Born to be wild: effects of rearing density and environmental enrichment on stress, welfare, and smolt migration in hatchery-reared Atlantic salmon

Abstract

Hatchery-reared salmonids released into the wild generally have poor survivability compared with wild conspecifics. To assess potential hatchery rearing improvements, behavioral and physiological effects of reducing animal density and adding in-tank shelter were investigated. Atlantic salmon (Salmo salar) parr were placed in barren or shelter-enriched tanks at high or low density up until release as smolts. Lowered density rendered positive effects on growth and intestinal barrier function, and both lowered density and shelter decreased conspecific aggression, as inferred by fin damage. Furthermore, while the presence of shelter decreased stress hormone levels following human disturbance, it also decreased growth and smolt migration success, an effect particularly pronounced at high densities. Therefore, we suggest that this type of structural enrichment should be avoided for Atlantic salmon smolts held at high densities and conclude that a lowered animal density with or without shelter has the highest potential in producing a more resilient smolt for stocking.

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

“…We found that enriched fry tended to have a slower growth than controls during pre-release rearing in the hatchery. This is in accordance with at least one previous experiment on S. salar in enriched environments (Rosengren et al, 2017), but in contrast to Brockmark et al (2007) finding of no difference between the size of enriched and control individuals reared at high densities. Rosengren et al (2017) hypothesised that growth differences could be a result of a preference for hiding instead of feeding if shelters are available.…”

“…This is in accordance with at least one previous experiment on S. salar in enriched environments (Rosengren et al, 2017), but in contrast to Brockmark et al (2007) finding of no difference between the size of enriched and control individuals reared at high densities. Rosengren et al (2017) hypothesised that growth differences could be a result of a preference for hiding instead of feeding if shelters are available. Because we found S. trutta predators to feed mostly on smaller prey, there is a possibility that enriched fish could have been more prone to size-selective feeding by predators, since the size distribution of fry from enriched rearing seemed to include larger proportions of small individuals than the control-reared fry.…”

“…Rosengren et al . () hypothesised that growth differences could be a result of a preference for hiding instead of feeding if shelters are available. Because we found S. trutta predators to feed mostly on smaller prey, there is a possibility that enriched fish could have been more prone to size‐selective feeding by predators, since the size distribution of fry from enriched rearing seemed to include larger proportions of small individuals than the control‐reared fry.…”

“…Other and additional practices might be needed for improving post-release survival. For example, earlier experimental works found positive effects of reduced rearing density (Brockmark & Johnsson, 2010;Rosengren et al, 2017), predator conditioning (Brown & Laland, 2001;Olla & Davis, 1989), large-scaled acclimatisation or habituation procedures (Brennan et al, 2006;Sparrevohn & Støttrup, 2007;Strand & Finstad, 2007), alternative or more enrichment types (Roberts et al, 2014), or they stocked fish at a size where they are not as prone to predation (Hyvärinen & Vehanen, 2004;Svåsand et al, 2000).…”

An Evaluation of Vertically Suspended Environmental Enrichment Structures during Rainbow Trout Rearing

“…Other studies have reported less fin erosion in fish reared under environmental enrichment regimes (Berejikian and Tezak ; Rosengren et al. ; Kientz et al. ).…”