Introduction
Yellowtail kingfish is a fast-growing athlete that requires sufficient water flow over the gills to meet the oxygen demands. It is successfully being farmed in marine RAS but energy economy under these conditions poses challenging metabolic issues. Moreover, swim-training at optimal speed enhances body growth, lowers feed conversion ratio, increases cardiac output capacity (Palstra et al., 2015) and represents a promising approach for application at the farm.
In this study, we attempted to gain more insight into the energy economy by investigating heart rate and acceleration, as determined by application of Starr-Oddi loggers, at increasing swimming speeds during a swim-fitness test, and during an induced stress challenge test by repeatedly lowering of water levels (Svendsen, 2021). Oxygen consumption was measured and locomotory behavior (including tailbeats and respiration frequencies) monitored by high speed camera recordings during the swim-fitness test and parameters were related to heart rate and acceleration. Ultrasound was used as additional determinator of heart rate, although under anesthesia.
Materials and methods
Fish – Yellowtail kingfish from the Kingfish Zeeland farm were transported to the animal experimental facilities of CARUS in Wageningen. N= 16 fish were used for surgical implantation of the loggers and N= 14 fish were controls without a logger. Implanted fish was allowed to recover for 10 days before experiments were started.
Loggers – DST milli-HRT ACT (Starr-Oddi, Gardabaer, Iceland) loggers were implanted (Fig 1AB). Heart rate was derived from the Electrocardiogram (ECG) signal at 200 Hz recorded every 10 min as beats per minute (bpm) and acceleration as average external acceleration value (AvEA in milli-g-force, which is an average of 600 measurements over 1 minute) recorded at 10Hz
Swim-fitness test – Fish were swum in series of three in three Blazka type swim tunnels (van den Thillart et al., 2004; Fig. 1C). Fish were kept at rest and then swimming at incremental swimming speeds of 0.2, 0.4, 0.6, 0.8 and 1.0 m s-1 for one hour each. The first N= 8 implanted fish were immediately dissected for the loggers after the swim test, the next N= 8 implanted fish were used for the stress challenge test.
Stress challenge test and cortisol – During the stress challenge, N= 8 implanted fish were kept with the controls in a tank and then exposed to four conditions: reducing water level and (1) filling up immediately, (2) after 1 min, (3) after 5 min, and (4) after 5 min with chasing the fish (Fig. 1DE).
Ultrasound – An Esaote MyLabFive Vet ultrasonography unit (Esaote Europe BV, Maastricht, the Netherlands) with a 18MHz LA435 ultrasound transducer and Pulsed Wave and color (CFM) Doppler was used as second methodology for determining heart rate, under anesthesia after swim fitness and stress challenge tests.
Results and discussion
When fish were subjected to the swim-fitness test, implanted fish measured 33.2 ± 2.2 cm total length (TL) and weighed 652 ± 152 g body weight (BW), control fish measured 34.7 ± 2.2 cm TL and weighed 729 ± 122 g BW. Optimal swimming speed (Uopt) for implanted fish was 0.84 ± 0.06 m s-1 at a Cost of Transport (CoT) of 169 ± 32 mg kg-1 km-1, for control fish Uopt was 0.82 ± 0.11 m s-1and CoT was 178 ± 40 mg kg-1 km-1. There was no impact of the logger on the oxygen consumption during swimming.
Base line heart rate of fish in the tank was 110 ± 19 bpm and accelerations were AvEA= 12 ± 4 milli-g. Heart rates of fish in the swim tunnel were higher but stable between 126 to 139 bpm up to swimming at 0.6 m s-1 and then increased up to 155 ± 2 bpm at 0.8 m s-1 and 162 ± 7 bpm at 1.0 m s-1. Accelerations increased linearly with swimming speed from AvEA= 11 ± 1 to 26 ± 4 milli-g. The first, second and third lowering of the water level increased heart rate up to 138-144 bpm and accelerations were up to AvEA= 26 milli-g during the first but decreased to AvEA= 19 and 15 milli-g during the second and third stress induction. The fourth lowering of the water level that included the chasing caused the highest heart rates at 186 bpm with accelerations of AvEA= 44 milli-g. When anaesthetized, the eight implanted fish that were subjected to the stress test had a heart rate of 47 ± 11 bpm as determined by ultrasound.
An integrative data analyses will be presented also including the locomotory behavior results which are currently being analyzed.
Acknowledgements: This study was funded by WUR’s own investment under the Next Level Animal Sciences innovation initiative https://www.wur.nl/en/Value-Creation-Cooperation/Next-Level-Animal-Sciences.htm.
References
Palstra, A.P., et al. (2015) Forced sustained swimming exercise at optimal speed enhances growth of yellowtail kingfish (Seriola lalandi). Frontiers Physiology 5:506
Svendsen, E., et al. (2021) Heart rate and swimming activity as stress indicators for Atlantic salmon (Salmo salar). Aquaculture 531, 735804.
Van den Thillart, G., et al. (2004). Endurance swimming of European eel. J. Fish Biol. 65, 312–318.