Trends in various biological parameters of the European sardine, Sardina pilchardus (Walbaum, 1792), in the Eastern Mediterranean Sea

Abstract

In this study we present the longest time series available concerning various biological parameters of sardine in Greek waters. Biweekly samples were collected within a single day in clusters from random collections of the total landed catch derived from different vessels operating in the Northwest Aegean Sea, between July 1996 and June 2000. Total length (TL) was measured for 51.246 individuals. In addition, for a sub-sample of 50 individuals per sampling period, gross body weight and gonad weight were also measured to the nearest 0.1 and 0.01 g respectively. The analysis of the biweekly length-frequencies showed that the dominant length classes ranged between 130 and 160 mm TL. The mean biweekly TL decreased significantly (r=-0.525, P<0.05) with time during the study period. The same was also true for the maximum TL (r=-0.46, n=77, P<0.001). In contrast, the exponent b of the length-weight relationship increased significantly (r=0.358, P<0.05) with time. No relationship was found between the gonadosomatic index and b (r=-0.13, P>0.05). The parameters of the seasonalized von Bertalanffy growth equation and mortality and exploitation rates, based on monthly length-frequencies, were also estimated. The decline in the mean TL and the increase in the slope b of the length-weight relationship are discussed in relation to the recent increase in the effort expended on sardine populations and within the framework of density-dependent effects on growth.