Effects of Growth and Reproductive History on the Egg Size-Fecundity Trade-off in Steelhead

The allocation of energy by females into different components of reproduction is critical and includes trade-offs between egg size and number. In anadromous salmonids, it is unclear whether the allocation patterns are determined by the growing conditions experienced when the fish are in freshwater or at sea or by the female's reproductive history. We collected data on length, egg size, and fecundity from female steelhead Oncorhynchus mykiss that had spent 1 year in freshwater and then 2 (designated 1.2) or 3 years at sea (1.3) before spawning for the first time. We also collected the same data from females spawning for the second time but of the same age as those spawning after 3 years for the first time (1.1S1). The older females were longer than the younger ones, but there was considerable overlap in length. The repeat-spawning fish were shorter than fish of the same age that were spawning for the first time, which reflected the reduction in growth associated with the previous spawning migration and reproductive event. Fish length was positively correlated with egg size and fecundity, but the 1.2 females had smaller eggs for their size than did the 1.3 fish, which implied an effect of growth rate at sea. We then measured scales from a subset of the fish as an index of individual fish size at the time of seaward migration and size after their first year of growth at sea. Fish that matured first after 2 years at sea were slightly larger as smolts but grew significantly more in their first year at sea than did those maturing a year later. However, neither measure was related to egg size or egg number after adjustment for length. The processes controlling the egg size-fecundity trade-off in steelhead and other salmonid species remain unclear, but the present evidence is incompatible with the hypothesis that growth in freshwater as juveniles determines reproductive trade-offs at maturity by females.