Can structural enrichment reduce predation mortality and increase recaptures of hatchery-reared Atlantic salmon Salmo salar L. fry released into the wild?

Captive-reared fish often have poor survival in the wild and may fail to boost threatened populations. Enrichment during the nursery period can in some circumstances generate a broader behavioural repertoire than conventional hatchery production. Yet, we do not know if enrichment promotes survival after release into the wild. We conducted a field experiment during three field seasons using age 0+ year Atlantic salmon Salmo salar to investigate if enrichment during rearing, in the form of structural complexity (shelters), reduced immediate (within 2 days after release) predation mortality by piscine predators (brown trout Salmo trutta) and if such rearing environments improved long-term (2-3 months after release) post-release survival. In addition, we investigated if predation mortality of released fry was size-selective. S. salar fry were reared in a structurally enriched environment or in a conventional rearing environment and given otolith marks using alizarin during the egg stage to distinguish between enriched and conventionally-reared fry. The outcome from the field experiments showed that structural enrichment did not consistently reduce immediate predation mortality and it did not improve, or had a negative effect on, the recapture rate of fry from the river 2-3 months after release. The data also showed that enriched rearing tended to reduce growth. Additionally, we found that S. trutta predators fed on small individuals of the released fry. Overall, the data suggest that structural enrichment alone is not sufficient to improve long-term survival of hatchery-reared fish after release and that other factors might affect post-release survival.

Are antipredator behaviours of hatchery Salmo salar juveniles similar to wild juveniles?

This study explores how antipredator behaviour of juvenile Atlantic salmon Salmo salar developed during conventional hatchery rearing of eggs from wild brood stock, compared with the behaviour of wild-caught juveniles from the same population. Juveniles aged 1+ years were tested in two unfamiliar environments; in one S. salar were presented with simulated predator attacks and in the other they were given the opportunity to explore an open-field arena. No difference was found in their spontaneous escape responses or ventilation rate (reflex responses) after simulated predator attacks. Hatchery-reared juveniles were more risk-prone in their behaviours than wild-caught individuals. Hatchery juveniles stayed less time in association with shelter. In the open-field arena, hatchery juveniles were more active than wild juveniles. Hatchery juveniles were also immobile for less time and spent a shorter amount of time than wild juveniles in the fringe of the open-field arena. Salmo salar size had no effect on the observed behaviour. Overall, this study provides empirical evidence that one generation of hatchery rearing does not change reflex responses associated with threats, whereas antipredator behaviour, typically associated with prior experience, was less developed in hatchery-reared than in wild individuals.

First records of reproductive behaviour and early development of the bearded goby Sufflogobius bibarbatus

Data on developmental characteristics from fertilization to 2 days post-hatching during captivity of the anoxia tolerant bearded goby Sufflogobius bibarbatus, one of the keystone prey species for many of the commercial fish populations, sea mammals and birds in south-west Africa, are presented.

Environmental variability in the early rearing environment generates behaviourally flexible cod: implications for rehabilitating wild populations

The release of hatchery-reared fishes for restoring threatened and endangered populations is one of the most controversial issues in applied ecology. A central issue has been to determine whether releases cause extinction of local wild populations. This may arise either through domesticated or non-local fishes hybridizing with wild fishes, or through inappropriate behavioural interactions; for example, many hatchery fishes show exaggerated aggressive and competitive behaviour and out-compete wild counterparts. The impact of the impoverished hatchery environment in shaping behaviour is only now receiving attention. Attempts to counteract hatchery-related behavioural deficiencies have utilized intensive training programmes shortly before the fishes are released. However, we show here that simple exposure to variable spatial and foraging cues in the standard hatchery environment generates fishes with enhanced behavioural traits that are probably associated with improved survival in the wild. It appears that fishes need to experience a varying and changeable environment to learn and develop flexible behaviour. Using variable hatchery rearing environments to generate suitable phenotypes in combination with a knowledge of appropriate local genotypes, rehabilitation of wild fishes is likely to succeed, where to date it has largely failed.