Abstract
Atlantic cod (Gadus morhua) aquaculture has historically been limited by suboptimal larval nutrition,
leading to variable growth, high mortality, and inconsistent production performance. Cod farming has
recently re-emerged in Norway, and future production will depend on the balance between production
costs and market prices. More research is thus needed on feed composition and nutritional physiology
in cod. This thesis aimed to improve current understanding of lipid nutrition in cod larvae, with a focus
on functional liver development and the effects of dietary phospholipids (PL) and bile salts (BS).
The liver is fundamental to lipid metabolism in cod. It produces bile that emulsifies lipids in the intestine
and accumulates lipid stores that are mobilized during fasting. Still, the functional ontogeny of the liver
remains poorly described in cod. A major finding was that the abundance of transport proteins mediating
bile secretion, specifically the BS transporter Abcb11 and the putative PL transporter Abcb4, increased
during the first 30 days post-hatching (dph). This shows that bile secretory capacity reaches functional
maturity before climax metamorphosis, consistent with an early prioritization of the biliary function to
support efficient lipid digestion. In contrast, lipid storage emerged as a dominant liver function at the
larval-juvenile transition, reflected by a rapid increase in vacuolization of the hepatocyte cytoplasm
between 45 dph (4.2 %) and 61 dph (35.1 %). This change did not appear to be related to dietary lipid
processing capability but instead coincided with upregulation of key genes involved in triacylglycerol
synthesis (dgat2) and lipid droplet formation (plin2). Together, these findings indicate a shift in hepatic
lipid metabolism toward a functional juvenile state during metamorphosis.
It is well established that dietary PL inclusion improves lipid utilization and larval performance across
fish species. Consistent with this, it was found that an increase in dietary PL level, from an average of
5.9 % to 7.2 % of dry matter, between 17 and 60 dph resulted in more complete ossification of median
fins and vertebrae. This effect was independent of larval size. Furthermore, larvae fed high-PL diets
exhibited higher specific growth rates and had larger pools of taurocholate (TC), the predominant BS
in gadoid bile. This latter result points to a possible interaction between PL nutrition and bile metabolism
in cod, which may have contributed to enhanced lipid digestion and larval performance. In the same
feeding trial, BS supplementation (0.04 % TC of dry matter) was tested to evaluate the hypothesis that
lipid digestive capacity during early life stages is constrained by insufficient endogenous BS synthesis.
Conversely, this had no measurable effects on any performance-related parameters or on liver function
and development. However, subsequent analyses revealed that the dietary intervention was confounded
by unexpectedly high levels of TC from fish meal (ca. 0.13 % of dry matter). Therefore, the significance
of BS supplementation in developing cod larvae remains unresolved.
Overall, these findings provide novel insights into liver physiology in developing cod larvae and show
that even moderate increases in dietary PL level can enhance larval performance. Furthermore, the work
highlights the need to account for fish meal-derived TC when designing BS supplementation studies.