© 2005 International Council for the Exploration of the Sea
Continuous light delays sexual maturation and increases growth of Atlantic cod (Gadus morhua L.) in sea cages
Institute of Marine Research PO Box 1870, N-5817 Bergen, Norway
*Correspondence to G. L. Taranger: tel: +47 55 23 6373; fax: +47 55 23 63 79. e-mail: geirt{at}imr.no.
Prevention of early sexual maturation is essential in Atlantic cod (Gadus morhua) farming because maturity results in reduced growth, affects flesh quality, and may lead to increased mortality. In farmed cod, almost 100% of the fish mature at two years of age and often at a size of 1.5–2 kg. Two pilot experiments were conducted with cod in sea cages at a commercial fish farm in western Norway (60°N) to test the effect of additional continuous light (LL) on the timing of sexual maturation and somatic growth compared with controls under natural light (NL). In the NL groups, 100% maturation was indicated during the natural spawning period from February to April at the age of two years. By contrast, LL treatment from 27 June (15-month-old cod) or 2 September (18-month-old cod) onwards delayed gonad development by three to five months, reduced reproductive investment, and enhanced winter growth compared with the controls. Fish held at NL decreased in body weight during the spawning season (February–April), whereas LL-exposed fish appeared to continue to grow during their spawning season (May–August). LL-treated cod reached mean body weights of 2.90–3.13 kg within 28 months of hatching, whereas the controls reached 2.20–2.42 kg during the same period.
Keywords: Atlantic cod, farming, gonad development, growth, photoperiod, sexual maturation
Received 13 June 2004; accepted 28 October 2005.
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Introduction |
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There has been growing interest in aquaculture of Atlantic cod (Gadus morhua L.) recently owing to high market prices and success with intensive juvenile rearing techniques. Precocious sexual maturation is one of the main problems in the on-growth farms. Nearly 100% of the cod mature at two years of age under normal farming conditions (e.g. Karlsen et al., 1995, 2000; Svåsand et al., 1996), whereas wild populations of Norwegian coastal cod typically mature at a median age of four to six years (Berg and Albert, 2003), and Arcto-Norwegian cod mature with a median age of six to eight years in the Barents Sea (Godø and Moksness, 1987).
Onset of maturity leads to a marked loss of body weight during the spawning season in both male and female cod (Kjesbu et al., 1996). This also affects the quality of the fish, as it reduces the proportion of edible flesh of whole body weight and alters the proximate composition of the fillet, including higher water content (Eliassen and Vahl, 1982; Kjesbu et al., 1991). The loss of growth during the spawning season lengthens the time required to reach desired harvest size, and the energy expenditure in gonad development, spawning, and recovery increases the amount of feed needed to produce fish of a certain size.
Karlsen et al. (2000) and Hansen et al. (2001) found that exposure of one-year-old cod in indoor seawater tanks to continuous light (LL) from summer solstice onwards delayed or arrested gonad development and inhibited spawning at the age of two years. Davie et al. (2003) also found that LL treatment from 15 months of age in indoor tanks delayed maturation age to three years, when only 7.2% of the cod were found to be spawning. Dahle et al. (2000) indicated that LL treatment with either 100 or 1600 lux intensity superimposed on ambient light in outdoor tanks was almost as effective in delaying gonad development in cod as LL treatment in indoor tanks. The latter observation suggests that LL treatment also may be effective in delaying or arresting sexual maturation in cod in sea cages. Although the practice of LL treatment has been adopted recently in commercial cod farming in sea cages to some extent, there is little information available on the effectiveness of this treatment on gonad development and maturation or on its effect on growth and reproductive investment.
The present study is the result of two pilot studies in commercial sea cages, the purpose of which was to test whether LL treatment superimposed on ambient light can delay gonad development in cod in sea cages, and to correlate this to associated somatic growth, liver size, and indices of reproductive investment.
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Material and methods |
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Fish stocks and rearing conditions
Two pilot experiments were carried out at the commercial farm Tveit Fish Farm AS in Tysnes, Hordaland, Norway (60°N). Atlantic cod hatched in March 1993 and 1994 were used in Experiments 1 and 2, respectively. The broodstock used were first generation farmed fish of western Norway coastal cod origin, spawning naturally in closed bags at the Institute of Marine Research, Austevoll Aquaculture Research Station (60°N) (Holm and Andersen, 1989). Throughout the spring of 1993 and 1994, the cod larvae were fed first on natural zooplankton in Parisvannet according to the method described by Blom et al. (1991). During both years, the cod juveniles were harvested from the pond after weaning to dry feed, size graded, vaccinated against Vibrio anguillarium (Norwax Cod, Intervet Norbio AS, Bergen, Norway), and transferred to sea cages during July and August.
For the duration of both experiments, the fish were fed a commercial dry diet (FK torsk, Noraqua AS, Norway) containing 53% protein, 10% lipid, 17.7% carbohydrate, 9.2% ash, and 10.1% water. Gross energy content was 19.5 MJ kg–1. Fish were fed to satiation by hand, daily or every second day depending on fish size and appetite. The monthly mean water temperature at 5-m depth ranged from 3.5 to 16.3°C during the experiments (Figure 1).
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Experiment 1
Five thousand cod (mean body weight = 77 g) were transferred to Tveit Fish Farm AS on 8 October 1993 and reared under natural light in a sea cage until September 1994. On 2 September 1994, the fish were randomly distributed between two sea cages: 600 fish into a control cage (5 m x 5 m x 5 m), and 3700 fish into the experimental cage (12 m x 12 m x 5 m). The experimental cage was exposed to additional continuous light (LL) from 2 September 1994 until 21 June 1995 (LL group), thereafter natural light (NL) until the end of the experiment in September 1995. The control cage (NL group) was maintained under NL throughout the study.
Additional light was supplied by 4 x 150 W metal halide lights (Euroflood, Siemens AS, Norway, 150 W bulb Osram HQI-TS; 11 250 lumen) mounted 1.5 m above sea level in the periphery of the cage, giving a night-time light illumination of approximately 50 lux in the middle of the cage at 3-m depth. The lamps were turned off during daylight hours. The cages were separated to avoid stray light from the illuminated cages to the control cages.
Experiment 2
Eight thousand cod (mean body weight = 60 g) were transferred to Tveit Fish Farm AS on 21 September 1994 and reared under NL in a sea cage until 27 June 1995, and thereafter LL until 24 July 1995, using the same lamps as in Experiment 1. On 24 July 1995, the cod were randomly distributed between two sea cages: 5300 fish in the experimental cage and 2500 fish in a control cage, both with dimensions of 12 m x 12 m x 5 m. The experimental cage received LL until 21 June 1996, and thereafter NL until the end of the experiment in November 1996 (LL group). The control cage was held under NL from 24 July 1995 until the end of the experiment (NL group).
Data sampling and analyses
During the experimental periods, with intervals from one to three months, random samples of 100–120 fish from each group were anaesthetized with metomidate (Mattson and Riple, 1989), measured for total length (±1 cm), and weighed (±10 g). No growth measurements were taken on 28 August, 8 October, and 11 November 1996 in Experiment 2 as a result of latent vibriosis infection.
On 6 December 1994, 9 February, 30 March, 29 May, 4 July, and 20 September 1995, random samples of 15–34 fish from each group in Experiment 1 were killed with a blow to the head for determination of gonad development and liver size (±1 g). In Experiment 2, the same samples were obtained from 19–27 randomly collected fish from each group on 8 November, 12 December 1995, 13 February, 19 March, 16 April, 15 May, 11 June, 9 July, 28 August, and 8 October 1996. An additional sample was collected on 11 November 1996 from the LL group. The sacrificed fish were bled, sexed, and the gonads were excised and weighed (±0.1 g), except on 6 December 1994 when no gonad weights were taken. A small piece of the ovary was fixed in 3.7% phosphate buffered formaldehyde in saline for subsequent morphometrical determinations. From each of 10 females in every group, the short and long axes of the most advanced oocytes (n = 10) were recorded (±10 µm). Subsequently, the overall mean diameter of the ten oocytes was calculated. This measure is called the group 1 (G1) oocyte diameter (Kjesbu, 1994). The following equations were used in calculating the data:
- Fulton's condition factor (K) = 100(BW x L–3), where BW is live body weight (g) and L is total length (cm).
- Gonadosomatic index (GSI) = 100(gonad weight x bled BW–1).
- Hepatosomatic index (HSI) = 100(liver weight x bled BW–1).
- Gonadosomatic index (GSI) = 100(gonad weight x bled BW–1).
Statistics
Data were analysed by Statistica 5.1 (StatSoft Inc., Tulsa, USA). Growth data and HSI are presented as mean values ± s.d. (standard deviation) in text and mean values ± s.e. (standard error of mean) in figures, whereas data on GSI and G1 diameters are presented as median ± one quartile and minimum/maximum values in the figures. Data on L, BW, K, and HSI were checked for normal distribution using a normal distribution plot (Sokal and Rohlf, 1995), and for homogeneity of variance by Levenes test (Brown and Forsythe, 1974). The effects of photoperiod treatment on L, BW, and K were tested using a one-way ANOVA, whereas the effects on GSI and G1 diameter were tested with a Mann–Whitney U-test (Sokal and Rohlf, 1995). The combined effects of photoperiod treatment and sex on HSI were tested by one-way ANOVA, followed by Students–Newman–Keuls (SNK) multiple comparisons test (Zar, 1996). A significance level (
) of 0.05 was applied in all tests except where indicated.
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Results |
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Experiment 1
The controls (NL group) reached full sexual maturity at the normal spawning time in February to April, as indicated by GSI values above 8% for all fish in February (Figure 2). By contrast, no females in the LL group had GSI above 1.9% in February, and only two males had GSI above 1.4% at this time. In May and July, only two of 29 LL-treated females had GSI above 8%. Only five of 14 LL-treated males had GSI above 8% in May, and no LL-treated males had GSI above 8% in July or September. The GSIs of both sexes were significantly lower in the LL group than in the NL group in February, but in contrast to the declining GSI values in the NL group from March onwards, GSI increased to a peak in May in the LL group, before a gradual decline until September. This resulted in significantly higher GSIs in the LL group than in the NL group from May onwards for both sexes.
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; males: n = 5–13) or continuous light from 2 September 1994 (
; females: n = 10–18, 
; males: n = 5–14) in Experiment 1. Boxes denote ±one quartile and whiskers denote minimum and maximum values. Asterisks denote significant differences using a Mann–Whitney U-test within a sampling date (*p < 0.05, **p < 0.01, ***p < 0.001, n.s. = not significant).In the NL group, median G1 diameter increased from 410 µm in December to a peak of 1060 µm in March (Figure 3a). All NL females had G1 diameters above 530 µm by February (n = 10), while females with G1 diameters above 1000 µm were only found in March (n = 5 of 8). By contrast, LL females had a median G1 diameter of 270 µm in December, reaching 390 µm in March, and peaking at 450 µm in July. No females with G1 diameters above 1000 µm were found in the LL group throughout the experiment, and only seven of 28 LL females had G1 diameters above 530 µm in May and July. Apparently spent females were characterized by G1 oocytes
250 µm and were found from March onwards in the NL group (n = 3 of 8 in March), and from July onwards in the LL group (n = 6 of 18 from July to September). From May onwards, all NL females appeared to be spent (n = 45).
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The NL group had a significantly higher average BW (860 ± 225 g) than the LL group (770 ± 200 g) in September 1994 (Figure 4a). The LL group appeared to grow faster than controls from September to March, with significantly higher BWs than the NL group from December 1994 onwards. The NL group exhibited a decline in BW from February to March, whereas the LL group continued to gain weight throughout the study. At the end of the experiment, in September 1995, the LL group had a mean BW of 3270 ± 250 g, being significantly higher than the NL group (2590 ± 550 g).
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In the NL group, mean L increased from 39 ± 4 cm in September 1994 to 58 ± 4 cm in September 1995 (Figure 4b). There was no significant difference in L between the two groups in September 1994, whereas the LL group grew faster in L until December 1994, showing significantly higher L than the NL group from this date onwards, reaching a mean L of 62 ± 4 cm in September 1995.
Mean K of the NL group increased to a peak of 1.59 ± 0.24 in February, with a subsequent sharp decline to 1.31 ± 0.17 in March (Figure 4c). The K of the LL group was significantly lower than the NL group in September 1994, but increased steadily until March, reaching a peak of 1.61 ± 0.17. Thereafter, K decreased gradually in the LL group, reaching 1.38 ± 0.13 in September 1995. In spite of this decline, K stayed significantly higher than in the NL group from March to September 1995.
HSI did not differ significantly between the NL and LL groups in September 1994 (Figure 5); a mean above 11% in both groups (p = 0.97, n = 14–15). From December onwards, HSI was tested for effects of both photoperiod treatment and sex. No significant differences were found among the subgroups in December (p = 0.64, range of means: 12.2–13.0%), whereas NL males had significantly lower HSI than the others in February and March. NL females had significantly lower HSI than LL males in February, but were significantly different from the LL females. LL females showed a gradual decline in HSI from March onwards, being significantly lower than the other subgroups in September 1995.
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Experiment 2
The median GSI values in the NL group increased from November 1995 to reach peaks of 10.9% and 19.0% in February and March for males and females, respectively (Figure 6). The maximum individual GSI value (29.8%) was reached in a NL female in February. The GSIs of LL females were significantly lower than those for NL females from November to April, whereas LL females had significantly higher GSI values than those for NL females from May until August. Peak median values in LL females were lower than those seen in NL females earlier in the year. In a similar manner, LL males had significantly lower GSIs than NL males from November to March, whereas LL males had significantly higher GSIs from April to August. Although the peak median values were less in LL males than in NL males, these differences were less pronounced than between the female groups. In February, the lowest GSI in a NL female was 8.9%, while all NL males had GSIs above 7.6%. At this time, only one of 11 LL females had a GSI above 1.6%, and only two of nine LL males had a GSI above 1.7%. Females with a GSI above 8% were found from May to October in the LL group, 15 of 64 females during this period. In LL males, such observations were found in 14 of 46 cases from March to July.
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Median G1 diameter in NL females increased from 310 µm in November 1995 to 1200 µm in March 1996, and individuals with G1 diameters above 1000 µm were found from February to May (Figure 3b). All NL females had a G1 diameter above 520 µm in February. By contrast, LL females had a median G1 diameter of 190 µm in December, increasing slowly to 440 µm in July. From May to July, a few LL females had G1 diameters above 1000 µm (six of 28), and only seven of 28 females had G1 diameters above 520 µm during this period. Apparently spent females (G1 diameters
250 µm) were detected from April onwards in the NL group, and from May onwards in the LL group (six of 39 LL females during May to August). From June onwards, all NL females appeared to be spent (n = 30). Mean BW in the NL group increased from 650 ± 170 g in July 1995 to 2420 ± 470 g in July 1996 (Figure 7a). Within this period, weight loss was observed from February to April, giving a temporary reduction in BW from 2090 ± 544 to 1640 ± 480 g. The increase in BW of the LL group was positive in all sampling periods, except for the few weeks between April and May. The LL group had significantly higher BWs than the NL group from February onwards.
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In the NL group, mean L increased from 37 ± 3 cm in July 1995 to 55 ± 4 cm in July 1996 (Figure 7b). There were no significant differences between the two groups before December 1995, when the LL group had the highest L. Thereafter, the LL group remained significantly longer than the NL group during the remainder of the experiment, reaching a mean of 59 ± 5 cm in July 1996.
The mean K of the NL group increased from 1.26 ± 0.13 in July 1995 to a peak of 1.70 ± 0.25 in February 1996 (Figure 7c). Thereafter, K declined to 1.26 ± 0.19 in April, with a subsequent increase. The K of the NL group was significantly higher than the LL group from September until February, while the LL group had a significantly higher K during the remainder of the experiment. The K of the LL group declined slowly from February onwards, and did not show a similar increase in K as in the NL group at the end of the experiment.
There were no significant differences in HSI among photoperiod treatments and sexes in November 1995 (Figure 8; p = 0.58, range of mean: 10.8–11.3%). The HSI of the NL males declined from November, reaching its lowest value in March and being significantly lower than the other subgroups from December to April. A decline in HSI was also seen in NL females from February to March, being significantly lower than LL females from February to May, and than LL males from February to April. HSI declined in LL females and males from April to August. As a consequence, no significant differences were found among the subgroups from June onwards.
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Exposure of one-year-old Atlantic cod in sea cages to additional continuous light (LL), from midsummer or late summer onwards, delayed gonad development, enhanced somatic growth, and appeared to reduce reproductive investment in these two pilot experiments. The experiments had several limitations in their designs, as they were conducted on a small commercial farm with limited access to cages and fish. On the other hand, the consistent results between the two separate experiments strongly indicate the effectiveness of the LL treatment in delaying gonad development and enhancing growth in cod in sea cages.
The proportion of sexually maturing fish appeared to be close to 100% in the NL groups in both experiments as judged by the GSI values and G1 diameters in February and the preceding months. G1 oocytes above 1000 µm are only found in ovulating females, whereas oocytes between 250 and 1000 µm are found in maturing fish (Kjesbu, 1994). In contrast, in the LL groups in both experiments, no maturation was indicated by the GSI values and G1 diameters during the normal spawning period from February to April. The low proportion of LL-treated fish with GSI above 8% and G1 diameters above 1000 µm throughout the experimental period, as well as a low proportion of apparently spent females, suggest that a lower proportion of fish were progressing to full sexual maturity in the LL groups compared with NL controls. On the other hand, we may have missed peak observations of both GSI and G1 diameters from LL-treated individuals, particularly in Experiment 1, as a result of infrequent sampling. Cod females typically spawn multiple batches at around three-day intervals (Kjesbu, 1989), and GSI and G1 oocyte diameters increase rapidly immediately before the spawning of each batch (Kjesbu et al., 1991).
Although no evidence of maturation was seen during the normal spawning season from February to April in the LL groups, a large proportion of LL-treated fish had GSIs in the range of 2–8% from May onwards in both experiments, suggesting that they were progressing into maturity during the summer. Also, an increasing proportion of LL-treated females had G1 diameters above 400 µm from March onwards in Experiment 1 and from April onwards in Experiment 2, with a peak in July in both experiments. This further suggests that maturation was in progress. Moreover, the observations of G1 oocyte diameters above 1000 µm from May onwards in LL-treated females in Experiment 2, and the presence of apparently spent LL females in both experiments, strongly indicate that ovulation occurred in some females in the LL groups. This suggests that the LL treatment only delayed the process of gonad development and maturation in most individuals rather than fully arresting this development, and that a substantial proportion of the LL-treated fish became sexually mature during the summer.
In Experiment 1, the GSI profiles indicated a delay in the LL group of three to four months in both sexes compared with NL controls, whereas delays of four to five months were indicated in both sexes in Experiment 2. In a similar manner, the seasonal profile of G1 diameters indicated a delay of four months or more in the LL groups in both experiments. However, the LL-treated fish never reached the same peak median levels of GSI seen in the NL groups during the spawning season, indicating that either a lower proportion of the LL fish actually became fully mature or the maturing LL individuals invested less in gonad size. In a similar manner, the median G1 diameters did not reach the same peak levels during the summer as in March in the NL groups, which may imply that a lower proportion of LL females became fully mature.
In contrast to the present study where gonad development seemed to be delayed by three to five months following LL treatment in sea cages, Hansen et al. (2001) detected low GSI values and apparently arrested oocyte development in fish maintained under LL in 5-m-circular indoor tanks from 15 to 36 months of age. Moreover, only a small proportion of the fish maintained under LL were noted to spawn at the age of 36 months, 12 months after the controls under natural light had spawned (Hansen et al., 2001). In a similar manner, a delay of gonad development by more than nine months was indicated by very low GSI values in most cod maintained under LL from 15 months of age in 3-m-circular indoor tanks compared with controls under ambient light (Karlsen et al., 2000). This further delay and/or arrest of gonad development observed by Hansen et al. (2001) and Karlsen et al. (2000), compared with the present study, could be the result of differences in the relative importance of natural and artificial light between the studies. In the study of Hansen et al. (2001), the LL treatment was applied in indoor tanks receiving only small amounts of natural light through windows in the roof, and in the study by Karlsen et al. (2000) the LL-treated fish were maintained in completely light-proof tanks preventing any signals from the ambient light. By contrast, in the current study, the LL treatment had to compete with the strong ambient light cycle in the cages. It is possible that the constant light in the indoor tank situation provided a stronger signal to stop or delay maturation than the situation in the sea cages, where the continuous light was superimposed on the much stronger ambient light cycle. This, in turn, may have given the cod confusing photoperiod signals in the sea cages, resulting in a less effective LL treatment. More studies are needed to reveal the importance of the intensity of LL treatment in sea cages.
Similar delays of gonad development were indicated in both experiments in the current study where the LL treatment commenced on either 27 June (15-month-old cod) or 2 September (18-month-old cod), suggesting that the LL treatment can be effective even when applied as late as September. By contrast, Davie et al. (2003) indicated that the LL treatment had to commence at or before the age of 15 months in indoor tanks to be effective in arresting maturation at the age of two years in cod. Thus, it is recommended that further studies be conducted with different timings of the LL treatment in sea cages to establish the optimum time for the initiation of the LL treatment.
The current experiments indicate that the LL treatment stimulated winter growth in length and body weight in a manner similar to that reported by Hansen et al. (2001) in cod exposed to LL from the summer solstice onwards in indoor tanks. The apparent growth enhancement in the LL-treated groups in the present study, and in the study of Hansen et al. (2001), were most likely a consequence of the delayed gonad development and spawning, thereby delaying and/or minimizing the impaired somatic growth normally derived from sexual maturation. However, the growth-promoting effects of LL indicated in these studies could also be the result, at least partially, of a stimulatory effect of LL on somatic growth, independent of sexual maturation, as seen in other fish species such as Atlantic salmon (Salmo salar) (e.g. Oppedal et al., 1997).
A negative correlation between sexual maturation and growth was indicated by the low, and to some extent negative, growth indicated in the NL groups during the spawning seasons in these experiments. During the spawning season, mean BW was reduced by 21.5% from 13 February to 16 April in the NL groups in Experiment 2, while a reduction of 11.3% was seen between 9 February and 30 March in the NL group in Experiment 1. By comparison, Karlsen et al. (1995) found the reduction in total body weight to be 24% in males and 34% in females during the spawning season in farmed cod.
The energy costs of reproduction in the NL groups in the present study were further indicated by marked decreases in K and HSI during the spawning season, reaching their lowest levels in late March and April near the end of the spawning season. The decrease in HSI was more pronounced in NL males than females in both years, possibly suggesting that the reproductive cost was higher in males than females in these experiments. By contrast, Karlsen et al. (1995) found that cod females had the largest reduction in HSI during the spawning season, with a decrease of 75% from January to May, compared with 66% in males.
In the LL groups, declining HSI was seen during spring and summer in both experiments, probably indicating energy allocation to gonad development and spawning. However, LL males did not reach HSI levels as low as were seen in NL males during their respective spawning seasons. Also, during the spawning months, the LL groups did not demonstrate values of K as low as those seen in the NL groups in March/April. Taken together, these results suggest that the LL groups invested less energy in sexual maturation than the NL groups. This inference was also corroborated by lower peak median GSI values in the LL groups than in the NL groups during their respective spawning seasons.
The overall growth in the present study, with mean body weights ranging from 1.7 to 2.4 kg at the age of 23 months from hatching, exceeds the growth reported in other experiments with Atlantic cod (e.g. Jobling, 1988; Karlsen et al., 1995; Svåsand et al., 1996; Hansen et al., 2001). The LL groups in the present study reached mean body weights of 2.9–3.1 kg, which is a reasonable harvest size, within 28 months from hatching. This is comparable to the production time for Atlantic salmon under favourable growth conditions (e.g. Oppedal et al., 1997), indicating the potential for commercial on-growth of cod in sea cages. A further delay of maturity until the age of three years in sea cages, as indicated with LL treatment in tanks (Dahle et al., 2000; Karlsen et al., 2000; Hansen et al., 2001; Davie et al., 2003), could potentially enable fish to reach 5 kg before onset of maturity, thereby facilitating year-round supply of market sized, immature cod.
In conclusion, the two current pilot experiments indicate the potential of using LL treatment to delay gonad development and sexual maturation in cod in sea cages. However, further studies should be conducted to verify the effects of LL treatment on cod in sea cages, and to further optimize the LL treatments, e.g. in terms of timing, light intensity, and spectral distribution of the LL, or to test alternative photoperiods to LL.
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Acknowledgements |
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The authors are indebted to Henrik Johan Tvedt and the rest of the staff on Tveit Fish Farm AS for running the experiments and providing excellent technical assistance, as well as Laura Rey for oocyte measurements. This study was financed by the Ministry of Fisheries and the Norwegian Research Council (110094/120).
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References |
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