ALBERT

Description: The present study investigated the response of an Arctic keystone species, Polar cod, Boreogadus saida, to hypoxia and warming. We measured the respiratory capacity (standard, routine and maximum metabolic rates, SMR, RMR, MMR, aerobic scope, critical oxygen saturation (Pcrit)) and swimming performance of Polar cod under progressive hypoxia at 2.4 °C and after warm acclimation to close to the species' thermal limit (10.0 °C) via intermittent-flow and swim tunnel respirometry. Polar cod for the experiments in this study were caught in October 2018 during RV HEINCKE expedition HE519 in Billefjorden (78°34'59.99 N 16°27'59.99 E). In total 46 fish were selected for this experiment and divided into two groups. The first group (n = 30, group C) measured 19.7 ± 1.3 cm and 39.6 ± 9.5 g and was kept at habitat temperatures. The second group (n=16, group WA) with an average size of 19.8 ±1.9 cm and weight of 41.3 ± 10.1 g, was progressively acclimated to 10.0 °C (warming rate: 1.5 °C month-1) and then kept at this temperature for several months. For both temperatures, SMR and RMR were measured using seven fully automated respiration chambers (Loligo Systems ApS, Denmark), submerged in two connected thermoregulated tanks (170 L). Oxygen consumption was measured in 12 different PO2 conditions, 100, 75, 65, 55, 45, 35, 30, 25, 20, 15, 10 and 5% air saturation (n = 7 per conditions). Each oxygen saturation was maintained for two days and two nights, containing approximately 80-100 measurement phases during which fish were left undisturbed to ensure proper determination of the SMR as they habituated to the experimental conditions. Only the metabolic rate data from the second night were used for SMR calculation. The respirometers' measurement cycles were 5 min flush, and 30.5 min measurement in the cold group (C) and 2.5 min flush, and 17.5 min measurement in the WA group. The metabolic rate and swimming performance of B. saida under hypoxia were recorded following a critical swimming speed (Ucrit) protocol (Brett (1964), modified after Kunz et al. (2018)) applying the same PO2 steps as in the respiration chambers (100, 75, 65, 55, 45, 35, 30, 25, 20, 15 and 10 % air saturation). A Brett-type swim tunnel respirometer of 5 L (test section 28 x 7.5 x 7.5 cm, Loligo Systems ApS, Denmark) was used to measure the swimming performance of B. saida (n=6-7 per PO2 treatment). The fish were transferred to the swim tunnel, seven days (C) or three (WA) days after the last feeding. After an acclimatisation period of 1.5 h in stagnant water, water velocity was increased to 1.2 BL (body length)/sec for 25 min. Afterwards, the velocity was increased to the first measurement velocity of 1.4 BL/sec before starting the Ucrit protocol. Each velocity step contained an ṀO2 measurement cycle, comprising a 60 s flush phase followed by 120 s of stabilisation and an 8 min measuring period, after which water velocity was increased by 0.15 BL/sec. Each measurement period thus returned 480 single oxygen recordings from which oxygen consumption (ṀO2) was calculated. The swim chamber was covered to minimise disturbance. The step-wise water velocity increase was performed until exhaustion, when the fish completely refused to swim and remained inactive for more than three minutes. The maximum metabolic rate was determined for each PO2 level inside the swim tunnel. To determine the gait‐transition speed Ugait (the switch from strictly aerobic to anaerobically supplemented swimming) (Drucker and Jensen, 1996), kick-and-glide swimming (so called "bursts") (Videler, 1981) was documented. In kick-and-glide swimming, thrust generation is supplemented by anaerobic muscle contractions and mainly white muscle is used. All bursts were counted and the corresponding time during the swim trial was documented. The critical swimming velocities (Ucrit) of the fish were calculated according to Brett (1964). After Ucrit was reached, the velocity was immediately decreased to the basic weaning velocity of 1.4 BL/sec and the fish stayed in the swim tunnel for another 10 min before being transferred back into their tanks. Blank respiration in the swim tunnel accounted for less than 2% ṀO2. The swim tunnel was cleaned daily.