The Potential for Cryptic Population Structure to Sustain a Heavily Exploited Marine Flatfish Stock

Abstract

AbstractComplex spatial structure is widely viewed as an attribute that can contribute to stability in fish populations. Depending on aspects of stock demography and productivity, the existence of complete or partial spatial refugia can enable population persistence when faced with high rates of exploitation. The Southern Flounder Paralichthys lethostigma has been harvested extensively by inshore fisheries in North Carolina for more than three decades, with estimates of fishing mortality (F) surpassing 2.0 year−1 in several years and systems. Potential hypotheses to explain population persistence under heavy exploitation include high levels of recruitment at low stock sizes (high steepness) and/or the maintenance of adult biomass in offshore habitats that remain cryptic to the fishery. We constructed and applied an age‐structured matrix model to evaluate the potential for cryptic stock structure, combined with varying levels of steepness in the stock–recruit relationship, to sustain the Southern Flounder stock across a range of plausible exploitation scenarios. Model simulations predicted that both high steepness in the stock–recruit relationship and large fractions of cryptic biomass were necessary to maintain even modest levels (~25%) of unfished biomass at the high estuarine harvest rates that have likely occurred since the inception of the fishery. When the estuarine fishery exploited the stock at an elevated rate (F > 1.0), high steepness in the stock–recruit relationship alone was insufficient to maintain population biomass at sustainable levels. Only maintaining a large fraction of the adult stock within the spatial refuge afforded by offshore habitats could offset high rates of estuarine exploitation and maintain more conservative levels (40% of unfished biomass) of population biomass. Future efforts should be focused on the identification and preservation of spatial structure within the Southern Flounder stock to maintain both age structure and fishery yield.