Sandeels with a full stomach swim for a longer time

The sandeel outnumber all other fish species in the North Sea during spring and early summer. Every day, massive numbers of the small fish forage for copepods in large schools but during the night, the sandeels seem to disappear. The explanation for their sudden disappearance is that when the sandeels are done foraging, the schools split up and the sandeels dig themselves into the sandy seabed where they spend the night well hidden from predators. During the colder months, the majority of the sandeels are usually permanently buried in the sand.

This peculiar behaviour gives fishery biologists a hard time. The sandeel is an important industrial fish and a major food source for many marine mammals, birds and commercially important fish like cod and salmon. Therefore, it is important to make stock assessments as accurate as possible. But the sandeels' game of hide-and-seek makes it difficult for fishery biologists to estimate how many sandeels are in an area and to predict when the sandeels are available to both fisheries and their natural predators.

"We make stock assessments based on a combination of how many fish are caught per day in the sandeel fishery and data from our own surveys. The problem is, that catch rates vary heavily from week to week because it is only possible to catch the sandeels that have left the sand, and we do not know how many that is" says postdoc Mikael van Deurs from the National Institute of Aquatic Resources (DTU Aqua) in Denmark who has done research on which factors determine how much time the sandeels spend out of the sand.

"Our results show that the more food that is available for the sandeels to eat, the longer it will be before they dig themselves into the sediment and are no longer available to fisheries and natural predators. Furthermore, the sandeels seem to respond to how much food has been around the last couple of days. That is, when food becomes scarce, the sandeels stay buried in the sand throughout the day," says Mikael van Deurs who has published the results in the peer-reviewed journal, Marine Biology, with co-authors Thomas Warnar, DTU Aqua, Jane W. Behrens, DTU Aqua, and John Fleng Steffensen, University of Copenhagen.

A research study at DTU Aqua shows that the amount of time the sandeels spend swimming in schools is determined by how much food they have in their stomachs. The sandeels in the picture are of the species Ammodytes tobianus. Photo: Thomas Warnar, DTU Aqua.

Ignores risk of being eaten

"It is a widely held view that foraging activity of fish is regulated by temperature, how much food is available and the circadian rhythm. This view is mainly build on statistical correlations that does not allow one to separate direct and indirect factors. In this study, we have tried to get round this by doing experimental studies in the lab where we can control all factors," says Mikael van Deurs.

In order to find out which factors determine how long the sandeels will be out of the sand and available to fisheries and predators, Mikael van Deurs and his colleagues caught two whole schools of sandeel of around 900 sandeels each and kept them in large video-monitored fish tanks in the laboratory. Here the researchers did different experimental studies in which they varied the amount of food as well as the water temperature.

The researchers found that the sandeels are willing to accept being out of the sand and vulnerable to predators for a longer time when they are rewarded with food.

"The sandeels pay an appreciable cost in terms of energy and predation risk when they swim back and forth from their refuge to go foraging. Therefore, it makes sense for the sandeels to take full advantage of good feeding conditions whenever they arise," explains Mikael van Deurs.

Sandeels do not bother to swim if no food is around

Interestingly, consecutive days of starvation made the sandeels spend their time buried in the sandy bottom.

"My guess is, that it is an adaptation to save energy and reduce the risk of being eaten while they wait for better times. In the lab, we observed that small schools of sandeels occasionally appeared from the bottom, for short periods, after many days of inactivity to explore the surroundings and assess whether food availability had improved," says Mikael van Deurs and continues:

"Apparently, the sandeels do not observe the temperature or how much food is present. Instead, their activity is determined by how much food they have eaten that day. That is, how full they are. The fuller, the more active they will be."

That means that even if the water temperature is optimal, the sandeels will reduce their activity if food is absent and they have empty stomachs. Furthermore, there can be times when there is plenty of food available but it does not result in the sandeels being out of the sand. For instance, if the visibility is poor and the sandeels cannot find the food, their stomachs will not be full and they will not be active. Likewise, if food is plentiful, the sandeels can be as active in the cold water in winter as in the warmer water in the spring.

"If routine analyses of the sandeels' stomach contents were done, it could potentially be used to predict how large a fraction of the sandeels would be out of the sand and available to both the fisheries and our surveys. It would be interesting to test our hypothesis from this study in the field in order to clarify whether analyses of stomach contents can be used to make the stock assessments even more accurate," says Mikael van Deurs.