Young Scientists Summer Program 2008 Six Research Projects

Goal To estimate probabilistic maturation reaction norms (PMRNs) for age and size at maturation of chum salmon in South Korea. It has long been realized that the proportion of mature fish at any given age and size affects a stock's population dynamics and thus influences the potential yield that can be extracted from the stock. It is thus important to disentangle the environmental and evolutionary effects that influence this proportion over the long run. Individual fish typically follow different growth trajectories and their maturation is influenced by these, so that growth-related phenotypic plasticity in maturation and the evolution of maturation schedules often occur together. Evolution of maturation schedules is expected to result from fishing, especially when such fishing is size-selective, since all fish species are genetically adapted to ancestral environmental conditions that are strongly altered by modern fishing. Researches on fisheries-induced evolutionary responses in maturation schedules have suggested that these schedules have often shifted towards smaller sizes and younger ages, an effect that should be taken into account for the effective management of current fisheries (Olsen et al. 2004; Barot et al. 2005). Probabilistic maturation reaction norms (PMRNs) characterize maturation schedules by describing the age-and size-dependent probability of maturation during one season. Their estimation thus helps to separate the fundamental biological processes of maturation, growth, and survival. Whereas the average age and size at maturation observed in a population is significantly affected by all three of these processes, PMRNs describe maturation probabilities conditional on an individual having grown to, and survived until, a given age and size, thus enabling a closer inspection of the maturation process itself. In other words, PMRNs reveal the confounding effects that variations in growth and survival have on maturation (Kraak 2007). Specifically, changes over time in a PMRN could be interpreted as being suggestive of evolutionary or genetic changes, often driven by changes in environmental conditions including the extra mortalities imposed by fishing. For example, studies on Northeast Arctic cod show that long-term shifts in PMRNs are accompanied by maturation changes due to phenotypic plasticity in response to short-term environmental fluctuations, e.g., in feeding conditions (Jørgensen 1990; Heino et al. 2002b). Chum salmon are semelparous fish in the North Pacific, spawning in freshwater and maturing once during their life time. Bigler et al. (1996) proposed that chum salmon have more than doubled during the past 20 years due to favorable ocean con-3 ditions, population management …