The impact of exotic trout on native charr in a Japanese stream

Abstract

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.

The impact of non-native rainbow trout within Afro-montane streams in eastern Zimbabwe

“…The rainbow trout could have altered mountain catfish habitat associations and abundances through direct predation, as reported from previous studies (Butler & Marshall, 1996;Marriott et al, 1997;. Because deeper habitats provide refugia and optimal feeding habitats for trout (McIntosh, 2000;Morita et al, 2004;, particularly the larger-sized individuals (pers. obs), predation pressure on the mountain catfish is likely to be highest in the deeper pools.…”

“…It is likely, however, that the mountain catfish with trout-invaded reaches may occupy sub-optimal habitat, which may influence their foraging, recruiting, and reproductive success (Ngugi et al 2009). Cases where native taxa have been forced to occupy marginal or sub-optimal habitats have been documented in other studies for native fishes (Morita et al, 2004;Ngugi et al, 2009) and amphibians (Bosch et al, 2006). These marginal habitats may, however, be less optimal and could further expose the catfish to terrestrial predators such as otters and predatory birds, and therefore may not be suitable for long-term sustenance of large catfish populations.…”

Plasticity in probabilistic reaction norms for maturation in a salmonid fish

“…Parr densities were estimated in a 100 m reach of each site using the three-pass removal method (model M (b) , program CAPTURE, available at http://www.mbr-pwrc.usgs.gov/software/index.html). At the same time, four physical characteristics (river width, depth, velocity and substrate code) were measured in each 100 m reach of each site following Morita et al (2004). Water temperatures were measured at each site at hourly intervals using data loggers ( A PMRN method based on logistic regression (Heino et al 2002) was used to analyse the relationships between body size and the probability of becoming mature for age 1þ year male parr.…”

The disruption of dominance hierarchies by a non-native species: an individual-based analysis

“…Invaders can directly influence native population dynamics, abundance and persistence through biological interactions such as competition, predation and/or parasitism (e.g. Blackburn et al 2004, Morita et al 2004, Gozlan et al 2005, Ortega et al 2006. Numerous studies have also revealed the ecological effects of invaders on native species at higher levels of organization, such as at the community and/or the ecosystem level (e.g.…”