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

Abstract

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.

Ontogenetic Change in Behavioral Responses to Structural Enrichment From Fry to Parr in Juvenile Atlantic Salmon (Salmo salar L.)

“…Studies that have released enriched reared early stages of salmonids (i.e., younger than 18 weeks; the fry life stage) did not find higher survival after release ( 56 – 58 ). Solås et al ( 59 ) who released 12- to 17-week-old fry in three different years report higher survival from predation mortality 48 h after release only in the year when 17-week-old fry were released, but enriched fish did not have higher survival 12 weeks after release of fry. A few release experiments using older Atlantic salmon parr at release have, however, demonstrated higher smolt migration survival of enriched reared compared to 11-month-old ( 50 ) and 24-month-old ( 49 ) control fish.…”

Does Vaterite Otolith Deformation Affect Post-Release Survival and Predation Susceptibility of Hatchery-Reared Juvenile Atlantic Salmon?

“…We used juvenile Atlantic salmon Salmo salar reared at the Voss hatchery using eggs originating from the Vosso River population housed at the Haukvik live gene bank. Individuals were group-marked in the otoliths at the eyed egg stage using Alizarin Red-S, as described in Solås et al ( 25 ), allowing us to differentiate between fry that would later be reared in an enriched rearing tank or in a conventional control rearing tank. Individuals of the enriched group were marked with two alizarin rings in their otoliths, while those in the control group were marked with one.…”

“…Structural enrichment was introduced to one of the tanks (hereafter referred to as enriched) at the onset of feeding (hereafter called start feeding), while the fish in the other tank continued being reared in conventional hatchery environments (no additional structures, hereafter referred to as control). The enrichment consisted of four plastic tube constructions and one green box to provide shelter, both with nylon ropes and plastic shreds attached, to simulate river flora [Figures 1A,B of ( 25 )]. The structures were cleaned approximately every other week during rearing in June and every week during rearing in July and August.…”

“…The stunned predators were collected with hand nets and transferred ashore in containers of river water for further examination. To collect the salmon fry that had been consumed by the brown trout predators, the sampled trout were either anaesthetised with MS-222 to enable evacuation of stomach contents in the field or euthanised with an overdose of MS-222 for later examination of stomach contents in the lab (see ( 25 ) for details).…”

“…In the present paper, we have scrutinised otoliths from the same field experiment as of Solås et al ( 25 ), and from enriched and control reared fish sampled in the rearing tanks on three occasions (July, August, September). We tested the effect of structural enrichment on vaterite presence and coverage using data from rearing tanks, and the effect of vaterite on survival using predation and release–recapture field experiments.…”