Developing a feeding protocol for grow-out Atlantic halibut based on growth, physiological and behaviour responses to feeding frequencies

Atlantic halibut have been a part of Norwegian aquaculture since the 80s, but due to its complex biology, it has been proven challenging for the halibut farmers to succeed commercially. One of the industry’s remaining challenges is the slow growth of the fish during the grow-out phase, which is thought to be mainly due to its low appetite/feed intake. Feeding fish in aquaculture is a complex process, since the farmer must balance overfeeding and underfeeding to obtain the best fish growth and at the same time limit negative impacts on water quality and the environment. When food is abundant, food intake is physically limited by the capacity of the stomach. The digestion and passing of food throughout the digestive system is affected by several factors including fish size, meal size, temperature and food quality. As the food is digested, and the stomach empties, the motivation to resume feeding increase. Thus, knowing the food transit time across the digestive system is valuable information to determine the time to the next meal and size of meal. To determine how much and how often halibut should be fed, this project will investigate how different feeding frequencies affect feed intake, growth, behaviour, appetite and digestion. As a first approach, the project determined the food transit speed across the digestive system, and based on these results, different feeding frequencies was tested and evaluated in a subsequent trial. The best feeding frequency was then tested and validated at a commercial-scale trial. In the first experiment, it was observed that 1 kg halibut at 8 °C emptied half their stomach contents after 8.5 hours, while full emptying took 74 hours. In the second trial 2 kg halibut were fed either every day, every second, or every third day. Therefore, it was expected to find feed remnants in the stomach before the next day's meal in the second experiment. However, to our surprise, all the fish had empty stomachs before feeding, regardless of the experimental feeding frequency. Even more surprisingly, since the stomachs were empty, it was expected that the fish would all eat similar meal sizes, irrespective of feeding frequency, and all fully utilize the capacity of the stomach. However, fish fed every second and third day ate meals twice and three times as large meals as those fed daily. By the end of the trial, all groups had similar growth. The intestine seems to play an important role in sending satiety signals during digestion of the feed, indicating a central role in appetite regulation for halibut. Gene expression analysis from all experiments showed that the digestive hormone pyy increased in fish with more feed in their intestines, while expression of the appetite hormone agrp in the brain decreased. In humans, it’s also known that pyy indirectly (via nerve pathways to the brain) inhibits agrp secretion, maintaining satiety and reducing the motivation to eat. The results show that there is no automatic link between frequent feeding and increased feed intake, growth, or profitability for halibut. The biology of farmed halibut suggests that one meal as seldom as every third day may be sufficient for halibut over 1 kg at 8°C. This can have several positive effects: more predictable appetite, reduced feed waste (and therefore less pollution in tanks), improved water quality, and equally good growth and economics. Reducing feeding frequency can free up labour for other tasks, making fish monitoring more efficient. The potential to optimize operations through less frequent feeding can contribute to both economic and environmental benefits in halibut farming.

Prosjektet har vist hvordan Atlantisk kveite sin appetitt påvirkes av ulike fôringsfrekvenser og gir fôringspersonell mulighet til å designe kunnskapsbaserte fôringsstrategier. Tubulah implementerte i 2021 (da Sogn Aqua) fôring hver andre dag på fisk over 1 kg. Sjeldnere fôring gir færre fôringsdager, hvor lokalt vannmiljø påvirkes negativt av fôrspill. Dette kan være gunstig for både fisken, mht bedre vannkvalitet (spesielt i lukkede systemer), og miljøet. Færre fôringsdager per enhet gir også mulighet for bedre oppfølging av fisken som fôres av fôringspersonell.

Feeding is one of the most fundamental aspect of rearing animals, as the farmed animal must grow in order to turn a profit. Slow growth of Atlantic halibut (Hippoglossus hippoglossus) above 1kg (grow-out) is viewed by halibut farmers as one of the remaining bottlenecks hindering growth in the industry. The development of optimized feeding strategies for grow-out halibut will therefore contribute to increase growth rates, minimize feed waste and reduce production cost. This has the potential to advance Atlantic halibut farming towards a more sustainable, economical viable and competitive aquaculture industry. Thus, this project will address current knowledge gaps regarding optimal feeding strategies for grow-out Atlantic halibut. The main goal of the PhD project is to determine how different feeding strategies can affect feeding behaviour, appetite, food intake and digestive performance, and identify the best-suitable feeding strategy which results in an improved growth and reduced food waste in grow-out Atlantic halibut. It will be important to investigate the gastrointestinal transit time in Atlantic halibut (WP1), as food intake is generally limited to the stomach capacity, and the time for the next meal limited by gastric evacuation time (GET). This will provide valuable information to design an optimal feeding regime regarding feeding frequency. The optimized feeding regime based on GET, will be further explored in terms of impact on water quality, feeding behaviour, migration patterns, performance, appetite control, digestibility, and fillet quality in a small-scale feeding trial (WP2). Finally, the effect from the optimized feeding regime will be assessed in a commercial large-scale rearing scenario as opposed to standard feeding protocols (WP3). The main findings will be available to the end-users and public through the planned peer reviewed articles and industry feeding protocol (WP4).