Survival analysis of tethered juvenile sea scallops Placopecten magellanicus in field experiments: effects of predators, scallop size and density, site and season

Abstract

The impact of predation on survival of juvenile sea scallops Placopecten magellanicus has important implications for the population dynamics and aquaculture of this species. In field experiments using tethered juvenile scallops, we examined the effect of biological and physical factors, such as prey size, predator and prey densitv, water temperature (season) and site, on mortality rates of scallops in Lunenburg Bay, Nova Scotia, Canada. The major predators of juvenile scallops were crabs Cancer irroratus and sea stars Asterias spp. In an experiment manipulating scallop density, crab predation rate increased significantly with scallop density, but crab density did not, suggesting a functional response. Sea star predation rate on scallops was not affected by scallop density. In a series of experiments with different slze classes of scallops tethered at different sites and seasons, and In different densities of surrounding scallops and predators, the effects of covariates on scallop survival times were assessed using the statistical procedure of survival analysis. Since the tethering technique affected crab and sea star predation differently, the competing causes of scallop mortality were analysed separately. Water temperature, site, and scallop density were the important variables affecting crab predation. There was a significant interaction between temperature and site, such that crab predation on scallops increased with temperature at one site and was independent of temperature at the other site. Predation by crabs also increased with scallop density and, to a lesser extent, with crab density. Water temperature and scallop size were the primary variables affecting sea star predation. Sea star predation on scallops increased with temperature and decreased with scallop size. The results of this study suggest options for minimizing predation of scallops by crabs and sea stars in bottom culture operations.

Why do juvenile fish utilise mangrove habitats?

“…Tethering techniques have the shortcoming of potentially producing experimental artifacts that complicate meaningful ecological interpretation of results (Peterson and Black, 1994;Micheli, 1996;Curran and Able, 1998). Artefacts are generally related to interference of tethers with escape responses causing increased susceptibility to predators (Barbeau et al, 1994;Zimmer-Faust et al, 1994) or to tethers inducing behavioural changes in response to habitat which may make prey more or less susceptible to predators (Curran and Able, 1998). Peterson and Black (1994) also suggested that artifacts induced by tethering could be habitat dependent due to differences in predator assemblages.…”

Size-related changes in vulnerability to predators and spatial refuge use by juvenile Iceland scallops Chlamys islandica

“…After 14 d , we noted whether scallops were living or dead, and in the latter case, whether the valves were crushed or intact. Crushed valves were assumed to result from decapod predation and intact valves from asteroid predation or other sources of mortality (Barbeau et al 1994, Stokesbury & Himmelman 1995. Vulnerability to predators was calculated as the proportion of scallops which had been preyed upon by each predator group.…”

A preliminary study of the behaviour and vitality of reseeded juvenile great scallops, of three sizes in three seasons

“…Apart from the quality of animals, the success of seeding depends on the size of the juvenile and on the seeding season (Barbeau et al, 1994). The aim was to monitor the seeding stress of three batches of different sizes in three different seasons: spring, summer and autumn (winter seedings were not advised due to adverse weather conditions).…”