The impact of intra- and interspecific interactions on young-of-the-year brook charr, in temperate lakes

The abundance, growth, spatial distribution, and feeding habits of five allopatric brook charr, Salvelinus fontinalis, populations (young-of-the-year, 0+ juveniles; YOY) were compared with five other populations living sympatrically with white sucker, Catostomus commersoni. The study was made in oligotrophic lakes of the Laurentian Shield (Québec, Canada) during three sampling periods in 1989 (July, August and September). The abundance of YOY charr was significantly higher in allopatric than in sympatric populations (45·3 ± 3·8 vs 3·4 ± 3·8 fish/lake caught in 1773 m² of gillnets; P<0·005). The mean length of YOY charr did not differ among allopatric and sympatric populations at each sampling period; July: 60·2 ± 3·0 vs 60·0 ± 4·5 mm; August: 61·9 ± 4·5 vs 63·2 ± 4·1 mm; September: 77·9 ± 8·7 vs 77·3 ± 7·8 mm respectively. Horizontal distribution of allopatric YOY charr did not differ from that of sympatric charr, 65% of the fish being captured within the first 2 m depth and the rest between 2 and 7 m depth. In contrast, the vertical distribution of allopatric YOY charr from both communities was significantly different; 81% of allopatric charr were captured within 0·5 m from the substrate compared to 64% for sympatric charr (P<0·001). Differences in vertical distribution of the fish were related to differences in diet; allopatric charr fed mainly on benthic and large planktonic organisms whereas sympatric charr fed less on these organisms and more on terrestrial organisms. In the lake where YOY charr were most abundant, individuals were spatially segregated into two groups; one ‘littoral’, found in 0–2m depth, and one ‘profundal’, found in 3–6 m depth. Growth, condition, and feeding habits of charr from the two groups were different, especially during the last sampling period.