Sea scallops exhibit strong local spatiotemporal structure associated with seabed stability and high flows

Abstract

AbstractSea scallops (Placopecten magellanicus) live along the Northwest Atlantic continental shelf and are aggregated over a broad range of spatial scales (m2–105 km2). However, little is known about the spatial distribution of local scallop neighborhoods, the scale at which spawning, intra‐specific competition, and predator–prey interactions occur. We surveyed 30,995 km2 of Georges Bank and the Great South Channel annually from 2003 to 2010 counting the number of scallops in 54,016 locally replicated 3.24‐m2 quadrats with underwater video. There were about 4 billion scallops occupying 11,200 km2 of the study area in concentrations from 1 to 41 scallops per scallop. Scallop spatial distribution switched from dispersed to aggregated at a concentration of 3–4 scallops in every year. High concentrations (>3) occurred in 11% but were only persistent in 4% (449 km2) of scallop habitat. There were 13 persistent high‐concentration aggregations (7–217 km2), and all but two occurred on gravel‐dominated sediments left by prehistoric glacial retreat. Model‐derived benthic boundary shear stresses in the persistent aggregations averaged two times higher than in scallop habitat, but the seabed was about two times more stable because it had significantly less sand and more granule–pebble and cobble sediment. The area occupied by scallops each year varied little (9%) despite a 49% increase in total scallop abundance between 2005 and 2007, suggesting Georges Bank scallops have a proportional density population structure. Most scallops occurred alone or at low concentrations (≤2) where fertilization success is probably poor. The persistent high‐concentration aggregations we identified may be critical for successful reproduction and sustainable harvest. Observing organisms at scales corresponding to their individual interactions reveals important processes shaping their landscape‐scale spatiotemporal distributions. These processes are obscured by the methods typically employed to estimate abundances of commercial fishery species. The spatiotemporal structure of sea scallop distribution has important implications for the design of abundance surveys and the assumptions underlying stock assessment methods. These should be investigated. Finally, this work suggests that natural disturbance (sediment stability) may play a substantial role in shaping spatiotemporal distributions and dynamics of benthic marine populations.