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