Density-dependent individual growth of marble trout (Salmo marmoratus) in the Soca and Idrijca river basins, Slovenia
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Although the main features of salmonid life cycles are currently well known, marble trout (Salmo marmoratus) populations have been scarcely studied due to the present limited geographical distribution of the species. In this work we tested the hypothesis of density-dependent individual growth of marble trout with data gathered from multi-year on-going monitoring started in 1996 in three streams (Zakojska, Gorska and Gatsnik) in the Soca and Idrijca river basins (Slovenia). As observed for other salmonid species, marble trout exhibits high plasticity in body size and growth in response to environmental conditions. Age-specific mean lengths are significantly different among the three streams. Despite this variability, the statistical analyses outlined some clear patterns. Mean lengths of marble trout cohorts aged ≥1+ are correlated with total population density in the first year of life of the cohorts; the age-specific relationship between mean length and total density is well explained by negative power curves. Length of marble trout at age 1+ is significantly correlated with length at subsequent years up to age 4+. ANCOVAs performed on the stream-specific datasets showed a significant effect of total density of marble trout on annual age-specific individual growth rates. Von Bertalanffy’s body growth models were calibrated on length–age data of Zakojska and Gorska marble trout. Our analysis shows the existence of density-dependent effects on growth of marble trout that might be crucial in regulating population dynamics.