General surface circulation controls the interannual fluctuations of anchovy stock biomass in the Central Mediterranean Sea

Abstract

The sustainable exploitation of small pelagic fish populations, characterized by short life span and early age at first reproduction, is typically more influenced by the success of annual recruitment rather than by fishing mortality. Recruitment strength, in turn, is related to the high environmental variability characterizing the pelagic fish habitats, able to strongly affect the survival of early stages, from hatching to recruitment. Here, we consider the case study of anchovy (Engraulis encrasicolus) stock in the Strait of Sicily (Central Mediterranean). The interannual fluctuations exhibited over an 18-year long period by this fish population was found to be mainly linked to surface circulation patterns, as far as they are able to control retention/dispersal processes of larval stages. We firstly used Lagrangian simulations to reproduce the fate of anchovy early stages during their planktonic phase. Larval retention indices constructed from the output of the simulations were able alone to explain a large proportion of variance (up to 70%) in yearly biomass of the anchovy population, outclassing the other environmental factors considered in this study. Such results are relevant for fisheries management, for all fish stocks characterized by potentially high vulnerability of early life stages.

Seascape connectivity of European anchovy in the Central Mediterranean Sea revealed by weighted Lagrangian backtracking and bio-energetic modelling

“…Also sea surface temperature might have a potential role on assessing the likelihood of trajectories. However, during the summer spawning in the Strait of Sicily, this environmental parameter is never a limiting factor for anchovy larva 21 , 35 and, therefore, we did not incorporate sea surface temperature filter in the Lagrangian modelling.…”

“…Existing literature does not offer clear evidence of connectivity of stocks of European Anchovy among different GSAs in the Strait of Sicily. Using forward Lagrangian simulations from observed egg concentration points Patti et al 21 highlighted the existence of a limited exchange rate of reproductive material between both sides of the Strait. However, this exchange was thought to be negligible (on average, < 1% of total larval abundance) in the light of expected mortality rates of anchovy early stages.…”

“…Beyond the importance of the frontal area off Cape Passero for the anchovy reproductive ecology, it is also worth noting how the generally more favorable environmental conditions over the whole continental shelf areas of the Strait may be relevant in determining strength of annual anchovy recruitment. Recent studies evidenced a strong relationship between the estimated rate of occurrence of anchovy late larval (pre-juvenile) stages in continental shelf areas of the Strait of Sicily, resulting from surface current advection from spawning sites, and hydro-acoustic biomass estimates of the adult fraction of the population in the following reproductive season one year later 21 .…”

“…In this framework, the effect of the environmental conditions occurring during the planktonic dispersion of anchovy larvae within the whole Strait of Sicily has never been evaluated, and it needs to be explored thoroughly. Inspired by previous studies 15 , 17 , 18 , 21 and by the patchy (although stimulating) datasets 22 we summarized in Fig. 1 , here we explore Lagrangian connectivity between the European and African coastal zones in the Central Mediterranean Sea by using a novel approach, based on back-trajectory analysis.…”

“…This innovative approach allows recognizing the original spawning regions by going back to larva “age zero” and also accounting for egg drifting, enlightening dynamical connections among different spawning and recruitment areas. In summary, novelty of this study is, therefore, given by the fact that connectivity is not explored by tracking the evolution of Lagrangian trajectories departing from spawning areas 17 , 18 , 21 , 23 ; but rather from the statistical reconstruction (i.e., estimation of probability distribution functions) of spawning areas that represent arrival targets of back-in-time trajectories departing from anchovy larvae, collected in a specific region and having a specific age. Such an investigation might reveal insights on biomass dynamics for this important fish population, since recruitment strength in GSA 15 and 16 (South of Sicily) may be affected by connectivity with the surroundings GSAs and, in particular, by transport rates of eggs and early larval stages released form the continental shelf off northern Tunisia coast (i.e., GSA 12).…”