Abstract
Herring (Clupea harengus L.) and smelt (Osmerus eperlanus (L.)) are common pelagic species in the Baltic Sea and have different coastal spawning areas. The spawning period of herring is more protracted and occurs further offshore in the archipelago, while smelt prefers inshore areas, rivers, and estuaries. After hatching the large finfold probably promotes larval dispersal by currents; larval vertical migrations also probably affect dispersal directions since the distribution areas of the larvae soon overlap. The decrease in finfold surface area, change in phototaxis, and vertical migrations seem to guide larval retention in the shallow water nursery areas where herring appear later while moving inshore. Herring and smelt larvae resemble each other, both having elongate bodies, but differing in yolk sac content and location, fin development, ventral pigmentation, and especially in larval developmental schedule. In herring the changes in body form are more pronounced, occur later, and last longer than in smelt. Some morphological dissimilarities seem to refer to ecological differences. Smelt and herring larvae extend their feeding areas offshore after the most intensive period of development in shallow warm water areas and earlier in smelt than in herring.