A new approach to recruitment overfishing diagnosis based on fish condition from survey data

Francesc Ordines,Roberto Carlucci,Josep Lloret,Luis Gil De Sola,Pilar Tugores,Angélique Jadaud,Enric Massutí,Marina Sartini,Chiara Manfredi,Beatriz Guijarro,Igor Isajlović,Cristina Porcu

doi:10.3989/scimar.04950.03a

Abstract

A new approach to recruitment overfishing diagnosis is presented. We hypothesize that condition of recruits should increase when recruitment failures are caused by fishing activity. This would be a consequence of the increase in trophic resource availability, because the population is smaller than that which the ecosystem could support. Temporal series of hake recruit condition were calculated from MEDITS survey data collected in Mediterranean geographical sub-areas (GSAs) 1, 6, 17 and 19 from 1994 to 2015. Multiple linear regressions were used to analyse the relationship between mean annual condition and abundance of recruits and climatic indices in each GSA. Significant correlations were only detected in GSA 6, where 69% of condition variability was explained by the negative correlation with recruit abundance, and with two climatic indices, the Western Mediterranean Oscillation and the standardized air temperature anomaly at surface from the Gulf of Lions. Despite the differences in recruit abundance among GSAs, their mean annual condition oscillated around the same basal value during most of the time series, pointing to density-dependent mortality rates as an important mechanism stabilizing hake recruitment to levels close to the carrying capacity when populations do not suffer recruitment overfishing. This pattern changed when the decreasing recruit abundance trend drove GSA 6 condition values persistently above those of the rest of the GSAs. According to our hypothesis, hake in GSA 6 is in recruitment overfishing.

Highlights

The Food and Agriculture Organization adopted the definition of recruitment overfishing in Restrepo (1999): “A situation in which the rate of fishing is such that annual recruitment to the exploitable stock has become significantly reduced
In the present study we aim to explore a new approach to the assessment of recruitment overfishing based on condition of hake recruits collected during the Mediterranean International Trawl Surveys (MEDITS) since 1994
The maps representing the mean somatic condition (SC) per haul for the years 2012 to 2015 show high variability depending on the year for all geographical sub-areas (GSAs) (Fig. 2)

Summary

Introduction

The Food and Agriculture Organization adopted the definition of recruitment overfishing in Restrepo (1999): “A situation in which the rate of fishing is (or has been) such that annual recruitment to the exploitable stock has become significantly reduced. Recruitment overfishing can lead to stock collapse, under unfavourable environmental conditions”. This definition takes into account most of the aspects considered in previous definitions of this type of overfishing [see Sainsbury and Polacheck (1993) and references therein]. A recruitment failure has two main causes: i) low egg production due to a reduced spawning stock and/or a low spawning activity of adults; and/or ii) a high mortality in the early life stages before recruitment to fishing stocks, which may be related to unfavourable oceanographic conditions (Watanabe et al 1995). Emphasizing the importance of the environmental influence on recruitment variability may lead stock assessments to ignore an underlying recruitment overfishing (Walters and Maguire 1996)

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Discussion

Conclusion