Abstract
The Alboran Sea is a hydrographically complex and variable area in which wind-driven coastal upwelling occurs. Its most abundant small pelagic resource is the sardine ( Sardina pilchardus ), which is subject to major interannual oscillations. Postflexion stages of sardine larvae were sampled in their main nursery grounds in the north Alboran Sea, and sardine juveniles were sampled periodically throughout the recruitment season. Daily growth analysis was used to identify periods favourable for larval survival and to assess the evolution of daily increment widths during the first months of life. The results showed that juveniles born later in the spawning season grew relatively faster than those born earlier. Two main growth phases were observed in juveniles: an initial one in which daily increment widths increased progressively, and a second one in which widths fluctuated, showing a decreasing trend. The beginning of the second phase was almost synchronous among different sub-cohorts, suggesting that it was triggered by environmental factors. The estimated mean daily growth rates during the larval phase were higher in surviving juveniles than in postflexion larvae born in the same month, supporting the “bigger is better” hypothesis in relation to larval survival. The influence of the environmental regime on growth was explored, and the evolution of a growth-related index, derived from otolith increment width variability, was compared with a single environmental descriptor, a “wind index” based on wind stress and direction. This analysis suggested that larval survival and larval and juvenile growth rates showed a positive correlation with NW component winds, associated with upwelling events on the continental shelf and calm sea weather conditions in the inshore nursery grounds. Conversely, Levantine and southern wind periods, which interrupt the upwelling and create rough seas in the inshore bays, lead to a decrease in growth rates in juveniles and low larval survival.
Highlights
It is well-known that population size, especially in the case of small pelagic species, can exhibit large interannual fluctuations (Kawasaki, 1991; Wyatt and Larrañeta, 1988)
As the preliminary results showed that the daily growth rates of juveniles belonging to different subcohorts appeared to be synchronised, mainly from April onwards, suggesting that they were strongly influenced by some environmental factor, we explored the relationships between daily growth rates in juveniles and an environmental index based on wind direction and speed
Postflexion larvae sampled in Málaga bay were born between 7 and 27 November, with 61% of birthdates concentrated between 15 and 23 and peaking on 20 November (Fig. 2a)
Summary
It is well-known that population size, especially in the case of small pelagic species, can exhibit large interannual fluctuations (Kawasaki, 1991; Wyatt and Larrañeta, 1988). Following the SARP approach (Bakun et al, 1991), daily growth studies of surviving juveniles were used to identify the periods when juveniles were born, and to determine the environmental conditions favourable for larval survival This methodology was first applied in the CALCOFI area (Methot, 1983), and later in the SE Atlantic (Painting et al, 1998; Van der Lingen, 2002), SW Atlantic (Anonymous, 1989) and NE Atlantic (López-Jamar et al, 1995), in the latter case to analyse the recruitment process of European sardine (Sardina pilchardus Walbaum 1792). The present study was undertaken within the framework of a research project one of whose main objectives was to define the Optimal Environmental Windows (OEWs) (Cury and Roy, 1989; Guisande et al, 2001) for Alborán Sea European sardine populations, which have showed major fluctuations in the last few decades (Abad et al, 1998)
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