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Changes in size and age at maturity in a population of kokanee Oncorhynchus nerka during a period of declining growth conditions

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Trends in size distributions and age at maturity of spawning kokanee Oncorhynchus nerka during a 5 year period of declining growth conditions at Bucks Lake, California, U.S.A. were consistent with the hypothesis that reductions in growth rates in successive cohorts induce a shift to an older age at maturity. This forestalls decreases in size at maturity during a transitional period characterized by an increasing proportion of individuals that delay maturation. During the course of the study, kokanee first began declining in size at maturity, and then shifted from a 3 year to a 4 year egg to adult cycle. Individuals that spawned during their fourth year (age 3 years) were significantly larger, on average, than members of their cohort that spawned during their third year (age 2 years). This difference was greatest when age 2 year adults were smallest. The shift to an older age at maturity prevented a steady decline in size at maturity, even though age‐specific size was steadily declining over time. Size at maturity, however, began to decline again once the transition to a 4 year cycle was complete. In addition, there was a general trend of decreasing length‐specific mass. The data indicate that there is a range of growth trajectories over which delayed maturity can prevent a temporal pattern of decreasing size at maturity as growth rates decline.

Keywords: Oncorhynchus nerka; delayed maturation; growth; kokanee; life history; size at maturity

Document Type: Regular Paper


Affiliations: Feather River College, 570 Golden Eagle Ave., Quincy, California 95971, U.S.A. and Department of Biology, Southern Utah University, Cedar City, Utah 84720, U.S.A.

Publication date: 2005-01-01

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