Physiological preparedness and performance of Atlantic salmon Salmo salar smolts in relation to behavioural salinity preferences and thresholds

This study investigated the relationships between behavioural responses of Atlantic salmon Salmo salar smolts to saltwater (SW) exposure and physiological characteristics of smolts in laboratory experiments. It concurrently described the behaviour of acoustically tagged smolts with respect to SW and tidal cycles during estuary migration. Salmo salar smolts increased their use of SW relative to fresh water (FW) from April to June in laboratory experiments. Mean preference for SW never exceeded 50% of time in any group. Preference for SW increased throughout the course of smolt development. Maximum continuous time spent in SW was positively related to gill Na⁺, K⁺‐ATPase (NKA) activity and osmoregulatory performance in full‐strength SW (measured as change in gill NKA activity and plasma osmolality). Smolts decreased depth upon reaching areas of the Penobscot Estuary where SW was present, and all fish became more surface oriented during passage from head of tide to the ocean. Acoustically tagged, migrating smolts with low gill NKA activity moved faster in FW reaches of the estuary than those with higher gill NKA activity. There was no difference in movement rate through SW reaches of the estuary based on gill NKA activity. Migrating fish moved with tidal flow during the passage of the lower estuary based on the observed patterns in both vertical and horizontal movements. The results indicate that smolts select low‐salinity water during estuary migration and use tidal currents to minimize energetic investment in seaward migration. Seasonal changes in osmoregulatory ability highlight the importance of the timing of stocking and estuary arrival.     

Physiological and molecular endocrine changes in maturing wild sockeye salmon, <Emphasis Type="Italic">Oncorhynchus nerka</Emphasis>, during ocean and river migration

Maturing adult sockeye salmon Oncorhynchus nerka were intercepted while migrating in the ocean and upstream in freshwater over a combined distance of more than 1,300 km to determine physiological and endocrine changes associated with ionoregulation. Sockeye migrating through seawater and freshwater showed consistent declines in gill Na⁺/K⁺-ATPase (NKA) activity, plasma osmolality and plasma chloride concentration. In contrast, plasma sodium concentration became elevated in seawater as fish approached the river mouth and was then restored after sockeye entered the river. Accompanying the movement from seawater to freshwater was a significant increase in mRNA for the NKA α1a subunit in the gill, with little change in the α1b subunit. Potential endocrine signals stimulating the physiological changes during migration were assessed by measuring plasma cortisol and prolactin (Prl) concentrations and quantifying mRNA extracted from the gill for glucocorticoid receptors 1 and 2 (GR1 and GR2), mineralocorticoid receptor (MR), growth hormone 1 receptor (GH1R), and prolactin receptor (PrlR). Plasma cortisol and prolactin concentrations were high in seawater suggesting a preparatory endocrine signal before freshwater entry. Generally, the mRNA expression for GR1, GR2 and MR declined during migration, most notably after fish entered freshwater. In contrast, PrlR mRNA increased throughout migration, particularly as sockeye approached the spawning grounds. A highly significant association existed between gill PrlR mRNA and gill NKA α1a mRNA. GH1R mRNA also increased significantly, but only after sockeye had migrated beyond tidal influence in the river and then again just before the fish reached the spawning grounds. These findings suggest that cortisol and prolactin stimulate ionoregulation in the gill as sockeye salmon adapt to freshwater.     

Late migration and seawater entry is physiologically disadvantageous for American shad juveniles

Juvenile American shad Alosa sapidissima were subjected to isothermal transfers into sea water (salinity 24)‘early’(1 September; 24° C) and ‘late’(10 November; 10° C) in the autumn migratory season. Early acclimation resulted in a modest osmotic perturbation that recovered rapidly. Haematocrit declined by 14% at 24 h, recovering within 48 h. Plasma osmolality increased by 6% at 4 h, recovering within 8 h. Early acclimation caused a two‐fold increase in gill Na⁺, K⁺‐ATPase activity by 24 h and a four‐fold increase by 4 days. The number of chloride cells on the primary gill filament increased two‐fold by 4 days. Chloride cells on the secondary lamellae rapidly decreased from 22 cells mm^?1 to <2 cells mm^?1 within 4 days. Late acclimation resulted in a severe and protracted osmotic perturbation. Haematocrit levels declined by 23% at 4 days, recovering by 14 days. Plasma osmolality increased by 36% by 48 h, recovering by 4 days. Initial gill Na⁺, K⁺‐ATPase activity was two‐fold greater than in ‘early’ fish and did not change during acclimation. Initial numbers of chloride cells on the primary filament were two‐fold greater than ‘early’ fish and did not increase during acclimation. Initial number of chloride cells on the secondary lamellae was five‐fold greater than ‘early’ fish (116 v. 22 cells mm^?1) and declined to negligible numbers over 14 days. Differences between initial measures for ‘early’ and ‘late’ fish reflect previously described physiological changes associated with migration. These data indicate that late migrants face a greater physiological challenge during seawater acclimation than early migrants. Physiological performance apparently limits the observed duration of autumnal migration.     

Seaward migrating Atlantic salmon smolts with low levels of gill Na+, K+ -ATPase activity; is sea entry delayed?

Two groups of migrating wild Atlantic salmon (Salmo salar) smolts caught within a 1 week interval in the River Alta, northern Norway, were tagged with acoustic transmitters and measured for gill Na⁺, K⁺ -ATPase activity in order to compare their smolt status with timing of sea entry. The first group of smolts had low levels of gill Na⁺, K⁺ -ATPase activity and resided in the lower part of the river twice as long as the second group that had high levels of gill Na⁺, K⁺ -ATPase activity. This indicates that early migrating smolts may not be completely physiologically adapted for salt water and delay their sea entry, thereby also synchronizing their seaward migration with the later migrating smolts.     

Movers and shakers: nutrient subsidies and benthic disturbance predict biofilm biomass and stable isotope signatures in coastal streams

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Evidence for episodic acidification effects on migrating Atlantic salmon Salmo salar smolts

Field studies were conducted to determine levels of gill aluminium as an index of acidification effects on migrating Atlantic salmon Salmo salar smolts in the north‐eastern U.S.A. along mainstem river migration corridors in several major river basins. Smolts emigrating from the Connecticut River, where most (but not all) tributaries were well buffered, had low or undetectable levels of gill aluminium and high gill Na⁺/K⁺‐ATPase (NKA) activity. In contrast, smolts emigrating from the upper Merrimack River basin where most tributaries are characterized by low pH and high inorganic aluminium had consistently elevated gill aluminium and lower gill NKA activity, which may explain the low adult return rates of S. salar stocked into the upper Merrimack catchment. In the Sheepscot, Narraguagus and Penobscot Rivers in Maine, river and year‐specific effects on gill aluminium were detected that appeared to be driven by underlying geology and high spring discharge. The results indicate that episodic acidification is affecting S. salar smolts in poorly buffered streams in New England and may help explain variation in S. salar survival and abundance among rivers and among years, with implications for the conservation and recovery of S. salar in the north‐eastern U.S.A. These results suggest that the physiological condition of outmigrating smolts may serve as a large‐scale sentinel of landscape‐level recovery of atmospheric pollution in this and other parts of the North Atlantic region.     

Changes in branchial and intestinal osmoregulatory mechanisms and growth hormone levels during smolting in hatchery-reared and wild brown trout

Evidence of smolting was studied in Danish hatchery-reared brown trout Salmo trutta L. Twenty-four hour seawater (SW) challenge tests (28‰, 10°C) at regular intervals showed that maximal hypo-osmoregulatory ability developed within a 3–4-week period in March and April. The improved ability to regulate plasma osmolality, muscle water content and plasma total [Mg] developed asynchronously, indicating that developmental changes in the gill, the gastrointestinal system and the kidney may not necessarily concur during smolting. Gill Na⁺, K⁺-ATPase activity peaked in April at the time of optimal hypo-osmoregulatory ability. Na⁺, K⁺-ATPase a -subunit mRNA level in gills was unchanged from January until April, but decreased in May in parallel with a decrease in the activity of the enzyme. In the middle region of the intestine, Na⁺, K⁺-ATPase activity increased in February and remained high until April. In the posterior region of the intestine, the activity was stable from January until April after which it decreased. In vitro fluid transport capacitity, J_v, in the middle intestine fluctuated throughout the spring. In the posterior intestine, J_v was low until late March, when it increased fivefold until early May. Drinking rate in fish transferred to SW for 24 h surged during spring. Na⁺, K⁺-ATPase activity in the pyloric caeca was elevated from March until May, and increased in response to SW transfer in June, suggesting a hypo-osmoregulatory function of the pyloric caeca. Plasma GH levels surged in FW trout during spring, concurring with the increase in gill Na⁺, K⁺-ATPase activity and SW tolerance, but peaked in May when gill Na⁺, K⁺-ATPase activity and SW tolerance were regressing. GH levels were generally low in SW-challenged fish, and there was no consistent effect of 24-h SW exposure on GH levels. In wild anadromous trout, gill Na⁺, K⁺-ATPase activity varied seasonally as in hatchery-reared fish, but peaked at higher levels suggesting a more intense smolting in fish living in their natural environment.     

Carcass analogues provide marine subsidies for macroinvertebrates and juvenile Atlantic salmon in temperate oligotrophic streams

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Plasticity of smolting in spring chinook salmon: relation to growth and insulin-like growth factor-I

In two year classes of Willamette River spring chinook salmon, reared at the Willamette Hatchery, and two groups of Yakima River spring chinook salmon, one sampled from the Yakima River and the other reared in a hatchery, fish which had relatively high growth rates in the summer–autumn period smolted in the autumn as measured by increases in gill Na⁺ K⁺ AT Pase activity. In contrast, groups with relatively low growth rate did not smolt in the autumn. Plasma levels of insulin-like growth factor-I (IGF-I) showed discrete differences between groups, with high levels associated with increased gill Na⁺ K⁺ AT Pase activities. These results demonstrate that smolting is plastic in spring chinook salmon, occurring in the autumn or the spring. In addition, smolting appeared to be related to growth rate; however, the relationships shown were correlational and causal mechanisms were not elucidated. Yet, the results do indicate a relationship between growth, an endocrine growth factor and smolting, suggesting a mechanistic link between developmental plasticity and the environment mediated by the endocrine system.     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

Bimodal length frequency distribution in 0+aged masu salmon,Oncorhynchus masou,in a natural stream of southern Hokkaido

The frequency distribution of the fork length of 0⁺ aged masu salmon,Oncorhynchus masou, changed from unimodal to bimodal distribution in autumn of the years from 1982 to 1984 in the Mogusa River of southern Hokkaido, Japan. The bimodal distribution consists of two (upper and lower) modal groups. These two groups resulted from a difference in growth rate of 0⁺ aged individuals in autumn. Fish belonging to the upper modal group are assumed to be potential 1⁺ smolts. Whether 0⁺ aged parr transform into smolt or remain as parr in the following spring may be related to the growth rate of fish in the first autumn.     

Early life stage salinity tolerance of wild and hatchery-reared juvenile pink salmon Oncorhynchus gorbuscha

Salinity tolerance in wild (Glendale) and hatchery (Quinsam) pink salmon Oncorhynchus gorbuscha (average mass 0·2 g) was assessed by measuring whole body [Na⁺] and [Cl^?] after 24 or 72 h exposures to fresh water (FW) and 33, 66 or 100% sea water (SW). Gill Na⁺, K⁺‐ATPase activity was measured following exposure to FW and 100% SW and increased significantly in both populations after a 24 h exposure to 100% SW. Whole body [Na⁺] and whole body [Cl^?] increased significantly in both populations after 24 h in 33, 66 and 100% SW, where whole body [Cl^?] differed significantly between Quinsam and Glendale populations. Extending the seawater exposure to 72 h resulted in no further increases in whole body [Na⁺] and whole body [Cl^?] at any salinity, but there was more variability among the responses of the two populations. Per cent whole body water (c. 81%) was maintained in all groups of fish regardless of salinity exposure or population, indicating that the increase in whole body ion levels may have been related to maintaining water balance as no mortality was observed in this study. Thus, both wild and hatchery juvenile O. gorbuscha tolerated abrupt salinity changes, which triggered an increase in gill Na⁺, K⁺‐ATPase within 24 h. These results are discussed in terms of the preparedness of emerging O. gorbuscha for the marine phase of their life cycle.     

Dietary lipid composition affects the gene expression of gill Na+/K+-ATPase α1b but not the α1a isoform in juvenile fall chinook salmon (Oncorhynchus tshawytscha)

We assessed the effects of dietary fatty acid composition on sodium–potassium ATPase (Na⁺/K⁺-ATPase) activity and isoform expression in the gills of juvenile fall chinook salmon, Oncorhynchus tshawytscha by supplementing diets with either anchovy oil (AO) or AO blended with canola oil (CO) so that CO comprised 0% (0CO), 11% (11CO), 22% (22CO), 33% (33CO), 43% (43CO), or 54% (54CO) of the measured dietary lipid content. The effects of diet were assessed in freshwater (FW) following 104 days of diet manipulation, in response to 24-h seawater (SW) transfer at this time, and following an additional 35 days of SW acclimation. Gill Na⁺/K⁺-ATPase activity was not significantly affected by diet at any sampling time, and there were no consistent effects of diet on the expression of the Na⁺/K⁺-ATPase α1a isoform. As dietary CO increased, Na⁺/K⁺-ATPase α1b mRNA decreased in fish held in FW, with the 43CO and 54CO diet groups having significantly lower levels than fish fed the 0CO and 11CO diets. Twenty-four-hour SW challenge did not affect the expression of the Na⁺/K⁺-ATPase α1a isoform in any diet group, but this isoform was down-regulated in all diet groups following 35 days of SW acclimation. Na⁺/K⁺-ATPase α1b expression levels increased in response to 24-h SW transfer and SW acclimation only in fish fed the 54CO diet. The effects of the two extreme diets (0CO and 54CO) were also assessed at various time points during 104 days of rearing in FW. Na⁺/K⁺-ATPase α1b mRNA levels were greater in fish fed diet 0CO versus those fed diet 54CO at all times during the FW culture period. These data demonstrate that dietary fatty acid composition can influence the gill Na⁺/K⁺-ATPase isoform physiology of juvenile fall-run chinook salmon prior to SW transfer.     

Ion-poor water and dietary salt deprivation upregulate the ghrelinergic system in the goldfish (Carassius auratus)

Because the ghrelinergic system in teleost fishes is broadly expressed in organs that regulate appetite as well as those that contribute to the regulation of salt and water balance, we hypothesized that manipulating salt and water balance in goldfish (Carassius auratus) would modulate the ghrelinergic system. Goldfish were acclimated to either freshwater (FW) or ion-poor FW (IPW) and were fed either a control diet containing 1% NaCl or low-salt diet containing 0.1% NaCl. Endpoints of salt and water balance, i.e., serum Na⁺ and Cl⁻ levels, muscle moisture content and organ-specific Na⁺-K⁺-ATPase (NKA) activity, were examined in conjunction with brain, gill and gut mRNA abundance of preproghrelin and its receptor, growth hormone secretagogue receptor (ghs-r). Acclimation of fish to IPW reduced serum osmolality and Cl⁻ levels and elevated kidney NKA activity, while FW fish fed a low NaCl diet exhibited a modest reduction in muscle moisture content but otherwise no apparent osmoregulatory disturbance. In contrast, a combined treatment of IPW acclimation and low dietary NaCl content reduced serum osmolality and Cl⁻ levels, elevated muscle moisture content and increased gill, kidney and intestinal NKA activity. This intensified response to the combined effects of water and dietary ion deprivation is consistent with an increased effort to enhance ion acquisition. In association with these latter observations, a significant upregulation of preproghrelin mRNA expression in brain and gut was observed. A significant increase in ghs-r mRNAs was also observed in the gill of goldfish acclimated to IPW alone but a reduction in dietary NaCl content did not impact the ghrelinergic system of goldfish in FW. The results support the hypothesis that the ghrelinergic system is modulated in response to manipulated salt and water balance. Whether the central and peripheral ghrelinergic system contributes to ionic homeostasis in goldfish currently remains unclear and warrants further research.     

A bioenergetic assessment of the influence of stocking practices on rainbow trout (Oncorhynchus mykiss) growth and consumption in a New Zealand lake

Summary

• 1 To investigate the carrying capacity and factors affecting growth of rainbow trout in Lake Rotoiti, we employed a bioenergetics model to assess the influence of stocking rates, timing of releases and prey abundance on growth and prey consumption. We hypothesised that stocking rates and prey abundance would affect growth and prey consumption by influencing per‐capita prey availability, and that the environmental conditions encountered by fish at the time of stocking would affect growth and consumption.
• 2 Prey consumption of stocked rainbow trout was calculated with the Wisconsin bioenergetics model. We calculated growth trajectories of released trout based on data from stocked trout that were released in spring and autumn from 1993 to 2009 and then re‐captured by anglers. Diet, prey energy density, body mass lost during spawning and lake temperature were measured locally.
• 3 Stocking timing had no effect on return rates to anglers or length or weight of caught fish. Although trout released in autumn were smaller than those released in spring, autumn‐released trout grew at a faster rate and had similar lengths and weights to spring cohorts after 2 years of growth in the lake. Modelled consumption parameters were negatively correlated with trout population size, suggesting that stocking rates (347–809 fish ha^?1 year^?1) caused density‐dependent effects on growth. Although common smelt (Retropinna retropinna) accounted for 85% of total prey consumption, no significant relationship was found between prey consumption by individual trout and adult smelt abundance, possibly because trout are targeting smaller smelt that our abundance estimate did not account for.
• 4 Releasing trout in autumn appears to be advantageous for growth, possibly because (i) temperature is more suitable for growth in autumn–winter than in spring–summer and (ii) prey for small trout is abundant in autumn. Mild winter conditions appear to enhance overwinter survival and growth of rainbow trout in warm‐temperate lakes compared to higher latitudes. This implies that moderately productive warm‐temperate lake ecosystems are highly suitable for trout growth in winter, but less so in summer, when lake stratification and high nutrient levels may create conditions suitable for algal blooms and hypolimnetic deoxygenation. High growth rates of trout in warm‐temperate lakes can therefore be supported by timing releases to coincide with favourable winter conditions.

     

Physiological, Endocrine, and Genetic Bases of Anadromy in the Brook Charr, Salvelinus Fontinalis, of the Laval River (Québec, Canada)

Brook charr, Salvelinus fontinalis, often display alternate life history styles in coastal areas. In the Laval River, some brook charr remain freshwater residents, while others undergo seasonal migrations between freshwater and saltwater environments. In the present paper, we examined physiological (electrolyte concentrations, gill Na⁺, K⁺-ATPase activity, and thyroid hormone levels) as well as genetic differences (neutral genetic markers) between anadromous and river-resident fish from the Laval River. We also examined how artificial rearing conditions affected seasonal variations in the osmoregulatory physiology of a domestic strain derived from wild anadromous fish. Sympatric anadromous and resident forms of brook charr of the Laval River exhibited differences in gill Na⁺, K⁺-ATPase activity, plasma thyroxine (T₄), and triidothyronine (T₃) concentrations. In domestic anadromous charr, rearing conditions during development had no negative impact on osmoregulatory ability or on gill Na⁺, K⁺-ATPase activity. These results argued for an important hereditary component of gill Na⁺, K⁺-ATPase activity. However, the spring increase in T₄ was present only in wild fish. Significant differences observed at microsatellite loci further suggested that at least some level of reproductive isolation may have occurred between anadromous and resident charr in the Laval River.     

Stable isotope evidence indicates the incorporation into Japanese catchments of marine‐derived nutrients transported by spawning Pacific Salmon

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A comparison of cystatin activity in the various tissues of chum salmon Oncorhynchus keta between feeding and spawning migrations

• 1.1. Cystatin was found to be widely distributed in various tissues of chum salmon. Most of the cystatins in the salmon tissues appeared to have a molecular weight between 10,000 and 20,000. They were considered to belong to the low molecular weight form cystatin, the type 1 and/or type 2 cystatins.
• 2.2. The activity in the liver of the salmon in spawning migration was significantly higher than that of the fish in feeding migration, while the activities of the serum, kidney, intestine, stomach, gill, skin and white muscle in spawning migration were apparently lower than that of the fish in feeding migration.
• 3.3. Such differences in the cystatin activity were considered to relate closely to the physiological conditions such as sexual maturation and/or starvation during spawning migration of the fish.

     

Visiting the hyporheic zone: young Atlantic salmon move through the substratum

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Behavioural changes during the parr–smolt transformation in coho salmon Oncorhynchus kisutch: is it better to be cool?

Behavioural changes that occur during the parr–smolt transformation were investigated in juvenile coho salmon Oncorhynchus kisutch. Fish from two populations were examined from the Fraser River catchment in British Columbia, Canada; a short and a long-distance migrating population. Fish showed a significant decrease in condition factor and significant increase in gill Na⁺K⁺-ATPase activity during the spring indicating that they became competent smolts, but no difference between populations. Temperature preference trials were conducted using a shuttlebox system throughout the spring. Mean temperature preference did not differ between the two populations, but preferred temperature decreased with development from 16.5 ± 0.3°C for parr to 15.5 ± 0.4°C for smolts. Mean swimming velocity was also greater in smolts than parr, but there was no difference between the two populations. The preference for warmer water temperature observed for parr in early spring may be advantageous for stimulating smolt development. Preference for slightly cooler temperatures observed for smolts would sustain elevated seawater tolerance during the smolt window by a short time and may ensure successful transition to the marine environment.