Tolerance of seahorse Hippocampus kuda (Bleeker) juveniles to various salinities

In line with current conservation efforts, some success in the captive breeding of the seahorse Hippocampus kuda (Teleostei: Syngnathidae) has been achieved. To evaluate the salinity tolerance of these hatchery‐bred juveniles, 9‐week‐old H. kuda were transferred without prior acclimatization from ambient full strength seawater (32–33 ppt) to salinities ranging from freshwater to 85 ppt. Survival, growth, and total body water content were determined after 4 and 18 days of exposure. Juvenile H. kuda are able to survive in dilute seawater (15 ppt) for at least 18 days without any compromise in growth (both wet and dry body weight), survival, and total body water. Fish abruptly transferred to freshwater succumbed within 4–24 h, while survival of 5 ppt‐reared fish decreased to ca. 65% in 18 days. Although 10 ppt‐reared seahorses had growth and survival comparable with the control (30 ppt seawater), total body water was significantly elevated indicating reduced adaptability. The upper limit of H. kuda salinity tolerance was 50 ppt. Fish reared at salinities ≥55 ppt succumbed within 24 h. Like several other marine teleosts, growth and survival of juvenile H. kuda tended to peak in diluted seawater salinities of 15 and 20 ppt. These results indicate the possibility of growing hatchery‐bred H. kuda in brackishwater environments.     

Physiological effects of saline waters on zander

Rapid transfer of zander Stizostedion lucioperca to hypoosmotic brackish water (mean osmolality 230 mOsmol kg–1 , c. 8 psu) significantly increased plasma chloride concentrations after 24 h compared to those transferred to fresh water, although plasma osmolality was not significantly affected. After 6 days, plasma osmolality was slightly elevated but stable plasma glucose and cortisol concentrations and blood haematocrit and haemoglobin suggest a lack of hormonal stress responses and resultant secondary effects. Rapid transfer of zander to a more saline environment, hyperosmotic to plasma (mean osmolality 462 mOsmol kg^‐1, c. 16 psu) induced a greater increase in plasma osmolality and chloride concentrations within 24 h, with a further rise after 6 days exposure, but all fish maintained a state of hypo‐osmoregulation both 24 h and 6 days after transfer. The initial osmotic disturbance (at 24 h) was accompanied by increased plasma glucose, blood haematocrit and haemoglobin and a decreased mean cell haemoglobin concentration (MCHC), suggesting an adrenergic stress response, but these parameters fully recovered within 6 days of exposure to this hyperosmotic environment with MCHC rising to exceed the level in freshwater fish. Zander did not survive rapid transfer to more hyperosmotic conditions (750 or 1001 mOsmol kg^‐1, 26‐35 psu), but they did survive exposure to simulated‘tidal cycles’ of rising and declining salinity, peaking after 6 h at c. 29 or 33 psu. Although osmotic disturbance was apparent after 6 h exposure and other physiological parameters suggested both adrenergic and corticosteroid components of a stress response, rapid recovery was apparent after return to fresh water. The results indicate that the zander, a non‐indigenous species in the U.K., has a high level of osmotic tolerance and a degree of hypo‐osmoregulation in saline environments not found in most stenohaline freshwater teleosts. This osmoregulatory ability could enable invasion of new U.K. river systems by using inshore marine environments of low salinity as saltwater bridges.     

Does body size affect the response to exercise in shortnose sturgeon (Acipenser brevirostrum)?

The effect of body size on various hematological variables was examined in juvenile shortnose sturgeon (Acipenser brevirostrum) ranging in mass from 38 to 730 g. The blood was examined for differences in plasma ionic composition (Na⁺, K⁺, Cl^?), blood oxygen carrying capacities (hemoglobin, hematocrit), and plasma metabolite concentrations (lactate, glucose), before and following a standard 5‐min chasing stress. All measured resting hematological variables were size independent in shortnose sturgeon. After exercise, levels of plasma lactate, potassium, and hemoglobin increased in all fish. Only post‐exercise levels of chloride and hemoglobin changed in a size‐dependent manner; however, the relationships were weak. The general lack of a relationship between body size and hematological variables might reflect the narrow range of fish sizes used in the present study. From a practical perspective, the results suggest that when examining the hematological stress response in juvenile shortnose sturgeon, a range of fish sizes could be used. This is important considering the variability in the growth rates of juvenile shortnose sturgeon under laboratory conditions.     

Impairment of the stress response in matrinxã juveniles (Brycon amazonicus) exposed to low concentrations of phenol

In this study we measured plasma cortisol, plasma glucose, plasma sodium and potassium, and liver and gill hsp70 levels in juvenile matrinxã (Brycon amazonicus) subjected to a 96 h exposure to phenol (0, 0.2, and 2.0 ppm), and the effect of this exposure on their ability to respond to a subsequent handling stress. Fish were sampled prior to initiation of exposure and 96 h, and at 1, 6, 12, and 24 h post-handling stress. During the 96 h exposure, plasma cortisol and glucose levels remained unchanged in all treatments. While plasma sodium levels were significantly reduced in all groups, plasma potassium levels only decreased in fish exposed to 0 and 0.2 ppm of phenol. Liver hsp70 levels decreased significantly at 96 h in fish exposed to 2.0 ppm of phenol. All groups, except fish exposed to 0.2 ppm of phenol, were able to increase plasma cortisol and glucose levels after handling stress. Fish exposed to 2.0 ppm of phenol showed decreased gill and liver hsp70 levels after the handling stress. Our data suggest that exposure to phenol may compromise the ability of matrinxã to elicit physiological responses to a subsequent stressor.     

Seasonal changes in sea-water tolerance of Arctic charr (Salvelinus alpinus)

Summary Groups of Arctic charr,Salvelinus alpinus, which had been acclimated to water with a salinity of 7 g·l^–1 and natural temperature and photoperiod, were exposed to water with different salinities and temperatures in June, September and February. At a salinity of 15 g·l^–1, plasma osmolality, plasma Na⁺, Cl^–, Mg²⁺ concentrations and the activity of gill Na-K-ATPase were stable, irrespective of temperature and season. In June, the charr were able to regulate blood plasma ionic levels within narrow limits when exposed to a salinity of 34 g·l^–1 (sea water) and a temperature of 8°C. The hypo-osmoregulatory capacity was less, but sufficient if the temperature was only 1°C during the seawater exposure. At the start of the experiment, the gill Na-K-ATPase activity was significantly higher in June than corresponding enzyme activities in September and February. Furthermore, an increase in gill Na-K-ATPase activity during the seawater exposure (8°C) was seen in June. Irrespective of ambient temperature and salinity, no fish died during the June experiments. In September and February, exposure to sea water produced marked increases in plasma osmolality and plasma ion concentrations. There were no changes in gill Na-K-ATPase activity. Consequently, the fish became dehydrated and were moribund after a short period of seawater exposure. Highest mortality was recorded when charr were exposed to winter sea conditions (34 g·l^–1 and 1°C) in February. The results indicate that an increase in daylength induce a hypo-osmoregulatory capacity in the Arctic charr during summer. In fall and winter, however, reduced daylength are accompanied by poor hypo-osmoregulatory capacity. This leads to high mortality as a result of increased electrolyte levels and dehydration.     

Effect of short term exposure to the anaesthetic 2-phenoxyethanol on plasma osmolality of juvenile dusky kob, Argyrosomus japonicus (Sciaenidae)

The plasma osmolality of early juvenile dusky kob, Argyrosomus japonicus , exposed to 2-phenoxyethanol and control fish that were pithed prior to sampling, was investigated. Exposure to 2-phenoxyethanol, after 2, 4, 6, 8 and 10 min, did not alter plasma osmolality (Friedman test; P = 0.976). There was no significant relationship between the size of fish within the range 133–170 mm SL (40–85 g) and plasma osmolality. Finally, there was no significant difference in plasma osmolality between anaesthetized fish and the control group that were pithed directly after removal from the tanks. Anaesthetizing juvenile dusky kob with 2-phenoxyethanol prior to blood sampling did not affect plasma osmolality.     

Summer seawater tolerance of small-sized Arctic charr, Salvelinus alpinus, on Svalbard

Migrating Arctic charr (Salvelinus alpinus) parr (118 ± 34.4 mm) were caught close to the rivermouth of the Dieset river on Spitsbergen (79°10′N), Svalbard. When subjected to a seawater tolerance test (34 ppt at 6°C) their blood plasma osmolality and sodium and magnesium concentrations increased significantly. After 90 h of exposure, average plasma osmolality was 410 (±54.1) mOsmol. Corresponding sodium and magnesium concentrations were 207 (±35.9) mmol l⁻¹ and 2.7 (±1.36) mmol l⁻¹, respectively. Survival at this time was only 12.5%. When smaller fish (96 ± 26.6 mm) were exposed to seawater, mortality was 100% within 72 h. We conclude that small-sized Svalbard charr may survive only short periods in seawater. Therefore, the lack of adequate hypoosmoregulatory capacity limits their access to marine food resources. Received: 29 December 1997 / Accepted: 12 April 1998     

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.     

Laboratory test of suspended sediment effects on short‐term survival and swimming performance of juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus,Mitchill, 1815)

Tested was the hypothesis that juvenile Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) would exhibit no response in short‐term survival or swimming performance when exposed to varying concentrations of suspended sediment simulating dredge plumes in waterways where this species may be impacted by dredging operations. Sediment collected from Savannah Harbor, South Carolina, USA was used to simulate a worst‐case scenario. Juvenile sturgeon were contained for a 3‐day period in flow‐through aquaria, with limited opportunity for movement, in sediment of varying concentrations (100, 250 and 500 mg L^?1 total suspended solids [TSS]) mimicking prolonged exposure to suspended sediment plumes near an operating dredge. Of the 90 fish exposed, 86 (96%) survived the test. Of the four fish that died, one was exposed to 250 TSS and three to 500 TSS. Swimming performance results indicated that nearly all fish were positively rheotactic. Critical swim speeds (U_crits) were moderate, whether measured as absolute values (21–31 cm s^?1) or as relative values (1.4–2.1 body lengths s^?1), with no significant differences among treatments (F < 0.83, P ≥ 0.4874). Behavior was dominated by contact‐based locomotion and station‐holding. Absence of substantial or significant immediate effects on survival and swimming performance suggest that impacts of sediment plumes in nature, where fish have freedom of movement and the power to escape rapidly, are minimal.     

The effect of reduced temperature and salinity on the blood physiology of juvenile Atlantic cod

The osmolality and ionic composition of the blood of juvenile Atlantic cod Gadus morhua and their response to conditions of reduced temperature and salinity in summer‐ and winter‐acclimated individuals was investigated. Haematocrit percentage was relatively stable throughout the experimental procedures. Summer‐acclimated juvenile Atlantic cod had higher plasma osmolality than winter‐acclimated fish in ambient conditions. Plasma Na⁺ levels were, however, higher in winter conditions, while Cl⁻ did not vary between seasons. Temperature reduction (12, 9 and 6° C in summer and to 6 and 4° C in winter) induced a significant response in plasma osmolality and Na⁺ levels in summer, but only in Na⁺ levels in winter‐acclimated fish. A pronounced effect was seen in the summer 6° C treatment. Salinity treatments (24, 16 and 8) had a significant effect on almost all the variables in both summer and winter and resulted generally in dilution of ionic and osmotic concentrations of the plasma. This effect was pronounced in the lowest temperature treatments, with the greatest reduction observed in the summer 6° C treatment. This could suggest that winter‐acclimated fish are physiologically adapted to cope with lower seawater temperatures as opposed to summer‐acclimated fish.     

Intestinal osmoregulatory acclimation and nitrogen metabolism in juveniles of the freshwater marble goby exposed to seawater

The objective of this study was to elucidate the role of the intestine from juveniles of the marble goby, Oxyeleotris marmorata, during seawater (SW) exposure. It has been reported elsewhere that SW-exposed juvenile O. marmorata exhibits hypoosmotic and hypoionic regulation, with the induction of branchial Na⁺/K⁺-ATPase (NKA), Na⁺:K⁺:2Cl⁻ cotransporter (NKCC), and cystic fibrosis transmembrane receptor-like chloride channels. Here, we report that SW exposure also led to significant increases in the activity and protein abundance of NKA in, and probably an increase in Na⁺ uptake through, its intestine. Additionally, there was an increase in apical NKCC immunoreactivity in the intestinal epithelium, indicating that there could be increased Cl⁻ uptake through the intestine. These results suggest that absorption of ions, and hence water, from the intestinal lumen could be an essential part of the osmoregulatory process in juvenile O. marmorata during exposure to SW. Furthermore, there were significant increases in the glutamate content, and the aminating activity and protein abundance of glutamate dehydrogenase (GDH) in the intestine of fish exposed to SW. Since the intestinal glutamine synthetase activity and protein abundance decreased significantly, and the intestinal glutamine content remained unchanged, in the SW-exposed fish, excess glutamate formed via increased GDH activity in the intestine could be channeled to other organs to facilitate the increased synthesis of amino acids. Taken together, our results indicate for the first time that, besides absorbing ions and water during SW exposure, the intestine of juvenile O. marmorata also participated in altered nitrogen metabolism in response to salinity changes.     

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.     

Renal,respiratory and ionic regulation in Atlantic salmon (Salmo salar L.) kelts following transfer from fresh water to seawater

Summary Atlantic salmon may return to the sea after spawning in fresh water. These fish, known as kelts, reportedly show a limited ability to hypoosmoregulate. However, this study shows that fresh-water-adapted kelts exposed to seawater demonstrate rapid adaptation (within 48 h) in osmoregulatory parameters to values characteristic of seawater-adapted salmonids. The urine flow rate falls from 1.2 to 0.2 ml·kg^-1·h^-1 within 24 h. Over the same period, urine osmolality increases from 48 mosmol·kg^-1 to become isosmotic with the plasma, and Mg²⁺ secretion by the kidney tubules elevates the urine concentration from 0.5 to 100 mmol·l^-1. As is characteristic for marine teleosts, kelts drink seawater and process the ingested water in the gut to replace body water lost by osmosis to the hyperosmotic medium. Seawater exposure causes a marked hypoxia, arterial oxygen tension falling by 43% within minutes and persisting for at least 4 days at this low level. This is associated with large changes in blood pH and acid-base balance. The physiological mechanisms involved in adaptation to a hyperosmotic external medium are discussed, and the osmoregulatory capacity of kelts is compared with that of salmon at other stages of the life cycle.Abbreviations FW fresh water - GFR glomerular filtration rate - Hb haemoglobin - Hct haematocrit - MCHC mean cell haemoglobin concentration - pH_a pH in arterial blood - P _aO₂ partial pressure of oxygen in arterial blood - SEM standard error of mean - SW seawater - UFR urine flow rate     

The Effect of Ration on Acclimation to Environmental Acidity in Rainbow Trout

Freshwater salmonids exposed to low environmental pH typically suffer a net loss of ions, primarily Na⁺ and Cl^–, across the gills, resulting in reduced plasma and tissue ion concentrations. However, in recent experiments in our laboratory, juvenile rainbow trout, Oncorhynchus mykiss, fed a ration of 1% body weight d^–1 or greater showed no ionoregulatory disturbance during chronic, sublethal acidification. This raised the possibility that these fish had acclimated to low pH in that they would be better able to withstand further, more severe acidification than fish that had no prior experience of acid conditions: previous studies had concluded that such acclimation does not occur. This hypothesis was tested by measuring unidirectional ion fluxes during a 24h acute acid challenge (pH 4.2) in juvenile rainbow trout that had previously been exposed to either ambient pH 6.2 (naive fish) or sublethal low pH 5.2 (acid pre-exposed fish) for 90 days, and fed a ration of either 1.0 or 0.25% d^–1 (wet basis). No mortalities were observed during the acute acid challenge in the fish fed the higher ration and no differences between the two groups in the response of Na⁺ fluxes were observed. Sodium influx in both groups was significantly inhibited throughout the challenge and Na⁺ net flux was significantly stimulated over the first 6h. Prior to the acute acid challenge, the fish fed the lower ration that had previously been exposed to pH 5.2 had significantly lower plasma ion concentrations than those fish previously exposed to pH 6.2. Both groups suffered mortalities; those of the naive fish (22% by 24h) being markedly lower than those of the acid pre-exposed fish (68% by 24h). However, there were no significant differences in either Na⁺ or Cl^– fluxes between the two groups of fish during the acid challenge: both showed significant inhibition of ion influxes and significantly greater net ion losses, resulting in reduced plasma ion concentrations. These results indicate that rainbow trout are unable to acclimate to environmental acidification irrespective of the availability of dietary salts.     

Growth and physiological parameters of whiting (Sillaginodes punctata) in relation to salinity

This study assessed the impact of salinity on whiting (Sillaginodes punctata) in an attempt to understand the mechanisms by which salinity could potentially influence habitat selection and growth of King George whiting in southern Australia. The experiment included whiting of two age classes, young of the year (YOY) and 2⁺ year, at three salinities (30, 40, 50 ppt). YOY whiting showed no significant difference in length or weight gain, specific growth rate, feed intake, food conversion ratio or condition factor when exposed to the three salinities for 72 day. Plasma osmolality of YOY whiting was not significantly different at any salinity, although it was significantly lower than that of 2⁺ year whiting. The 2⁺ year whiting showed significantly higher plasma osmolality than the YOY. Blood plasma potassium and chloride levels of 2⁺ year fish at 50 ppt were significantly higher than those at 30 ppt and 40 ppt. Blood sodium levels at 50 ppt were significantly higher than at 30 ppt but the sodium level at 40 ppt was not different from 30 ppt or 50 ppt. Haematocrit of 2⁺ whiting was significantly higher at 30 than at 50 ppt while haematocrit at 40 ppt was not different from 30 or 50 ppt. The 2⁺ year‐old whiting had a more pronounced increase in plasma osmolality and plasma ions at high salinities, indicating poorer osmoregulatory capacity in older fish. This study provides physiological evidence to partially explain habitat occupancy and growth in relation to salinity of different age groups of whiting in southern Australia.     

Ontogeny of salinity tolerance and evidence for seawater-entry preparation in juvenile green sturgeon, <Emphasis Type="Italic">Acipenser medirostris</Emphasis>

We measured the ontogeny of salinity tolerance and the preparatory hypo-osmoregulatory physiological changes for seawater entry in green sturgeon (Acipenser medirostris), an anadromous species occurring along the Pacific Coast of North America. Salinity tolerance was measured every 2 weeks starting in 40-day post-hatch (dph) juveniles and was repeated until 100% survival at 34‰ was achieved. Fish were subjected to step increases in salinity (5‰ 12 h⁻¹) that culminated in a 72-h exposure to a target salinity, and treatment groups (0, 15, 20, 25, 30, 34‰; and abrupt exposure to 34‰) were adjusted as fish developed. After 100% survival was achieved (134 dph), a second experiment tested two sizes of fish for 28-day seawater (33‰) tolerance, and gill and gastrointestinal tract tissues were sampled. Their salinity tolerance increased and plasma osmolality decreased with increasing size and age, and electron microscopy revealed three types of mitochondria-rich cells: one in fresh water and two in seawater. In addition, fish held on a natural photoperiod in fresh water at 19°C showed peaks in cortisol, thyroid hormones and gill and pyloric ceca Na⁺, K⁺-ATPase activities at body sizes associated with seawater tolerance. Therefore, salinity tolerance in green sturgeon increases during ontogeny (e.g., as these juveniles may move down estuaries to the ocean) with increases in body size. Also, physiological and morphological changes associated with seawater readiness increased in freshwater-reared juveniles and peaked at their seawater-tolerant ages and body sizes. Their seawater-ready body size also matched that described for swimming performance decreases, presumably associated with downstream movements. Therefore, juvenile green sturgeon develop structures and physiological changes appropriate for seawater entry while growing in fresh water, indicating that hypo-osmoregulatory changes may proceed by multiple routes in sturgeons.     

Altered burst swimming in rainbow trout Oncorhynchus mykiss exposed to natural and synthetic oestrogens

Juvenile rainbow trout Oncorhynchus mykiss were exposed to two concentrations each of 17β‐oestradiol (E2; natural oestrogen hormone) or 17α‐ethinyl oestradiol (EE2; a potent synthetic oestrogen hormone) to evaluate their potential effects on burst‐swimming performance. In each of six successive burst‐swimming assays, burst‐swimming speed (U_burst) was lower in fish exposed to 0·5 and 1 µg l^?1 E2 and EE2 for four days compared with control fish. A practice swim (2 days prior to exposure initiation) in control fish elevated initial U_burst values, but this training effect was not evident in the 1 µg l^?1 EE2‐exposed fish. Several potential oestrogen‐mediated mechanisms for U_burst reductions were investigated, including effects on metabolic products, osmoregulation and blood oxygen‐carrying capacity. Prior to burst‐swimming trials, fish exposed to E2 and EE2 for 4 days had significantly reduced erythrocyte numbers and lower plasma glucose concentrations. After six repeated burst‐swimming trials, plasma glucose, lactate and creatinine concentrations were not significantly different among treatment groups; however, plasma Cl^? concentrations were significantly reduced in E2‐ and EE2‐treated fish. In summary, E2 and EE2 exposure altered oxygen‐carrying capacity ([erythrocytes]) and an osmoregulatory‐related variable ([Cl^?]), effects that may underlie reductions in burst‐swimming speed, which will have implications for fish performance in the wild.     

Seawater tolerance in first-time migrants of anadromous Arctic charr (Salvelinus alpinus)

Anadromous juvenile Arctic charr (Salvelinus alpinus) were caught in the Sila River (66°21'N, 13°10'E, Nordland County, Norway) by use of a fish trap during their migration towards the sea in May/June. The absence of tags (not captured before) and examination of otoliths from sacrificed fish revealed that none of the experimental charr had previously encountered seawater. During 4 days of exposure to seawater, only minor changes in blood plasma osmolality, and blood plasma concentrations of Na⁺, Cl^- and Mg²⁺ were detected. An increase in Na⁺-K⁺-ATPase activity was measured during the exposure period. These results verified that first-time migrants exhibit hypoosmoregulatory capacity similar to that of smoltified Atlantic salmon (Salmo salar). Accordingly, the Arctic charr first-time migratory behaviour, including late descent and a possible stay within an estuary, cannot be taken as evidence of a poorly developed seawater tolerance.     

Rapid digestion of fish prey by the highly invasive ‘detritivore’Oreochromis mossambicus

Stomach residence time was tested over 24 h in three size classes of Oreochromis mossambicus using juvenile Lates calcarifer. In all 63 observations, the fish prey was digested within 24 h of consumption and most probably within 1 h, suggesting a need to re‐evaluate the trophic status and potential effects of this highly invasive species.     

Differential expression of Na+, K+-ATPase α-1 isoforms during seawater acclimation in the amphidromous galaxiid fish Galaxias maculatus

Inanga (Galaxias maculatus) is an amphidromous fish with a well-known capacity to withstand a wide range of environmental salinities. To investigate the molecular mechanisms facilitating acclimation of inanga to seawater, several isoforms of the Na⁺, K⁺-ATPase ion transporter were identified. This included three α-1 (a, b and c), an α-2 and two α-3 (a and b) isoforms. Phylogenetic analysis showed that the inanga α-1a and α-1b formed a clade with the α-1a and α-1b isoforms of rainbow trout, while another clade contained the α-1c isoforms of these species. The expression of all the α-1 isoforms was modulated after seawater exposure (28 ‰). In gills, the expression of the α-1a isoform was progressively down-regulated after seawater exposure, while the expression of the α-1b isoform was up-regulated. The α-1c isoform behaved similarly to the α-1a, although changes were less dramatic. Physiological indicators of salinity acclimation matched the time frame of the changes observed at the molecular level. A 24-h osmotic shock period was highlighted by small increases in plasma osmolality, plasma Na⁺ and a decrease in muscle tissue water content. Thereafter, these values returned close to their pre-exposure (freshwater) values. Na⁺, K⁺-ATPase activity showed a decreasing trend over the first 72 h following seawater exposure, but activity increased after 240 h. Our results indicate that inanga is an excellent osmoregulator, an ability that is conferred by the rapid activation of physiological and molecular responses to salinity change.