Integrating the effects of connectivity and climate on recruitment into fisheries management

A European interdisciplinary research team (including MIO researcher Vincent Rossi) has used an innovative strategy to assess the impact of the environment on the dynamics of fishery resources. It has shown that a large part of the inter-annual variability observed in the repopulation of hake populations over the last 25 years is due to simultaneous changes in climate and ocean circulation. These changes have an impact on the dispersion and survival of larvae in several management units in the western Mediterranean. These results will have important implications for the assessment and management of fish stocks, most of which are currently overexploited.

Stocks of commercial fish species, most of which are currently overexploited, are regularly assessed independently in regional management units. Along with other biological and fisheries data, annual estimates of 'recruitment' (the degree to which the adult population has been repopulated by juveniles) enable scientists to estimate the current state of stocks and managers to regulate fishing in each region.
At the same time, marine populations tend to be heterogeneously distributed across the seascape and respond in a non-linear fashion to oceanic and atmospheric variability. Yet fisheries management does not take these complex processes into account when estimating stocks, which makes it less effective.

A research team1 has studied the factors affecting the interannual variability of recruitment, the ecological cornerstone of fisheries science, in several management units. The researchers drew on the growing ability of high-resolution circulation models to simulate ocean dispersion and on previous work on a widely distributed and exploited population of demersal fish (showing territoriality in relation to the seabed on which they live).

They were thus able to demonstrate that the variations in recruitment observed in each of the management units considered could be explained by the variability of the climate and ocean currents, this variability affecting the dispersal and survival of the early life stages of the species (eggs and larvae). Simulated indicators of connectivity (the latter being defined here as the transport of larvae by ocean currents, which creates 'links' between different regions) and a hydroclimatic index (a composition of atmospheric measurements that materialise in an index the interactions between the ocean and the atmosphere) synthesise most of the environmental influences that affect fish populations, creating new opportunities for integrating environmental effects into the management of fish stocks.

The researchers focused their study on European hake (Merluccius merluccius) in the north-western Mediterranean, because of its socio-economic importance and its proven status as an overexploited species. Using an innovative methodology, they measured larval retention (confinement) and exchanges of larvae between six sub-populations in three Geographical Sub-areas (GSAs) independently assessed by the General Fisheries Commission for the Mediterranean (GFCM). In each of the three regions and over the last two decades, the researchers have demonstrated a link between the inter-annual variability of hake recruitment (estimated from fishing data) and that of the climate and a new connectivity indicator, larval self-recruitment (the ratio between the larvae produced locally, confined to the area, and those from neighbouring GSAs, which also contribute to local repopulation).
This result suggests that oceanographic processes generated by the atmosphere have strong and heterogeneous impacts on hake recruitment. It also implies that recruitment levels in these GSAs are interdependent and linked to larval connectivity. For example, 1989 was marked by the predominance of a south-westerly movement of larvae due to the Liguro-Provençal current, with no connection with the Balearic archipelago, where recruitment is, in such a situation, determined more by local processes. In contrast, 2005 was characterised by a reduction in south-westerly transport and closer connections between the Gulf of Lion (GSA7), the Catalan continental shelf (GSA6) and the Balearic Islands (GSA5). In this case, recruitment levels in these GSAs are governed by a combination of more or less advantageous larval dispersal routes and hydroclimatic conditions that may or may not be favourable to their survival.

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