Investigating the low O2 tolerance and likely response of Hāpuku (Polyprion oxygeneios) to hypoxia

Abstract

Over the last 50 years anthropogenic activities have lead to an increase in the frequency and severity of low oxygen events (hypoxia) in coastal marine ecosystems. Hypoxia can affect fish in many ways, either at the individual level by, for example, affecting stress, growth and/or swimming speed, or at the population level due to changes in species distribution and survival. Hypoxia may therefore pose a serious threat to fish held under inescapable conditions in seacage aquaculture. This study investigated the hypoxia tolerance of the wreckfish Hāpuku (Polyprion oxygeneios) to generally add understanding on the effects of hypoxia on the physiology and behavior of teleost fish, but to also provide a range of physiological and behavioural O2 thresholds for potential aquaculture endeavors. The metabolic rate of P. oxygeneios in response to declining O2 saturations was estimated at 14oC by measuring the rate of O2 consumption in terms of standard metabolism (SMR), maximal metabolic rate (MMR) and aerobic metabolic scope (AMS). These measures were subsequently used to resolve the critical O2 saturation level (Scrit) of the species, which is considered a useful measure of hypoxia tolerance. The onset of physiological stress during hypoxia was measured according to changes in plasma cortisol, lactate and osmolality. The behavioural swimming response of fish was assessed using video tracking software. Results indicate that the critical oxygen saturation level of Hāpuku, Scrit, was 34.9% which preceded a small but significant increase in swimming speed between 20% and 30% O2. Cortisol levels increased significantly at 50% O2 (P-value < 0.05), lactate rose at 20% (P-value < 0.05), but no significant change in plasma osmolality was observed and all measures of stress during hypoxia were considered sub-maximal. As the Scrit level measured in this study is relatively high compared with other species (e.g. the Scrit of Atlantic Cod, Gadus morhua, at comparable temperatures is ~ 20% O2 saturation), the results suggest that Hāpuku is not a hypoxia tolerant species. However, since Hāpuku did not show maximal levels of stress below their apparent Scrit limit, even with an increase in swimming speed (i.e. energy demand), the hypoxia tolerance Scrit limit of Hāpuku resolved in the current study is suspiciously high. Evidence from another published study suggests that the SMR of the 448.8 (±50.6) g fish used in the current study may have been overestimated and that a Scrit value of ~ 20% may be more realistic. The caveats surrounding this analysis are discussed but, from a broad perspective, the overall conclusion is that Hāpuku do not appear to be very tolerant to hypoxic conditions. The current study provides a first insight into P. oxygeneios potential hypoxia tolerance. By investigating both the physiological and behavioural responses it is possible to understand how a change in one can affect the other in an environment of low DO.