Evaluation of spawning frequency in a Mediterranean sardine population (Sardina pilchardus sardina)
The postovulatory follicle method was used to assess the frequency of spawning of sardine (Sardina pilchardus sardina) in the Central Aegean Sea (eastern Mediterranean) in November-December 1999 and 2000. A field-based aging key for postovulatory follicles (POFs) was developed, and preliminary evidence is presented of a temperature influence on the degeneration of POFs. Bias in the fraction of day-0 spawners (females that had, were, or would spawn the night of sampling) with respect to sampling time was identified and attributed to spawning behavior. An appraisal of consistency in spawners' fractions and sex ratio, based on four sets of sequential trawl hauls, showed a considerably high small-scale variability, which suggested that spawning schools are structures limited in space and/or ephemeral in time. Sampling gear, i.e. commercial purse seines and research pelagic trawls, did not differ significantly with respect to spawners' fractions. The fractions of day-l and day-2 females were the same during both years, and their combination resulted in more precise, composite estimates of spawning frequency. In both years, the Mediterranean sardine spawned every 11-12 days. The incidence of spawning increased significantly with fish size. When compared to all estimates available for other sardine genera, species and subspecies, the spawning frequency of the Mediterranean sardine was lower; this was attributed to the synergetic effect of oligotrophy and smaller body sizes in the Mediterranean.