Effect of rearing density on the growth and welfare indices of juvenile spotted wolffish,Anarhichas minor (Olafsen)

The goal of this study was to determine the optimal stocking density for rearing juvenile spotted wolffish, Anarhichas minor (Olafsen), at two different sizes and assess the welfare status in relation to density. No major growth impairment was observed, although smaller fish (50–100 g) were significantly affected by density during the 120 days of the experiment, with final mean weights of 119.6 ± 11.6, 118.0 ± 5.8 and 88.7 ± 0.6 g for initial rearing densities of 10, 20 and 40 kg m^?2 respectively. No effect of rearing density was seen for larger fish (100–160 g) during the 90 days of the experiment, with final mean weights of 160.2 ± 5.9, 159.7 ± 3.7 and 163.7 ± 11.5 g at fixed rearing densities of 20, 30 and 40 kg m^?2 respectively. Our results suggest that the optimal rearing density of juvenile spotted wolffish is below 40 kg m^?2 for smaller size fish (～50–100 g) and probably ≥40 kg m^?2 for the larger fish (100–160 g). Furthermore, it appears that the range of rearing density used did not have a significant effect on a selection of stress indicators (Na⁺ and K⁺ concentration, haematocrit, hepatosomatic index, total amount of plasma proteins and liver and muscle water content) and immunity response (plasma lysozyme activity) of juvenile spotted wolffish, making it a very tolerant species to crowding.     

Growth and survival of first-feeding spotted wolffish (Anarhichas minor Olafsen) at various temperature regimes

In order to define temperature regimes that could benefit successful production of spotted wolffish (Anarhichas minor) juveniles, experiments with offspring from two different females were carried out. The larvae were fed a new formulated feed or a commercial start‐feed for marine fish, both of which have given high survival rates. In the first experiment newly hatched larvae were fed at constant 6 °C, 8 °C, 10 °C and 12 °C as well as at ambient seawater temperature (2.9–4.5 °C) during 63 days. High survival, 90% to 96%, was registered at ambient and most constant temperature regimes, whereas in the 12 °C groups survival was reduced to 80%. Growth rate (SGR) was very low, 1.8% day^?1, at the low ambient temperatures. Growth rate was positively correlated with temperature and varied between 3.1% day^?1 to 4.7% day^?1, from 6 °C to 12 °C. In the second experiment, set up to include potential detrimental temperatures and study beneficial effects of a more restricted, elevated first‐feeding temperature regime, the larvae were fed at constant 8 °C, 10 °C, 12 °C, 14 °C and 16 °C until 30 days post hatch, followed by constant 8 °C for the next 33 days. In this experiment, low survival, 25% and 2.0%, was registered at 63 days post hatch when larvae were reared initially at 14 °C and 16 °C respectively. The survival of the larvae at the other temperature regimes varied from 47% to 64%, highest survival rate (64%) was found at 8 °C. The lowest specific growth rate, 2.6% day^?1, was noted in the 16 °C group. At constant 8 °C to 14 °C (regulated to 8 °C), the SGR varied from 4.45% day^?1 to 5.13% day^?1. The larvae grew faster in the experiment when initially comparable temperatures (8 °C, 10 °C and 12 °C) were regulated to constant 8 °C after 30 days compared with the first experiment where feeding was carried out at the same constant temperatures (8 °C, 10 °C and 12 °C) during the whole experimental period.     

Early ontogeny of the spotted wolffish (Anarhichas minor Olafsen)

This study illustrates the embryo development of the spotted wolffish (Anarhichas minor Olafsen), an interesting candidate for cold‐water aquaculture. The egg morphology (semitransparent, yellow‐white with numerous oil droplets in the yolk), size (5.4–6.5 mm) and long embryogenesis (c. 800–1000 d°, depending on temperature) of A. minor are very similar to Anarhichas lupus. Cleavage is slow, and the first cell divisions take place at 12 h at 8°C. After 12 days the 2‐mm embryo with the first somites is laid down and the blastopore starts closing. The fat globules in the yolk fuse into one after 22 days, and after 30 days eye pigmentation is noticeable. After 44 days, eye pigmentation is strong, the digestive tract folded and a green gall bladder can be noted in the now 11‐mm‐long embryo. One week later the blood is brightly red, the intestine is pigmented and the lower jaw is well developed. Premature hatching may occur from this stage. After 58 days vascularization of the yolk is complete, capillaries are noted in the fin fold, the first ray rudiments are established in the tail and pectoral fins, and the four gill arches are covered by the operculum. The preanal finfold is reduced after 72 days, stomach and gill filaments are formed, and six pigmented rows are noted on the 17‐mm‐long embryo body. After 86 days all fin rays are seen and the digestive tract is intensely pigmented and folded. Hatching (normal) starts after 110 days and may last for 2–3 weeks. Late embryos and early larvae of A. minor have more distinct bands of pigment along the body compared with the closely related A. lupus. An increase in both length and weight of the embryos in individual batches occurs during the hatching period.     

Metabolic and digestive enzyme activity profiles of newly hatched spotted wolffish (Anarhichas minor Olafsen): effect of temperature

Three groups of newly hatched spotted wolffish (Anarhichas minor) were held at three different temperatures in order to determine relationships between metabolic, digestive and growth response in rapidly developing larvae. Growth rates were successfully modulated by temperature (5, 8 and 12 °C). Activity levels of trypsin and lactate dehydrogenase (LDH) were positively linked to specific growth rates at all temperatures. Trypsin showed a positive compensation (higher activity at lower temperature) whereas glycolytic enzymes (pyruvate kinase and Lactate dehydrogenase) and aspartate aminotransferase (AST) showed a negative compensation (lower activity at lower temperature). Citrate synthase was not affected by growth rate, indicating that the level of aerobic capacity was adequate in sustaining the high energetic needs associated with rapid growth early in the life of the spotted wolffish. In light of our results, it is suggested that protein digestion, as demonstrated by the activity profile of trypsin in relation to growth rate and temperature, is likely a key growth‐limiting agent during the early‐life stages of wolffishes. Our results are discussed in comparison with A. lupus, a closely related species displaying different temperature preferences and growth capacities.     

Effects of incubation temperature on development and yolk sac conversion efficiencies of spotted wolffish (Anarhichas minor Olafsen) embryos until hatch

Two egg batches of spotted wolffish, Anarhichas minor Olafsen, were incubated at 4, 6 and 8°C. Embryo samples were fixed and compared on each 100th daydegree until hatching (up to 1000 daydegrees). Embryos, yolk sacs and chorions were dissected and the sizes, wet and dry weights were recorded separately. Comparisons of gross morphologies and measured parameters showed increasing and generally significant differences with time between the incubation temperatures. Lower temperatures produced longer and more differentiated larvae at hatch with a smaller yolk sac. Even though some unexpected deviations were registered among batches and experimental groups, it was clear that temperature affected embryo survival and time of hatching. Overall survival was best at 6°C, in agreement with results from earlier studies. Yolk conversion efficiencies measured around the hatching point were generally high, ranging from 60% to 78%, varied between the two batches and probably reflected the developmental variations between embryos and larvae at the respective ages (daydegrees). The hatching process was apparently an energy‐demanding period; yolk conversion efficiencies of unhatched embryos of similar age at each temperature were always higher. Temperature is one environmental factor that can be manipulated in hatcheries to induce hatching of viable larvae at an optimal stage of differentiation with respect to first‐feeding success and early survival.     

Cortisol and Behavioral Response to Handling (Acute) and Confinement (Chronic) Stressors in Juvenile Spotted Wolffish,Anarhichas minor

The purpose of this study was to determine the cortisol response of spotted wolffish (Anarhichas minor) after exposure to air immersion and netting (acute stress) under a range of increasing densities (chronic stressor). In addition, the presence of a cumulative negative impact from chronic stress was assessed by the application of an additional acute stressor following rearing at various densities, and proxies of behavior were evaluated (feeding and aggression data). The stress response to air immersion and netting led to increasing plasma cortisol values at 0.5, 13, and 37 h post-stress, with the highest value achieved after 13 h. At 168 h, cortisol concentrations returned to similar pre-stress value. Changes in cortisol concentrations were relatively low after acute stress (five to six fold increases) and recovery time long lasting (>37 h). Prolonged rearing at 20, 30, and 40 kg m^?2 showed a non-significant trend of increasing cortisol values with increasing density, and the cortisol response after the application of an additional acute stressor was significantly altered, i.e., higher cortisol levels were measured in unstressed (chronic stress only) compared to stressed fish (chronic stress + acute stress). Our results suggest that (1) spotted wolffish is a low responder that additionally displays a long-lasting cortisol elevation following an acute stressor; (2) fixed densities did not lead to significant differences of plasma cortisol concentration or aggression levels, although involved marked changes in daily feed intake and food conversion efficiency were observed; and (3) cortisol levels reached acute stress values after prolonged chronic stress. Although this species displays indications of being relatively resistant to handling disturbances during aquaculture practices, it is nevertheless responsive to chronic stress factors that could alter the physiological response to subsequent acute stressor.     

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.     

Graded environmental hypercapnia in juvenile spotted wolffish (Anarhichas minor Olafsen): effects on growth, food conversion efficiency and nephrocalcinosis

Growth performance and food conversion efficiency (FCE) were investigated in juvenile spotted wolffish (Anarhichas minor Olafsen), mean (S.D.) initial weight 15.7 (4.8) g, reared at four levels of carbon dioxide (CO_2(aq)) for 10 weeks at 6 °C and 33‰. CO₂ levels averaged 1.1 (control), 18.1 (low), 33.5 (medium) and 59.4 (high) mg l⁻¹, with corresponding pH values of 8.10, 6.98, 6.71 and 6.45, respectively. In addition, kidneys from sampled fish were examined macroscopically for gross signs of calcareous deposits, i.e. nephrocalcinosis, at the start and end of the experiment. Growth was significantly reduced at the highest concentration (P<0.0001), as compared to all other groups, while no overall differences in growth rate or mean weight were seen in the range of 1.1–33.5 mg CO₂ l⁻¹ at the end of the experiment. Daily feeding rates and total food consumption were reduced at the highest concentration (P<0.001), whereas food conversion efficiency did not vary significantly between groups. Plasma chloride levels displayed a significant decrease with increasing CO₂ levels, from 151.3 mmol l⁻¹ (1.1 mg CO₂ l⁻¹) to 128.3 mmol l⁻¹ (59.4 mg CO₂ l⁻¹) at the end of the experiment, whereas plasma osmolality in the high CO₂ group was significantly higher compared to the control group at the end of the experiment (371.4 and 350.8 mOsmol kg⁻¹, respectively). Nephrocalcinosis was observed in all groups at the end of the experiment, but was most pronounced in the medium and high CO₂ group.     

Effects of hypoxia and hyperoxia on growth and food conversion efficiency in the spotted wolffish Anarhichas minor (Olafsen)

Abstract The effect of water oxygen content on growth and food conversion efficiency was evaluated for juvenile spotted wolffish, mean (± SD) initial weight 68.5 (± 17.5) g, reared at oxygen levels of 4.0, 6.0 (hypoxia), 9.6 (normoxia) and 14.5 (hyperoxia) mg L^?1 for 11 weeks at 8 °C. Mean weights and total food consumption were significantly higher in the control and hyperoxic groups compared with the hypoxic groups at the end of the experiment. The 9.6 and 14.5 mg L^?1 groups exhibited significantly higher overall specific growth rates (0.90 and 0.86% day^?1 respectively) compared with the groups on 4.0 and 6.0 mg L^?1 (0.46 and 0.71% day^?1 respectively). In the hyperoxic group, growth was only limited in the first period and, in the hypoxic groups, growth rates increased throughout the experiment, with the 6.0 mg L^?1 group performing equally well compared with the control in the last period. Overall, our findings suggest that the species will adapt to both high and low ambient water oxygen content given a period of adaptation. After the adaptation phase, growth and food conversion efficiency are comparable in the oxygen level range of 6.0–14.5 mg L^?1.     

Effects of egg disinfection and incubation temperature on early life stages of spotted wolffish

Eggs of spotted wolffish (Anarhichas minor Olafsen)were incubated at constant 4, 6 and 8 °C, and disinfected withglutaric dialdehyde (150 p.p.m. for 5 min) once ortwice a month during two thirds of the incubation period, to prevent growth ofmicroorganisms. Hatching of apparently normal larvae started earlier when eggswere disinfected twice a month compared to once a month at all incubationtemperature regimes. The time to 50% hatch was 900 and 920 day-degrees (16 and16,5 weeks) at 8 °C, 835 and 880 day-degrees (20 and 21 weeks)at 6 °C and 725 and 800 day-degrees (26 and 28,5 weeks) at 4°C, in the egg groups disinfected twice or once a month,respectively. The best survival until hatching was noted when eggs weredisinfected twice a month and incubated at 6 and 8 °C.Survivalwas very low at 4 °C. Prematurely hatched larvae wereregistered in all egg groups disinfected twice a month and the highestfrequencywas noted in the 8 °C groups. The larval weight at normalhatching in the 6 and 8 °C groups was negatively correlatedwith incubation temperature and intervals of disinfection during the incubationperiod, but after 42 days feeding with live feed (unenrichedArtemia) the weights of the larvae were not significantlydifferent. The specific growth rates of the larvae from the eggs incubated at 6°C and 8 °C were 3.0% and 3.2%, respectively.The mean survival of larvae was between 88% and 96% at 42 days post-hatching.Young wolffish originating from the 6 °C incubation groupsshowed lowest mortality.     

Cultivation of the spotted wolffish Anarhichas minor (Olafsen) – a new candidate for cold-water fish farming

The spotted wolffish Anarhichas minor (Olafsen), with its rich and tasty fillets, popular skin and high growth rates in captivity, is a promising candidate for cold-water aquaculture. We have established a production line for the wolffish during 5 years of active research on the biology of the species. Our broodstock, sampled from the Barents Sea, has mainly spawned during autumn. The females must be stripped as soon as possible after ovulation. The best egg batches have shown c. 100% fertilization and survival rates between 60% and 80% during the 800- to 960-daydegrees-long incubation period. Rearing temperatures of 6 °C, 8 °C, ambient and decreasing temperatures have turned out to be satisfactory for survival. Treatment with 150 p.p.m. glutardialdehyde twice a month is recommended to control microorganism growth on eggs. Premature hatching has been a problem in individual egg batches; the reasons are not fully understood. Normally hatched individuals are ≈ 22 mm long and well developed, with a small yolk sac, which is completely resorbed after about 4 weeks at 6–8 °C. The fry have been successfully start fed on formulated feed as well as Artemia. Highest early growth rates have been noted at 8 °C, but overall survival was best at 6 °C. On-growth of spotted wolffish juveniles fed formulated dry floating feed at low temperatures in shallow raceways has been promising. After 2, 3 and 4 years, the mean weights of the first generation produced in aquaculture were 0.7, 2.7 and 5.1 kg respectively. These individuals now make up the broodstock at a recently established commercial production facility.     

Influence of egg lipids and fatty acids on egg viability, and their utilization during embryonic development of spotted wolf-fish, Anarhichas minor Olafsen

The changes in egg lipids and fatty acid compositions that occur during embryonic development of spotted wolf‐fish, Anarhichas minor, were examined by monitoring individual egg batches from the time of spawning (egg stripping) until hatching. The lipids, present as 3.7±0.1% of the wet mass of the freshly stripped eggs, contained high percentages of monoenes (monounsaturated fatty acids (MUFAs), ca. 33%) and polyenes (ca. 43%) and approximately 20% saturated fatty acids (SFAs). The fatty acid profiles were dominated by a small number of fatty acids. The major SFA was 16:0 (ca. 14%), the dominant MUFA was 18:1 n‐9 (ca. 21%), and among the polyenes, the n‐3 highly unsaturated fatty acids (HUFAs) 22:6 n‐3 docosahexaenoic acid (DHA) and 20:5 n‐3 eicosapentaenoic acid (EPA) were present in the highest concentrations (EPA, ca. 16%; DHA, ca. 19%). The n‐6 HUFA 20:4 n‐6 arachidonic acid (AA) was present as ca. 1% of the total fatty acids in the freshly stripped eggs. This resulted in an AA:EPA of ca. 0.07, which is lower than reported for eggs of many other fish species. As embryonic development progressed, the percentage contribution of AA to the total fatty acids almost doubled. There were also increases in the relative proportions of SFAs (due mainly to an increase in the percentage of 16:0 to ca. 16% at hatch) and DHA (to ca. 23%), and there was a corresponding decrease in the percentage of MUFAs (mostly brought about by a decrease in the percentage of 18:1 n‐9 to ca. 18% at hatch). The most marked changes occurred towards the end of incubation. The percentage of EPA changed little during incubation. This implies that there was selective retention of DHA, 16:0 and AA, and these fatty acids were probably incorporated into cell membranes. MUFAs, particularly 18:1 n‐9, seem to have been catabolized to provide energy for the developing embryo, and some EPA also seems to have been utilized as an energy source. Survival of eggs to the eyed stage (range ca. 10–80%) and to hatch (ca. 5–75%) was negatively correlated with the %AA, %EPA and AA:DHA of the freshly stripped eggs. There was also a negative correlation between AA:EPA and egg survival, which implies that there is not a universal requirement for a high AA:EPA to ensure high rates of survival of fish eggs.     

Physiological stress responses in spotted wolffish (Anarhichas minor) subjected to acute disturbance and progressive hypoxia

The spotted wolffish (Anarhichas minor) is considered a promising species for coldwater aquaculture. It is a sedentary, bottom-dwelling fish which exhibits a calm and “non-stressed” behaviour in captivity. There are, however, no reports on the physiological responses to stressors in this species. In the present study we investigated primary (cortisol secretion) and secondary (glucose mobilization) responses to common aquaculture stressors like disturbance and hypoxia. Pre-stress plasma cortisol levels were within those generally considered representative for unstressed fish (~ 10 ng ml^− 1), whereas basal glucose levels were unusually low (0.3–0.4 mM). After exposure to a short, but severe disturbance challenge (emptying the tank of water for 10 min), the increase in plasma cortisol level was slow and relatively weak, reaching a peak level of 25 ng ml^− 1 4 to 8 h after disturbance. When the fish were exposed to a gradually decreasing oxygen level in the tank, a significantly elevated plasma cortisol level (35 ng ml^− 1) was seen in the fish that remained in the tank until oxygen saturation had decreased to 20% oxygen saturation after 2.5 h. However, a two-fold higher plasma cortisol level (~ 70 ng ml^− 1) was seen in all fish exposed to reduced oxygen levels (60, 40 and 20% oxygen saturation) after 3.5 h recovery in normoxic water. Plasma glucose levels showed only moderate increases (~ 70%) following disturbance and hypoxia challenges. An in vivo injection of ACTH caused a strong elevation of plasma cortisol (peak level ~ 170 ng ml^− 1), demonstrating a high capacity for interrenal steroidogenesis in the spotted wolffish. The slow and relatively weak cortisol response to stressors, and low plasma glucose levels, may relate to the sedentary lifestyle of the spotted wolffish. The stress-response is characterised by a passive (reactive) coping style, which is considered adaptive for farming of this species.     

Do protein hydrolysates improve survival and growth of newly-hatched spotted wolffish (Anarhichas minor), a non-metamorphic aquaculture fish species?

Despite larval robustness characterized by the absence of metamorphosis and readiness for exogenous feeding based on commercial feed, the spotted wolffish (Anarhichas minor) displays highly variable survival at first-feeding. In this study, we investigated the use of three dietary concentrations of protein hydrolysates (PH, pre-digested proteins) (0, 10 and 20%) when newly hatched juvenile wolffish were held at three different rearing temperatures namely, 5, 8 and 12 °C to determine whether digestion of protein was a limiting factor for fish growth and survival. Final weights for fish at 5, 8,and 12 °C at day 60 were respectively 0.35, 1.19 and 2.02 g. Mean specific growth rates were 1.97, 4.01 and 4.88%/day and survival rates were 49.8, 53.4 and 33.2% respectively. No significant effects of PH were observed on growth or survival at any time during the experiment. However, as a general trend, fish survival was always higher when the diet contained 20% PH. We suggest that the degree of hydrolysis of the PH used may have been insufficient to induce specific digestive enzyme stimulation for promoting larval growth. Moreover, precocious ontogeny of the digestive system may have precluded any significant effect of using dietary protein hydrolysates. Our results are discussed in comparison with metamorphic species which characteristically display incomplete development of the digestive system.     

The effects of temperature on growth rate and growth efficiency of juvenile common wolffish

The effects of temperature on the survival, growth rate and growth efficiency of larval and juvenile common wolffish, Anarhichas lupus L. were studied at 0–31 days and 9–12 months post-hatching, respectively. The influence of temperature regime during egg incubation on subsequent survival and growth was also examined. The fish were reared at constant water temperatures of 5, 8, 11 and 14°C, and all groups were fed dry pellets. At age 1 month, maximum growth rates were observed at 11 and 14°C. Growth rates and survival of early juveniles were dependent upon incubation history, high growth being obtained only if rearing temperature exceeded the temperature of egg incubation. In juveniles at age 9–12 months, the relationships between temperature and growth, and temperature and growth efficiency were parabolic: the optimum water temperatures for growth (Topt.G) and growth efficiency (Topt.GE) were 11°C and 9.7°C respectively. The growth rate and growth efficiency at these water temperatures were 0.9% day–1 and 0.45 g weight gain per g food offered, respectively. © Rapid Science Ltd. 1998     

Steroid synthesis in follicular cells and induction of germinal vesicle breakdown in spotted wolffish (Anarhichas minor) oocytes

Incubation of follicular cells from postvitellogenic spotted wolffish ovaries with tritiated steroid precursors revealed that granulosa cells were able to convert 17-hydroxyprogesterone (17-P) to 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) and androstenedione. Theca cells had limited ability to synthesise additional steroids from 17-P but converted 17,20β-P to 17,20β-P sulphate. Neither of the two cell layers was able to synthesise 5β-pregnane-3α,17,20β-triol-20-sulphate (3α,17,20β-P-5β-S) which is found in high concentrations in plasma. 17,20β,21-trihydroxy-4-pregnen-3-one (17,20β,21-P), 17,20β-dihydroxy-5β-pregnan-3-one (17,20β-P-5β) and 17,20β-P were most potent in inducing germinal vesicle breakdown (GVBD). Sulphation of 17,20β-P resulted in loss of GVBD inducing activity.