Testing spatial heterogeneity with stock assessment models.

Ernesto Jardim,Margit Eero,Lionel Pawlowski,Finlay Scott,Alexandra Silva,Andres Uriarte,Isabel Riveiro,Erwan Duhamel,Steven J Holmes,Iago Mosqueira,Clara Ulrich,Leire Ibaibarriaga,José A A De Oliveira,Pablo Carrera,Nekane Alzorriz,Leire Citores

doi:10.1371/journal.pone.0190791

Abstract

This paper describes a methodology that combines meta-population theory and stock assessment models to gain insights about spatial heterogeneity of the meta-population in an operational time frame. The methodology was tested with stochastic simulations for different degrees of connectivity between sub-populations and applied to two case studies, North Sea cod (Gadus morua) and Northeast Atlantic sardine (Sardina pilchardus). Considering that the biological components of a population can be partitioned into discrete spatial units, we extended this idea into a property of additivity of sub-population abundances. If the additivity results hold true for putative sub-populations, then assessment results based on sub-populations will provide information to develop and monitor the implementation of finer scale/local management. The simulation study confirmed that when sub-populations are independent and not too heterogeneous with regards to productivity, the sum of stock assessment model estimates of sub-populations’ SSB is similar to the SSB estimates of the meta-population. It also showed that a strong diffusion process can be detected and that the stronger the connection between SSB and recruitment, the better the diffusion process will be detected. On the other hand it showed that weak to moderate diffusion processes are not easy to identify and large differences between sub-populations productivities may be confounded with weak diffusion processes. The application to North Sea cod and Atlantic sardine exemplified how much insight can be gained. In both cases the results obtained were sufficiently robust to support the regional analysis.

Highlights

The spatial structure of fish and shellfish stocks is a major issue for fisheries management and assessment of stocks’ status
If each subpopulation is closed, in the sense of not having significant migrations across sub-populations, the estimates of N obtained from stock assessment models fitted to each sub-population will add up to the estimates obtained from the meta-population fits
The quantities of interest (QoI) are compared and if the estimates are considered similar, e.g. by having overlapping confidence intervals, the process continues with the analysis of the sub-population dynamics

Summary

Introduction

The spatial structure of fish and shellfish stocks is a major issue for fisheries management and assessment of stocks’ status. Amid the panoply of processes that induce spatial structure of some kind in fish populations, spatial heterogeneity [6] is one of the most common, in particular in stocks that span large areas These stocks are likely to cross different ecosystems with different oceanographic characteristics, food availability, etc. Such conditions are likely to have an impact on biological processes at a local scale, creating sub-populations with potentially distinct dynamics, organized in a network of sub-populations, much like the concept of meta-populations [9].

Methods

Results

Conclusion