Implications of late-in-life density-dependent growth for fishery size-at-entry leading to maximum sustainable yield

Abstract

Currently applied fisheries models and stock assessments rely on the assumption that density-dependent regulation only affects processes early in life, as described by stock–recruitment relationships. However, many fish stocks also experience density-dependent processes late in life, such as density-dependent adult growth. Theoretical studies have found that, for stocks which experience strong late-in-life density dependence, maximum sustainable yield (MSY) is obtained with a small fishery size-at-entry that also targets juveniles. This goes against common fisheries advice, which dictates that primarily adults should be fished. This study aims to examine whether the strength of density-dependent growth in actual fish stocks is sufficiently strong to reduce optimal fishery size-at-entry to below size-at-maturity. A size-structured model is fitted to three stocks that have shown indications of late-in-life density-dependent growth: North Sea plaice (Pleuronectes platessa), Northeast Atlantic (NEA) mackerel (Scomber scombrus), and Baltic sprat (Sprattus sprattus balticus). For all stocks, the model predicts exploitation at MSY with a large size-at-entry into the fishery, indicating that late-in-life density dependence in fish stocks is generally not strong enough to warrant the targeting of juveniles. This result lends credibility to the practise of predominantly targeting adults in spite of the presence of late-in-life density-dependent growth.

Saving large fish through harvest slots outperforms the classical minimum‐length limit when the aim is to achieve multiple harvest and catch‐related fisheries objectives

“…Moreover, letting the fish growth until a cohort reaches its maximum biomass before harvest constitutes a suitable approach to achieve high biomass yields (Froese et al, ). A minimum‐length limit set well above size‐at‐maturation is thus generally predicted in models to maximize yields (Allen, ; Ayllón et al, ; Beverton & Holt, ; Clark et al, ; Dunning, Ross, & Gladden, ; Gwinn et al, ; Jensen, ; Lenker et al, ; Prince & Hordyk, ; Reed, ; Ricker, ; Saila, ; van Gemert & Andersen, ). In support of this classical perspective, our model similarly predicted that minimum‐length limits often are a suitable harvest regulation if the aim is to achieve high biomass yields.…”

“…Single‐species age‐structured population models widely support the prediction that biomass yield is maximized by implementing sharp size‐selectivity in harvest and direct exploitation on ages/sizes where a cohort's biomass peaks (Froese et al, ). One common harvest regulation able to safeguard such selectivity is a minimum‐length limit (Allen, Ahrens, Hansen, & Arlinghaus, ; Clark, Alexander, & Growing, ; Jensen, ; Maceina, Bettoli, Finely, & DiCenzo, ; Ricker, ) or more generally size‐ and/or age‐dependent sigmoidal selectivity where small, young and immature fish are protected from harvest and large, old and mature fish are aggressively harvested (Beverton & Holt, ; van Gemert & Andersen, ).…”

Accounting for the bin structure of data removes bias when fitting size spectra

“…Size-spectra parameters have been selected by the European Union as one of several indicators used to evaluate sea-floor integrity (European Commission 2010, Rice et al 2012), and size-spectrum models continue to be widely applied (e.g. Taniguchi et al 2014, Álvarez et al 2016, Stasko et al 2016, Mindel et al 2018, van Gemert & Andersen 2018, Woodson et al 2018, Zhang et al 2018, Zhou et al 2019. Such applications require consistent calculation methods, especially when used for providing practical advice to fisheries (or ecosystem) managers.…”

Density‐ and size‐dependent mortality in fish early life stages

“…When in the pre‐recruitment period the density dependence occurs, warrants further investigation. Quantifying at which life stage density dependence occurs is important, for example, to assess population consequences of environmental influences on abundances of fish eggs and larvae, as such environmental effects tend to be dampened if the subsequent juvenile stages show strong compensatory density dependence (van Gemert & Andersen, ; Ohlberger, Rogers, & Stenseth, ).…”