Sigurd O. Stefansson

The interaction of temperature and salinity on growth and food conversion in juvenile turbot (Scophthalmus maximus)

Abstract The effects of temperature and salinity on growth and feed conversion of juvenile turbot (initial mean weight 14 g) were investigated by rearing fish at (mean±SD) 10±0.2°C, 14±0.2°C, 18±0.3°C and 22±0.2°C and 15±0.4‰, 25±0.4‰ and 33.5±0.1‰ for 3 months. Growth, food consumption, and food conversion efficiency were highest at 15‰, and lowest at 33.5‰. There was an interactive effect of temperature and salinity at the two highest temperatures (18°C and 22°C) but not at 10°C and 14°C. The optimal temperature for growth (Topt. G) varied with salinity: Topt. G at 33.5‰ was 19.6±0.3°C (±SEM), whereas the Topt. G at 15‰ was 22.9±1.0°C, and at 25‰ was 24.7±2.1°C. A similar trend was found for food conversion efficiency (FCE). The optimal temperatures for FCE were 17.4±0.5°C, 17.9±1.0°C and 19.0±0.9°C at 33.5‰, 25‰ and 15‰, respectively. Overall, we found the optimal temperature–salinity combination for growth to be 21.8±0.9°C and 18.5±0.8‰. The optimal temperature–salinity combination for food conversion efficiency was found to be 18.3±0.6°C and 19.0±1.0‰. It is concluded that growth and food conversion efficiency of juvenile turbot can be improved by rearing them at intermediate salinities in the upper temperature range.

Differential expression of gill Na + ,K + -ATPase - and -subunits, Na + ,K + ,2Cl - cotransporter and CFTR anion channel in juvenile anadromous and landlocked Atlantic salmon Salmo salar

SUMMARY This study examines changes in gill Na+,K+-ATPase (NKA) α- and β-subunit isoforms, Na+,K+,2Cl- cotransporter (NKCC) and cystic fibrosis transmembrane conductance regulator (CFTR I and II) in anadromous and landlocked strains of Atlantic salmon during parr-smolt transformation, and after seawater (SW) transfer in May/June. Gill NKA activity increased from February through April, May and June among both strains in freshwater (FW), with peak enzyme activity in the landlocked salmon being 50% below that of the anadromous fish in May and June. Gill NKA-α1b, -α3, -β1 and NKCC mRNA levels in anadromous salmon increased transiently, reaching peak levels in smolts in April/May, whereas no similar smolt-related upregulation of these transcripts occurred in juvenile landlocked salmon. Gill NKA-α1a mRNA decreased significantly in anadromous salmon from February through June, whereas α1a levels in landlocked salmon, after an initial decrease in April, remained significantly higher than those of the anadromous smolts in May and June. Following SW transfer, gill NKA-α1b and NKCC mRNA increased in both strains, whereas NKA-α1a decreased. Both strains exhibited a transient increase in gill NKA α-protein abundance, with peak levels in May. Gill α-protein abundance was lower in SW than corresponding FW values in June. Gill NKCC protein abundance increased transiently in anadromous fish, with peak levels in May, whereas a slight increase was observed in landlocked salmon in May, increasing to peak levels in June. Gill CFTR I mRNA levels increased significantly from February to April in both strains, followed by a slight, though not significant increase in May and June. CFTR I mRNA levels were significantly lower in landlocked than anadromous salmon in April/June. Gill CFTR II mRNA levels did not change significantly in either strain. Our findings demonstrates that differential expression of gill NKA-α1a, -α1b and -α3 isoforms may be important for potential functional differences in NKA, both during preparatory development and during salinity adjustments in salmon. Furthermore, landlocked salmon have lost some of the unique preparatory upregulation of gill NKA, NKCC and, to some extent, CFTR anion channel associated with the development of hypo-osmoregulatory ability in anadromous salmon.

Leptin and leptin receptor genes in Atlantic salmon: Cloning, phylogeny, tissue distribution and expression correlated to long-term feeding status.

The present study reports the complete coding sequences for two paralogues for leptin (sLepA1 and sLepA2) and leptin receptor (sLepR) in Atlantic salmon. The deduced 171-amino acid (aa) sequence of sLepA1 and 175 aa sequence for sLepA2 shows 71.6% identity to each other and clusters phylogenetically with teleost Lep type A, with 22.4% and 24.1% identity to human Lep. Both sLep proteins are predicted to consist of four helixes showing strong conservation of tertiary structure with other vertebrates. The highest mRNA levels for sLepA1 in fed fish (satiation ration=100%) were observed in the brain, white muscle, liver, and ovaries. In most tissues sLepA2 generally had a lower expression than sLepA1 except for the gastrointestinal tract (stomach and mid-gut) and kidney. Only one leptin receptor ortholog was identified and it shares 24.2% aa sequence similarity with human LepR, with stretches of highest sequence similarity corresponding to domains considered important for LepR signaling. The sLepR was abundantly expressed in the ovary, and was also high in the brain, pituitary, eye, gill, skin, visceral adipose tissue, belly flap, red muscle, kidney, and testis. Fish reared on a rationed feeding regime (60% of satiation) for 10 months grew less than control (100%) and tended to have a lower sLepA1 mRNA expression in the fat-depositing tissues visceral adipose tissue (p<0.05) and white muscle (n.s.). sLepA2 mRNA levels was very low in these tissues and feeding regime tended to affect its expression in an opposite manner. Expression in liver differed from that of the other tissues with a higher sLepA2 mRNA in the feed-rationed group (p<0.01). Plasma levels of sLep did not differ between fish fed restricted and full feeding regimes. No difference in brain sLepR mRNA levels was observed between fish fed reduced and full feeding regimes. This study in part supports that sLepA1 is involved in signaling the energy status in fat-depositing tissues in line with the mammalian model, whereas sLepA2 may possibly play important roles in the digestive tract and liver. At present, data on Lep in teleosts are too scarce to allow generalization about how the Lep system is influenced by tissue-specific energy status and, in turn, may regulate functions related to feed intake, growth, and adiposity in fish. In tetraploid species like Atlantic salmon, different Lep paralogues seems to serve different physiological roles.

Abrupt changes in photoperiod affect age at maturity, timing of ovulation and plasma testosterone and oestradiol-17β profiles in Atlantic salmon, Salmo salar

Abstract Atlantic salmon ( Salmo salar L.), reared in sea cages for 18 months (age 36 months from hatching), were exposed to natural light (NL, 61°N), or continuous additional light from January (ALJ) or March (ALM) until July. On July 13, the fish were moved to indoor raceways with brackish water (2–19‰) and ambient temperature (declining from 13.0 to 5.6°C). Fish from each treatment were subjected to either simulated natural photoperiod (SNP), continuous light (24L), or short photoperiod (8L=8 L:16D), creating a total of nine experimental groups with approx. 50 fish in each. The proportion of sexually maturing females was reduced from 91% in the NL groups, to 67% and 9% in the ALM and ALJ groups, respectively ( p ≤0.005). A similar reduction was observed among the males, from 74% in the NL groups, to 57% and 16% in the ALM and ALJ groups, respectively ( p ≤0.001). Ovulation commenced in late October in the control group (NL-SNP). Compared with control, median ovulation time was advanced by 5, 4 and 3 weeks in the ALM-8L, NL-8L and ALM-SNP groups, respectively, whereas ovulation was delayed by 1 and 6 weeks in the ALM-24L and NL-24L groups, respectively. The altered timing of ovulation among the groups was paralleled by similar shifts in the seasonal plasma oestradiol-17 β and testosterone profiles. Survival of eggs to the eyed stage was lower in the ALM-8L group (mean=64.2%) compared with the NL-SNP group (mean=92.5%), indicating a negative effect on egg quality in the most advanced group. Although abrupt changes in photoperiod can be used to control timing of ovulation in Atlantic salmon to obtain off-season eggs, the decrease in egg survival and proportion of maturing fish may set constraints on how much maturation can be advanced by use of continuous light during winter and spring. However, the effect on age at maturity may also be exploited to reduce the problem with unwanted early maturation in salmon farming.

Growth, oxygen consumption and activity of juvenile turbot (scophthalmus maximus L.) reared under different temperatures and photoperiods

Abstract Juvenile turbot (5–125 g) were reared under two experimental temperatures: 10°C and 16°C, and three experimental photoperiods: LDN (natural photoperiod), LD 16:8 (16 h light: 8 h darkness), LD24:0 (continuous light), to study effects of temperature and photoperiod on growth, activity and oxygen consumption. Growth was strongly affected by temperature and was higher at 16°C than at 10°C. Continuous light had a growth-promoting effect at 10°C from mid-December to late March, while at 16°C this effect was restricted to December and January. A seasonal change in the condition index was found. The groups reared in continuous light had higher condition indices in winter. The experimental groups held at 16°C had higher O2 consumption than those at 10°C. The LD24:0 groups invariably had a higher overall O2 consumption than did the LDN and LD16:8 groups, the differences being caused by reduced O2 consumption in the latter groups during darkness. The LD24:0 groups displayed higher activity than the LDN groups, in which activity was very low at night.

Effects of photoperiod and temperature on growth and parr-smolt transformation in Atlantic salmon (Salmo salar L.) and subsequent performance in seawater

Potential Atlantic salmon (Salmo salar L.) 1 + smolts were reared during late winter and spring in a factorial design combining elevated, 12–13°C (e), or ambient (a) temperature with four photoperiod regimes: simulated natural photoperiod, 60°N (LN60, control); simulated natural photoperiod, 70°N (LN70); simulated natural photoperiod, 1 month phase delayed (LD60); and continuous light (LL). Growth rate in fresh water was enhanced by elevated water temperature and continuous light. No consistent differences were seen among the LN70 and LN60 groups at the two temperature regimes, whereas the LD60e group had a lower growth rate than LN60e. Condition factor decreased between February and April on LL under both thermal regimes. For the other groups on elevated temperature, condition factor decreased from March onwards, while no clear reduction was seen among the LN60a, LN70a or LD60a groups. Based on morphological changes, increase in hypo-osmoregulatory ability and gill Na+,K+-ATPase activity, the LN60e, LN70e and LD60e groups developed smolt characters in April or early May, while corresponding groups on ambient temperature did not complete smoltification until late May or early June. Fish were transferred to seawater in mid-June and measured again in mid-November. Groups reared on ambient temperature had higher growth rates in seawater than corresponding groups from elevated temperature. Smolts from LD60 had lower growth rate than did those from LN60 and LN70, indicating that the delay in photoperiod influenced the completion and/or timing of smolting. The LLe group had the lowest growth rate in seawater, suggesting that the continued exposure to LL and elevated temperature interfered with the normal course of parr-smolt transformation.

The influence of temperature and ration on growth, feed conversion, body composition and nutrient retention of juvenile turbot (Scophthalmus maximus)

Abstract The influence of two water temperatures (16 and 22 °C) on growth, feed conversion, body composition and nutrient retention was investigated in juvenile turbot fed to satiation (0.9% and 1.1% bw day−1 at 16 and 22 °C, respectively) and at restricted rations of 65% and 35% of the satiation level at each temperature. Fish fed the same % rations at 16 and 22 °C did not differ in final mean weight or specific growth rate, which decreased at restricted rations. Feed restriction did not result in an increase in size heterogeneity over time at any temperature, as indicated by the stability of the coefficients of variation of weight (ΔCVw=1.00–1.13%). At both temperatures, the best feed conversion efficiency (FCE) was found at the 65% ration, and the FCE from fish fed the same rations was higher at 16 °C (1.30 g g−1) than at 22 °C (1.17 g g−1). A similar trend was found in energy and protein retention levels. At both temperatures, fish fed 35% rations had lower body lipid and higher ash and moisture content compared to fish fed to satiation, with the most pronounced effects on lipid (4.8% vs. 7.8% bw) and ash (4.1% vs. 3.6% bw) at 22 °C. Feeding ration proved to be the main differentiating factor in all growth, feed conversion and body composition parameters, whereas additional temperature and/or interaction effects were found in FCE, whole body protein, lipid, moisture and energy contents. Between fish fed 100% and 65% rations, only minor differences were found, but at 35% ration, the rearing temperature of 22 °C had a pronounced negative influence, and resulted in a reduction of available anabolic energy for growth and adaptive responses.

Environmental endocrinology of salmon smoltification.

Smolting is a hormone-driven developmental process that is adaptive for downstream migration and ocean survival and growth in anadromous salmonids. Smolting includes increased salinity tolerance, increased metabolism, downstream migratory and schooling behavior, silvering and darkened fin margins, and olfactory imprinting. These changes are promoted by growth hormone, insulin-like growth factor I, cortisol, thyroid hormones, whereas prolactin is inhibitory. Photoperiod and temperature are critical environmental cues for smolt development, and their relative importance will be critical in determining responses to future climate change. Most of our knowledge of the environmental control and endocrine mediation of smolting is based on laboratory and hatchery studies, yet there is emerging information on fish in the wild that indicates substantial differences. Such differences may arise from differences in environmental stimuli in artificial rearing environments, and may be critical to ocean survival and population sustainability. Endocrine disruptors, acidification and other contaminants can perturb smolt development, resulting in poor survival after seawater entry.

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.

Effects of continuous additional light on growth and sexual maturity in Atlantic salmon, Salmo salar, reared in sea cages.

Abstract Individually tagged Atlantic salmon postsmolts (body weight 243±0.9 g [mean±SE], n =1800) were distributed randomly among four sea cages, two cages received continuous additional light (AL) from November to July, and two received natural light (NL) only (controls). Equal numbers of fish (150 from each cage) were moved between the AL and NL cages in December and January, creating a total of six experimental groups. The fish were fed to excess during the hours of NL. Compared with the control group, exposure to AL from November, December or January until July (Nov–Jul, Dec–Jul and Jan–Jul groups, respectively) resulted in a 48–52% reduction in specific growth rate (SGR) during the subsequent 6 weeks, followed by a higher SGR during the next 4 to 5 months. A similar growth response as in the Nov–Jul group occurred in the groups receiving only 6 or 12 weeks of AL from November (Nov–Dec and Nov–Jan groups, respectively). The SGR of the Nov–Jul group was higher than in the Nov–Jan and Nov–Dec groups at the end of the experiment. Between May and July, groups exposed to AL from November (Nov–Jul, Nov–Jan and Nov–Dec) grew significantly less than the groups initially receiving NL (Control, Jan–Jul and Dec–Jul). In July, the body weight (mean±SE) of the fish depended on the duration and timing of AL exposure; Nov–Jul: 1072±26 g, Dec–Jul: 995±23 g, Jan–Jul: 977±20 g, Nov–Dec: 930±23 g, Nov–Jan: 870±22 g and Control: 815±17 g. The proportion of sexually maturing males increased with early exposure and duration of AL; Control: 6.5%, Jan–Jul: 8.0%, Dec–Jul: 14.7%, Nov–Dec: 15.5%, Nov–Jan: 21.7% and Nov–Jul: 37.6%. The study provides evidence that AL superimposed on NL enhances growth of Atlantic salmon in sea cages during winter and spring, with timing and duration of the exposure affecting growth and the proportion of early maturing males.