Development in Salmo trutta at different temperatures, with a quantitative scoring method for intraspecific comparisons

The period from fertilization to the end of yolk resorption in the brown trout Salmo trutta was described as a series of 40 successive steps, each describing the maturation of multiple characters during an interval of development. An associated quantitative scoring system was devised, whereby individual embryos were assigned points ranging from 1 to 1000, on the basis of the assessment of a wide range of developmental features. Developmental score increased linearly with embryonic age at all temperatures studied (2, 6 and 10° C), indicating that the relative durations of successive intervals of development were maintained. At lower temperatures, the beginning of heartbeat, trunk muscle contractions, bile production, pectoral fin movements and hatching were all delayed relative to the rest of development. Therefore these features have been excluded from the scoring system. Maternal migratory type did not affect early developmental patterns.