95% during the first weeks after settlement appeared to have prevented the establishment of an adult population at the study site as few adults were found during three consecutive sampling years. While macroalgae provided settlement substrate for spat, higher macroalgal biomass did not offer increased protection from predation during the juvenile phase. This appears to be linked to the recessing behavior of P. fumatus, which may be hindered or prevented when algal biomass is high. When recessed into the sediment scallops are assumed to be less vulnerable to detection by predators. Thus the interplay between prey behavior and substrate characteristics was considered important in determining scallop survival. Pecten fumatus is a simultaneous hermaphrodite with a protracted spawning season from October to March supported by stored energy reserves early in the spawning season, whereas later in the spawning period energy from oocyte breakdown provides an energy source for development of new oocytes when primary productivity levels were low. Protracted spawning represents a bet-hedging strategy that would ensure some recruitment by increasing the probability of offspring survival when environmental conditions are unpredictable. However, for low density populations the advantages of protracted spawning may not be fully realized because of the negative relationship between density of spawning stock, aggregation patterns, and synchronization of spawning. Areas with lower densities of scallops had less small scale aggregation and increased nearest neighbour distances. Spawning synchronization was highly variable throughout the season, with 3.5-59.8% of individuals spawning at a given time. Pecten fumatus was more likely to spawn when present in high density aggregations and when in closer proximity to conspecifics. Reducing densities not only reduces the number of individuals contributing to the production of gametes, but also reduces the synchronization of spawning and rate of gamete release. At density levels currently observed in the D‚ÄövÑvºEntrecasteaux Channel it is probable that most individuals are at distances too great for fertilisation. Also, this study suggests that despite being a hermaphrodite, P. fumatus seems to be favouring cross-fertilization over self-fertilization. By examining biological and ecological factors, this research identified a number of factors that may hinder the effective recovery of Pecten fumatus stocks within the area despite protracted closures to fishing. Specifically, this research highlights the benefits of maintaining areas with a minimum density of 0.2 ind.m-2 of P. fumatus, which could be achieved under the current rotational harvesting system in south-eastern Tasmania. In the case of the D‚ÄövÑvºEntrecasteaux Channel, however, the populations are currently severely depleted and, being most likely self-recruiting, alternative restoration efforts such as transplantation or restocking may be warranted. These efforts would benefit from targeting species specific habitat characteristics, for example areas with greater sponge cover for M. asperrima, greater seagrass cover for E. bifrons and lower macroalgal cover for P. fumatus.