Early enrichment effects on brain development in hatchery-reared Atlantic salmon (Salmo salar): no evidence for a critical period

Abstract

In hatcheries, fish are normally reared in barren environments, which have been reported to affect their phenotypic development compared with wild conspecifics. In this study, Atlantic salmon ( Salmo salar ) alevins were reared in conventional barren hatchery trays or in either of two types of structurally enriched trays. We show that increased structural complexity during early rearing increased brain size in all investigated brain substructures. However, these effects disappeared over time after transfer to barren tanks for external feeding. Parallel to the hatchery study, a group of salmon parr was released into nature and recaptured at smoltification. These stream-reared smolts developed smaller brains than the hatchery reared smolts, irrespective of initial enrichment treatment. These novel findings do not support the hypothesis that there is a critical early period determining the brain growth trajectory. In contrast, our results indicate that brain growth is plastic in relation to environment. In addition, we show allometric growth in brain substructures over juvenile development, which suggests that comparisons between groups of different body size should be made with caution. These results can aid the development of ecologically sound rearing methods for conservational fish-stocking programs.

Early experience and reproductive morph both affect brain morphology in adult male Chinook salmon (Oncorhynchus tshawytscha)

“…Interestingly the overall size effects of hatchery rearing did persist through adulthood in the current study, similar to what has been seen for Atlantic salmon (Salmo salar) (Näslund et al 2012). Although the mechanisms behind increased brain size in hatchery fish are unknown, the environmental conditions of a hatchery could unintentionally promote an increase in energy allocation toward brain growth.…”

“…A fish's brain grows continuously throughout its lifetime, so its sensitivity to its environmental conditions and environmental feedback contributing to brain development that could alter behaviour and habitat preference in adulthood (Zaunreiter et al 1991) may negate hatchery effects. Thus, in fish, developmental plasticity could be a contributing factor to neural phenotype (Näslund et al 2012); however, the degree and timing of influence of environmental conditions on brain growth remains unknown (Hofmann and Fernald 2000;Lema et al 2005).…”

“…As such, in the wild fish are provided with a more complex environment that likely requires brain use in a different way than in a bare hatchery. There is accumulating evidence that salmonid brains can in fact reflect differences in habitat and early experience (Marchetti and Nevitt 2003;Burns et al 2009), although there is still some debate as to how long early experience can affect brain morphology (Näslund et al 2012).…”

Environmental effects on fish neural plasticity and cognition

“…Together, these studies show that brain size does not necessarily reflect functionality or development status. Future research should include studies describing the normal allometric growth in brain substructures over different salmonid life stages (Nässlund et al, 2012) combined with detailed changes in circuitry and cognitive development. These studies may help to guide the growing number of fish biologists who work with brain size and morphology in choosing the most appropriate method for comparison and analysis.…”

Microhabitat Use Affects Brain Size and Structure in Intertidal Gobies

“…Numerous studies have found larger telencephalons in mobile fish foraging within spatially complex environments compared to fish that forage in less complex environments [Huber et al, 1997;Kotrschal et al, 1998;Pollen et al, 2007;Gonda et al, 2009Gonda et al, , 2011Costa et al, 2011]. Controlled laboratory studies have demonstrated that salmon [Kotrschal et al, 2012;Näslund et al, 2012;Salvanes et al, 2013] and zebra fish [von Krogh et al, 2010] show an increased telencephalon size when raised in spatially enriched environments compared to barren environments. Similarly, in Azorean rock pool blennies (Parablennius sanguinolentus) the variation in telencephalon size appears to be closely related to the spatial challenges associated with structurally complex environments [Costa et al, 2011].…”