Aquaculture moves off-shore: using sensor tags to study the physiological performance of fishes in unsteady flow

In the light of climate change and environmental sustainability challenges, marine fish farms are compelled to move from traditional near-shore sites to more exposed open ocean areas. These offshore locations are characterised by unsteady flow conditions, which deviate from the relatively stable environments that near-shore fish farms typically offer. Although it is known that fish (and other aquatic organisms) display wide variations in energy usage in uncontrolled open ocean environments, not much is known about how the flow conditions in these areas will affect the swimming capabilities and overall energy expenditure of farmed fish.

“Water flow patterns in nature are typically complex,” says Wisdom Agbeti, first author of the paper and PhD student at the Animal Breeding and Genomics (ABG) group. “There may be variations in the speed and direction of the flow, or in turbulences of varied strengths. Knowledge on how these changing flow conditions affect the physiological performance of cultured fish is lacking, as performance of cultured fish species is typically studied under steady flow conditions.”

Steady vs unsteady water flow conditions

To gain more insight into the effect of unsteady water flows on the swimming performance of cultured fish species, Wisdom and his colleagues from Wageningen and the Plant and Food Research Institute in New Zealand investigated the effects of swimming in steady and unsteady flows at increasing swimming speeds on Atlantic salmon. They used a Loligo swim tunnel and implanted sensor tags to compare the oxygen consumption (MO₂), locomotor behaviour, and overall dynamic body acceleration (ODBA) of the fish in steady and unsteady flow conditions.

“Energy expenditure resulting from movement is regulated by muscle contractions that generate accelerations in the body,” says Wisdom. “We used tri-axal accelerometer sensors¹ to monitor these accelerations, which gave us valuable information about the activity-related energy expenditure of the fish. Because an unsteady flow will require a constantly alternating pattern of acceleration and deceleration, we hypothesised that swimming in unsteady flow would increase the energy cost of movement.”

Results

Results of the study showed that the MO₂ of the fish whilst swimming in unsteady flows was significantly higher (15-53%) than when swimming in steady flows. Also, Wisdom and his colleagues found significant interaction effects of ODBA with flow conditions and swimming speed: ODBA was strongly and positively correlated with swimming speed and MO₂ in both unsteady and steady flow. In addition, results showed that in an unsteady flow condition, ODBA increased twice as fast with MO₂ compared with steady flow conditions. Put simply, it was energetically more costly for the fish to swim in unsteady flow than it was swimming in steady flow. Data are currently used for designing a salmon digital twin, modelling the individual energy economy, within the Next Level Animal Sciences innovation initiative.

Read the full publication for more detailed information about the results.

¹Tri-axial accelerometers allow for remote in situ assessments of animal movements and are often used in the marine realm.