Sigurd O. Handeland

Seawater adaptation by out-of-season Atlantic salmon (Salmo salar L.) smolts at different temperatures

Abstract Atlantic salmon smolts ( Salmo salar L.) were transferred to full-strength seawater for 0 (initial control group), 0.5, 1, 2, 4, 8, 14, 30, 42 and 60 days at four different temperatures (4.6, 9.1, 14.4 and 18.9°C). Water temperature in each tank was adjusted during the last 5 days in freshwater (ambient 8°C) to gradually establish test conditions of 4.6, 9.1, 14.4 and 18.9°C 24 h prior to transfer. Thereafter, the water in all tanks were changed from freshwater to seawater of identical temperature, and full salinity (33‰) was reached within 60 min. Physiological adaptation was measured as changes in plasma growth hormone levels, gill Na + ,K + –ATPase activity, plasma chloride levels and muscle water content. The fatty acid composition of gill tissues was determined after 30 days in seawater. Ion and water balance following seawater transfer were significantly affected by temperature. Exposure to high temperatures (18.9°C) resulted in a rapid increase in plasma chloride levels and a rise in tissue dehydration within 24 h, whereas low temperatures (4.6°C) resulted in a delayed osmotic disturbance and a prolonged period of osmotic stress. Least osmoregulatory disturbance was observed at 9.1°C. Gill Na + ,K + –ATPase activity did not increase after seawater exposure at 4.6 and 9.1°C, whereas a gradual increase was observed with increasing temperatures at 14.4 and 18.9°C during the first 48 h in seawater. Plasma GH increased in all groups during the first 24 h of seawater exposure. GH levels decreased during long-term adaptation in the 4.6, 9.1 and 14.4°C groups, whereas a significant increase was observed in the 18.9°C group. Growth increased with increasing temperature between 4.6 and 14.4°C, but decreased significantly between 14.4 and 18.9°C demonstrating that 18.9°C is above optimum for growth and development in seawater.

Seawater adaptation and growth of post-smolt Atlantic salmon (Salmo salar) of wild and farmed strains

Atlantic salmon smolts (Salmo salar L.) of wild (Namsen) and farmed (AquaGen) strains were transferred to full-strength seawater (33‰) for 0 (initial control group), 0.5, 1, 2, 4, 8, 14, 30, 42 and 60 days at three different temperatures (4.3, 9.4 and 14.3 °C). Freshwater temperature in each tank was adjusted to test conditions 10 days prior to transfer. Physiological adaptation was monitored as changes in plasma growth hormone levels, gill Na+,K+-ATPase activity, plasma chloride levels and survival in seawater. Overall, smolts from the wild strain were better able to tolerate transfer to seawater than smolts from the hatchery strain. A delay in the osmotic disturbance and a prolonged period of osmotic stress were observed at the low temperature. Circulating GH levels increased transiently in all groups during the first 12–48 h in seawater and long-term GH levels were positively correlated with seawater temperature. Growth rates were influenced by temperature and strain, with the farmed smolts showing a higher growth than the wild smolts. Food Conversion Efficiency (FCE) was higher in smolts from the farmed strain, whereas no differences in daily food consumption were observed. Optimum temperature for FCE was calculated to be 10.5 °C, whereas the optimal temperature for growth in seawater was calculated to be 13.0 °C. We suggest that the observed differences in seawater tolerance, growth and food conversion probably are genetic and may reflect the fact that the hatchery fish have been bred for rapid growth for several generations.

Effects of temperature and salinity on osmoregulation and growth of Atlantic salmon /Salmo salar L. smolts in seawater

Abstract One of the main developmental events of the smoltification process of Atlantic salmon ( Salmo salar L.) is the pre-adaptation to an increase in salinity. Seawater acclimation involves a series of physiological changes which are critical for subsequent performance. The aim of this study was to monitor some important physiological mechanisms involved in seawater adaptation under different salinity (28 and 34‰) and temperature (4 and 8°C) regimes. An increase in plasma chloride levels and a decrease in muscle water was observed in all groups after 24 h of seawater exposure. Salinity did not affect plasma chloride levels nor tissue moisture, and no interactions between temperature and salinity were found. Temperature affected plasma chloride levels significantly after 12 h of seawater exposure, with the 4°C groups having lower levels than the 8°C groups. Between days 1 and 14, muscle water was observed to increase and then stabilise in the 8°C groups, while the low temperature groups required a further 14 days until tissue moisture was at levels similar to the freshwater group. After an initial reduction, both groups at 8°C showed elevated and stable gill Na + ,K + -ATPase activities compared with the low temperature groups, which showed a long-term decrease. Salinity did not affect gill Na + ,K + -ATPase activity and no interactions between temperature and salinity were found. During the first 2 months of seawater exposure, the growth pattern was affected by temperature only, while higher growth rate in brackish water at low temperature (4°C) in the period between days 64 and 90 indicates that a reduction in salinity may improve long-term growth in the sea.

Photoperiod control and influence of body size on off-season parr–smolt transformation and post-smolt growth

To examine the combined effects of photoperiod and body size on off-season parr–smolt transformation and post-smolt growth, groups of small and large graded potential Atlantic salmon smolts (initial size 10 and 23 g) were raised under four different photoperiods between 26 July and 14 November. Photoperiod treatments included a 10 months phase-advance of simulated natural photoperiod (SNPa, from LD12:12 to LD24:0 over 16 weeks); two square wave photoperiod regimes; which were continuously light interrupted by LD12:12 for 9 (9) or 12 (12) weeks, respectively; and continuous light (LD24:0). Photoperiod manipulation significantly influenced growth and hypo-osmoregulatory ability. Between July and November, smolts raised on SNPa and LD12:12 (9) showed an increase in seawater tolerance, concurrent with an increase in Na+, K+-ATPase activity and a decrease in condition factor (CF). No differences in plasma chloride levels were seen among the small and large graded smolts (SG and LG), whereas a higher Na+, K+-ATPase activity was recorded in the small smolts at weeks 12, 14, and 16. Higher growth rates were seen in the small smolts during the freshwater period. Continuous light increased growth in freshwater, but interfered with smolt development. Following transfer to seawater, both SG and LG previously held under SNPa and LD12:12 (9) grew better than the fish reared under LD24:0, which showed a decrease in growth. The high growth rates in seawater in the SNPa and LD12:12 (9) groups concur with the good smolt status prior to transfer. Our findings indicate that both SNPa and LD12:12 (9) can be recommended in the production of off-season salmon smolts and among the smolt parameters monitored, gill Na+, K+-ATPase activity appears to be the most reliable indicator of smolt status.

New clade of betanodaviruses detected in wild and farmed cod ( Gadus morhua ) in Norway

Betanodaviruses have been isolated and detected in both farmed and wild fish species worldwide. They are classified in five clusters, and all are connected to mortalities in farmed fish. The clusters do not represent specific geographical areas or host species, but one cluster, barfin flounder nervous necrosis virus (BFNNV), is mainly associated with cold water fish species. This study presents the first species-specific clade within the BFNNV cluster. This clade consists of six isolates from wild and farmed Atlantic cod in Norway and is genetically distinct from other betanodaviruses in the North Atlantic. Screening of farmed and wild cod in Norway shows that betanodaviruses are present in wild fish on the west coast of Norway, including migratory cod, but so far we have not detected any betanodavirus-positive wild cod in northern Norway. The presence of significant amounts of betanodaviruses in wild cod represents a serious challenge for the management of viral nervous necrosis in farmed cod in Norway. Betanodavirus-positive farmed cod were present both in western and northern Norway. Mortalities in three cod farms were suspected to be caused by betanodaviruses; however, in two of these, other pathogens may have been responsible for or strongly contributed to the mortalities.

Osmoregulation, feed intake, growth and growth hormone levels in 0+ Atlantic salmon (Salmo salar L.) transferred to seawater at different stages of smolt development

We studied the physiology and endocrinology of seawater acclimation in two size-groups (body weight [b.wt.] at transfer 38 and 53 g, respectively) of 0+ Atlantic salmon (Salmo salar L.) smolts transferred to seawater (33‰) in autumn at three different times following the completion of the photoperiod-induced smoltification process. Smoltification was monitored by use of 24-h seawater challenge tests, and measurements of gill Na+, K+-ATPase activity and condition factor in freshwater acclimated fish. Parr–smolt transformation in large fish was slightly delayed compared to small fish. In late October 1999, when both groups displayed full smolt characteristics, each size group was transferred to four tanks (trial I) supplied with either running fresh water (FW) or running seawater (SW), both at 8.8 °C. In all groups of fish, individual feed intake, body weight, gill Na+, K+-ATPase activity, and plasma levels of chloride and growth hormone (GH) were monitored on days 3 and 30 following transfer. The same protocol was repeated with starting dates December 3 (trial II) and January 11, 2000 (trial III). At the start of trial III, both size-groups displayed reduced hypoosmoregulatory ability (seawater challenge tests) and decreasing gill Na+, K+-ATPase activity. Mortality was low (≤4.4%) in all groups during the trials. For the small SW-smolts, the highest rates of feed intake and specific growth (SGR) were recorded in trial I (SGR: 1.2% b.wt. day−1), whereas feed intake and growth of the larger SW-smolts were highest during trials II and III (SGR: 0.9% b.wt. day−1). Plasma chloride levels in small SW-fish on day 3 increased slightly with date of transfer (from 143 mM in trial I to 154 mM in trial III), but were within the normal range by day 30 across all size-groups and trials (140–144 mM). Following seawater transfer, plasma GH levels increased transiently, whereas gill Na+, K+-ATPase activity increased permanently in all groups. The different patterns of growth displayed by the two size-groups of fish following seawater transfer suggest that the optimal time for seawater transfer may depend on the body size of the fish. Fish transferred past their peak in smolt status showed high hypoosmoregulatory capacity and retained, at least in part, their ability to increase plasma GH levels and gill Na+, K+-ATPase activity following exposure to seawater. Growth rates in seawater may indicate that in commercial farming of underyearling Atlantic salmon, seawater transfer can be delayed for a considerable period of time after smolting, provided that seawater temperature is favourable.

The influence of first-feeding diet on the Atlantic cod Gadus morhua phenotype: survival, development and long-term consequences for growth

Atlantic cod Gadus morhua larvae reached four-fold (at low larval density) to 11 fold higher body mass (high larval density) at 50 days post hatch (dph) when fed zooplankton rather than enriched rotifers. A short period (22-36 dph) of dietary change affected larval growth positively if changed from enriched rotifers to natural zooplankton and negatively if prey type changed vice versa. Overall survival did not differ between the two larval groups at low larval density, but at high density the rotifer group had a higher overall survival (10.8% v. 8.9%). Long-term growth was affected significantly by larval diet in favour of the zooplankton diet; juveniles reached a 23% higher mass in a 12 week growth period. No difference in growth performance was found between juveniles fed natural zooplankton during the larval period for 36, 22 or 14 days, but all these juveniles performed significantly better compared with the rotifer-fed group. These findings suggest that optimal diet during a short period in the larval period can result in improved growth in both the larval and juvenile period. Improved rotifer quality may, therefore, hold a large potential for growth improvement in this species.

Osmoregulation and growth in a wild and a selected strain of Atlantic salmon smolts on two photoperiod regimes

Abstract Atlantic salmon ( Salmo salar ) smolts originating from wild (Imsa) and selected (AquaGen) strains were maintained on simulated natural photoperiod (SNP) or constant light (LL) in replicate tanks between 3 December 1998 and 26 May 1999. Gill Na + ,K + -ATPase activity, plasma growth hormone (GH) levels, condition factor (CF) as well as individual food intake, growth and food conversion were monitored regularly in all groups during the experimental period. Freshwater temperature was kept constant at approximately 12 °C. In both strains, GH levels and gill Na + ,K + -ATPase activity rose steadily in the SNP groups, whereas the abrupt increase in day-length caused a significant increase in GH levels within 4 weeks, concurrent with an increase gill Na + ,K + -ATPase activity. The close relationship between GH levels and gill Na + ,K + -ATPase activity indicates that GH plays and important role in the development of hypo-osmoregulatory ability during smolting. Overall, a higher growth rate was seen in smolts from the AquaGen strain compared with the Imsa strain. No differences in food consumption were seen between strains, whereas an improved food conversion was observed in smolts from the AquaGen strain. Photoperiod was found to enhance growth through stimulation of food intake. It is concluded that the observed differences in growth and food conversion between wild and selected smolts are genetic, and probably reflect the fact that the AquaGen fish have been bred for rapid growth for several generations.

Effects of salinity acclimation on pre-smolt growth, smolting and post-smolt performance in off-season Atlantic salmon smolts (Salmo salar L.)

Abstract The main objective of this study has been to investigate the effect of salinity acclimation on parr–smolt transformation and post-smolt performance in off-season Atlantic salmon smolts ( Salmo salar L.). Four groups of potential off-season salmon smolts were raised under a phase advanced simulated natural photoperiod between 26 July and 14 November. After 9 weeks of photoperiod treatment, salinity in three of the tanks was changed from freshwater to a salinity of 6.0, 13.1 and 20.0‰. All groups were transferred to full strength seawater on 14 November and monitored regularly during the next three months. Hypo-osmoregulatory ability improved in all groups during the first 9 weeks of photoperiod treatment. A significant increase in gill Na + ,K + -ATPase activity was observed between weeks 9 and 16, with a higher activity in the 20.2‰ group at week 16. Growth and condition decreased in all saline groups and no positive effects on post smolt performance were observed. Final weight after three months in seawater was 13% lower in the 6.0 and 13.1‰ groups, and 28% lower in the 20.2‰ group, compared with controls. Our findings suggest that acclimating Atlantic salmon to saline water during off season smolting has no positive consequences, and is probably not necessary to stimulate the development of hypo-osmoregulatory ability and increase gill Na + ,K + -ATPase activity during smolting or improve post-smolt growth.

Aluminum exposure impacts brain plasticity and behavior in Atlantic salmon (Salmo salar).

SUMMARY Aluminum (Al) toxicity occurs frequently in natural aquatic ecosystems as a result of acid deposition and natural weathering processes. Detrimental effects of Al toxicity on aquatic organisms are well known and can have consequences for survival. Fish exposed to Al in low pH waters will experience physiological and neuroendocrine changes that disrupt homeostasis and alter behavior. To investigate the effects of Al exposure on both the brain and behavior, Atlantic salmon (Salmo salar) kept in water treated with Al (pH 5.7, 0.37±0.04 μmol 1−1 Al) for 2 weeks were compared with fish kept in under control conditions (pH 6.7, <0.04 μmol 1−1 Al). Fish exposed to Al and acidic conditions had increased Al accumulation in the gills and decreased gill Na+, K+-ATPase activity, which impaired osmoregulatory capacity and caused physiological stress, indicated by elevated plasma cortisol and glucose levels. Here we show for the first time that exposure to Al in acidic conditions also impaired learning performance in a maze task. Al toxicity also reduced the expression of NeuroD1 transcript levels in the forebrain of exposed fish. As in mammals, these data show that exposure to chronic stress, such as acidified Al, can reduce neural plasticity during behavioral challenges in salmon, and may impair the ability to cope with new environments.