Effect of prey type and size on the growth,survival and pigmentation of cod (Gadus morhua L.) larvae

The aim of this study was to investigate the effect of natural zooplankton versus rotifers and the effect of prey size on the growth and survival of cod larvae. At 20 days post hatch (DPH) myotome height, standard length and dry mass were significantly higher in larvae fed zooplankton compared to those fed rotifers. The dry mass at age 25 DPH was 135 μg (±45), 331 μg (±114), 391 μg (±121) for larvae fed rotifers, small size and large size plankton, respectively. At 25 days post hatch, the survival rates were 41.8% (±10.5), 90.7% (±2.3) and 91.4% (±1.7) for larvae reared on rotifers, small size and large size plankton, respectively. The limited growth and survival of cod larvae reared on rotifers were not mainly caused by the small size of rotifers. Large differences in skin coloration between larvae in the rotifer group and the two zooplankton groups were observed, probably caused by the large difference in astaxanthin levels in rotifers and natural zooplankton. We suggest that the nutritional composition of rotifers is a limiting factor for cod larvae growth and survival.     

Behavioural responses to visual environment in early stage Atlantic cod Gadus morhua L. larvae

Marine fish larvae exhibit species‐specific sensitivity to light, which changes during ontogeny. Our aim was to explore how visual environment, characterized by light intensity, microalgae addition and wall colour, affects spatial distribution of Atlantic cod larvae. We applied two different small‐scale factorial designs, each of them as a series of independent short‐term trials at fixed larval ages from 5 to 35 days post hatch (DPH). Positive phototaxis increased in larvae from 5 to ≈24 DPH, followed by reduced response at later ages. Effects from the experimental factors on larval behaviour changed with age, and significant interactions between factors were present. Algae significantly affected larvae only at 5 DPH, increasing phototactic response. White wall colour induced distribution of larvae close to the tank walls at all ages. The experimental approach demonstrated clear effects from visual environment on larval distribution, and revealed both relative importance of individual factors, interactive effects and temporal changes due to larval ontogeny. Our results suggest that by fine‐tuning the visual environment in tanks, the larval distribution may be managed to improve overall rearing conditions through a more even distribution with reduced closeness to tank walls.     

Environmental influences on melatonin secretion in Atlantic cod (Gadus morhua L.) and their relevance to commercial culture

This study investigated the effects of photoperiod and temperature on plasma melatonin secretion in Atlantic cod (Gadus morhua L.). Initial work confirmed the presence of a diel profile of melatonin synthesis, with elevated levels during the dark phase. Unusually for fish, the peak in plasma melatonin occurred towards the end of the dark phase, which is indicative of a type `A' melatonin profile. When exposed to 60 hours of continuous darkness a clear endogenous rhythm of melatonin synthesis was observed, which continued for 4 cycles with a periodicity which, approximated to 24 h. When acclimated to varying temperatures (4, 8, 12 or 16 °C) no variation in melatonin production was seen, however, body size appeared to be an important influence, with the smallest fish exhibiting significantly higher levels of dark phase melatonin. Finally, the application of additional night-time illumination to cod maintained in sea cages i.e. without blackout, did not significantly reduce dark phase plasma melatonin, suggesting that cod are less sensitive to photoperiod manipulation in cages than salmonids.     

Inter‐individual variation in the dominant intestinal microbiota of reared Atlantic cod (Gadus morhua L.) larvae

The diversity of the microbiota in 15 Atlantic cod (Gadus morhua L.) larvae from three different hatcheries with variable food type and water treatment was characterized with culture dependent and independent approaches. The culture‐dependent approach was phenotypic characterization of bacterial isolates based on 16 tests, and the culture‐independent approach was terminal restriction fragment length polymorphism (T‐RFLP) of PCR amplified 16S rRNA. Both approaches revealed large inter‐individual differences in the microbiota of cod larvae. Data from real time PCR indicate substantially lower variation in bacterial number larva^?1 both within and between hatcheries than the data on colony forming units, resulting in an estimated four orders of magnitude variation in culturability between larvae. Considerably higher richness and diversity were observed with T‐RFLP than with phenotypic characterization, and no correlation between the two approaches was found for diversity indexes calculated at the individual level. Dissimilarity in the composition of the microbiota between individuals from the same rearing environment was comparable to the dissimilarities between hatcheries. The large inter‐individual variation is important to consider when designing experiments, as pooling of individuals will completely mask this variability. We conclude that colonization of individual cod larvae by bacteria seems to be a stochastic and not a deterministic process, which is not well understood.     

The Effect of Photoperiod on Sexual Maturation,Appetite and Growth in Wild Atlantic Cod (Gadus morhua L.)

An experiment was conducted to examine (a) the effects of photoperiod on timing of sexual maturation (b) the relationship between plasma steroid levels, appetite and growth in male and female Atlantic cod (Gadus morhua L.). Wild caught Norwegian coastal cod were subjected to either a 6L/18D photoperiod typical of January at 60° N-(Short day group) or a simulated natural photoperiod (Normal day group) from June 2000 until spawning started. Appetite of individual fish were measured twice weekly, while weight, length and plasma levels of the sex steroids testosterone (T), 11 keto-testosterone (11-KT) and estradiol-17β (E2) were monitored bimonthly. Cod in the Short day group matured 3 months ahead of the cod in the Normal day group and started spawning in early November. Appetite decreased in both sexes 2–3 months prior to spawning in both groups, but this reduction was stronger among males. In both sexes, length growth was reduced concurrently with the appetite loss. Overall, females had significantly higher somatic growth, put relatively less energy into length growth and had developed larger livers compared to males at the time of spawning in the Short day group. Plasma steroid levels increased in both groups throughout the experiment, reaching peak levels of ca 10 ng ml⁻¹ (T) and 15–20 ng ml⁻¹ (11-KT) in males, and 1.5–2 ng ml⁻¹ (T) and 12–18 ng ml⁻¹ (E2) in females at the onset of spawning. Steroid levels increased more rapidly among Short day cod verifying the earlier onset of maturation. These results confirm that photoperiod is a major cue to maturation in cod and imply that the high cost of spawning for females incur differences in appetite between the sexes.     

Immunostimulation of larvae and juveniles of cod, Gadus morhua L

Cod larval culture is currently hampered by high mortalities in the first 2-3 weeks after hatching, often due to infectious diseases. The immune system of cod is not fully competent until 2-3 months after hatching. Conventional vaccination is, therefore, not of value before this time, and the larvae are wholly reliant on non-specific parameters for their defence against infection. A range of substances, generally derived from bacterial, fungal or plant origin, can activate these non-specific parameters. During three hatching seasons, 2001-2003, at the Marine Institute's Experimental Station, Stadur, Grindavik, Iceland, the effects of several immunostimulants on survival and disease resistance of cod larvae and juveniles were examined. Both bathing treatments and administration in the feed were used. One of these substances, lipopolysaccharide (LPS), isolated from the bacterium Aeromonas salmonicida (ssp. salmonicida or achromogenes), appeared in some instances to improve survival and have a beneficial effect on disease resistance. Other substances tested had limited effects. The results emphasize the need for further work in this field.     

Movements and spatiotemporal distribution of escaped farmed and local wild Atlantic cod (Gadus morhua L.)

Commercial farming of Atlantic cod (Gadus morhua L.) is now being developed in several countries. The ecological consequences of cod culture are poorly understood, but recent research suggests that Atlantic cod are more prone to escape from net pens than Atlantic salmon. Here, we describe the movements and the spatiotemporal distribution of farmed cod after escape relative to wild cod, both during and outside the natural spawning season. The experimental design included simulating escape incidents of farmed cod tagged with acoustic transmitters and using an array of automatic listening stations to monitor their dispersal and distribution. For comparison, local wild cod were monitored using the same array of receivers. The farmed cod dispersed rapidly after a simulated escape, they randomly distributed over large areas and their distribution overlapped with local wild cod. Moreover, escaped farmed fish were found at local cod spawning areas during the spawning season. The study also indicated that the recapture rate of escaped farmed cod was high compared with that of escaped farmed salmon. Thus, while our results showed that there is a considerable potential for ecosystem effects caused by escaped farmed cod, mitigating actions such as an efficient recapture fishery for escapees may be possible.     

Development and evaluation of microparticulate diets for early weaning of Atlantic cod Gadus morhua larvae

Early weaning trials were conducted with cod larvae to investigate the effectiveness of microparticulate diets (microbound and microcoated) with and without lipid-walled capsules (LWCs). The microparticulate diets were evaluated by measuring physical parameters of the diet in the water column (leaching and settling rate), palatability (intestinal fullness), performance of the diet (survival and growth), and examination of the diet in the larval intestine (histological analysis). A feeding trial was conducted using four experimental diets (carrageenan microbound diet, carrageenan microbound diet with LWCs, zein microcoated diet and zein microcoated diet with LWC), one commercial diet (BioKyowa: A-250) and a live feed control (rotifers and Artemia ). Survival of cod larvae to 39 days post-hatch ranged from 5 to 10% with the experimental diets, 22.9% with the BioKyowa diet, and 36.5% with live prey. There was evidence of food absorption with all diets in the form of lipid vacuoles in the midgut and supranuclear vacuoles in the hindgut. Large vacuoles in the midgut were more abundant in the enterocytes of larvae fed the experimental diets compared with larvae on the BioKyowa diet and the live feed control. Based on observations of intestinal fullness, the experimental diets appeared to be less palatable than the BioKyowa diet. As a result, it took longer to wean the larvae and higher mortality was experienced during weaning. Once successfully weaned, the experimental diets yielded growth rates equivalent to larvae feeding on the commercial diet for the remainder of the experiment.     

Proteome profile comparison of two differently fed groups of Atlantic cod (Gadus morhua) larvae

Proteome analysis was used to study the effects of feeding early Atlantic cod (Gadus morhua) larvae with a saithe (Pollachius virens) protein hydrolysate (SPH). Protein hydrolysates have previously been shown to beneficially affect fish larval development. Feeding was initiated on day 2 post hatch (ph) or as soon as the larvae opened their mouth and the protein expression was monitored 4 days later or in 6‐dph cod larvae. The results demonstrated changes in the abundance of 13 protein spots in the cod larvae fed SPH. Of these, seven protein spots were up‐regulated and six protein spots showed down‐regulation. Five of the up‐regulated proteins in cod larvae are known to be involved in energy metabolism. A few early larval specific proteins were down‐regulated in the SPH‐fed cod larvae possibly because of an enhanced development in this group relative to the control group. Two trypsin isoforms were detected within the cod larval proteome. The detection of the trypsin spots was made possible by co‐electrophoresis of known cod trypsins with the cod larval protein extract. Surprisingly, no difference in trypsin content was observed between the SPH‐fed and the control larval groups.     

Early introduction of an inert diet and unenriched Artemia enhances growth and quality of Atlantic cod (Gadus morhua) larvae

The effects of two weaning diets and different weaning protocols on growth, survival, skeletal deformity and gut morphology of Atlantic cod larvae were studied in four groups from 16 to 45 days posthatch (dph). Cod larvae in groups 1 (early weaning with control diet) and 2 (early weaning with experimental diet) were used to evaluate the effects of different polar lipid content of weaning diets on larval and juvenile performance. Cod larvae in groups 2, 3 (early weaning with experimental diet + cofeeding with Artemia) and 4 (earlier weaning with experimental diet and earlier cofeeding with Artemia) were used to evaluate the effects of early introduction of dry diet and Artemia. From 45 to 170 dph, cod juveniles from all four groups were reared using a standard feeding protocol. No significant differences in growth, survival, deformities and gut morphology were found between cod larvae and juveniles from groups 1 and 2. Cod larvae fed on cofeeding regime with Artemia nauplii (groups 3 and 4) were bigger and had lower frequencies of jaw and neck deformities and higher foregut microvillus circumference than cod larvae from group 2. Our results demonstrate the importance of proper weaning protocols in producing better quality cod juveniles.     

Growth,development and survival of European sea bass (Dicentrarchus labrax) larvae cultured under different light spectra and intensities

We assessed the effects of light intensity and spectrum on the growth, development and survival of Dicentrarchus labrax larvae from 30 to 96 days after hatching. Twelve lighting regimes were applied using 0.3, 1 and 2 W/m² full spectrum white, blue, red and green light. By day 66 of exposure, growth performance was significantly better in larvae reared under green or red light compared with white or blue light (p < 0.05), while it was significantly better at 2.0 W/m² compared with 1.0 or 0.3 W/m² (p < 0.05). Larvae reared under white or blue light had significantly more deformities (p < 0.05) than did larvae reared under light of other spectra, while larvae reared under 0.3 W/m² light intensity had a significantly higher proportion of jaw malformations than reared under 1.0 and 2.0 W/m² (p ? 0.05). Dicentrarchus labrax larvae subjected to green light showed significantly lower survival compared with the other light spectra (p < 0.05). Moreover, light significantly affected brain aanat1a, aanatt1b, mellc, mt1, mt2, gh, tsh and crf mRNA expression. Therefore, the red and green light groups (or under 2.0 W/m²) could improve the growth performance and enhance secretion of melatonin, gh and tsh, and reduce the proportion of jaw malformations.     

Spawning‐related mortality in captive Atlantic cod (Gadus morhua L.)

Two year‐classes of Atlantic cod were reared in indoor tanks from November 2002 to November 2004. The average annual mortality was 21% and 36% for males and females respectively. The highest monthly mortality rates in females were observed around and during the spawning periods from March to July, maximally 7% and 20% for year‐class 2001 and 18% and 34% for year‐class 2000. The mortality in males was less dependent on season, reaching a maximum of 7% and 5% month^?1 for year‐class 2001 and 2000 respectively. The maximum mortality during spawning increased with age from: 7%, 19% and 34% per month for 2‐, 3‐ and 4‐year‐old females respectively. The average gonadosomatic indices in 3‐ and 4‐year‐old mature females that died during the spawning periods were 22% and 26% respectively and most of them were spawning when they died. The results suggest that egg‐retention may have been the main cause of death in females during spawning.     

A general biophysical model of larval cod (Gadus morhua) growth applied to populations on Georges Bank

Field-derived growth rates (RNA-DNA based) of cod (Gadus morhua) larvae collected on the southern flank of Georges Bank were higher on average in May 1993 than May 1994, despite the apparent higher abundance of potential prey in 1994. A biophysical modeling study is presented here in which factors are examined that may have led to the difference in population mean growth. A one-dimensional physical model, forced by winds and tides, was used to simulate the vertical structure (of currents, temperature field, and turbulent kinetic energy dissipation rate) following a column of water in a Lagrangian sense at a site on the southern flank of Georges Bank over 5-day periods in late May of 1993 and 1994. The biophysical model and observed zooplankton abundance allowed us to explore the vertical structure and temporal (hourly) evolution of feeding and growth for cod larvae in relation to environmental conditions. Our trophodynamic model is improved over previous versions and now includes the effect of light on larval feeding response, as well as the effect of temperature on larval metabolic costs, ingestion, and digestion. Larval prey profiles, comprising four copepod species, were used from a time series of 1/4-m² MOCNESS tows to define the prey field. Data from a collateral time-series of larval gut contents (1-m² MOCNESS tows) was used to define maximum ingestion (satiation level) and prey selection. Model outputs provide depth-dependent estimates of growth, prey biomass ingested, larval length, and larval weight. Water-column growth-rate profiles were made for four size classes of larvae (5, 6, 7 and 9 mm) under the environmental conditions observed in May 1993 and 1994. A weighted-mean growth rate based on the mean vertical distribution of larvae was estimated for each size class. In all cases, when using all available potential prey, the model-derived 1994 growth rates were higher (by 3–6% day⁻¹) than those for 1993. However, simulations in which 7-mm larvae followed the field-derived weighted mean depth over the sampling period, and were limited to their preferred Pseudocalanus prey, resulted in average growth of 12.2% day⁻¹ for 1993 and 9.7% day⁻¹ for 1994. These compared closely to the field growth means of 11.3% day⁻¹ in 1993 and 9.8% day⁻¹ in 1994. Thus, the lower observed growth in May 1994 may have resulted from depth-dependent food limitation and prey-selectivity coupled with the greater metabolic costs induced by the higher temperature that year.     

Changes in Atlantic cod (Gadus morhua L.) sperm quality during the spawning season

The biology of cod reproduction is well described in the scientific literature. However, sperm biology and spermatozoa management are poorly studied in this species. Because of its recent farming expansion, a better knowledge of cod gametes is becoming especially useful. This work aimed at establishing tools to study sperm biology in cod, and also investigated the existence of changes in cod sperm quality during the spawning period. We showed that sperm concentration could be assessed using spectrophotometry at 260 nm. Sperm motility significantly decreased after a 168‐h storage at 4 °C. A 1:9 dilution of sperm in a non‐activating medium (1/3 seawater and 2/3 freshwater, osmotic pressure: 360 mOsm kg^?1) improved sperm storage. Sperm concentration, sperm velocity and storage capacity at 4 °C peaked during the medium period of the spawning season and then decreased to values close to those observed at the beginning of the reproductive period. The measured values of osmotic pressure, pH, protein, Na⁺, Cl^? and Ca²⁺ concentrations of the seminal fluid were modified along the spawning period. Cell damage was noted at the end of the spawning period: local blebs were observed on the flagellum but also loops at its distal part. On the other hand, spermatocrit did not vary with the sampling date. In conclusion, cod sperm quality is modified during the spawning period, the highest‐quality samples being collected during the medium part of this season.     

Effects of pre-rigour filleting on quality aspects of Atlantic cod (Gadus morhua L.)

Aquaculture is promising in increasing the amount of fresh Atlantic cod available all year around. The aim of this study was to investigate the effects of pre‐rigour filleting on fillet quality of wild and farmed cod. Pre‐rigour filleting resulted in significantly less fillet gaping than in fillets produced post rigour. The pre‐rigour fillets were, however, 12–13% shorter post rigour. In spite of the significantly lower water content of the muscle 6 days post mortem, a significantly higher liquid loss was seen in the pre‐rigour group. Differences in gaping score and muscle pH were dependent on whether the fish were wild or fed. Fillet gaping increased with storage time and was particularly pronounced for well‐fed cod. Fed cod had a significantly higher hydroxyproline (Hyp) content and significantly lower amount of glycosaminoglycans (GAGs) than wild cod. The ratio between GAGs and Hyp was thus significantly lower in fed cod. A connection between connective tissue components and fillet gaping may exist. However, further investigations are required.     

Temperature influences on growth of unfished juvenile Northern cod (Gadus morhua) during stock collapse

This study examined growth of unfished juvenile Northern cod (Gadus morhua) off Newfoundland concomitant with stock collapse in the cold early 1990s. Two unpublished data sets were examined, one from collapse‐period trapping sites along the northeast coast of Newfoundland and one from a post‐collapse inshore trawl survey. Cumulative surface and bottom temperatures were significant predictors of growth rates of the young fish with year‐classes born during collapse experiencing slower growth than those born during subsequent warming. Relationships between accrued temperature and growth were consistent across periods, with slow growth of collapse‐period fish reflecting slower accumulation of temperature‐at‐age. Temperature influences were spatially broad‐based with no significant differences in growth rates for fish captured along the entire northeast coast of Newfoundland. Predicted differences in growth rates for collapse versus recovery year‐classes were proportional to cumulative surface temperatures but not cumulative bottom temperatures. Although significant, temperature effects on growth were relatively unimportant at youngest ages. Overall, growth differences between periods were small but large differences occurred between slowest and fastest growing year‐classes. The results suggest initial responses to increasing temperatures were delayed following collapse. We conclude that although temperature was an important determinant of dampened productivity that it alone cannot account for the collapse and slow recovery of the stock. This is the first known study to directly quantitatively link temperature impacts to an unfished component of the Northern cod stock complex during collapse, removing need for implicit assumptions about whether or not cold conditions contributed to the collapse of this iconic fish stock.