Effects of freshwater hyperoxia and hypercapnia and their influences on subsequent seawater transfer in Atlantic salmon (Salmo salar) smolts

Abstract

Atlantic salmon (Salmo salar) presmolts, smolts, and postsmolts compensate for a respiratory acidosis associated with 96 h of exposure to hyperoxia (100% O 2 ; hO 2 ), hypercapnia (2% CO 2 and 98% air; hCO 2 ), and combined hO 2 /hCO 2 in freshwater (FW) by increasing strong ion difference, predominantly through a reduction in plasma [Cl -] (presumably via branchial Cl -/HCO 3 -exchange). In smolts, compensation during hO 2 or hCO 2 occurred within 24 h, whereas that in combined hO 2 /hCO 2 was much slower, resulting in 33% mortality by 96 h. FW hO 2 and combined hO 2 /hCO 2 appeared to impair gill function, likely through oxidative cell damage. This resulted in reduced hypoosmoregulatory ability following subsequent transfer to seawater (SW), as indicated by changes in plasma ion levels, osmolality, and muscle water content, resulting in considerable mortalities. Interestingly, FW hCO 2 appeared to enhance hypoosmoregulatory ability during subsequent SW transfer. Smolts are often transported from FW to a subsequent SW release site, and these data indicate that care should be taken to minimize the degree of hyperoxia experienced by the smolts. Hypercapnia, which results from metabolic CO 2 production and inadequate water aeration, does not impair SW transfer, provided it does not occur in conjunction with hyperoxia.Résumé : Les pré-smolts, smolts et post-smolts de saumon atlantique (Salmo salar) compensent l'acidose respiratoire associée à une exposition de 96 h à une hyperoxie (100% O 2 ; hO 2 ), une hypercapnie (2% de CO 2 et 98% d'air; hCO 2 ) et une combinaison de hO 2 /hCO 2 en eau douce en augmentant la différence des ions forts, principalement par une ré-duction du [Cl -] plasmique (vraisemblablement par échange branchial de Cl -/HCO 3 -). Chez les smolts, la compensation pendant hO 2 ou hCO 2 se produisait dans les 24 h, tandis qu'elle était beaucoup plus lente pour la combinaison hO 2 /hCO 2 , qui causait une mortalité de 33% en 96 h. Des conditions de hO 2 en eau douce et la combinaison hO 2 /hCO 2 semblaient entraver la fonction branchiale, vraisemblablement par dégradation oxydative des cellules. Cela provoquait une réduction de la capacité d'hypoosmorégulation suite au transfert dans l'eau de mer, comme le montraient des changements dans les concentrations plasmiques d'ions, l'osmolalité et la teneur en eau des muscles, qui causaient des mortalités considérables. Il est intéressant de constater que la condition de hCO 2 en eau douce semblait améliorer la capacité d'hypoosmorégulation pendant le transfert subséquent en eau salée. Les smolts sont souvent transportés de l'eau douce à un point de lâcher en eau salée, et nos données indiquent qu'il faut prendre soin de minimiser le degré d'hyperoxie qu'ils peuvent connaître. L'hypercapnie, qui provient de la production métabolique de CO 2 et d'une aéra-tion insuffisante de l'eau, ne nuit pas au transfert en eau salée à condition qu'elle ne survienne pas en même temps que l'hyperoxie.[Traduit par la Rédaction] Brauner et al. 2064

Vacuolar-Type H+-ATPase and Na+, K+-ATPase Expression in Gills of Atlantic Salmon (Salmo salar) during Isolated and Combined Exposure to Hyperoxia and Hypercapnia in Fresh Water

“…Introduction). This was also the case in the present exposures (reported in Brauner et al, 2000), as large decreases were seen in plasma [Cl -] during hypercapnia and/or hyperoxia. The present data using a homologous V-type H + -ATPase Bsubunit probe demonstrate that compensation of an internal respiratory acidosis induced by exposure to hypercapnic water is associated with a consistent decrease in V-type H + -ATPase B-subunit expression in gills of Atlantic salmon after 1-4 days (Fig.…”

“…This trend was also obvious in the present study ( Fig. 2 and Brauner et al, 2000). Additionally, smolts exposed on May 5 were more fully developed than smolts on April 27, as reflected by increased gill Na + ,K + -ATPase activity levels (Fig.…”

“…The upper-mode Atlantic salmon used in the present study clearly underwent the parr-smolt transformation during the course of this study as judged from changes in gill Na + ,K + -ATPase activity and mRNA levels in FW (this study), and from plasma osmolality and ion deflections in fish transferred to SW for 24 hr (reported in Brauner et al, 2000). The basic level of gill V-type H + -ATPase B-subunit mRNA (i.e.…”

“…1A, B, D). The less pronounced decrease in gill V-type H + -ATPase B-subunit mRNA levels in the fish exposed to hyperoxic water as compared to hypercapnic exposure correlates with lower HCO 3 -accumulation in this situation (see Brauner et al, 2000). Atlantic salmon exposed to combined hyperoxia and hypercapnia showed depressed V-type H + -ATPase B-subunit mRNA levels but generally not to the same degree as fish exposed to hypercapnia (Fig.…”

“…Little is known about the physiological consequences of this treatment to the salmon in relation to the stage of the parr-smolt cycle. Recently we reported on the organismal responses of exposing Atlantic salmon pre-smolts, smolts and post-smolts to hyperoxia and hypercapnia and its influence on subsequent SW-tolerance (Brauner et al, 2000). The aim of the present study was to investigate at the mRNA level, how vacuolar-type H + -ATPase B-subunit and α-and β-subunits of the Na + ,K + -ATPase were affected in the gills of just the same fish as used in the study of Brauner et al (2000).…”