A retrospective approach to fractionize variation in body mass of Atlantic cod Gadus morhua.

Eggs of a single spawning batch from wild-caught Norwegian Atlantic cod Gadus morhua were hatched and first fed on either natural zooplankton or enriched rotifers Brachionus plicatilis during the larval period. Juvenile G. morhua (initial mass 14·2 g) from the two first-feeding groups were then reared for 3 months under a variety of temperature (10 and 14° C) and salinity (15 and 32) combinations. All fish were individually tagged and microsatellite markers were used in a multiplex to trace the pedigree of all fish and body mass variation analysed according to different environmental and genetic sources. After the termination of the laboratory trial, the fish were transferred to land-based tanks and later to sea pens and reared at ambient conditions for 26 months until they were harvested in March 2009. Growth gain from the larval and juvenile periods was persistent during the 26 months of sea pen ongrowing. The final mass of the zooplankton group was 12% higher compared to the B. plicatilis group. Similarly, rearing under a temperature of 14° C and salinity of 15 during the initial 3 month period during the early juvenile stage resulted in 7-13% larger size at harvesting compared to the other three temperature and salinity combinations. The study indicates that the first-feeding method and temperature and salinity manipulation explain nearly 90% of the body mass variation explained by the model. The genetic effect (measured as body mass variation within the families studied) only accounted for c. 2% during the initial rearing period, whereas it has a large effect on growth variation (30%) during the long-term rearing at ambient conditions. Sex proportion and final maturation did not differ between family groups, and no interaction between sex and family group was seen.

The influence of first-feeding diet on the Atlantic cod Gadus morhua phenotype: survival, development and long-term consequences for growth.

Atlantic cod Gadus morhua larvae reached four-fold (at low larval density) to 11 fold higher body mass (high larval density) at 50 days post hatch (dph) when fed zooplankton rather than enriched rotifers. A short period (22-36 dph) of dietary change affected larval growth positively if changed from enriched rotifers to natural zooplankton and negatively if prey type changed vice versa. Overall survival did not differ between the two larval groups at low larval density, but at high density the rotifer group had a higher overall survival (10.8% v. 8.9%). Long-term growth was affected significantly by larval diet in favour of the zooplankton diet; juveniles reached a 23% higher mass in a 12 week growth period. No difference in growth performance was found between juveniles fed natural zooplankton during the larval period for 36, 22 or 14 days, but all these juveniles performed significantly better compared with the rotifer-fed group. These findings suggest that optimal diet during a short period in the larval period can result in improved growth in both the larval and juvenile period. Improved rotifer quality may, therefore, hold a large potential for growth improvement in this species.

Distribution of cholecystokinin-immunoreactive cells in the gut of developing Atlantic cod Gadus morhua L. larvae fed zooplankton or rotifers.

One of the main gastrointestinal hormones, cholecystokinin (CCK), was studied in order to advance understanding of the control of the digestive process in Atlantic cod Gadus morhua larvae after onset of first feeding. Larvae were fed either natural zooplankton or enriched rotifers in similar rearing systems and sampled from hatching to 22 days post-hatch (dph). CCK was visualized by immunohistochemistry and the first CCK-immunoreactive (IR) cells were detected at 8 dph corresponding to 6 days after first feeding. The CCK-IR cells were mostly found in the anterior midgut, and the number of CCK-IR cells was lower in the posterior midgut. They were also present in the hindgut of some of the larvae, but not in the foregut. No clear differences were found in the ontogenetic appearance and the distribution pattern of CCK-IR cells between the two dietary treatments. This indicates that the onset of CCK production in the gut as well as the spatial distribution of the CCK-IR cells is not differentially affected by these diets. To what extent the hormone production itself is influenced by dietary factors needs to be studied by more sensitive methods.

Expression of the oligopeptide transporter, PepT1, in larval Atlantic cod (Gadus morhua).

The intestinal absorption of di- and tri-peptides generally occurs via the oligopeptide transporter, PepT1. This study evaluates the expression of PepT1 in larval Atlantic cod (Gadus morhua) during the three weeks following the onset of exogenous feeding. Larval Atlantic cod were fed either wild captured zooplankton or enriched rotifers. cDNA was prepared from whole cod larvae preceding first feeding and at 1000 each Tuesday and Thursday for the following three weeks. Spatial and temporal expression patterns of PepT1 mRNA were compared between fish consuming the two prey types using in situ hybridization and quantitative real-time PCR. Results indicated that PepT1 mRNA was expressed prior to the onset of exogenous feeding. In addition, PepT1 was expressed throughout the digestive system except the esophagus and sphincter regions. Expression slightly increased following first-feeding and continued to increase throughout the study for larvae feeding on both prey types. When comparing PepT1 expression in larvae larger than 0.15-mg dry mass with expression levels in larvae prior to feeding, no differences were detected for larvae fed rotifers, but the larvae fed zooplankton had significantly greater PepT1 expression at the larger size. In addition, PepT1 expression in the zooplankton fed larvae larger than 0.15-mg dry mass had significantly greater expression than rotifer fed larvae of a similar weight. Switching prey types did not affect PepT1 expression. These results indicate that Atlantic cod PepT1 expression was slightly different relative to dietary treatment during the three weeks following first-feeding. In addition, PepT1 may play an important role in the larval nutrition since it is widely expressed in the digestive tract.