HYPOXIC THRESHOLD FOR BIGHEAD CARP (Hypophthalmichthys nobilis) JUVENILES: SIZE DOES REALLY MATTER
Bighead carp (Hypophthalmichthys nobilis) is a Cyprinid species that endures a wide range of environmental oxygen saturation, which has resulted in many morphological and physiological adaptations to survive. However, size specific information on critical tolerance limit of oxygen level is not well defined in scientific literature. A set of experiments was conducted with small (S, mean weight 0.22g), medium (M, 1.08g) and large (L, 10.1g) bighead carp to determine if there is an ontogenetic (size) impact on resistance to hypoxia. Two 3L respirometry flasks equipped with two side openings and one headspace opening were used for the experiment. Side openings were connected with water inlet and outlet pipes to measure the oxygen consumption (μmol O2/g fish/hour) from water (WO). The headspace opening was connected with nitrogen (N2) cylinders and Micro-Oxymax computerized respirometer (Columbus Instruments, OH) to measure the oxygen consumption from air (AO). Of the two flasks, one was used with fish (RF) and the other as control (atmospheric air in headspace prior to N2 infusion) or without fish (RC). Oxygen consumption by fish was measured every 40 minutes. Two stages were selected, first by keeping the oxygen level constant and measuring the consumption over time (S1) and second by gradually lowering the oxygen level until surface respiration or loss of equilibrium were observed (S2).
In S1, no significant differences (P<0.05) were observed in average WO over time (within the same fish size) by S (RC, 0.31±0.11; RF, 0.33±0.25) and L fish (RC, 0.59±0.06; RF, 0.47±0.11). However, M fish (RC, 0.56±0.05; RF, 0.27±0.11) differed significantly (P<0.05). In S2, WO differed significantly (P<0.05) in all sizes of fish when RC and RF were compared. Moreover, in S2, average WO and AO over time were compared (between size groups) where only AO in L differed significantly (S, 0.21±0.03; M, 0.20±0.1; L, 0.03±0.01, respectively). L fish are incapable of AO. Critical tolerance limit (surface respiration) and loss of equilibrium (LOE) were monitored and found that L fish reach LOE level earlier followed by M and S, respectively. After reaching LOE, fish were transferred to plastic tank and kept in 6.4 mg O2/L for 30 minutes. The recovery rate varied between size, S (65%), M (80%) and L (89%), respectively. The results indicate that WO and AO are size dependent; L fish reach LOE faster than M and S, but recovered at a higher rate.