The Response of Juvenile Snapper, Pagrus auratus, to Environmental Hypoxia and Oxygen Profiles in the Mahurangi Estuary, Auckland Region

Abstract

Hypoxia is not uncommon in estuarine systems, but its frequency and magnitude is increasing as a result of anthropogenic activities (e.g. eutrophication). This is a global cause-for-concern given that even non-critical levels of hypoxia can have significant effects on the abundance, distribution and productivity of marine fishes living in these systems. Despite the general definition of hypoxic water being ~25% oxygen saturation, detrimental effects occur at or near levels of 50% oxygen saturation. The aim of this research was to assess whether the fish nursery, the Mahurangi Estuary, is susceptible to seasonal hypoxia, and address whether there may be a link between hypoxic events and the distribution patterns of a commercially important teleost, the snapper Pagrus auratus. In order to infer the snapper population response to estuarine hypoxia, the individual response of snapper to hypoxia must be characterised. This was achieved by integrating the aerobic physiological performance, and the behavioural avoidance and physiological stress response of snapper to hypoxia. The Mahurangi Estuary is hypothesised to become hypoxic at depth during summer months as a result of thermal stratification, shifts in oxygen demand and supply (respiration/photosynthesis) and prolonged residence of nutrient loaded waters. The Mahurangi Estuary was monitored from November 2009 to October 2010, with measurements of dissolved oxygen (DO), temperature and salinity taken throughout the estuary. Hypoxic waters of ≤ 50% DO was not observed at any time or site during the entire sampling period. Respirometry was used to determine the critical oxygen tension (Pcrit) of juvenile snapper where fish make a transition from being an oxygen regulator to oxygen conformer. The Pcrit of 70- 150g and 10-30g snapper were 28.7% and 35.8% at 15°C and 19°C respectively, and represents a moderate degree of hypoxia tolerance. The behavioural response of snapper was assessed using a choice chamber experiment where individuals where given a choice between hypoxic and normoxic environments. Snapper of both size classes had avoidance thresholds (Pavoid) below their Pcrit (70- 150g Pavoid= 16%, 10-30g Pavoid= 12%). The 70-150g also had significantly higher lactate levels (P<0.05) indicating the use of anaerobic metabolism presumably because metabolic scope limits were surpassed. Residence time in hypoxia was important for survival in severe hypoxia as regular excursions into normoxia allowed for physiological recovery. Swimming did not assist avoidance behaviour. These results suggest that snapper employ a non-adaptive high-risk behavioural strategy to hypoxia. From an ecological perspective, juvenile snapper may not adapt to extensive severe hypoxia in the wild. Fortunately, the pristine water quality of the Mahurangi Estuary does not currently present a physiological challenge for this species.