The smell of water changes fish behaviour

A team of researchers carried out a novel experiment in which they observed the behaviour of ocellated wrasse (Symphodus ocellatus) juveniles, a fish very abundant in coastal waters of the Mediterranean, exposed to different olfactory stimuli.

The study included scientists from the Balearic Oceanographic Centre of the Spanish Oceanographic Institute (IEO), in collaboration with the Marine and Aquaculture Research Laboratory (LIMIA) of Balearic Islands Government and the Mediterranean Institute of Advanced Studies (CSIC-UIB IMEDEA).

This work, published in the Journal of Experimental Marine Biology and Ecology, is the first description of the use of chemo-receptors in a Mediterranean species in a selection of water masses according to the odour they give off.

In total, the responses of the fish to five different water masses with different odours were tested: odourless control water, posidonia oceanica-scented water, algae-scented water, predator-scented water and a last one with same species smell.

Although the fish did not show clear preferences for any water body in terms of the time they remained in each of them, there were clear differences in their behaviour. Fish moved faster and with more abrupt movements in water bodies containing chemical information from the habitat of oceanic posidonia, algae and predator and more slowly in the water containing the chemical information of individuals of the same species.

"It is possible that the odours of posidonia, algae and predator will cause a more active behaviour, related to the search for food and the escape of the predator, respectively", explains Adam Gouraguine, predoctoral researcher at the University of Essex at the Oceanographic Centre of Baleares of IEO and first author of the work.

"In the case of water with the chemical information of the same species, it is possible that individuals felt calmer thus showing a slower movement," he added.

In order to carry out the experiment, a flow selection system was used, which has the capacity to pass two masses of water of different chemical properties to a small sized compartment without mixing them.

The device used in the water mass selection experiment was equipped with a video camera that allows recording and following the movement of the fish inside the selection compartment. The analysis consisted in testing the responses of each fish within the compartment both in the time they stayed in each water body, and the speed and number of sudden movements.

There are numerous studies on the olfactory detection of chemical compounds by different species of fish, but the importance of olfactory detection of these compounds in the selection of habitats in the marine environment is not yet clear.

This work makes a significant contribution to the study of the selection of water bodies by fish, as it provides a new analysis methodology.

Besides the conventional analysis based on the time spent in each water body as the main indicator of preference, this study also analyzed the speeds and the number of sudden movements of each fish. Thus scientists have been able to demonstrate that juveniles of Symphodus ocellatus are able to detect and react to chemical information present at sea, however the behavioural response is complex and is not reflected only in residential times, as it has been seen in previous studies of tropical species.

"It is therefore imperative to include speed analysis in future studies in order to study habitat selection using this type of methodology," Gouraguine concludes.