Spawning of AnchovyEngraulis encrasicolusin the Mondego Estuary, Portugal

Estuaries of the Iberian Peninsula west coast are preferential spawning areas of anchovyEngraulis encrasicolus(L.). This is an unusual reproductive strategy, since pelagic spawnings inside estuaries are uncommon. In order to contribute to the understanding of this reproductive strategy, a study of adult and planktonic phases of anchovy was undertaken in the Mondego river estuary (Portugal), where this species spawns almost exclusively in a brackish area of the upper estuary. This paper focuses on the daily period of spawning, the health condition of spawners, the batch fecundity, the spawning rate and the parental investment on reproduction. The daily spawning of the anchovy took place at dusk and during the first hours of darkness. Photoperiod appears to be the most important factor determining the beginning of the daily spawning. The differences found in the absolute batch fecundity between age classes were only due to the differences in the average weight of each age class: no differences were found in the relative batch fecundity. During the reproductive season, the batch fecundity decreased, which reflects a reduction in the parental investment in reproduction due to the exhaustion of energetic reserves. With only one exception, the majority (>85%) of females caught in each sampling date spawned or would spawn simultaneously, i.e. in the same day. This high fraction of simultaneously spawning females, together with both the predominance of males in almost all samples and the absence of immature and atretic females, indicated that sampled anchovies only represent a small fraction of the overall anchovy population, from which spawning schools migrate frequently to the upper estuary and return.