Introduction
In recent years, commercial farming of Atlantic cod (Gadus morhua) has resumed in Norway. However, the industry continues to face significant challenges related to fish welfare and economic losses, primarily due to intestinal disorders recently identified as strangulating obstruction (Skedsmo et al., 2024). Contemporary cod diets still rely heavily on marine-derived ingredients, in contrast to the plant-based formulations used in salmonid aquaculture, despite the growing global scarcity of marine resources. The natural diet of Atlantic cod is rich in chitin-containing organisms such as crustaceans and echinoderms, including crabs and shrimps (Lall and Nanton, 2002). This suggests that marine shell meals may represent promising side-stream candidates for inclusion in cod feeds (Tibbetts et al., 2006). Given the critical role of gastrointestinal function in overall health and nutrient absorption, a deeper understanding of cod gut physiology is necessary to identify the underlying causes of intestinal pathologies. In this context, dietary chitin has previously been reported to exert immune- and microbial modulatory effects with positive outcomes for fish gut health. The primary objective of this study was to enhance our knowledge of the Atlantic cod gastrointestinal system by evaluating two experimental diets incorporating crab and shrimp shell meals, assessing fish gut health and the digestibility of the formulated diets.
Materials and methods
The present study evaluated the nutritional digestibility and gastrointestinal health and function of Atlantic cod with an average body weight of 300 g, fed isoenergetic diets incorporating alternative feed ingredients. Three experimental diets were formulated to be isoenergetic (20–21 kJ/g) with a digestible protein-to-energy ratio of 2.5 and approximately 50% crude protein and 17% lipid content. The control diet included 15% sunflower meal, while the other two substituted this fraction with 15% crab shell meal or 10% shrimp shell meal. Following a 27-day feeding period, fish were sampled for growth performance evaluation. Individual body weight and length, liver weight and pooled intestinal contents were recorded to determine dry matter and amino acid digestibility, and growth performance indices calculated. An additional subset of six fish per diet group was sampled for comprehensive physiological assessments. These included macroscopic examination of the digestive system in the abdominal cavity, histological analysis of intestinal tissue, gene expression profiling of the gastrointestinal tract (focusing on immunological, osmoregulatory and digestive markers), measurement of intestinal length and organ indices, bile salt and digestive enzyme assays and plasma biomarker analysis. The entire digestive system was dissected, and the intestine was sectioned into distinct anatomical regions: stomach, pyloric region with caeca, midgut (subdivided into three segments) and distal intestine. Statistical analyses were performed to assess the effects of dietary treatments and differences between anatomical segment differences, with significance determined at p < 0.05.
Results and discussion
In addition to assessing digestive capacity and morphological characteristics, this study provided new insights into the spatial expression of gut-associated gene transcripts related to immune response, osmoregulation and digestion along the gastrointestinal tract of farmed Atlantic cod. A positive correlation was observed between fish body size and relative intestinal length, which may have implications for the development of digestive complications later in life. Fish fed the shrimp shell meal diet exhibited a tendency toward increased feed intake during the initial phase of the trial, resulting in a higher feed conversion ratio compared to the other dietary groups, despite overall comparable growth rates and total feed intake. The group fed crab shell meal demonstrated significantly higher amino acid digestibility than the other groups. However, elevated ash and calcium levels in the shell meal were associated with increased faecal output and reduced dry matter digestibility, in line with findings by Albrektsen et al. (2009). Biochemical analyses revealed elevated levels of maltase activity and bile salts in the pyloric caeca of fish fed the crab meal diet compared to the control group. No significant differences were observed among diet groups in terms of mortality or blood biomarkers associated with nutrient metabolism and only one case of strangulating obstruction occurred during the trial. Histological examination revealed increased lamina propria cellularity in the distal intestine of fish fed the control diet containing 15% sunflower meal, suggesting a potential adverse effect of sunflower meal not observed in the shell meal diets. Overall, the intestinal health of the experimental fish was assessed as normal, functional and free of significant pathological alterations.
Conclusions
This study provided new insights into the mapping of functional gut gene transcripts in relation to intestinal morphology and digestive capacity, serving as a reference for future research addressing intestinal strangulation in farmed Atlantic cod. Although further investigation is needed to determine whether this condition is genetically determined, diet-induced or specific to farmed populations, the present findings are essential for improving our understanding of normal gastrointestinal structure and function in this species. Shell meal from marine raw materials represents a side stream that can be used as ingredients in cod feed, helping to reduce reliance on fish meal from wild stocks and contribute to increased sustainable use of local resources.
Acknowledgements
This work was funded by the project ‘Development of feeding strategies for cod for improved production and fish health’, led by senior researcher Thor Magne Jonassen (Akvaplan-niva, Norway), and supported by the Research Council of Norway (grant no 337056).
References
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Skedsmo, F. S., Løkka, G., Chikwati, E., Jacobsen, J. V., Espenes, A., and Kortner, T. M. (2024). Intestinal strangulation in farmed Atlantic cod (Gadus morhua): Pathological changes and possible predisposing anatomical features. J. Fish Dis. 47, e13955.
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