A review of dissolved oxygen and fish species in Waikato rivers revealed very low dissolved oxygen levels in some areas, which were impacting local fish populations. NIWA has now produced revised safe levels of dissolved oxygen for fish, which will help inform future environmental planning and resource consents and help to keep New Zealand rivers full of healthy fish.
The problem
Lowland streams are important habitats for lowland fish species, but because many of New Zealand’s native freshwater fish species are diadromous, i.e. have to migrate between the sea and freshwater to complete their life cycle, lowland streams are also critical migratory pathways to and from upstream habitats. Consequently, maintenance of suitable conditions in lowland rivers and streams, for both fish passage and resident fish, is important.
The concentration of dissolved oxygen in water is a key component of fish habitat quality and a critical measure of stream health. Evidence is beginning to emerge showing that, for some lowland streams, water quality, including dissolved oxygen, is declining during summer months. This issue is exacerbated by high water temperatures, proliferation of aquatic weeds, pollution and reduced stream flows. However, little was known about the extent of these changes or whether fish and fish habitats in small streams were being compromised. Nor was it known whether a low dissolved oxygen environment could inhibit migrations of fish through these reaches.
Knowledge of these factors is important for water resource managers to determine appropriate protection levels when making decisions on allocating water and granting consents for discharges.
The results
Monitoring of dissolved oxygen, flow and fish communities was carried out in three catchments in the Waikato region during late summer low flow conditions in 2010. Results showed that at some sites hypoxic (low dissolved oxygen), or even anoxic (no dissolved oxygen) conditions, occurred and lasted in some cases for several weeks during the monitoring period.
The ability of fish to survive in hypoxic environments depends on how low the dissolved oxygen goes, how long they are exposed to low dissolved oxygen, the type of fish, their age and how healthy they are. They will also be affected by other environmental conditions e.g., temperature.
At the sites where dissolved oxygen was poorest, resident fish communities were typically characterised by a low diversity of species and often a relatively low abundance of fish. These communities were also dominated by the most tolerant fish species and frequently had a higher proportion of non-native pest fish species present. Where dissolved oxygen levels were high, sensitive species were more common. However, sensitive species were also caught at some of the impacted sites, albeit in lower numbers than the less impacted sites. Sensitive species were also captured upstream of impacted sites. It appears therefore that some fish are able to make temporary use of the impacted habitats, probably by adapting their behaviour or because migration occurs before the onset of low dissolved oxygen conditions.
A review of the literature indicated that knowledge on the specific responses of New Zealand native fish species to reduced dissolved oxygen is relatively limited and focuses on lethal effects. The measurements also only cover a limited range of temperatures, typically 15°C, whilst temperatures recorded during this study frequently reached 20°C or greater. This will impact on the survival success of fish under low dissolved oxygen conditions. Sub-lethal effects, such as reduced growth or poor reproduction, may be more important in determining long term responses to low dissolved oxygen, but were infrequently considered. Despite these limitations, the magnitude and duration of hypoxic conditions at some of the stream sites surveyed were enough to exceed the lethal thresholds identified for all but the most tolerant of New Zealand fish species.
The solution
Based on a combination of the known lethal tolerances of New Zealand species and the more detailed research into the effect of low dissolved oxygen on production in trout, revised dissolved oxygen criteria are proposed for New Zealand’s fish communities (Table 1). The proposed criteria have been designed to address both how long dissolved oxygen can be low, and how low it can go without causing significant adverse effects.
Table 1: Recommended dissolved oxygen levels for protection of freshwater fish. Imperative protection level is the minimum recommended protection level. Guideline protection level should be target protection level or minimum where salmonid species are dominant.
| Dissolved Oxygen | Early life stages | Adults | |
|---|---|---|---|
| 30-day mean (mg L-1) | Guideline | 9.0 | 8.0 |
| Imperative | 6.5 | 6.0 | |
| 7-day mean (mg L-1) | Guideline | 7.5 | 6.5 |
| Imperative | 5.5 | 5.0 | |
| 7-day mean minimum (mg L-1) | Guideline | 6.0 | 5.0 |
| Imperative | 5.0 | 4.0 | |
| 1-day minimum (mg L-1) | Guideline | 6.0 | 4.0 |
| Imperative | 4.0 | 3.0 |
Two levels of protection are proposed to account for different fish communities and to provide both minimum and optimum levels of protection. Imperative targets are the minimum acceptable criteria for minimising the likelihood of significant detrimental effects for the majority of fish species. The guideline criteria target optimising protection of fish communities, but should also be the minimum requirement for communities dominated by salmonids or other sensitive species. The proposed criteria should be achieved most of the time, but it is recognised that natural variation may result in some deviation beyond these thresholds. Where natural conditions alone result in dissolved oxygen concentrations less than the proposed criteria, an imperative criterion of 90% of the natural concentration is proposed, with a guideline criterion of no reduction in the natural concentration.
The significance of conditions that fail to meet the recommended levels will depend on the magnitude, duration, frequency of occurrence and spatial extent of the breach, along with the biological significance of the site. Evaluation of an event’s significance will typically be on a case-by-case and site-by-site basis. When the dissolved oxygen criteria are not met on a regular basis, action will be required to prevent deterioration of ecological communities. This should include consideration in the process of ecological flow determination.
