Tom Hansen

Control of puberty in farmed fish

Puberty comprises the transition from an immature juvenile to a mature adult state of the reproductive system, i.e. the individual becomes capable of reproducing sexually for the first time, which implies functional competence of the brain-pituitary-gonad (BPG) axis. Early puberty is a major problem in many farmed fish species due to negative effects on growth performance, flesh composition, external appearance, behaviour, health, welfare and survival, as well as possible genetic impact on wild populations. Late puberty can also be a problem for broodstock management in some species, while some species completely fail to enter puberty under farming conditions. Age and size at puberty varies between and within species and strains, and are modulated by genetic and environmental factors. Puberty onset is controlled by activation of the BPG axis, and a range of internal and external factors are hypothesised to stimulate and/or modulate this activation such as growth, adiposity, feed intake, photoperiod, temperature and social factors. For example, there is a positive correlation between rapid growth and early puberty in fish. Age at puberty can be controlled by selective breeding or control of photoperiod, feeding or temperature. Monosex stocks can exploit sex dimorphic growth patterns and sterility can be achieved by triploidisation. However, all these techniques have limitations under commercial farming conditions. Further knowledge is needed on both basic and applied aspects of puberty control to refine existing methods and to develop new methods that are efficient in terms of production and acceptable in terms of fish welfare and sustainability.

Growth, gonadal development and spawning time of Atlantic cod (Gadus morhua) reared under different photoperiods

Abstract Individually tagged Atlantic cod ( Gadus morhua L . ) (mean body weight 459±6 g) were reared in 20-m 3 tanks supplied with running sea water. During the first year the fish were exposed to one of four photoperiod regimes: Natural light (LDN), continuous light from June 23, 1993 (LL), natural light from June 23, 1993 and continuous light from December 22, 1993 (LDN/LL) or continuous light from June 23, 1993 and natural light from December 22, 1993 (LL/LDN). In July 1994, the LDN and LDN/LL groups were terminated. The LL and LL/LDN groups were reared on until May 1995. From July 1994 to the termination of the experiment in May 1995, the LL group was reared under continuous light and the LL/LDN group was reared under natural photoperiod. Cod reared under natural photoperiod spawned in the period between January and April. Photoperiod manipulation changed the incidence of sexual maturation, spawning time, fecundity and egg size. Cod that were transferred from natural photoperiod to continuous light in December spawned earlier, having lower fecundity and smaller eggs than the cod reared under natural photoperiod. Oocytes of females reared under continuous light were arrested in the cortical alveoli stage, and even in their second year on continuous light very few females matured. When transferred to natural photoperiod in December, females ovulated within 4–5 months, approximately 3 months delayed compared to the natural photoperiod group. These cod had a higher fecundity and smaller eggs than natural photoperiod cod. When reared on for another year under natural photoperiod these cod spawned again the next year at their normal spawning time, e.g. 9 months after their first spawning. The pattern of sexual maturation influenced the somatic growth pattern. Differences in growth were explained by differences in the timing of the spawning and the relative spawning investment. At an age of 26 months, the weight of the cod reared under natural photoperiod and continuous light were 1.5 and 2.5 kg, respectively. From this, it is concluded that a reduction in daylength is a vital environmental signal regulating the maturation and spawning of cod, and that sexual maturation may be arrested or considerably delayed in its absence.

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.

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.

Growth performance and sexual maturation in diploid and triploid Atlantic salmon (Salmo salar L.) in seawater tanks exposed to continuous light or simulated natural photoperiod

Abstract Triploid Atlantic salmon are considered to be functionally sterile and the use of these in intensive fish farming would potentially reduce interactions between escapees and wild stocks. However, variable performance under commercial production conditions in seawater has been reported. The experiment described here is part of a larger comprehensive study on triploid Atlantic salmon, and investigates seasonal growth performance in diploid and triploid salmon under simulated natural light, and an accelerated production regime using continuous light during winter and spring. Four full-sib families of diploid and triploid Atlantic salmon postsmolts were held in six 20-m3 circular indoor tanks in the January–October period after their transfer to seawater in July. Two tanks were given continuous light (LL) from January to June and subsequently put on simulated natural photoperiod (SNP), while four tanks were kept on SNP throughout. Variation in feed intake was monitored by waste feed collection, and fish were sampled monthly for weight and length. A depression in feed consumption was noted in the LL groups during the first 6–8 weeks after onset of light. As expected, the LL groups outperformed their SNP counterparts, but displayed an unexpected equal incidence of sexual maturation. Triploids displayed distinctly enhanced feed intake during their second sea autumn, which resulted in their being heavier and significantly longer than diploid fish in October. This increased feed intake in triploids was advanced in the LL treated fish. Only males showed signs of becoming ‘sexually mature’ (increased GSI) as grilse (after 1.5 years in seawater), and equal proportions of the diploid and triploid fish were characterised as sexually mature. Thus, lack of ‘sexual maturation’ could not explain the improved triploid growth. There were no distinct differences in mortality or occurrence of deformities between ploidy types. Both triploids and diploids responded to the accelerated light regime with an advanced seasonal development of growth and condition factor, long-term enhanced growth and an indicated improved feed conversion. In general, all sibling groups responded to the ploidy and light treatments. Growth differences between sibling groups within treatments were mainly ascribed to different proportions of sexual maturation. This study indicated that triploid Atlantic salmon performed equally, or better, compared to diploid conspecifics reared in indoor seawater tanks.

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.

Pinealectomy induces malformation of the spine and reduces the mechanical strength of the vertebrae in Atlantic salmon, Salmo salar

Abstract:  This study describes the long‐term effects of surgical ablation of the pineal gland on the spine of 3‐yr‐old Atlantic salmon (Salmo salar L.) with a mean weight of 3.2 kg. Radiographic examinations showed that 82% of the pinealectomized fish developed marked lateral (scoliosis) and dorso‐ventral spinal curvatures. The proportions of the individual vertebral bodies and their mechanical properties were also altered. The stiffness, yield limit and resilience of the vertebral bodies, as measured by compression in the cranio‐caudal direction, were significantly lower in the pinealectomized than in the sham‐pinealectomized group. Calcium, phosphorous and total mineral content of the vertebral bodies were also significantly lower in the pinealectomized fish, while these parameters were similar in scales in the two groups. Alterations of the spinal curve accompanied by changes in the proportions, mechanical strength and mineral content of the vertebral bodies of the pinealectomized salmon indicate that melatonin has several functions related to vertebral bone growth. As the lesions found in salmon are similar to the spinal malformations observed in avian species and mammals after pinealectomy, this study strengthens the hypothesis of a phylogenetically conserved function of the pineal gland related to skeletal development.

Heat shock during early somitogenesis induces caudal vertebral column defects in Atlantic salmon (Salmo salar).

In several terrestrial vertebrates, heat shock (HS) during somitogenesis causes vertebral deformities. To determine if vertebral deformities can occur due to sudden temperature changes during early development in fish, Atlantic salmon embryos were HS treated during somitogenesis. Ten months later these individuals displayed a high prevalence of caudal vertebral column condensations (27–34%). The defects were located caudally of the abdominal cavity, displaying an even distribution in this region independent of time of HS. To determine if HS disturbed vertebral development during somitogenesis, two genes coding for markers of skeletal development were identified, namely, the secreted protein Shh (Sashh) and the transcription factor Twist (Satwist). These proteins are involved in the proliferation and specification of presumptive skeletal cells (sclerotome) in vertebrates. The spatial expression pattern of sashh and satwist in salmon indicated a functional conservation of these proteins. Furthermore, HS embryos displayed expressional disturbance in both sashh and satwist, indicating an effect of HS on sclerotomal cell patterning. However, the HS-protecting ability in embryos seems to be individually regulated because reduction in gene expression was not detected at all stages; in addition, HS did not induce somitic disturbance and vertebral deformity in all embryos.

Continuous light increases growth rate of Atlantic salmon (Salmo salar L.) postsmolts in sea cages

Abstract Groups of Atlantic salmon ( Salmo salar L.) postsmolts were reared in sea cages under natural light or continuous additional light from 20 February to 24 June. The two light regimes were combined with two feeding regimes in a 2 by 2 factorial design. No significant differences in growth rate were found between the feeding regimes. Growth rate and incidence of grilsing were highest in the groups subjected to continuous additional light.

Intestinal transport mechanisms and plasma cortisol levels during normal and out-of-season parr–smolt transformation of Atlantic salmon, Salmo salar

Abstract The intestine is one of the major osmoregulatory organs in fish. During the salmon parr–smolt transformation, the intestine must change its functions from the freshwater (FW) role of preventing water inflow, to the seawater (SW) role of actively absorbing ions and water. This development can be assessed as an increased intestinal fluid transport (Jv) during the parr–smolt transformation. The developmental changes taking place during parr–smolt transformation are governed by a number of endocrine systems, of which cortisol is the main stimulator of Jv. In order to further elucidate the mechanisms behind the elevation of Jv during parr–smolt transformation, juvenile Atlantic salmon were followed during natural (1+age) as well as photoperiod-induced (0+age) smoltification. Plasma cortisol levels, gill and intestinal Na + ,K + -ATPase activity, Jv (only during natural smoltification) and intestinal paracellular permeability were measured. In natural smolting as well as in photoperiod-induced smolting, normal patterns of plasma cortisol levels and gill Na + ,K + -ATPase activity, with clearly defined, transient peaks were obtained. When fish were transferred to SW, a second elevation in plasma cortisol levels and gill Na + ,K + -ATPase activity occurred, whereas Jv remained at similar levels as in FW fish. As to the mechanisms behind the increased Jv during parr–smolt transformation, the intestinal Na + ,K + -ATPase activity increases in the anterior intestine and the paracellular permeability, as judged by transepithelial resistance (TER), appears to decrease in the posterior intestine. These events correspond with the increase in Jv seen during this developmental stage. Furthermore, the increase in the physiological parameters follows the changes in plasma cortisol levels, shifted by a couple of weeks. When the fish were transferred to SW, a further increase in Na + ,K + -ATPase activity was apparent in both anterior and posterior intestine and the paracellular permeability decreases. To summarize, the increased Jv seen during the parr–smolt transformation of Atlantic salmon may be due to an increase in the paracellular water flow of the posterior intestine. When the fish enter SW, the water flow appears to be directed from the paracellular pathway towards a more transcellular route with increased intestinal Na + ,K + -ATPase activity as the main driving force.