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Abstract. The relationship between absence of functional swim‐bladder, calculosis and larval mortality of hatchery‐reared sea bream, Sparus aurata L., was observed. There was a temporal variation in the main causes of mortality, i.e. not inflated swim‐bladder appears to be the main cause up to the 47th day, and the presence of calculi around the 55th day. Histological sections, semi‐quantitative analysis and scanner electron microscope photographs were carried out.

Atlantic salmon, Salmo salar L., brown trout, 5. trutta L. and their hybrids are normally identified in the field by empirical differences in maxilla length, thickness of the caudal peduncle, degree of forking of the tail and overall body conformation. This study quantifies these characters and analyses their variations in electrophoretically identified hatchery-reared individuals. Means and variances of the various measures are presented for salmon, trout and hybrids separately. The morphometric characters do not satisfactorily distinguish hybrids from the pure species, and even within samples of the pure species, some individuals will be misclassified as hybrids, or as members of the opposite species, if single characters are used on their own. Hybrids often resemble one or other parent species in one or more characters and are less often intermediate in phenotype. Triploidized hybrids are more like salmon than diploid hybrids are, and triploidized salmon are not different from diploid salmon. The results confirm that frequencies of hybrids of these species cannot be reliably assessed by morphological characters alone, and even for individuals of the pure parental species, independent confirmation of species status is advisable. Early reports of hybrid frequencies in wild stocks should be treated with caution, and apparently higher levels of hybridization in more recent studies compared with older, traditional surveys may simply reflect the greater precision of electrophoretic identification.Correspondence: Dr N.P. Wilkins.