Intraspecific competition can cause density-dependent growth (DDG) when resources are limited. Resources are often sheared among species, so DDG can also result from interspecific competition. The strength of this DDG can be affected by abiotic factors. In the Kushiro River system, northern Japan, white-spotted charr (WC) and masu salmon (MS) generally occur sympatrically and compete for food resources. In this study, we examined spatial variations in growth and densities of WC and MS parr, and abiotic characteristics in 21 tributaries (i.e., populations) in the river system to determine if interspecific competition and abiotic factors cause DDG in these species.

The growth of WC decreased as the densities of WC and MS increased. MS density was 1.5 times stronger effect on WC growth than WC density did. Similarly, the growth of MS decreased as the densities of WC and MS increased. In addition, the effect of WC density on MS growth increased as water temperature and gravel size increased. However, in older age class of WC, the effect of density on MS growth increased as gravel size decreased. In conclusion, the growths of WC and MS were density dependent across the species. The strength of inter-specific competition might be condition specific, that is, competitive superiority might depend on water temperature and gravel size.