Physiological response of smolts from two strains of Atlantic salmon, Salmo salar L., to a temperature increase on their migratory route

Résumé

In Belgium, the population of Atlantic salmon disappeared in the 1940’s. A restoration program in the Meuse Basin was launched in 1987. Thanks to many efforts, results are encouraging, but only few adult spawners have been captured yet. Environmental data show temperature differences, sometimes exceeding 5°C, between a tributary and a larger river during the downstream migration period. Since temperature is a primary cue in smolting, we investigated the potential effects of such thermal conditions on smoltification. We looked for differences of various physiological smoltification markers between two strains of fish and between early and late migrants. We also examined the transcriptional response of smoltification-related genes in two crucial organs, the liver and the gill. We then aimed at verifying our experimental results in the field by sampling smolts in two sites where temperature differences had been measured.
First, we compared two foreign strains, commonly used in Belgium for stocking, under simulated natural conditions based on the temperature and photoperiod of a tributary annually restocked. We observed strain-related differences of the influence of temperature and daylength on cortisol, GH and sodium plasma levels. Using Na+/K+ATPase activity as an indicator, both strains smoltified successfully and simultaneously under local conditions.
Then, we investigated the effect of a rapid temperature increase on hypo-osmoregulatory capacities in early and late migrants. The ability to hypo-osmoregulate was assessed by seawater challenges. We observed that a rapid temperature increase during the smoltification period similarly impaired osmoregulatory capacities in smolts of both strains (decreased NKA activity and increased plasma osmolality). After an early or late temperature increase, smolting indicators tend to be modified toward parr values within one week after the increase, suggesting that local temperature conditions may heavily compromise smolt survival chances at sea entry.
We also examined gene expression in the liver in response to a swift temperature increase. Results showed deleterious effects of temperature in all defined gene groups like endocrine regulation of smoltification, oxygen transport, iron metabolism, lipid and carbohydrate metabolism, immune response and cell cycle. Findings also revealed important changes occurring during smoltification. Differences between the strains are thought to be linked to water temperature and migration distances of the rivers of origin. Gene expression in the gill was also impaired after a temperature increase. Changes in the transcription of genes associated with the endocrine control of smoltification (igf1r and igf2) were identified and altered expression of genes linked to hypo-osmoregulation (nkaα1b and nkcc1a) was consistent with physiological markers. Data suggests dual roles in the smoltification and desmoltification process for GH and IGF1 and points to the implication of genes, previously unstudied (nbc) or with little data available (igf2), in the smoltification process. This study gives further insights on the molecular processes underlying smoltification and desmoltification in Atlantic salmon and possible responses to human-related water temperature increase.
Field work presented numerous challenges that had to be overcome. Environmental conditions and infrastructure-linked issues caused many setbacks and only limited data were obtained. However, differences emerged compared to laboratory conditions, notably early migrants seemed much less affected than late migrants three days after transfer into warmer water while early and late migrants were similarly affected under simulated natural conditions. Our results suggest a deleterious effect of human-linked temperature increase on migrating salmon.
All in all, a rapid temperature increase arising between two rivers seems to strongly influence the smoltification. Considering the response of early migrants in the field experiment, early migrating strains may have better chances to reach the sea with high capacity for hypo-osmoregulation. Effects of modelled temperature increase due to climate change, stocking management and strain selection were discussed in regard to our results. Early migrating strains and efforts to facilitate downstream migration may promote smolt survival under local conditions.

la date de réponse	26 avr. 2018
langue originale	Anglais
L'institution diplômante	Universite de Namur
Sponsors	Fund for Research Training in Industry and Agriculture (FRIA) & Region Wallonne
Superviseur	Patrick Kestemont (Promoteur), Robert Mandiki (Copromoteur), Frederic Silvestre (Président), Michaël Ovidio (Jury), Isabelle Leguen (Jury) & Sigurd Stefansson (Jury)