This study investigated diverse reproductive types in complex mating systems of steelhead Oncorhynchus mykiss. Postspawned steelhead (kelts) were sampled during attempted downstream migration over Lower Granite Dam on the Snake River, U.S.A. Multilocus microsatellite genotypes (14 loci) were used to assign unknown origin, kelt individuals to upstream populations of origin. Results indicated that iteroparity is a life-history trait that remains in several tributaries of the Snake River basin despite strong selection against downstream adult passage because of hydroelectric dams. The largest populations of steelhead in the Snake River, however, were only weakly represented (Clearwater River ¼ 7Á5% and Salmon River ¼ 9Á4%, respectively) in the kelt steelhead mixture relative to the Grande Ronde River (18Á2%), Imnaha River (17Á4%), Pahsimeroi Hatchery (25Á2%) and Asotin Creek (22Á2%). A lack of correlation between population escapement size and kelt proportions (P > 0Á05) suggests that iteroparity was not uniformly expressed across populations, but was significantly negatively correlated with body size (P < 0Á05). Iteroparity may be a valuable source of genetic variability and a conservation priority, especially in years with poor recruitment or in recently bottlenecked populations.

Biological and statistical properties of spacing of the marginal circuli of scales (CIRC) were examined in an experimental population of tilapia (Oreochromis mossambicus/hornorum) hybrids. Biological, or "causal," interrelationships of body length, growth, and CIRC were analysed by maximum likelihood solution of a simultaneous equation model involving the three variables. Spacing varies directly with growth rate and inversely with fish length, and growth varies inversely with length. Consequently, spacing is relatively constant along the radial axis of the scale except where variation in ration level or other factors varies the rate of growth as a function of length. The correlation of CIRC with current growth rate in populations of mixed parentage, sizes, and ages is sufficiently high (r = 0.65–0.71) to render it a useful growth estimator in aquaculture and natural situations where ages are unknown and sequential size measurements are difficult to obtain. Statistical and procedural conditions for maximizing the utility of the growth estimator in several applications are examined: (a) comparison of individual growth rates within a population, (b) comparison of mean growth of populations in different environments, (c) indirect selection for growth in aquaculture genetic programs, and (d) response of growth to exogenous variables such as parasite load, behaviour, and microhabitat.