Detection of density-dependent growth at two spatial scales in marble trout (Salmo marmoratus) populations

Density-dependent body growth has often been observed in freshwater salmonid populations. Several studies suggest this compensatory pattern as a potential mechanism of population regulation. The choice of the spatial scale is important for the detection of densitydependent growth, as study areas need to be of the appropriate size to capture the density of conspecifics actually experienced by individuals over the preceding growth period. Here, we used four marble trout (Salmo marmoratus) populations (Gatsnik, Gorska, Huda and Zakojska) living in Slovenian stream to study the relationships between early density of marble trout and subsequent body growth. As streams are divided in sectors delimited by natural barriers that prevent or strongly limit movement of individuals, we tested the relationship between early density and body size through the lifetime at two spatial scales, that is, sector level (for Gatsnik and Zakojska) and whole stream level (the four populations were pooled). Sector length in Gatsnik and Zakojska ranged from 113 to 516 m. At both sector and whole stream level, temporal data were pooled. Growth declined significantly with increasing density both at the sector and whole stream levels, and the density-dependent relationship was described by negative power curves. However, at the sector level the density-dependent pattern was stronger in Gatsnik, a stream in which fish could move across sectors, than in Zakojska, where upstream movement across sectors is prevented by waterfalls. S. Vincenzi, A. J. Crivelli, D. Jesensek, G. A. De Leo Dipartimento di Scienze Ambientali, Università degli Studi di Parma, Viale G.P. Usberti 33 ⁄ A, I-43100 Parma, Italy, Station Biologique de la Tour du Valat, Le Sambuc,F-13200, Arles, France, Tolmin Angling Association, Modrej 26a, 65216 Most na Soci, Slovenia