Colin Brauner

Fecal hormone metabolite analysis is increasingly used as a non-invasive method to evaluate physiological stress in free-ranging animals, especially those of conservation concern. In this study, it is applied to investigate stress in the Northeast Pacific southern and northern resident killer whale (Orcinus orca) populations. The southern resident killer whale population is listed as endangered, as the population is small and declining. Despite sharing a similar diet and environment, the northern resident population shows greater reproductive success and has increased in recent years. It has been hypothesized that the southern residents are more physiologically and nutritionally stressed than the northern residents, due to anthropogenic threats of reduced prey availability, acoustic and physical disturbance, and environmental contaminants. Fecal samples were collected from both populations along the coast of British Columbia, Canada, in July and August of 2018-2019. I first validated four commercially available enzyme-linked immunosorbent assays for quantifying steroid (progestogens, androgens, and glucocorticoid) and thyroid hormone metabolites in killer whale feces, then compared concentrations of fecal glucocorticoid and thyroid metabolites between the southern and northern resident populations as indicators of stress. Mean fecal progestogen metabolites (fPM) were highest in pregnant females, higher in females (non-pregnant) than males, and were successfully used to determine pregnancies. Mean fecal androgen metabolites (fAM) were highest in pregnant females and sprouter (adolescent) males, and were greater in males relative to non-pregnant females. Mean fecal glucocorticoid metabolites (fGCM) were higher in pregnant females than non-pregnant females and males, but were not significantly different between the southern and northern resident populations. There were no significant differences in mean fecal thyroid metabolites (fTHM) among any demographics, but mean fTHM was significantly lower in the southern resident population than the northern resident population (P
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Freshwater fish such as white sturgeon (Acipenser transmontanus) are particularly vulnerable to the effects of anthropogenic global warming; however, little is known about how acclimation to higher temperatures or rate of temperature increase affects their thermal tolerance. The Kenney dam on the Nechako River is home to the northern-most population of white sturgeon and is mandated to maintain water temperatures below 20°C for migrating sockeye salmon, but it remains unclear whether 20°C is an appropriate threshold for developing white sturgeon. To address this, 37-51 days post hatch (dph) and 66-80 dph juvenile white sturgeon were acclimated to one of four ecologically relevant temperatures (15°C, 18°C, 21°C, and 24°C) for two weeks, following which thermal tolerance (CTmax), size, condition factor, and survival were assessed. White sturgeon displayed highly plastic CTmax in response to acclimation, illustrated by a positive relationship between acclimation temperature and CTmax and large acclimation response ratios compared to other fish species. Acclimation to temperatures above 18°C was found to negatively affect condition factor, which suggests the presence of a sub-lethal threshold between 18°C and 21°C. Their highly plastic response to temperature was further demonstrated when the effect of heating rate (0.3°C/min, 0.03°C/min, 0.003°C/min) on thermal tolerance, somatic indices, and Hsp mRNA expression was assessed. White sturgeon CTmax was highest in the slowest heating rate, contrary to what has been observed in most other fish species. Hepatosomatic index decreased in all heating rates relative to control fish, indicative of the metabolic costs of thermal stress. Expression of Hsp70 mRNA was increased in all heating rates relative to controls, whereas expression of Hsp90a and Hsp90b mRNA only increased in the two slower trials. Together these data indicate that while white sturgeon have a very plastic thermal response, acclimation to temperatures above 18°C may negatively affect overall health, indicated by lower condition factor. As such, in the best interest of white sturgeon conservation, the operators of the Kenney dam may want to reconsider whether the 20°C threshold is appropriate.

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Recent studies suggest that teleost fishes may be able to greatly enhance the amount of oxygen (O₂) unloaded to the tissues during a generalized acidosis associated with stress. This mechanism relies on pH-sensitive hemoglobin, a red blood cell (RBC) β-adrenergic Na⁺/H⁺ exchanger (β-NHE) activated by catecholamines to protect RBC pHi, and plasma-accessible carbonic anhydrase (paCA) at the tissues, but not at the gills, to short-circuit the β-NHE, acidify the RBC, and unload O₂ from hemoglobin. This system has been shown to increase tissue PO₂ (∆PO₂) by up to 30 Torr and has been proposed to play an important role both during early teleost evolution, as well as in modern teleost physiology. To date, most studies regarding this system have been conducted in the context of high stress and circulating catecholamines; little is known about this system under low catecholamine levels, such as following stress. In addition, despite being often extrapolated to all teleosts, this system has only been studied within the salmonids. Here, I investigate the time course of β-NHE short-circuiting by paCA in rainbow trout blood following in vitro stimulation by isoproterenol, a synthetic catecholamine, as well as natural catecholamines in vivo to determine how long after adrenergic stimulation this system may remain operational. A significant increase in plasma ∆PO₂ due to β-NHE short-circuiting by paCA was found in rainbow trout blood stimulated in vitro up to one hour after removal of isoproterenol; this was reduced to less than 30 min under natural catecholamines. A significant ∆PO₂ of approximately 5 Torr was determined even 6 h after stimulation, as well as in pharmaceutically blocked blood, suggesting that enhanced O₂ unloading may be operational in resting fish. The system was also investigated in two teleost species distantly related to the salmonids, cobia (Rachycentron canadum) and mahi-mahi (Coryphaena hippurus). In cobia and mahi-mahi, evidence of a RBC β-NHE was found, and it is predicted that cobia may exhibit up to a 61% increase in enhanced O₂ unloading with no change in blood flow through short-circuiting of RBC β-NHE, a value consistent with salmonids. This second phylogenetically distant group provides support that this system may be functional in teleosts in general.

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Land-based, closed containment salmon aquaculture involves rearing salmon from smolt to adult in recirculating aquaculture systems (RAS). Unlike in open-net pen aquaculture, rearing conditions can be specified in RAS in order to decrease physiological stress. The environmental conditions that yield optimal growth and physiological stress tolerance in salmon are, however, unknown. To address this knowledge gap, we reared Atlantic (Salmo salar) and coho (Oncorhynchus kisutch) salmon in 7 separate RAS for 400 days post-smoltification under 2 photoperiods (12 or 24 hours of light) and 4 salinities (2.5, 5, 10 or 30 ppt) and assessed the effects of these conditions on growth and thermal tolerance. We found that salinity and photoperiod had significant effects on growth of Atlantic and coho salmon, but optimal conditions were not determined. Secondly, we found Atlantic salmon generally grew best under 24 hours of light until day 400 when the trend was lost and that coho salmon grew best under 12 hours of light in the freshwater (2.5 ppt) treatment. Finally, we found higher levels of maturation in Atlantic salmon reared under 24 hours of light, whereas the 12:12 photoperiod triggered greater levels of early maturation in coho. In order to evaluate the effects of photoperiod and salinity on the physiological stress tolerance we used critical thermal maxima or CTmax tests as a performance proxy. We found that over the first 120 days post-smoltification, rearing coho under a 24 hour photoperiod resulted in a ~2°C lower CTmax than in coho reared under a 12:12 photoperiod. This photoperiod effect did not persist at 200 and 400 days, which was coincident with an overall decrease in CTmax in coho salmon relative to 120 days. Finally, Atlantic salmon had a higher CTmax (~28°C) compared to coho (~26°C) at 400 days post-smoltification. Overall, these findings are important for the future implications of RAS and for the aquaculture industry to help identify physiologically sensitive time stages.

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An organism’s phenotypic characteristics can be altered by environmental variations experienced during embryonic development. These changes can persist into adulthood. Increasing global temperatures are a current concern that may be particularly acute for species already threatened or endangered, such as the white sturgeon, Acipenser transmontanus. Given scant information on the effects of early embryonic temperatures on subsequent physiological parameters such as thermal tolerance, development, and growth in this species, the effects of global climate change on the future of white sturgeon populations is uncertain. To address this, white sturgeon embryos were incubated at 12, 15, and 18 °C until hatch, after which fish were reared at a common 15 °C for 80 days post-hatch. I conducted acute thermal tolerance tests through early ontogeny to determine how relative thermal tolerance (CTmax) changed and differed during this period while also sampling and evaluating various morphometric characteristics to determine the effects of embryonic temperatures on growth and development. Embryonic incubation at 12 °C with subsequent rearing at 15 °C increased larval development rate, growth rate, and the development of thermal tolerance; however, once developed CTmax averaged 29.9 for all embryonic rearing temperatures. For white sturgeon, embryonic temperatures appear to have discernable effects on growth and development up to 80 days post-hatch but not on thermal tolerance.

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Carbon dioxide (CO₂) is continuously produced as a result of aerobic respiration and must be excreted to maintain internal acid-base balance. Most CO₂ is carried in the blood as HCO₃- and must be converted back to molecular CO₂ at the respiratory surface to diffuse out into the environment. The uncatalyzed rate of HCO₃- dehydration is too slow to excrete CO₂ at a physiologically relevant rate and therefore it must be catalyzed by the enzyme carbonic anhydrase. The distribution of carbonic anhydrase in the blood and gills of fish therefore provides important information about general patterns of gas exchange and acid-base balance. Teleost fishes have a fast CA in the red blood cell (RBC), no extracellular CA activity, an endogenous plasma CA inhibitor and a relatively low plasma buffer value so HCO₃- dehydration is largely restricted to the RBC. Pacific spiny dogfish (Squalus suckleyi), however, have a slow RBC CA, extracellular CA activity, no endogenous plasma CA inhibitor, plasma accessible CAIV at the gills and a relatively high plasma buffer value, implying that both the RBC and plasma compartments may contribute to HCO₃- dehydration. This thesis uses biochemical assays, subcellular localization and immunohistochemistry on blood and gill samples from 13 chondrichthyan species to examine whether the characteristics of the dogfish model of CO₂ excretion apply to chondrichthyan fishes in general. Overall, the results of this study were consistent with the proposed chondrichthyan model of CO₂ excretion because most chondrichthyans had lower RBC CA activity than teleosts, some extracellular CA activity, no endogenous plasma CA inhibitor, higher plasma buffer values and type IV-like CA at the gills. Pacific spiny dogfish had 3x more microsomal CA activity (183 ± 13.2 µmol CO₂ min-1 mg protein-1) in the gills than the other three species examined for this trait, indicating that dogfish may not be a representative species to compare with other vertebrate groups. Overall, the results of this thesis suggest that all chondrichthyans have the capacity to use both the plasma and RBC compartments for CO₂ excretion and these data provide important information about general patterns of gas exchange and acid-base balance in fishes.

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Recirculating aquaculture systems (RAS) are an emerging technique in aquaculture to rear salmon in land-based facilities, but the systems are also associated with high costs, so rearing fish under optimal conditions for maximum growth is required for profitability. However, few systematic studies have been conducted to determine optimal conditions for growth of salmon from smolt through to market size in RAS and the related effects on physiological performance. To address this knowledge gap, the first part of my thesis investigated the effect of salinity on growth, metabolism and hypoxia tolerance of Atlantic and coho salmon. Smolts were reared at salinities of 0, 5, 10, 20 and 30 ppt under 24 h of light in RAS for up to 460 days. Between Days 200 and 400, respirometry was conducted to measure routine, maximum metabolic rate and aerobic scope, while time to loss of equilibrium at 10% air saturation was measured to determine hypoxia tolerance. The initial effect of salinity on growth was found in Atlantic and coho salmon at Day 295 and 59, respectively, after which growth was generally enhanced at intermediate salinities of 5 and 10 ppt. No clear relationship was found between salinity and metabolic measurements in either species. Hypoxia tolerance of Atlantic salmon was enhanced at 5 and 10 ppt, but salinity had no effect on hypoxia tolerance of coho salmon. The second part of this thesis aimed to (1) determine metabolism and hypoxia tolerance of coho salmon during their early growth stages where the effect of salinity on growth was the most profound and (2) explore the interactive effect of photoperiod. A new cohort of coho salmon smolts were reared in RAS at 2.5, 5, 10 and 30 ppt under 12:12 and 24:0 (light:dark) photoperiods, while respirometry and hypoxia trials (15% air saturation) were conducted at Day 60 and 120. No effect of salinity and photoperiod was found on metabolic measurements and hypoxia tolerance in the younger coho salmon during these periods. Overall, my data suggest that there is some potential to enhance growth of salmon by manipulating environmental conditions in RAS without compromising other physiological performance.

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Salmon are among the most popular seafood products and their culture continues to expand with improved aquaculture technology. Typical aquaculture production rears salmon from smolt to market size in net-pens in oceans, but this practice has been criticized due to potential environmental concerns such as eutrophication and interactions between escaped cultured fish and wild populations. Rearing fish in recirculating aquaculture systems (RAS) is a new technology to address some of the concerns over net-pen aquaculture, as well as enhance production of salmonids and other fish species. Salmon can inhabit a wide range of salinities with different osmoregulatory costs, but presumably these costs can be reduced if fish reside in water isosmotic to their blood plasma. Photoperiod has been shown to affect growth rates in salmon at different life stages but can also affect early maturation in salmon. To examine the effects of salinity and photoperiod on the growth of salmon, seven replicate RAS with salinities of 2.5, 5, 10 and 30ppt under 12:12 and 24:0 light:dark photoperiod were used to rear Atlantic and coho salmon from smolts onwards for 120 days. Salinity and photoperiod had an effect on Atlantic salmon growth, with those reared at 10ppt in 24:0 light showing the highest growth rates. However, neither photoperiod nor salinity affected coho salmon growth. To understand the effects of salinity and photoperiod on swimming performance and hematology, coho salmon from two separate studies underwent repeat maximum swimming speed (Umax) tests. In the first study, Umax was assessed in coho salmon that were reared for 350 days in 0 and 10ppt. In the second study, Umax was assessed in coho salmon that were reared for 60 and 150 days in 2.5, 10 and 30ppt under 12:12 and 24:0 light:dark photoperiod. In the first study, salinity had significant effects on resting plasma osmolality and chloride concentration. In the second year, salinity affected first Umax, but neither salinity nor photoperiod affected repeated Umax and recovery ratio. There were also significant effects of salinity on the hematocrit, hemoglobin concentration, MCHC and plasma osmolality and chloride concentration of exhausted salmon after repeated swimming tests.

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The Amazon contains 20% of the world’s freshwater fish species that are predicted to experience an increase in temperature by up to 2.2 to 7˚C within the next century. An increase in temperature will likely be associated with an increase in the frequency, duration, and magnitude of hypoxic bouts, creating an even greater challenge. Thermal tolerance may be limited by the ability to supply and deliver enough oxygen to tissues at critical temperatures, as is the case for hypoxia tolerance, thus both may be associated with similar mechanisms in fish. A direct relationship between thermal and hypoxia tolerance however, has not yet been investigated in a wide range of fish species. To address this, I conducted acute thermal tolerance (CTMax) and hypoxia tolerance (% air saturation at loss of equilibrium) assays in 20 species that spanned a broad phylogenetic range. In fish acclimated to the temperatures within the current temperature range of the Amazon River (28 or 31˚C), I found a positive relationship between CTMax and hypoxia tolerance. In fish acutely transferred to higher temperatures of 33 or 35˚C, there was a reduction in hypoxia tolerance relative to that at 28 or 31˚C. Acclimation (10 days or 4 weeks) to 33 or 35˚C did not increase hypoxia tolerance, and in some species there was a further reduction in hypoxia tolerance. Acclimation to 33 or 35˚C (10 days or 4 weeks) and exposure to hyperoxia (>200% air saturation) increased CTMax, although in most species only moderately. One of the most significant findings of my thesis was that most species failed to acclimate to the higher temperatures: of the 13 species investigated, 2 species did not survive 10 days or 4 weeks (chronic lethal maximum) at 31˚C, 9 species did not survive at 33˚C, and only 2 species survived 35˚C. Overall, acclimation to higher temperatures that are predicted to occur within the next century had little or no effect on thermal tolerance and reduced hypoxia tolerance indicating that the high fish biodiversity of the Amazon may be at risk given the predicted changes in temperature and hypoxia associated with climate change.

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Rising atmospheric carbon dioxide (CO₂) and by association pCO₂ in aquatic habitats (hypercarbia) has put increased focus on understanding the underlying mechanisms of acid-base regulation in fish. Aquatic hypercarbia results in a blood acidosis in fish, which is compensated for by the exchange of Na+ and Clˉ for its acid/base counterpart (H+, HCO₃ˉ) across the gill epithelium. Surprisingly, there are no studies on how a single species, capable of inhabiting both fresh and saltwater, responds to hypercarbia, and no existing studies examining how sustained exercise may affect hypercarbia recovery. The goal of this thesis was to examine how changes in ambient water ionic composition (soft-, hard-, and saltwater) affect the rate and degree of acid-base compensation in a rainbow trout, Oncorhynchus mykiss, during hypercarbia, at rest and during sustained exercise. Additionally, I sought to determine the effect of sustained exercise on the rate and degree of acid-base compensation during hypercarbia. Trout were acclimated to soft-, hard-, or saltwater and acid-base relevant blood parameters were measured during a 1% CO₂ hypercarbia exposure, both at rest and during sustained exercise (~60% Ucrit). After 48h of hypercarbia, resting hard-, and saltwater acclimated trout had fully restored blood pH, however soft water acclimated trout were only 60.6±10.5% recovered, and in all fish recovery was associated with an increase in plasma [HCO₃ˉ] and a decrease in plasma [Clˉ] of similar magnitude. Trout exposed to hypercarbia during sustained exercise had a similar response, and following 8h the saltwater acclimated fish had fully restored blood pH, while soft-, and hard water fish were 42±18.1 and 64±6.8% recovered, respectively. Furthermore, following 8h of hypercarbia there was a significant effect of exercise on the degree of recovery compared with resting fish, suggesting that sustained exercise results in a more rapid recovery from hypercarbia in trout, relative to rest. These results provide intra-specific support to previous studies that demonstrate marine fish compensate for hypercarbia faster than freshwater fish. This thesis not only demonstrates an important link between ambient water ion levels and the ability to recover from acid-base disturbances, and presents novel data suggesting sustained exercise enhances acid-base regulation.

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As a consequence of increasing atmospheric CO₂, the partial pressure of CO₂ (pCO₂) in the oceans is rising, causing a decrease in pH and carbonate ions known as ocean acidification (OA). Since freshwater systems have the same potential for atmospheric equilibrium with CO₂, similar scenarios of acidification will occur in freshwater, yet little is known about the potential impacts of climate change-related acidification on freshwater species and ecosystems. Moreover, virtually no research has investigated the effect of oscillating pCO₂ tensions on fish, which are more likely to reflect natural coastal conditions. The goal of this thesis is to address some of these knowledge gaps by studying the potential effects of climate change-related acidification on pink salmon (Oncorhynchus gorbuscha) at a sensitive and critical life-stage during development in both freshwater and seawater under future elevated and fluctuating pCO₂ tensions (freshwater: 400 μatm, 1000 μatm, 2000 μatm, 400-2000 μatm (over 24hr); seawater: 400 μatm, 1600 μatm, 400-1600 μatm). Growth, production efficiencies and aerobic scope were measured starting 2 weeks pre-hatch in freshwater up until 2 weeks post-seawater transfer. Growth was reduced during freshwater rearing and following seawater transfer. Specifically, size, production efficiencies and absolute growth rates were reduced at freshwater tensions of 1000 and 2000 μatm and seawater tensions of 1600 μatm. However, no significant effects on growth were seen in response to oscillating pCO₂ tensions. Similarly, aerobic scope was reduced at high pCO₂ following seawater transfer through a reduction in maximal oxygen consumption rate (ṀO₂max) (but not routine (ṀO₂routine)), indicating that exercise in pink salmon fry may be particularly affected by climate change-related acidification. Given that control fish exhibit a dramatic increase in ṀO₂max 7 days post-seawater transfer, which is likely associated with a change in life history from a sedentary to a migratory stage, elevations in seawater pCO₂ may have implications for their seaward migration success. Overall, this thesis demonstrates that pink salmon, under predicted future increases in pCO₂, may be faced with sublethal impacts of acidification on various aspects of their physiology at a very critical time in their life history.

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This thesis contributes to our general knowledge of ionoregulatory function in developing fish by characterizing the functional ontogeny of sodium (Na⁺) balance in salmonids reared in freshwater during early development. Chapter 2 investigated the plasticity of Na⁺ balance and transport capacity during a critical developmental transition from cutaneous-dominated to gill-dominated ionoregulation in the model teleost fish, the rainbow trout. Fish experienced very high resting unidirectional Na⁺ uptake rates early in development which were reduced to values typical of adults following yolk absorption. Maximal uptake rate (Jmax) for Na⁺ was high during early development and decreased following yolk absorption while uptake affinity decreased (Km increased) following hatch and increased following yolk absorption. It appeared that early in development, high Na⁺ uptake rates across cutaneous ionocytes were driven by high maximal uptake rate, while the gill ionocytes that dominate ionoregulation post-yolk absorption had an increased affinity for Na⁺. Following hatch, when ionoregulation occurs predominantly across cutaneous ionocytes, larval fish exhibited little ionoregulatory plasticity in their Na⁺ uptake rates and Na⁺ uptake kinetics. As ionoregulation shifts to the gill, developing fish exhibited increased uptake affinity for Na⁺ in low-[Na⁺] environments as observed in adult fish; however, maximal uptake rate for Na⁺ did not increase in low-[Na⁺] environments as seen in adult fish, suggesting that the capacity to overcome Na⁺-poor environments may be limited and still developing at this stage. Chapter 3 contributed to our understanding of Na⁺ transport during early salmonid development and explored Na+ transport characteristics employed by the early-migrating anadromous salmonid, the pink salmon. It was clear that heightened and increasing whole-body [Na⁺] during yolk absorption in freshwater was not a unique characteristic of developing pink salmon associated with preparation for early ocean entry. This trait was shared by the non-anadromous rainbow trout. Interestingly, the mechanism by which pink salmon and rainbow trout achieved high whole-body [Na⁺] during early development did not appear to be the same. Rainbow trout experienced increasing Na⁺ uptake rates during development while pink salmon did not alter Na⁺ uptake rates during development, suggesting that pink salmon regulated whole-body [Na⁺] via modulation of Na⁺ efflux rates

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Following yolk-sac absorption and gravel emergence pink salmon (Oncorhynchus gorbuscha) migrate into seawater (SW) at as small as 0.2 g. This life-history strategy is in contrast with most anadromous salmonid species that generally spend 1-2 years growing in fresh water (FW) and physiologically preparing for life in SW before they migrate to SW as smolts. This study characterized for the first time the ontogeny of SW tolerance in pink salmon around the period of yolk-sac absorption. Post-hatch juvenile pink salmon were either held in FW for 26 weeks or transferred to SW every two weeks for 20 weeks to follow % survival, whole body (WB) Na+ and water content, as well as changes in wet and dry mass, gill Na+K+ATPase (NKA) activity and α1a and α1b mRNA isoform expression. An increase in gill NKA activity and the ratio of the α1b/α1a isoform expression, a plateau in WB water and Na+ levels, and the switch from catabolic to anabolic growth were all observed at the time of yolk-sac absorption in fish retained in freshwater. At this time, morbidity following subsequent SW transfer fell to 0% from a high of 100% for newly hatched alevins, but then rose to 25% in older fry, suggesting that a window of increased salinity tolerance exists for pink salmon at the time of yolk-sac absorption. This proposed window of SW tolerance is similar to the smolt window that has been identified for other salmonids; but in pink salmon appears to be endogenously mediated, as fish were reared under constant (12L:12D) photoperiod and at 5˚C throughout the study. Moreover, smoltification is incomplete since transfer to SW further elevated gill NKA activity and increased gill NKA α1b/α1α isoform expression ratio 8-fold at yolk-sac absorption. Thus, even the most SW-tolerant fish were not fully prepared for SW before entry, but responded directly to SW by further increasing hypo-osmoregulatory ability. This study filled the previously existing void of knowledge regarding the acquisition of salinity tolerance in juvenile pink salmon.

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No abstract available.

Pink salmon (Oncorynchus gorbuscha) enter seawater (SW) following gravel emergence at a body mass of 0.2 g. Two hydromineral challenges associated with this remarkable early ocean entry were investigated: (1) initial exposure to a hyper-osmotic environment and (2) sea louse (Lepeophtheirus salmonis) parasitism. To survive SW, pink salmon were hypothesized to develop hypo-osmoregulatory abilities as larval alevins prior to natural SW entry as post-larval fry. To test this, alevins and fry were transferred from freshwater (FW) in darkness to SW under a simulated natural photoperiod (SNP). Ionoregulatory status was assessed at 0, 1 and 5 days post-transfer. Alevins showed no evidence of hypo-osmoregulation, marked by a loss of water balance, a 35% increase in body [Cl-], and no change in gill Na⁺/K⁺-ATPase (NKA) activity. Conversely, fry maintained water balance and increased gill NKA activity by 50%. Fry gill NKA activity also increased by 50% following exposure to SNP in FW, providing the first evidence of photoperiod-triggered smoltification for pink salmon. A 15% increase in fry body [Na⁺] was observed as well, perhaps representing a novel mechanism for maintaining water balance during ocean entry. Physical damage to the host epidermis is a primary proximal effect of louse infection. Such damage may exacerbate existing hydromineral flux in SW. To test this, ionoregulatory status was measured in pink salmon of varying size with and without attached-stage lice. In laboratory-infected fish (~1 wk SW; 0.2-0.4 g), body [Na⁺] increased by 12% when infected with 1 chalimus IV louse, and by 23% with 2-3 chalimus III lice. Mortality was 6%. In wild-infected fish (~4-12 wks SW; 0.5-1.5 g), body [Na⁺] did not differ from controls. Combining data sets revealed a “no effect” fish size threshold of 0.5 g for 1 chalimus IV louse. This threshold is partly due to increasing hypo-osmoregulatory ability. Pink salmon thus appear to possess a novel hypo-osmoregulatory strategy where ion balance is sacrificed to maintain water balance prior to maximum ion excretion capacity. Out-migrating fish are particularly vulnerable to sea louse parasitism at this time, and as such, BC fish farms have relocated to minimize interactions during this critical period.

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