Mortality of sea scallops Placopecten magellanicus in the Mid-Atlantic Bight: Comment on Stokesbury et al. (2011)

Abstract

Stokesbury et al. (2011; Mar Ecol Prog Ser 425:167−173) concluded that 10 billion sea scallops Placopecten magellanicus probably died from discarding during fishing operations between surveys in 2003 and 2004, based on the observed reduction in scallops between these 2 surveys. The reported mortality primarily occurred in a very large 2001 year class that was below commercial size in 2003−2004, whereas mortality rates were lower for larger commercial-sized scallops. This pattern is not consistent with mortality from discarding, because commercial scallop dredge gear is designed to retain commercial-sized scallops, allowing a substantial proportion of undersized scallops to escape through the dredge rings. Data from at-sea observers in the MidAtlantic indicate that only 319 million scallops were discarded during this period. Non-capture (incidental) fishing mortality was also too low to account for the observed mortality. However, juvenile scallop mortality due to predation increases with their density, primarily induced by crabs Cancer spp. The observed high mortality of juvenile scallops was therefore probably a result of increased predation, rather than discarding.KEY WORDS: Scallop · Cancer spp. · Predation · Fishery discards · Density dependence · Placopecten magellanicusResale or republication not permitted without written consent of the publisher

Larval Behavior and Protective Implications for Sea Scallops and Surf Clams within the Middle Atlantic Bight: Part One, a Sensitivity Study

“…Without considerations of larval mortality, released larvae were categorized into three groups, that is, those that successfully settled on the SWC within their lifespan (45/35 days for scallops/surf clam larvae) at a settlement size of 250 μm, those that still wandered within the SWC with a size smaller than the settlement size after their lifespan, and those that were lost into the deeper water column [6]. Settled larvae were generally analyzed for their settlement rate (percent of settled larvae in relation to all released larvae), drift distance, and redistribution percentage.…”

“…Sustainable populations of bivalves (e.g., sea scallops) are economically and ecologically important [1]. As the essential part of the population of bivalves, the larval population is generally determined by larval reproduction, settlement, fishing and predation, mortality, and area closure [2][3][4][5][6]. The initial stage for larval growth and the sustainable population of bivalves, larval settlement, is influenced by behaviors such as swimming with temperature-seeking beha, sinking, and growth rates [7][8][9][10][11][12].…”

“…A fast growth rate (e.g., for surf clams) makes larvae settle over a shorter period of time (e.g., ~35 days) and reduces the chance for circulation to transport larvae out of the shallow shelf, while slowly growing larvae (e.g., scallops) that fail to settle sooner (e.g., longer than 45 days) more than likely lose their chance of survival. From 1994 to 2005, area closures have helped increase the biomass of sea scallops within the Middle Atlantic Bight (MAB) by approximately eight fold [6,14].…”

Post-settlement predation by sea stars and crabs on the sea scallop in the Mid-Atlantic Bight

“…are predicted to result at an upper mortality rate of 62% at densities of 10 kg tow , and scallops are susceptible to crab predation over a broader range of sizes (Nadeau et al 2009). Additionally, crab predation rates on scallops are density dependent and may account for a recent scallop mortality event in this region (Wong & Barbeau 2005, Hart & Shank 2011. Given the broader range of scallop sizes that are susceptible to crab predation and that Cancer crabs have been recorded to eat up to 3 scallops d −1…”

“…Both models account for temporal and larger scale (across subregions) spatial variance with the random effects and, thus, analyze only the relationships between predator and prey at subregional spatial scales. Intra-cohort densitydependent mortality can occur in juvenile sea scallops (Wong & Barbeau 2005, Hart & Shank 2011, but this cannot be detected using our methods since we only have data on the abundance of sea scallop recruits after the density-dependent recruitment processes have occurred.…”