Muli‐faceted effects of land‐use change on streams

FINNBAR LEE - University of Auckland

(Environmental News #40)

The health of New Zealand’s rivers and streams has recently received considerable attention, with a strong focus on the effect of agricultural land‐use on water quality and the precarious state of many of our freshwater species. While land‐use change can have negative, direct effects on both water quality and biological communities, there are also less considered secondary effects that may turn out to be problematic.

Legacy of agricultural land‐use

Streams in catchments dominated by agricultural land‐use often have reduced riparian vegetation, altered flow regimes, higher and more variable water temperatures, elevated nutrient concentrations and increased primary productivity. The causal mechanisms by which agricultural land‐use alter in‐stream conditions are well understood. For example, agricultural land‐use can result in a reduction in riparian vegetation via clearing, which reduces stream shading and increases water temperature.

More light coupled with sediment and nutrient inputs from bank erosion and fertiliser application can increase stream productivity and macrophyte growth. Finally, proliferation of macrophytes can reduce instream velocities by choking flow in the channel.

Such changes typically lead to both habitat homogenisation and a reduction in water quality, which in turn affects the composition of biological communities. In degraded streams, community composition may shift but overall productivity often increases, resulting in resource‐rich systems that are not particularly suited to some of the original inhabitants, but that provide opportunities for potential invaders.

Mosquitofish ‐ the ‘plague minnow’

The western mosquitofish (Ganbusia affinis) is one of these. It is the most widely distributed invasive freshwater fish and ranks among the world’s 100 ‘worst’ invasive species.

It has been intentionally spread to control mosquito‐borne diseases as it preys on mosquito larvae, although its effectiveness is debated. Introduced to the Auckland domain in the 1930s, mosquitofish is now found throughout the North Island (the species made it to Nelson but was quickly exterminated), and favours the warmer waters of the northern half of the North Island.

Mosquitofish’s preferred habitat is warm, slow‐ flowing streams with dense macrophyte cover, exactly the type of conditions agricultural land‐ use results in. Recently, we showed that the distribution and abundance of mosquitofish is positively associated with the physiochemical and habitat changes to streams associated with agricultural land‐use.

Where conditions are suitable, mosquitofish can explode in numbers and end up dominating the invaded system (hence the moniker ‘plague‐minnow’).

They are veracious predators and internationally are associated with the decline of amphibians and fish (and they likely negatively affect many invertebrate species via predation). In New Zealand, they are known to compete with some whitebait species (Galaxiids), prey on invertebrates and affect ecosystem functioning.

Native freshwater fish

Like many of our other taxonomic realms, New Zealand’s freshwater species are unique and highly threatened. We have 72 species of fish (57 native, 15 introduced), of which 31 are endemic, a high ratio by global standards.

Forty of our 57 native freshwater fish, are threatened with extinction...

Seventy percent of native freshwater fish are threatened with extinction (again high by global standards) and one species, upokororo/ New Zealand grayling (Prototroctes oxyrhynchus) is now extinct.

The negative effects associated with agriculture and poor water quality are well known, but less obvious negative impacts, such as facilitating invasive species, can exacerbate the effects of land‐use change and further threaten our freshwater species.

While mosquitofish are not yet known to occur on Aotea, vigilance is essential as there is plenty of potential habitat for them on the island (Figure 1), including around Okiwi, Claris, Medlands and Tryphena, and anywhere else with significant land clearance.

These fish are frequently caught as pets by children and rereleased (technically illegal). If they were to make it here, several rare native fish (for example, shortjaw kokopu) would suffer from the mosquitofish’s presence.

Note: (1) Crow, S. 2018. New Zealand Freshwater Fish Database. Version 1.6. The National Institute of Water and Atmospheric Research (NIWA). Occurrence dataset https:// doi.org/10.15468/ms5iqu accessed via GBIF.org on 2018‐ 11‐12.

Native fish on Aotea

At least 10 of New Zealand’s 57 species of freshwater fish are found on Aotea, including multiple species of kokopu, inanga and long and short‐finned eels (Table 1). Four of the 10 species are recorded as declining (on a national level), and one, shortjaw kokopu, is listed as nationally vulnerable.

The first record of the Dart goby in New Zealand was made on Aotea, a self introduced species from Australia, classified as a coloniser. Four of the five whitebait species are present and the fifth species, giant kokopu, was formerly present but is now possibly locally extinct. Little information beyond occurrence is available, and the status of these species on Aotea is largely unknown. 

Native fish found on Aotea|Great Barrier Island and their conservation status, based on New Zealand Freshwater Fish Database records 1

Species (Scientific name): Conservation status

Banded kokopu (Galaxias fasciatus): Not Threatened

Bluegill bully (Gobiomorphus hubbsi): Declining

Common bully (Gobiomorphus cotidianus): Not Threatened

Dart goby (Parioglossus marginalis): Coloniser

Giant bully (Gobiomorphus gobioides): Naturally Uncommon

Inanga Declining Koaro (Galaxias maculatus): Declining

Longfin eel (Galaxias brevipinnis): Declining

Redfin bully (Anguilla dieffenbachii): Not Threatened

Shortfin eel (Gobiomorphus huttoni): Not Threatened

Shortjaw kokopu (Anguilla australis) Nationally Vulnerable