Early stages of Sardina pilchardus and environmental anomalies in the Northwestern Mediterranean

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Abstract

The influence of environmental conditions on the spatial distribution patterns of the early stages of Sardina pilchardus off the Catalan coast (Northwestern Mediterranean) during November 1998, February 1999 and November 1999 was investigated. As a general rule, eggs were found on the continental shelf, while larvae displayed a wider cross-shelf distribution. The local circulation during autumn–winter 1998–1999 was dominated by an anticyclonic eddy of new Atlantic Water that remained stationary at the centre of the Catalan Sea from September 1998 to the end of February 1999. Such a situation is very infrequent and the influence of these waters extended to the continental shelf zone, modifying the slope current by diverting the flow and even reversing the direction of the flow. On the third cruise, carried out at the beginning of the spawning season of 1999 (November), the circulation and water mass distribution in the region was typical, with the old Atlantic Water occupying the whole region. Abundance of pilchard eggs and larvae during this last cruise was higher than in the previous surveys. In November 1998, the abundance of pilchard eggs and larvae was relatively low in the zones affected by the new Atlantic Water. The most interesting result of these surveys was the very low concentrations of eggs and larvae found in February 1999, possibly due to the anomalous situation created by the eddy, which had remained almost stationary during the 1998–1999 spawning season.

Introduction

The European pilchard, Sardina pilchardus, is a coastal pelagic species found in the Western Mediterranean and Adriatic Sea (Whitehead, 1984). It is the most abundant small pelagic species in the Northwestern (NW) Mediterranean (Martı́n, 1991), though the biomass of the population has decreased since 1993 (Provisional standard forms reporting information on stock assessment in the GFCM areas, 2000). In spite of the fishery importance of the species, there is little information on the spatial distribution patterns of adults and recruits in this region.

Pilchard have a protracted spawning season, with some differences in the peak of spawning depending on the areas and the temperature regimes (Ré et al., 1990, Solá et al., 1992). In the NW Mediterranean, the spawning season extends from October to May and the peak of spawning occurs in January–February (Gómez-Larrañeta, 1960, Palomera and Olivar, 1996). This period coincides with that reported for other areas of the Mediterranean, like the Adriatic Sea (Dulcic & Grbec, 2000).

The major aspects of the dynamic conditions and the typical water mass structure in the study area are well known and have been described by several authors (e.g. Salat and Cruzado, 1981, Font et al., 1988, López-Jurado et al., 1996, Millot, 1999, Salat et al., 2002). The cyclonic circulation in the surface layer around the basin is controlled by two permanent density fronts: the Catalan front located over the Iberian Peninsula slope and the Balearic front located over the insular slope (Fig. 1). The Catalan front separates the saltier old Atlantic Water (AW)1 in the central part from fresher shelf waters near the Catalan coast. The Balearic front separates the old AW from the less saltier and warmer new AW incoming into the basin through the Balearic Island channels (Pinot, López-Jurado, & Riera, 2002). The circulation is subjected to meso-scale variability, which has been attributed to the action of open sea eddies (Garcı́a et al., 1998, Tintoré et al., 1990). Off the Catalan coast, fertilization of the shelf waters during the autumn–winter season is caused by local vertical mixing and shelf intrusions of slope water (Salat et al., 2002).

There have been many studies of the influence of oceanographic processes on temporal and spatial patterns of distribution of species reproducing during the spring/summer period off the Catalan coast (Sabatés, 1990, Palomera, 1992, Sabatés and Olivar, 1996). However, few studies have addressed the environmental effects on species that reproduce during autumn–winter, the most unstable period of the year (Olivar, Salat, & Palomera, 2001).

The aim of this work was to investigate the influence of environmental conditions on the spatial distribution patterns of S. pilchardus eggs and larvae in the NW Mediterranean.

Section snippets

Material and methods

The study area corresponded to the southern sector of the Catalan coast, and extended from near Barcelona to the north of the Ebro river Delta (Fig. 1, Table 1). Three oceanographic cruises were conducted: November 1998, February 1999 and November 1999. According to previous information, these months fall within the main spawning period for this species (Gómez-Larrañeta, 1960, Palomera and Olivar, 1996, Olivar et al., 2001). The total number of stations was not identical for the different

November 1998 cruise

The vertical structure of the water column was unusual for the autumn season, with a relatively shallow and narrow pycnocline (40–60 m) (Fig. 2). Temperature profiles showed great changes from surface to 50 m ranging from 20 to 15 °C, and 13.5 °C was reached at 125 m, below which the water column was almost homogeneous. Salinity was homogeneous in the first 40 m (37.85), but increased thereafter down to 300 m, where it reached 38.55. Fluorescence profiles still showed the importance of the deep

Hydrography

Hydrographic characteristics during autumn–winter 1998–1999 showed an anomalous situation. In contrast, in November 1999, the oceanographic features were more typical for the region. The unusual conditions encountered in autumn–winter 1998–1999 were characterized by the anomalous strong intrusion of new AW into the basin that was reflected in the appearance of warmer and less saltier waters near the Catalan shelf. During November 1998, the surface salinities observed in the study area were

Acknowledgments

We are very grateful to B. Aguilera, B. Moli and G. Quı́lez for sorting the samples and measuring the larvae, and to the participants on the cruises for their help and collaboration on board. We especially wish to thank Jordi Salat and Jean M. Pinot for their comments and advice. A. Lounds prepared the English version of the manuscript. This work was funded by the following Research Programs: ‘Fil’ Spanish CYTMAR, MAR 97 09-02 and ‘Llucet’ EU Contract FAIR CT-97-3522 (1998–2000).

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