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The Box-Balance Model: A new tool to assess fish larval survival, applied to field data on two small pelagic fish

Vanesa Raya*, Jordi Salat, Ana Sabat├ęs

*Corresponding author:

ABSTRACT: This work develops a new method, the Box-Balance Model (BBM), to assess the role of hydrodynamic structures in the survival of fish larvae. The BBM is applied in the northwest Mediterranean to field data, on two small pelagic fish whose larvae coexist in summer: Engraulis encrasicolus, a dominant species, and Sardinella aurita that is expanding northwards in relation to sea warming. The BBM allows to quantify the contribution of the circulation, with a significant mesoscale activity, on the survival of fish larvae, clearly separating the effect of transport from biological factors. It is based on comparing the larval abundances at age found in local target areas, associated to the mesoscale structures (boxes), with those predicted by the overall mortality rate of the population in the region. The application of the BBM reveals that dispersion/retention by hydrodynamic structures favours the survival of E. encrasicolus larvae. In addition, since larval growth and mortality rates of the species are required parameters for the BBM application, we present their estimates for S. aurita in the region for the first time. Although growth and mortality rates found for S. aurita are both higher than for E. encrasicolus, their combined effect give a lower survival to S. aurita larvae. Thus, despite the warming trend in the region would contribute to the expansion of the fast-growing species, S. aurita, we can confirm that E. encrasicolus is well established, with a better adapted survival strategy and continues to win the game.