Feeding pattern and diet of first feeding brown trout fry under natural conditions

Abstract

The feeding habits of brown trout Salmo trutta fry were studied during the critical first feeding period in a natural spawning and nursery stream. A low proportion of the fry initiated exogenous feeding before emergence from the gravel, and while having nearly 30% of the yolk sac remaining. This probably reflected low feeding motivation or limited feeding opportunities within the gravel environment. The majority of the fry started feeding after emergence, and after most or all yolk was absorbed. Some fry emerged with large amounts of yolk remaining, while others emerged after yolk exhaustion. The degree of stomach fullness revealed that feeding was more efficient after a territory had been acquired. The diets of the young fry were dominated by chironomid larvae, followed by zooplankton and Plecoptera larvae. Fry dispersing downstream and out of the nursery area were significantly smaller than resident fry, indicating displacement due to competition for territories. The majority of the downstream dispersing fry had initiated feeding, and there was at this point no evidence of starvation in any of the fry. It therefore appeared that the later emerging fry actively migrated out of the overpopulated nursery area to find available territories further downstream.

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“…One group takes station close to the nest, and the other, having delayed formation of static swimming behavior, drift downstream away from the nest (Cuinat and Héland 1979;Héland 1999). The downstream drifters have been suggested to constitute a group of individuals with the strategy of forming territories in areas where there are less competition (Héland 1999;Skoglund and Barlaup 2006). Trout fry show these different behaviors even if reared in isolation (Héland 1999), a finding which is supported by our results.…”

“…In other cases, like in the tributaries to the Norwegian river Daleelva, non-territorial drifting fry do not seem to starve and may thus not be outcompeted; instead this appears to be a specific dispersal strategy (Skoglund and Barlaup 2006). The possibility of coexistence of different behavioral types is likely positively influenced by territory availability and environmental complexity Hoogenboom et al 2012;Reid et al 2012), which likely differ among study sites and over time.…”

“…through effects on metabolic rate (Régnier et al 2012), this may be an indication of bet-hedging with different behavioral types performing well in different situations, utilizing different niches, or different competitive strategies (e.g. Grant and Noakes 1987;Skoglund and Barlaup 2006;Závorka et al 2015). In this way the offspring from a single female may have a wider total niche breadth.…”

“…The supplied food consisted of thawed chironomid larvae (Akvarieteknik, Sweden). Chironomids constitute a major part of the natural food eaten by brown trout at the early fry stage (Nilsson 1956;Skoglund and Barlaup 2006). Food rations were the same for all fish within a treatment.…”

“…It has been suggested that there are two, more or less distinct, behavioral strategies adopted by emerging salmonid fry (Héland 1999;Skoglund and Barlaup 2006). One behavioral strategy is to quickly establish and actively defend a territory (active and aggressive strategy), while the other is to hide and nocturnally disperse downstream from the nest and away from the main area of competition (passive and shy strategy).…”