Accounting for ocean connectivity and hydroclimate in fish recruitment fluctuations within transboundary metapopulations

Abstract

Marine resources stewardships are progressively becoming more receptive to an effective incorporation of both ecosystem and environmental complexities into the analytical frameworks of fisheries assessment. Understanding and predicting marine fish production for spatially and demographically complex populations in changing environmental conditions is however still a difficult task. Indeed, fisheries assessment is mostly based on deterministic models that lack realistic parameterizations of the intricate biological and physical processes shaping recruitment, a cornerstone in population dynamics. We use here a large metapopulation of a harvested fish, the European hake (Merluccius merluccius), managed across transnational boundaries in the northwestern Mediterranean, to model fish recruitment dynamics in terms of physics‐dependent drivers related to dispersal and survival. The connectivity among nearby subpopulations is evaluated by simulating multi‐annual Lagrangian indices of larval retention, imports, and self‐recruitment. Along with a proxy of the regional hydroclimate influencing early life stages survival, we then statistically determine the relative contribution of dispersal and hydroclimate for recruitment across contiguous management units. We show that inter‐annual variability of recruitment is well reproduced by hydroclimatic influences and synthetic connectivity estimates. Self‐recruitment (i.e., the ratio of retained locally produced larvae to the total number of incoming larvae) is the most powerful metric as it integrates the roles of retained local recruits and immigrants from surrounding subpopulations and is able to capture circulation patterns affecting recruitment at the scale of management units. We also reveal that the climatic impact on recruitment is spatially structured at regional scale due to contrasting biophysical processes not related to dispersal. Self‐recruitment calculated for each management unit explains between 19% and 32.9% of the variance of recruitment variability, that is much larger than the one explained by spawning stock biomass alone, supporting an increase of consideration of connectivity processes into stocks assessment. By acknowledging the structural and ecological complexity of marine populations, this study provides the scientific basis to link spatial management and temporal assessment within large marine metapopulations. Our results suggest that fisheries management could be improved by combining information of physical oceanography (from observing systems and operational models), opening new opportunities such as the development of short‐term projections and dynamic spatial management.

Influence of larval transport and temperature on recruitment dynamics of North Sea cod (Gadus morhua) across spatial scales of observation

“…A similar result was observed by Hidalgo et al. (2019) who found that retention influenced recruitment in European Hake populations characterised by high drift to and from other areas in the northwest Mediterranean.…”

“…While the effect of transport on recruitment has previously been explored using coupled biological‐oceanographic models (e.g. Daewel et al., 2015;Hinrichsen et al., 2016), direct inclusion of ELS dispersal model output in stock–recruitment models is less common (but see Hidalgo et al., 2019). Some studies have used proxies for larval transport such as wind speed (e.g.…”

“…Hare et al., 2015;Köster et al, 2003), water circulation indices (Zimmermann et al., 2019) or cumulative average depth of modelled particles (Baumann et al., 2006). We instead incorporate estimates of the proportion of cod larvae retained within a population and the influx of larvae from neighbouring populations, that is a more direct proxy for the effect of larval transport (Hidalgo et al., 2019), and apply this approach to the North Sea cod.…”

“…In fact, it has been suggested that ELS transport could be one of the drivers behind the unclear relationship between spawning stock size and recruitment (Huwer et al., 2016). While some studies have included proxies of larval transport in stock–recruitment relationships (Baumann et al., 2006; Zimmermann, Claireaux, & Enberg, 2019), few have included direct estimates of larval transport (but see Hidalgo et al., 2019).…”

“…Moreover, transport can influence connectivity among populations (e.g. through interannual variability in oceanographic current patterns, Huwer et al., 2016;Kvile, Romagnoni, Dagestad, Langangen, & Kristiansen, 2018) and recruitment dynamics across large geographic scales (Cadrin, Goethel, Morse, Fay, & Kerr, 2019;Henriksen et al., 2018;Hinrichsen, Von Dewitz, & Dierking, 2018) and thereby population management (Fogarty & Botsford, 2007;Hidalgo et al., 2019;Ramesh, Rising, & Oremus, 2019). Critically, the spatial scale of observation can affect the stock–recruitment relationship, providing contrasting results across scales (Chang, Chen, Halteman, & Wilson, 2016).…”