Variations in stress response in fish, and how this affects their appetite

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It has been shown that stressed fish have a reduced ability to develop new brain nerve cells, which in turn results in impaired memory and a poor sense of orientation. Using stress resistant fish in fish farms actually gives a double benefit: increased animal welfare and increased profits for the operator.

Rainbow trout on the road
In the summer of 2005, several hundred rather special rainbow trout were sent from Windermere in Northern England to the Norwegian University of Life Sciences (UMB) at Ås. Their journey went by tank truck under the English Channel, by ferry from Denmark and finally along the narrow and steep roads to the research station at Solbergstrand. Including sorting and marking prior to departure, the entire trip lasted 5 days. However, these fish have a much older history. These trout belong to distinct genetic lines, resulting from selection trials that were initiated by English fish physiologists in the early 1990s.

Fish personality
In these studies, it was discovered that in salmonoids, stress sensitivity can vary considerably between different individuals of the same species. If disturbed or frightened, some fish panic and show increased levels of the stress hormone cortisol. Under the same conditions, though, other individuals show a much calmer behaviour and low cortisol levels. Studies over several generations have shown that these traits remain relatively constant in individuals over a period of time, and that they are heritable.

These results were the basis for developing so-called high and low response lines of rainbow trout. Scientifically, these two lines are highly interesting, since the same biological mechanisms play a vital role in the development of various personality types in humans. A couple of recent review articles give a more detailed presentation of these issues. Here, we wish to present the research in this field currently being performed at the Department of Animal and Aquacultural Sciences (IHA). Among others, we are focussing on the potential benefits of selecting for stress tolerance for the fish farming industry.

Stressed fish lose their appetite
One of the most obvious behavioural differences between stress resistant and stress-prone fish is how stress and environmental disturbances affect food intake. Anyone that has gone fishing knows how easy it is to frighten fish and thus avoid making the “big catch”. For the cold-water salmonoids, this phenomenon is especially marked: they have a low metabolism, and are thus able to get by without food for longer periods if necessary. Appetite is to a large extent controlled by the same pheromones that regulate the physiological stress response, and individuals with high cortisol levels actually did show a greater and more prolonged reduction of food intake following stress.

Applied to the conditions in fish farms, this means that more feed is wasted if the operator does not carefully adjust the amount of feed after the stock has been exposed to some kind of stress. In fish farm operations, optimized utilization of feed resources is important both ecologically and economically. We therefore studied the difference in feed efficiency over time between the stress resistant and the stress-prone fish populations. Results showed that when both populations were given the same amount of feed, more feed was wasted in the unit containing stress-prone fish. After no more than 12 days of observations, the total feed wastage in the stress-prone population was twice as high (see Figure 1). Further, we observed that the feed that actually was ingested gave a higher weight gain in stress resistant fish. This may be linked to the metabolic effects of stress and cortisol, but it has also earlier been shown that stress affects the intestinal flora and the digestion in fish, much the same as in humans. Our further research aims to study how the selection for improved stress resistance can benefit the fish farming industry as a whole.

Fish memory also impaired
Our study also focuses considerably on aspects of fish welfare, since the combination of stress and steroid hormones such as cortisol have a significant impact on emotional and cognitive processes. For example, it has been shown that stressed fish have a reduced ability to develop new brain nerve cells, which results in impaired memory and a poor sense of orientation. It is difficult for the human observer to determine if and to what degree this affects fish welfare, but presumably the use of stress resistant farmed fish could have a double benefit: improved fish welfare and increased profits for the operator. And finally, perhaps we can learn something about ourselves by studying the causes of individual differences in fish.

Published: 10.11.06
Updated: 14.05.09
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