B. Th. Björnsson

The biology of salmon growth hormone: from daylight to dominance

The elucidation of the molecular structure of salmon growth hormone (GH) in the mid-1980s paved the way for a new era of endocrinological research. Establishment of homologous immuno- and receptor-assays have made studies of the secretion, tissue and plasma GH levels, GH turn-over and GH receptor concentrations possible. This overview attempts to summarize the present understanding of the biological roles of GH in salmon. Although the involvement of GH in the regulation of physiological processes throughout the salmon life history has yet to be comprehensively explored, the hormone has already been demonstrated to have several important functions. GH is a principal regulator of somatic growth in salmonids. The growth-stimulating effect of GH is probably integrated with that of insulin-like growth factor I (IGF-I), as in later vertebrates. GH stimulates protein synthesis and improves feed conversion during growth. The hormone also promotes lipid and glycogen breakdown as well as gluconeogenesis, functions which are probably of great importance during starvation when GH levels are seen to increase. During parr-smolt transformation of anadromous salmonids, circulating GH levels appear to be governed by environmental cues. Increasing springtime daylength elevates GH levels, and temperature modulates the photoperiod regulation of GH. The seawater-adapting role of GH during the parr-smolt transformation is complex. In freshwater, GH improves hypoosmoregulatory ability by stimulating branchial Na+,K+-ATPase activity and probably also acts in kidney and intestine. Following seawater entry, GH levels and turn-over increase transiently, probably to further increase seawater tolerance. Accumulating in vitro and in vivo data support the conclusion that GH is involved in the regulation of sexual maturation in salmonids although further studies are needed to establish the exact role of GH in this process. GH increases appetite but it is unclear whether the hormone effects the central nervous system directly, or acts indirectly through metabolic changes. GH increases swimming activity as well as dominant feeding behaviour and diminishes anti-predator behaviour of juvenile salmonids. The GH-induced changes of behavioural patterns imply that there exists an ecological trade-off between high growth rate and long-term survival which may explain why natural fish populations normally grow at sub-maximal rates. Current knowledge indicates that GH is an important and multi-functional hormone in salmon and a central mediator of seasonal changes in physiology and behaviour. The regulatory effects of GH are also of great applied interest as they are likely to affect both product quality in aquaculture and long-term survival of released fish.

Repeated acute stress reduces growth rate of Atlantic salmon parr and alters plasma levels of growth hormone, insulin-like growth factor I and cortisol

Abstract Atlantic salmon ( Salmo salar ) parr were subjected to acute handling stresses and growth-monitored for at least 30 days. In fish stressed twice daily, growth rate in weight was 61% lower than controls after 11 days (1.00 vs. 2.57% day −1 ) and over a 30 day period it was 50% lower than controls (1.53 vs. 3.07% day −1 ). In fish stressed once daily, growth rate was 18% lower than controls after 10 days (2.17 vs. 2.63% day −1 ) and over a 30-day period it was 34% lower than controls (1.71 vs. 2.59% day −1 ). In fish stressed once daily, food consumption was reduced by 62% and 37% after 17 and 37 days, respectively. At the end of 40 days of acute stress once daily, control and stressed fish were sampled 1 h prior to, 3 and 7 h after a stress event. Plasma growth hormone levels were significantly higher in the stressed group than in the controls prior to and 7 h after stress. Plasma insulin-like growth factor I (IGF-I) levels were higher in the stressed group only 3 and 7 h after stress. Plasma cortisol levels were lower in the stressed group prior to and 3 h after stress. The results indicate that acute stressors decrease growth of Atlantic salmon parr, with increasing frequency of stress having a more rapid and greater effect.

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 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.

Growth hormone response to seawater challenge in Atlantic salmon, Salmo salar, during parr-smolt transformation

Abstract The relation between plasma growth hormone (GH) levels and hypo-osmoregulatory ability during parr-smolt transformation was studied in 2-year-old immature parr and previously mature male parr of two different Baltic stocks of Atlantic salmon ( Salmo salar ). At several times between early May and the end of June fish were transferred to seawater (25‰ salinity) and sampled after 24 h. Among freshwater controls plasma GH levels increased only slightly in one stock during the smoltification period, whereas GH levels were significantly elevated in mid-June in the other stock. Seawater adaptability reached an optimum by the end of May to early June in one stock and 2 weeks later in the other one. Plasma GH levels in fish exposed to seawater were similar to those of freshwater fish until mid-May, but increased to significantly higher levels at the time of peak seawater adaptability. Changes in plasma sodium levels during seawater exposure were negatively correlated to plasma GH levels. At the time of peak seawater adaptability two groups were detectable among previously mature males: one group had plasma sodium levels comparable to those of immature fish, while the other had significantly higher sodium levels. In one stock plasma GH showed a similar bimodal distribution, where the group with high GH levels corresponded to low plasma sodium levels and the group with low GH levels had significantly higher sodium levels. The results indicate that some Baltic stocks of Atlantic salmon develop an ability to respond to seawater exposure with a rapid increase in GH levels during the parr-smolt transformation. It is suggested that increasing environmental salinity triggers the final development of hypo-osmoregulatory ability.

Growth and endocrine effects of recombinant bovine growth hormone treatment in non-transgenic and growth hormone transgenic coho salmon

To examine the relative growth, endocrine, and gene expression effects of growth hormone (GH) transgenesis vs. GH protein treatment, wild-type non-transgenic and GH transgenic coho salmon were treated with a sustained-release formulation of recombinant bovine GH (bGH; Posilac). Fish size, specific growth rate (SGR), and condition factor (CF) were monitored for 14 weeks, after which endocrine parameters were measured. Transgenic fish had much higher growth, SGR and CF than non-transgenic fish, and bGH injection significantly increased weight and SGR in non-transgenic but not transgenic fish. Plasma salmon GH concentrations decreased with bGH treatment in non-transgenic but not in transgenic fish where levels were similar to controls. Higher GH mRNA levels were detected in transgenic muscle and liver but no differences were observed in GH receptor (GHR) mRNA levels. In non-transgenic pituitary, GH and GHR mRNA levels per mg pituitary decreased with bGH dose to levels seen in transgenic salmon. Plasma IGF-I was elevated with bGH dose only in non-transgenic fish, while transgenic fish maintained an elevated level of IGF-I with or without bGH treatment. A similar trend was seen for liver IGF-I mRNA levels. Thus, bGH treatment increased fish growth and influenced feedback on endocrine parameters in non-transgenic but not in transgenic fish. A lack of further growth stimulation of GH transgenic fish suggests that these fish are experiencing maximal growth stimulation via GH pathways.

Propylthiouracil-induced hypothyroidism in coho salmon, Oncorhynchus kisutch: effects on plasma total thyroxine, total triiodothyronine, free thyroxine, and growth hormone

Thyroid hormones transiently increase during parr-smolt transformation in coho salmon, Oncorhynchus kisutch, and are believed to trigger morphological, physiological, behavioural, and neural changes. The effectiveness of propylthiouracil (PTU) to induce hypothyroidism in smolting coho salmon was determined by immersing coho salmon, Oncorhynchus kisutch, in 30 mg l−1 PTU from May 1, two weeks prior to the consistent annual total thyroxine (TT4) peak in mid-May, until the last sampling date. Plasma was obtained at two sampling dates from control and PTU -treated coho salmon: May 15, during the plasma TT4 peak; and May 26, after the TT4 peak. Radioimmunoassays were used to measure plasma TT4, total triiodothyronine (TT3), free thyroxine (FT4), and salmon growth hormone (sGH). The PTU -treatment inhibited the natural smoltification-related increases in plasma TT4, TT3 and GH levels compared with controls, but PTU-treatment did not affect these hormone levels when they were low. PTU -treatment increased FT4 and decreased TT3 and sGH levels in the May 26 sample. In the May 15 sample, FT4 levels were unaffected by PTU-treatment, whereas TT4 levels were decreased. These data demonstrate the ability of PTU to induce hypothyroidism in salmonids as shown by the decrease in TT4 and TT3. These data demonstrate that PTU treatment by immersion can induce hypothyroidism in salmonids as shown by: (1) the inhibition of the natural increases of TT4 and TT3; (2) the increase in FT4 levels corresponding to the lowered TT3 levels, suggesting an inhibition of thyroxine 5′-monodeiodinase activity. We also show for the first time that PTU treatment can lower plasma GH levels in salmonids. This lowering of plasma GH level is associated with the decrease in TT3 levels and the increase in FT4 levels. The PTU induced lowering in GH levels may contribute to the observed changes in FT4 and TT3, since GH is known to increase thyroxine 5′-monodeiodinase activity.

Plasma growth hormone‐binding protein levels in Atlantic salmon Salmo salar during smoltification and seawater transfer

Specific growth hormone (GH)-binding protein (Ghbp) was purified from Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss plasma with immunoprecipitation and characterized in cross-linking studies using autoradiography and western blots. The size of the Ghbp was estimated to be c. 53 kDa. A radioimmunoassay was established to measure Ghbp in salmonids, using antibodies specific against the extracellular segment of the S. salar growth hormone receptor 1 (grh1; GenBank AY462105). Plasma Ghbp levels were measured in S. salar smolts in fresh water and after transfer to seawater (SW; experiments 1 and 2), and in post-smolts kept at different salinities (0, 12, 22 and 34) for 3 months (experiment 3). A transient increase in plasma Ghbp, which lasted for 1 month or less, was noted in smolts after transfer to SW. Concomitantly, plasma GH and gill Na(+) -K(+) -ATPase activity increased during smoltification (in experiment 2). No difference in plasma Ghbp was evident between post-smolts kept at different salinities, although the fish kept at salinity 34 had higher plasma GH than the group kept at salinity 22 and higher hepatic ghr1 expression than post-smolts kept at salinity 12. This suggests that plasma Ghbp regulation may respond to salinity changes in the short term. The lack of correlation between Ghbp, plasma GH and hepatic ghr1 expression in the long-term post-smolt experiment indicates that Ghbp levels may be regulated independently of other components of the endocrine GH system in salmonids.

Long-term rearing of Arctic charr Salvelinus alpinus under different salinity regimes at constant temperature.

Arctic charr Salvelinus alpinus of the Hólar strain (mean ± s.e. body mass = 152·1 ± 3·1 g) were reared at four different salinity regimes at a constant temperature of 7·4° C. Two groups were given a three-month acclimation in salinity 18 before the salinity was increased to either 25 or 29 (groups called A25 and A29), and two groups were reared in salinities 25 or 29 over the full experimental period of 409 days (groups called F25 and F29). In the first 3 months, the A25 and A29 groups had the highest growth rates. By October 2011, there were no significant differences (two-way nested ANOVA, P > 0·05) in the mean body masses among A25, F25 and F29 (c. 1450 g), whereas A29 had a lower mean mass (1282 g). The growth in the last period from October 2011 to January 2012 was reduced by sexual maturation in the highest salinity regimes (A29 and F29), whereas fish in groups A25 and F25 showed high growth throughout the study. Males in all salinity groups had higher growth rates than females for the most part of the study, but the divergence between the sexes was most pronounced in the highest salinity regimes. All salinity groups showed distinct changes in Na(+) , K(+) -ATPase activity, with high activity in spring and summer, and lower activity in the autumn. Plasma sodium (Na(+) ) levels were stable indicating that none of the experimental groups had problems in maintaining hydromineral balance during the study. While plasma leptin levels were not affected by salinity regimes, it was noted that these levels were 13-30% higher in fish with empty guts compared with those having food in their gut at the time of sampling. This suggests a link between leptin levels and food intake, indicating that this hormone may play a role in food intake and energy allocation in fishes.

Effects of short‐day treatment on long‐term growth performance and maturation of farmed Arctic charr Salvelinus alpinus reared in brackish water

The effects of a 6 week short-day photoperiod followed by continuous light, applied during the juvenile phase of Arctic charr Salvelinus alpinus in fresh water on smoltification and on the long-term growth and maturity following transfer to brackish water (BW) (constant salinity of either 17 and 27 or increasing salinity in steps from 17 to 27) were investigated. Prior to salinity transfer, the juveniles were either reared at continuous light (C group) or reared for 6 weeks on a short day (8L:16D, S group) followed by continuous light (24L:0D). Increased salinity had negative effect on growth, with female fish reared at 17 salinity weighing 19 and 27% more than the salinity-step group (17-27) and the 27 salinity group, respectively. The stepwise acclimation to salinity had limited advantage in terms of growth rate. Short photoperiod for 6 weeks (November to January) followed by continuous light improved growth, but not seawater (SW) tolerance. Gill Na(+) , K(+) -ATPase activity and plasma Na(+) levels changed with time, indicating some variation in osmoregulatory capacity during the experimental period. Overall, there appear to be interactive effects on maturation from applying short-day photoperiod followed by rearing at higher salinities. Plasma leptin varied with time and may be linked to stress caused by the observed variations in osmoregulatory ability. It is concluded that changes in growth rates observed in this study are mainly related to rearing salinity with higher growth rates at lower salinities. Short-day photoperiod has some growth-inducing effects but did not improve SW tolerance. Farmers of S. alpinus using BW for land-based rearing should keep salinity at moderate and stable levels according to these results to obtain best growth.