Adaptive Variation in Body Size, Age, Morphology, Egg Size, and Developmental Biology of Chum Salmon (Oncorhynchus keta) in British Columbia

Abstract

Chum salmon (Oncorhynchus keta) stocks in British Columbia spawning from August through January and in rivers of different size were surveyed for variation in sex ratios, age composition, meristic characters, morphology, and developmental biology. Males were more abundant than females at the younger (2 and 3 yr of age) and older (5 and 6 yr of age) age-classes. Males had larger heads, thicker caudal peduncles, and larger dorsal fins than did females, but females had larger anal fins than did males. Stocks from large rivers had larger heads, thicker caudal peduncles, and larger fins than did those from small rivers. Chum salmon in an area characterized by a lower number of gillrakers also had a lower number of branchiostegal rays. At the same incubation temperature, alevin hatching time was similar for all stocks examined except those from the Queen Charlotte Islands, which was later than the other stocks. Early-spawning stocks had older fish, larger eggs, and later times of fry emergence than did late-spawning stocks. Late-spawning stocks had higher survival rates of embryos at 4 °C than did early-spawning ones. Stocks had different trends in alevin and fry length and weight with respect to incubation temperature. Northern stocks were more efficient than southern stocks at converting egg yolk to fry body weight at low incubation temperatures (4 °C). Selection has produced stocks adapted to the various environments that they encounter in their natal streams, allowing chum salmon to exploit a wide variety of spawning habitats in British Columbia.