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

Abstract

Complex 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.

First Observations of Long‐Distance Migration in a Large Skate Species, the Winter Skate: Implications for Population Connectivity, Ecosystem Dynamics, and Management

“…The AMPC hypothesis does not assume an underlying population structure but instead argues that regional dynamics, including observed regional biomass fluxes, are influenced and/or driven by the movement of adults. These results add to growing evidence for the importance of long-distance movements that control population connectivity and/or meta-population dynamics and control regional temporal abundance in marine teleosts and elasmobranchs (O'Leary et al 2013;Rochette et al 2013;Frisk et al 2014;Chapman et al 2015;Andr e et al 2016;Archambault et al 2016;Morse et al 2017;Midway et al 2018;Wang et al 2018). However, our observations showed only the capacity for long-distance migration patterns, and this study was not able to detect the large-scale and periodic movement of individuals as envisioned by Frisk et al (2008).…”

Changing climate associated with the range‐wide decline of an estuarine finfish

“…High steepness values have been estimated for similar species, such as summer flounder ( Paralichthys dentatus ), where recruitment was determined to be mostly independent of SSB (Maunder, 2012). Specific to southern flounder, Midway et al (2018) determined that in order to maintain population levels seen in North Carolina under high levels of fishing pressure, southern flounder not only were exhibiting a high degree of compensatory recruitment, but there also was likely a cryptic population offshore contributing to reproduction. The assumed high degree of density‐dependent compensation makes southern flounder more resilient to overfishing (Davis et al, 2015).…”

Simulating fish population responses to elevated CO2: a case study using winter flounder

“…A cautionary note is that the model-generated steepness values, especially in Decade 4 with Severe CO 2 conditions (0.35−0.42), were lower than reported values and, while reflective of the Severe effects within our virtual world, are unrealistic (see Rose et al 2001). Shertzer & Conn (2012) and Midway et al (2018) included values of steepness as low as 0.2 in their analyses for flatfish but noted that these were included as extremely low values to illustrate their analyses. Our values of steepness are meant for comparison of relative changes among simulations and do not represent empirically based realistic values in absolute terms and are not intended for management use.…”