Introduction
One of the main challenges for the Atlantic cod (Gadus morhua) farming industry is a fatal intestinal disorder that compromises animal welfare and causes high mortality. The production of Atlantic cod is experiencing a resurgence to fulfil market demand for a stable fish supply , and ambitions are high in expanding the industry for the future . The pathological gut conditions, undoubtedly painful for the fish, occur from small to large slaughter-size causing high economic losses and is considered a main obstacle for further increase in production. There has until now been a genetic selection proc ess for growth, behaviour and other favourable domestication traits in Atlantic cod , but gut anatomy and gut health have so far not been the focus of the cod breeding programs. T he current feeding strategy for cod is moreover based on knowledge and infrastructure from the salmon industry and is probably not yet optimally adapted for cod farming . The gut can show remarkable phenotypic flexibility in response to dietary and other environmental signals. T he overall goal of the ongoing research project FôrCod (RCN project no. 337056) is to red uce mortality in cod aqua culture related to intestinal disorders, by increasing our understanding of cod gut anatomy, prevalence and extent of gut disorders , and identifying potential underlying causes of pathological conditions.
Materials and methods
Several field surveys and controlled feeding trials have been carried out to study f actors to better meet the physiological requirements of Atlantic cod, including improvements of feeding regimes during on-growing stages (meal frequency, underwater feeding) and diet composition (ash and chitin content, moisture, raw material use). Field surveys in cooperation with commercial cod farmers have been performed to investigate the current status of the gastrointestinal system of farmed cod. Inclusion of c rab shell meal in the diet have been tested , both in a controlled short- term digestibility trial
and in a large- scale feeding trial in practical production . Cod at early developmental stages, obtained from the cod breeding program of Nofima, have moreover been studied. Studies have focused on gastrointestinal anatomy and organi zation, gut morphology and health, and nutritional status.
Results
We recently published the first in-depth pathological description of the intestinal disorder, which is correctly denoted ‘strangulating obstruction’
(Fig. 1A). In affected fish, part of the intestine is permanently entrapped (incarcerated) under fibrous strands in the mesentery. The entrapment interferes with blood flow and physically blocks the flow of the intestinal content, causing a strangulating obstruction with luminal fluid build-up , severe venous congestion causing ischemia of the intestinal wall.
We have observed a large variation in gut morphology observed in today’s farmed cod (Fig. 1B-C) where relative intestinal length increase with fish size (Fig. 1D ). Compared to wild cod, the farmed cod has much longer intestine and a more complex organization of the gastrointestinal tract and mesentery. C od farmers report that intestinal disorders primarily manifest during periods of rapid growth and high feed intake and increase in severity as the fish grow larger. This suggests an association between the long, complicated intestines of the farmed cod and the occurrence of fatal strangulating obstru ction. The long and complicated intestine seems to develop during early life stages of farmed cod (Fig. 1E) indicating that problems with intestinal strangulation can be rooted in the larval stage. Whether the complicated intestinal system in the farmed cod is related to the en vironmental conditions such as the feeding regime, a result of genetic breeding , or a combination, is not known. Results from several trials and field surveys performed as part of the project will be presented and discussed at the conference.
Acknowledgements
This project has received funding from Research Council of Norway (FôrCod project no. 337056).
References
1. Bogevik AS, Jonassen TM, Alvestad R, Chikwati E, Løkka G, Kortner TM: Testing av rekeskallmel i torskefôr-sluttrapport. In: Nofimas rapportserie. vol. 10/2024. Nofima’s report series: Nofima; 2024.
2. Skedsmo FS, Løkka G, Chikwati E, Jacobsen JV, Espenes A, Kortner TM: Intestinal strangulation in farmed Atlantic cod (Gadus morhua): Pathological changes and possible predisposing anatomical features. Journal of Fish Diseases 2024, 47(8):e13955.