This paper, authored by Brett Glencross, IFFO's Technical director, was published in the October edition of International Aquafeed (2024)

I recent read an excellent review paper in the leading journal Reviews in Fisheries Science and Aquaculture that examined a highly complex and often overlooked part of the nutrition equation – feed intake regulation. While most nutritionists argue the details over the levels of specific amino acids, omega-3, and vitamins among other things, the story rarely falls to the question “what do the animals actually like”? After all, if they won’t eat what you’re offering them, then those finer details of the nutrient balance become a secondary issue.

However, what the review in question points out is that the feed intake story is one of two main regulatory systems; homeostatic regulators, which are those things linked to maintaining energy balance (homeostasis) which for the most part is controlled by hormones like ghrelin, leptin and cholecystokinin. Those regulators tend to be longer term responses though; think here of days, weeks, months, and have the primary objective of survival. The other regulatory system is the hedonic regulators, which are those linked to sensory responses like sight, smell, taste and so on. These regulators tend to be shorter term responses; think here of seconds, minutes, hours, and days and have the primary objective of pleasure. So, we have two complementary regulatory systems governing how feed intake is controlled, with separate systems working on short and long-term objectives, but each far from being mutually exclusive of the other.

A key issue here from an aquaculture perspective is how to navigate those regulatory systems to optimise intake. And by optimise, we mostly mean to maximise intake, as most evidence supports that so long as we can ensure intake of feed, then we get the commensurate conversion of that feed into growth. Logic also contends that we must navigate the short-term before the long-term. Afterall you won’t get to the days-weeks-months, without navigating the shorter-term constraints first. So, what does influence those hedonic pleasure regulators in fish?

Most studies on factors that influence smell and taste in fish point to small molecular weight volatile compounds that tend to be soluble in water. Though there is some recent evidence that supports that omega-3 fatty acids can also influence this part of the story. Defining the nature of those volatile compounds depends on the responses by the animal to which they are exposed, but fortunately there is a decision hierarchy that allows some objectivity of this process which was published in another review a few years back [https://doi.org/10.1111/anu.13138]. That hierarchy helps to define stimulants from attractants from ingestants and so on. One notable aspect of that sensory response process is that while there are some common aspects across fish species, different species do sometimes appear to respond to different things. Afterall the palatability and attractiveness of the food we feed and how each species perceives that food comes down to a matter of taste.