Thor Magne Jonassen

Photoperiod regulation of growth of juvenile Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract Juvenile Atlantic halibut ( Hippoglossus hippoglossus L., initial weight (SD) 31.1 g (±5.8 g)) were reared for 147 days at a constant temperature of 11°C and subjected to four different light regimes from February 10: continuous light (LD 24:0), simulated natural photoperiod of Bergen (60°25′N, LDN), constant 8 h light:16 h dark (LD 8:16) and LD 8:16 switched to continuous light on May 4 (LD 8:16–24:0). Fish exposed to LD 24:0 had a significantly higher specific growth rate (1.05% day −1 ) than those on LDN (0.98% day −1 ). Fish exposed to LD 8:16 had the poorest growth (0.80% day −1 ). The group on LD 8:16–24:0 (overall growth rate 0.94% day −1 ) grew poorly at first but subsequently increased growth rate after having been exposed to continuous light. This response was delayed by at least 21 days, suggesting that the fish needed some time to adapt to the change in photoperiod. Sensitivity to continuous light became less pronounced under summer photoperiods (above approximately 18 h daylength), which suggested that continuous light could be used to improve growth in juvenile halibut when the natural daylength is less than approximately 18 h. Overall mortality was 11.9%, with significantly higher mortality being observed in the LDN (17.4%) and LD 8:16–24:0 (17.1%) groups than in the LD 24:0 (6.2%) and LD 8:16 (6.8%) groups. Size-dependent mortality was seen in LD 8:16–24:0 in all periods from May 4 to July 6, with mortality being higher amongst small fish.

Regulation of growth in turbot (Scophthalmus maximus Rafinesque) and Atlantic halibut (Hippoglossus hippoglossus L.): aspects of environment × genotype interactions.

This review is aimed atelucidating the mechanisms that regulate growthin cultured juvenile Atlantic halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maximus) by evaluating thesignificance of environmental factors(temperature, photoperiod) and the interactionsbetween them. In addition, we examine growthproperties in three populations of juvenilehalibut and turbot in order to elucidate anygeographical differences in growth and growthefficiency in these species. Both temperatureand photoperiod have a significant andpersistent effect on growth rate in bothspecies. Temperature tolerance, demonstrated bya wide temperature range supporting maximalgrowth, increased with size. Fish subjected tocontinuous light exhibited faster growth thanthose experiencing a natural photoperiod or aconstant short day. Moreover, when thephotoperiod increased naturally with day-lengthor when fish were abruptly switched from beingreared on short-day conditions to continuouslight, a subsequent increase in growth rate wasobserved. This growth enhancing effect ofextended photoperiods was more apparent in ashort time scale in Atlantic halibut than inturbot, but both species show significantlong-term effects of extended photoperiods.Enhanced growth in fish in continuous light waspartly explained by higher growth efficiency.In both species, there was a significantinteraction between temperature andphotoperiod, suggesting that thegrowth-enhancing effect of continuous light isrelatively stronger at lower temperatures.Growth rate in both species was alsosignificantly influenced by the interaction oftemperature and fish size, as the optimaltemperature for growth decreased rapidly withincreasing fish size. Differences in growth,food intake, food conversion efficiency,metabolism, ammonia excretion and RNA/DNAratios in white muscle were observed betweendifferent strains of halibut and turbot, withthe best growth properties being observed inthe northern populations. These findings onhalibut and turbot partly support the theory ofcountergradient variation in growth, suggestingthat populations from high latitudes havehigher growth capacity than populations fromlow latitudes.

Variations in growth in haemoglobin genotypes of Atlantic cod

In the present paper are described the growth properties of three different haemoglobin genotypes of juvenile Atlantic cod (Gadus morhua) reared at 7, 10, 13 and 16 °C. In addition one group was reared under “temperature steps” i.e. moved successively from 16 to 13 and 10 °C. The genotype Hb-I(2/2)displayed the overall highest growth rate in the temperature range 13–16 °C, whereas the Hb- I(1/1)genotype showed the highest overall growth at the lowest temperature (7 °C). Accordingly, we found a significant interaction between genotype and temperature. The differences in growth were largest when cod were reared under the temperature step regime where the Hb-I(2/2)genotype displayed 17 and 24% higher growth than Hb-I(1/1)and Hb-I(1/2),respectively. Optimal temperature for growth (Topt.G) varied between the genotypes with the genotype Hb-I(1/2)displaying the highest (mean ± SE) Topt.G (14.5 ± 1:0.8 °C) and Hb-I(1/1)the lowest (12.5 ± 0.2 °C). The biological significance of this link between biochemical genetic variation and physiological properties might be the influences on growth pattern, ultimate size and age at first maturity.

Growth and age at first maturity in turbot and halibut reared under different photoperiods

The effect of extended photoperiods on growth and age at first maturity was investigated in 166 (79 females and 87 males) individually tagged Atlantic halibut Hippoglossus hippoglossus and in 114 (50 females and 64 males) individually tagged turbot Scophthalmus maximus. The halibut were reared at 11 °C on four different light regimes from 10 February to 6 July 1996: simulated natural photoperiod, (LDN), continuous light (LD24:0), constant 8 h light and 16 h darkness (LD8:16) and LD8:16 switched to continuous light 4 May 1996 (LD8:16–24:0). From 6 July 1996 to 9 February 1998 the LD24:0 and LD8:16–24:0 were reared together under continuous light and the LDN and LD8:16 together under natural photoperiod. The turbot were reared at 16 °C on three different light regimes: constant light (LD24:0), 16 h light:8 h darkness (LD16:8), or simulated natural photoperiod (LDN). After 6 months on the different photoperiods, the turbot was reared together on LDN for approximately 12 months until first maturation. Juveniles subjected to continuous light (halibut) or extended photoperiods (halibut and turbot) exhibited faster growth than those experiencing a natural photoperiod or a constant short day. Moreover, when the photoperiod increased naturally with day-length or when fish were abruptly switched from being reared on short-day conditions to continuous light, a subsequent increase in growth rate was observed. This growth enhancing effect of extended photoperiods was more apparent on a short time scale in Atlantic halibut than in turbot, but both species show significant long-term effects of extended photoperiods in the form of enhanced growth. In both species lower maturation of males was seen in groups exposed to extended or continuous light compared to LDN and this could be used to reduce precocious maturation in males leading to overall increase in somatic growth.

The interrelation of growth and disease resistance of different populations of juvenile Atlantic halibut (Hippoglossus hippoglossus L.)

Abstract Growth of juvenile Atlantic halibut from three areas of the North Atlantic (Canada, Iceland and Norway) was studied in an experiment using individual tagged fish reared at 15°C for 85 days. Fish from each population were subsequently split into two groups and acclimatised to either 12°C or 18°C. The fish were then injected intra-peritoneally with a Vibrio anguillarum bacteria suspension and mortality monitored for 4 weeks. Growth rates of the Canadian population ranked lowest, whereas the Norwegian population had the highest mean growth rates (SGR=1.70% day −1 , 1.62% day −1 and 1.53% day −1 for the Norwegian, Icelandic and Canadian populations, respectively). The halibut from Norway had the best survival following bacterial challenge (80%, 50% and 55% survival for the Norwegian, Icelandic and Canadian populations, respectively). Mortality was higher at 18°C than at 12°C in the Icelandic (62% at 12°C and 27% at 18°C) and Canadian (56% at 12°C and 32% at 18°C) fish, whereas a smaller difference between temperatures was observed in the Norwegian fish (25% at 12°C and 13% at 18°C). Fish that survived the challenge test were those that had grown fastest in the growth trial. Low, but significant, correlations between survival and size and growth were seen, but these correlations varied between populations. In the Canadian population, no correlation between size and growth and survival were seen; only size was correlated ( r =0.27) with survival in the Icelandic population, whereas both size ( r =0.18) and growth ( r =0.17) were correlated with survival in the Norwegian population.

Effects of photoperiod, stocking density and diet on growth in young spotted wolffish (Anarhichas minor olafsen)

Full scale experiments in tanks were conducted to elucidate the effect of photoperiod regime, dietary fat level and stocking density on growth in spotted wolffish. The study showed that continuous light (LD24:0) did not give a higher final mean weight or faster overall growth rate in spotted wolffish compared to constant 8 h light and 16 h darkness (LD8:16). However, there were indications that the fish needed time to adapt to a new photoperiod regime. The feeding experiment indicated a negative relation between dietary fat level and growth, where fish given 15% fat in the diet had a 13% higher final mean weight compared to fish on a diet with 20% fat. However, no conclusions could be made. The final mean weight was 10% higher at 25 kg/m2 compared to 40 kg/m2, indicating a negative impact of high stocking density on spotted wolffish.

It works! Lumpfish can significantly lower sea lice infestation in large-scale salmon farming

ABSTRACT To assess the efficacy of lumpfish grazing on attached sea lice on Atlantic salmon, six large-scale sea cages, (130 m circumference, 37,688 m3 volume) each stocked with approximately 200,000 salmon 0+ smolts, were stocked with a 4, 6 and 8% density (8000, 12,000 and 16,000, respectively) of lumpfish. The sea cages without lumpfish acted as controls. Sea lice infestation levels on the salmon were monitored weekly and bi-weekly from 6 October to 17 May the subsequent year. Mortality of the lumpfish rose with decreasing sea temperatures to around 0.8% week−1 and did not vary between the lumpfish groups. There were clear signs of lumpfish grazing on sea lice, with significantly lower average levels of chalimus, pre-adult and adult female Lepeophtheirus salmonis and Caligus elongatus sea lice per salmon. Lumpfish in the high density (8%) group reduced the mature female L. salmonis to levels equal to or lower than the counts recorded prior to the start of the study. Overall, the present results indicate that lumpfish are a suitable cold-water option for biological delousing of Atlantic salmon in large-scale production conditions. Summary: Lumpfish are effective cleaner fish for biological delousing and can significantly lower average levels of pre-adult and female Lepeophtheirus salmonis and Caligus elongatus sea lice on Atlantic salmon.