Spring and summer in Austria for NOBROOK

This past May until August we finally ran the much anticipated experiments in the artificial stream channels in Lunz Am See, Austria!

Early spring until fall of 2023 for the NOBROOK project has been dedicated to the collaborative experiments with previous SEG member and project leader Libor Závorka, who currently works at Wassercluster Lunz. The facility we used for these experiments is situated with two separate stream channels where water from the nearby lake ran through and we could control parameters such as flow, water level and keep a somewhat constant temperature throughout the hot summer months. We subdivided each of these stream channels into four sections on each side, with buffer zones in between and at the beginning and end.

This allowed for us to have eight different enclosures where we placed different assemblages of either only brown trout or a combination of brown trout and brook trout. Furthermore, brown trout were either wild or hatchery-reared undergone different feeding regimes. All the brook trout we used were wild and caught in local streams.

Within the stream sections we created a gradient ranging from a “best” part of the enclosure upstream and the “worst” downstream, with a total of four different shelters. The upstream habitat had coarser substrate and more structures as well as presumably access to more invertebrates for the fish as we would put these in the buffer zones. We used portable antennas to assess which fish was inhabiting the best shelter upstream the most, i.e. holding the best position and most likely being the most dominant/best competitor of the group.

Additionally, each fish underwent several tests of cognition prior to entering the stream channels at a different lab facility where we could carefully monitor their movements and solution to tasks via camera footage. When they were done here, tagged and measured they were ready to enter the main experiment; the stream channels.

We then allowed for fish to interact freely for ten days under which we filmed them at several occasions during the morning, day, evening and night, as well as tracked them with both stationary and portable antennas. The behavioural interactions will be assessed from the video data, the antenna data has already given us valuable data on the dominance status and the social organization of the fish within the enclosures under both high and low water conditions (this was changed during the ten day run).

On the final day all the brown trout were sampled. They were weighed, measured, their stomach content collected, muscle samples for isotope analysis were taken as well as brain samples for fatty acid analysis. Additionally, we took their otoliths in order to examine the occurrence of vaterite (abnormal calcite polymorph, see picture below where most of the otolith consists of this).

All of this data will result in a lot of new insights into the interactions between brook trout and brown trout in terms of aggression and territoriality, as well as how different diets and hatchery conditions affect brown trout’s ability to compete and interact with others in a semi-wild environment.

Hopefully, a lot of interesting findings will come of this, but for now the massive amounts of data are being processed so stay tuned!