Summary1. Regression and isodar methods developed by community ecologists provide tools for evaluating the existence and magnitude of interspecific interactions in the field. In this study, we examined the usefulness of both methods to assess the impact of exotic trout on native charr. 2. Brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss have been introduced world-wide. They are invading many streams in Japan that also harbour native salmonids. We studied these species and the endemic Asian white-spotted charr Salvelinus leucomaenis in the Hekirichi River, south-western Hokkaido, Japan. We estimated the densities of all three species in pools and riffles from 27 stream reaches over 2 successive years. Data were analysed by regression and isodar methods to infer interspecific interactions. 3. The regression method identified a negative relationship between brown trout and white-spotted charr densities, implying that brown trout are replacing white-spotted charr. 4. The isodar method identified significant competition for habitat between rainbow trout and white-spotted charr. This method suggested that white-spotted charr preferred pools to riffles when rainbow trout were scarce but moved from pools to riffles as trout density in the pools increased. 5. Synthesis and applications. Although they are based on limited data, regression and isodar methods were able to evaluate the potential impact of exotic trout on native white-spotted charr. From these results, we were able to hypothesize mechanisms of displacement of native charr by exotic trout. However, according to the two methods the mechanisms underlying the negative impacts on charr differ between brown and rainbow trout; in other words, limitations exist for both methods. From a more practical viewpoint, the analyses conducted in this study may be useful tools for conservation biologists who have limitations on their time and effort but must assess the impacts of exotics on native species.

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The relationship between body size and the probability of maturing, often referred to as the probabilistic maturation reaction norm (PMRN), has been increasingly used to infer genetic variation in maturation schedule. Despite this trend, few studies have directly evaluated plasticity in the PMRN. A transplant experiment using whitespotted charr demonstrated that the PMRN for precocious males exhibited plasticity. A smaller threshold size at maturity occurred in charr inhabiting narrow streams where more refuges are probably available for small charr, which in turn might enhance the reproductive success of sneaker precocious males. Our findings suggested that plastic effects should clearly be included in investigations of variation in PMRNs.

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