The effect of trigger depth on self-feeder utilisation, growth and the vertical distribution of tank-held rainbow trout Oncorhynchus mykiss (Walbaum)

This study investigated the effect of placing a self-feeding trigger at either a) 1 cm below the water surface or b) mid-water (35 cm below the surface), on self-feeder usage, growth, feed wastage and fin damage of juvenile rainbow trout, Oncorhynchus mykiss (Walbaum). The vertical distribution of trout was also measured for the first 31 days of self-feeding. Group-held trout (n = 10 fish group^− 1, 5 groups treatment^− 1), weighing 51.06 g ± 8.09 g (mean ± SD) were held in 200 l cylindroconical tanks (depth = 75 cm) for 62 days. Trigger placement had no significant effect upon self-feeder conditioning, and upper trigger group (UTG) and lower trigger group (LTG) fish took an average of 16 ± 7.4 days and 14 ± 9.8 days to reach a stable level of self-feeding, respectively. Specific growth rate was significantly higher in UTG fish (1.83 ± 0.20 vs. 0.95 ± 0.21 for UTG vs. LTG fish, respectively) whilst condition factor and self-feeder utilisation (ration size, 1.21 ± 0.18 vs. 0.52 ± 0.15 kg tank^− 1 for UTG vs. LTG fish) were also significantly higher in UTG fish. Trigger depth had no significant effect on size heterogeneity, mortality, feed wastage, feed conversion ratio (FCR) or the incidence of caudal and dorsal fin splitting and fin erosion. Dorsal fin splitting decreased with time in UTG but not in LTG fish. Caudal fin erosion increased with time in UTG (upper lobe only) and LTG (both lobes). The vertical distribution of fish also differed between treatments and more LTG fish preferred the lower part of the tank. Placing a self-feeding trigger at a mid-water depth can significantly limit growth, condition factor and self-feeder utilisation in juvenile rainbow trout, in addition to hindering recovery from dorsal fin erosion.