Non-linearity in stock–recruitment relationships of Atlantic cod: insights from a multi-model approach

Abstract

The stock–recruitment relationship is the basis of any stock prediction and thus fundamental for fishery management. Traditional parametric stock–recruitment models often poorly fit empirical data, nevertheless they are still the rule in fish stock assessment procedures. We here apply a multi-model approach to predict recruitment of 20 Atlantic cod (Gadus morhua) stocks as a function of adult biomass and environmental variables. We compare the traditional Ricker model with two non-parametric approaches: (i) the stochastic cusp model from catastrophe theory and (ii) multivariate simplex projections, based on attractor state-space reconstruction. We show that the performance of each model is contingent on the historical dynamics of individual stocks, and that stocks which experienced abrupt and state-dependent dynamics are best modelled using non-parametric approaches. These dynamics are pervasive in Western stocks highlighting a geographical distinction between cod stocks, which have implications for their recovery potential. Furthermore, the addition of environmental variables always improved the models’ predictive power indicating that they should be considered in stock assessment and management routines. Using our multi-model approach, we demonstrate that we should be more flexible when modelling recruitment and tailor our approaches to the dynamical properties of each individual stock.

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

“…Worldwide, most stocks for which data are available follow a Beverton–Holt stock–recruitment relationship, but there are 17% of global stocks for which data are available that show Ricker recruitment (Szuwalski et al, ). In particular strongly piscivorous, and by the same token cannibalistic marine and freshwater species, such as pike‐perch ( Sander lucioperca , Percidae, Gröger, Winkler, & Rountree, ), walleye (Zhao, Kocovsky, & Madenjian, ), pike (Edeline et al, ) or cod (Sguotti et al, ), show evidence for Ricker recruitment. Hence, based on our model, in top predatory species modest harvest is recommended even when the goal is to maximize catch rate or catch of trophies, as some harvest releases the remaining fish from cannibalistic control, increasing recruitment into the fishery and growth of fish to reach memorable, large sizes.…”

Projected impacts of warming seas on commercially fished species at a biogeographic boundary of the European continental shelf

“…Varying depth and climatic conditions have already shaped marine species assemblages within the region (Hinz et al., 2011; ter Hofstede et al., 2010). Given the importance of temperature affecting species responses, incorporating temperature driven effects within projections from stock assessments is crucial for anticipating future climate effects on stock dynamics and informing management decisions to help consider broader ecosystem effects (Sguotti et al., 2019; Skern‐Mauritzen et al, 2015). Yet, for many stocks within this region, incorporating such environmental variability within assessments is still lacking.…”

Population variability under stressors is dependent on body mass growth and asymptotic body size

“…Five species of these families are indicated according to their position in the parameter space: anchovy, Atlantic cod, Atlantic herring, golden redfish and yellowfin tuna (table 3) Vert-pre et al [42] have shown that some fish stocks do not show the classical direct link between spawning biomass and recruitment (a stock-recruitment relationship). In this context, the environment seems to be an important driver of recruitment dynamics, as observed in several stocks of fish such as herring and cod [43,44]. In our model, we assumed an inherent stock-recruitment relationship.…”