Catlins FMU (Freshwater Management Unit)

The dominant soil is brown soils and podzols. Brown soils occur with moderate structure and drainage making these soils suitable for cultivation. Podzols occur where acid leaf litter and high rainfall combine, so mainly in indigenous forest. They have low natural fertility and root growth and require fertiliser for grassland farming.

Catlins FMU land use summary

Water use is relatively low compared with water availability, as rainfall is high and evapotranspiration low. Calculating naturalised flows for allocation is unnecessary and observed flows are used instead.

ORC monitors flow at two river sites in the Catlins and Owaka Rivers, with hydrological modelling providing flow estimates for remaining catchments. Most water use occurs within the Puerua River and Owaka River catchments, mainly for rural supply and dairy sheds. Water use compared to availability is the highest in the Puerua River catchment.

Good water quality is expected due to the intact nature of headwaters and native vegetation, however cleared valleys allow intensive farming activities and there are indicators of degraded water quality for some monitored parameters. ORC monitors water quality and rivers and stream ecology for four rivers in the Catlins: Catlins River, Owaka River, Maclennan River and the Tahakopa.

Combined results provide an indication of state of a river or stream. Suspended fine sediment and E. coli results didn’t meet the national bottom line according to the National Policy Statement for Freshwater Management in Owaka and Tahakopa sites categorised as degraded overall.

Trend analysis results for the Catlins River are mixed, the 20-year trend indicates high likelihood of degrading water quality for most attributes, while the 10-year trend indicates likely improvements for many of the attributes.

Catlins FMU Water Quality State and Trends Technical Report

Introduction to water quality

Catlins FMU water quality summary

Most groundwater is hosted in fractured rock. This differs from most of Otago, where groundwater is mainly found in alluvial deposits. However, groundwater in the Puerua catchment is found in alluvium of the Lower Clutha delta.

Groundwater use in the Catlins is generally low, with only 20 completed bores in the area, for community supply, stock, domestic supply, and industry. There are only five consents to take groundwater with low rates of take. 

The Catlins has extensive freshwater biodiversity values including rare and threatened species, and vulnerable ecosystems such as ephemeral wetlands, dune slacks, estuaries, lake margins and wetlands. These ecosystems contribute a lot to national biodiversity, however, are often threatened by processes such as land use change and invasive species. Little is often known about their extent and/or condition.

A diverse range of native freshwater fish, invertebrates, birds, plants, and a bat depend on freshwater ecosystems in the Catlins. The 19 native freshwater fish include two eels, five bullies, four migratory galaxias (whitebait), lamprey, smelt, torrentfish, estuarine triplefin, black flounder, and three non-migratory galaxias. The threatened freshwater fishes are non-migratory galaxias and the lamprey. Freshwater invertebrates include freshwater crayfish, freshwater mussels, and shrimp.

Many native birds depend on freshwater ecosystems, either as permanent or mobile residents. Threatened birds include Australasian bittern, the black stilt, and the at-risk, black-billed gull. Many plants are freshwater-dependent, including the threatened heart-leaved kohuhu. The introduced sports fishes found are brown trout and perch. Information is often missing at a species level, particularly for freshwater invertebrates, non-vascular plants, and algae. Many native freshwater species are under threat and continue to decline.

Estuaries are highly productive ecosystems but extremely sensitive to human activity. The Catlins contains four estuaries: Waipati/Chaslands Estuary, Tautuku River Estuary, Tahakopa Estuary, and the Catlins River Estuary. The Catlins River Estuary is most affected by human activity and experiences algal growth problems, increased sedimentation and poor sediment oxidation compared to the other three less-modified catchments.

The Catlins has some of Otago’s more unmodified estuaries such as the Tautuku and Waipati/Chaslands estuary which are fringed by native forest and contain a natural transition between estuary and dunes/salt marsh/wetland into native forest and low mud content. While some good examples of more “natural” estuaries exist, impacts from sedimentation and nutrients can accumulate and degrade these sensitive environments.