New Zealand has a freshwater fish fauna characterized by high levels of national and local endemism and which is threatened by anthropogenic stressors including habitat destruction or deterioration, commercial harvest, pollution and interactions with invasive exotic species. Significant expansion of New Zealand's dairy production has recently created further deterioration of lowland water quality and greater pressure for water allocation in drier eastern regions of the South Island. New Zealand has large freshwater resources and its climate is predicted to experience less dramatic changes in mean annual temperature and precipitation than many other regions of the world as a result of anthropogenic climate change. Predicted changes in regional climate and further expansion of the dairy industry, however, will impose similar pressures on freshwater resources in northern New Zealand to those already acting to threaten freshwater biodiversity in the eastern South Island.
It is now acknowledged that the global climate is warming at an historically unprecedented rate and that anthropogenic emissions of greenhouse gases are one of the key forces driving global warming (Pachauri & Reisinger, 2007). Various atmospheric and oceanic global circulation models (GCM) predict a range of warming during the 21st century with effects likely to be greater at temperate and polar latitudes than in tropical and subtropical regions, although such GCMs show poor ability to predict fine-scale regional change. Recent increases in annual temperature, since 1970, are greater in northern temperate latitudes than in other regions (Pachauri & Reisinger, 2007). Widespread acceptance of a warming climate is accompanied by acknowledgement that the Earth is currently experiencing a major anthropogenically induced extinction event, with freshwater biodiversity declining at a significantly greater rate than either terrestrial or marine biodiversity (Abell, 2002; Jenkins, 2003). Freshwater fishes comprise >13000 species and c. 45% of all fishes but occupy only 0·01% of the hydrosphere (Lévêque et al., 2008). Fishes in fresh waters must compete for habitat with multiple human stakeholders and acknowledged anthropogenic threats to freshwater fish biodiversity are overexploitation, flow modification, habitat destruction, invasion by exotic species and pollution (Dudgeon et al., 2006). Expectations associated with a warming climate, declining freshwater availability and quality and increased water withdrawal predict accelerating losses in freshwater fish diversity (Xenopoulos et al., 2005).
New Zealand is one of the most isolated major land masses and has a predominately temperate maritime climate. Situated on the boundary between the Australian and Pacific plates, the South Island is divided by the Southern Alps, which create a dramatic difference (c. 10 fold) in mean annual precipitation between east and west (Fig. 1). Although predictions of future climate change in New Zealand are less dramatic than elsewhere with an overall forecasted rise in mean annual temperature of c. 2·6° C by 2099, its natural ecosystems are regarded as one of the most vulnerable sectors to a warming climate (Reisinger et al., 2010). Changes in precipitation with a warming climate are expected to exacerbate the current west and east differential in rainfall with the west becoming slightly wetter and eastern areas of the North and South Islands becoming drier. New Zealand's most northern region, Northland, is also expected to become significantly drier. The combination of warmer temperatures and lower rainfall will combine to significantly increase the potential evapotranspiration in the east and north of the country (Fig. 2) greatly increasing annual drought risk in these areas (Mullan et al., 2005).
New zealand's freshwater fish fauna
While New Zealand's freshwater fish fauna is derived exclusively from nine diadromous families (Geotriidae, Anguillidae, Retropinnidae, Prototroctidae, Galaxiidae, Cheimarrhichthyidae, Eleotridae, Mugilidae and Pleuronectidae), only 19 of the currently recognized extant taxa are obligately (13) or facultatively (6) diadromous, and 92% of all taxa are endemic. Only one endemic species, the grayling Prototroctes oxyrhynchus Günther, is known to have become extinct since the first human settlement of New Zealand c. 700 years ago. Sixty-eight per cent of all extant taxa and 76% of non-diadromous taxa, however, are now considered threatened or at risk (Allibone et al., in press).
Australasia has low freshwater fish diversity compared with other global regions (Lévêque et al., 2008) although the understanding of New Zealand's fish diversity has recently been significantly expanded by the application of genetic techniques able to discriminate morphologically cryptic species (Wallis et al., 2009). A recent review of the conservation status of New Zealand's freshwater fishes recognized 50 genetically distinct, extant taxa (Allibone et al., in press), including a cluster of non-diadromous galaxiids in the south-eastern South Island thought to have evolved as a result of glacial or geomorphological vicariance during the Pleistocene (Wallis et al., 2009). This finding significantly elevates both the local species richness and the local endemism of the fauna (Fig. 3). Almost all the locally endemic taxa in Otago and Canterbury provinces are small-bodied galaxiids that are restricted to small tributary streams by downstream predatory exotic salmonids (McIntosh, 2000; McDowall, 2003, 2006), and episodic upstream incursions of salmonids, due to the failure of instream barriers, threaten many isolated galaxiid populations. In at least one area, introgressive hybridization has occurred between diverging genetic taxa due to historical human-induced mixing resulting from construction of extensive water races during the 19th century Otago gold rush (Esa et al., 2000). Throughout this region, the conflicting needs of biodiversity conservation, sport fisheries management and agriculture create ongoing pressures on scarce water resources, and the recent competition for water allocation in Canterbury has been called a ‘gold rush effect' (Creech et al., 2010). Creech et al. (2010) considered that the issue of freshwater management (both ground and surface water) is the single most significant issue facing the Canterbury Region.
New zealand's freshwater stock account and condition
New Zealand has an abundance of fresh water and is 12th in the world for its per capita renewable freshwater resource (Ministry for the Environment, 2006). New Zealand's freshwater resource is roughly equal to that of Australia's but New Zealand has only 20% of the human population and only 3·5% of the land area. On that basis, it could be expected that New Zealand's freshwater fisheries are in reasonably good condition by international standards. This is not the case, however. The country's economic dependence on agriculture and low degree of industrialization is reflected in water usage, with major freshwater allocations comprising 77% for irrigation, 11% for public use and 9% for industry. In addition, hydroelectricity generation contributes c. 65% of total electricity production with hydroelectricity abstraction accounting for 33% of total precipitation (Statistics New Zealand, 2005).
New Zealand's fresh waters have declined in quality in recent years. The most recent state-of-the-environment report showed a trend of increasing nutrient pollution and increasing biochemical oxygen demand in lowland rivers associated with intensification of agriculture and increasing the use of nitrogen fertilizers (Ministry for the Environment, 2007). The period from 1999 to 2006 also saw a 50% increase in water allocations, primarily for irrigation. Continuing degradation of lowland streams, rivers and lakes has been attributed in part to the significant expansion of New Zealand's dairy production over the past two decades. The New Zealand dairy herd, which remained stable throughout the 1970s and 1980s has increased from 3·3 × 106 cattle in 1989 to 5·8 × 106 in 2009, with a concomitant increase in annual production from 0·88 to 1·39 × 109 kg of milk solids, respectively. New Zealand is now the world's largest dairy exporter and eighth in terms of total production (Statistics New Zealand, 2010). Much of this growth has occurred due to conversions of dry-stock farms and exotic plantation forestry blocks in the South Island where dairy cattle numbers have risen from 312 000 in 1989 to 1·8 × 106 in 2010. Regions of the South Island experiencing the greatest growth in dairy production are the eastern and southern provinces of Canterbury, Otago and Southland. Accompanying the nationwide growth in dairy production during this period has been increased applications of nitrogen and phosphorus fertilizers by 1270 and 280%, respectively, and increased water allocations. Canterbury and Otago, in the rain shadow of the Southern Alps (see Fig. 1), have traditionally supported pastoral agriculture with irrigation. The rise in dairying in these regions, however, has massively increased the need for water. Although data on irrigation are difficult to obtain, these two regions accounted for 80% of all irrigated land in New Zealand by 2006 (Ministry for the Environment, 2006) with a 63% increase in area consented for irrigation since 1999. This increased need for water and the low annual rainfall combine to create considerable water allocation pressure on these catchments (Fig. 4) and increasing concern over the ability to achieve ecologically sustainable river flows. Some of the most vocal opposition to increasing water allocations in the region, however, has come from managers of recreational exotic salmonid fisheries, which are themselves implicated in declining native fish abundance (McDowall, 2006). A localized example of the rapid expansion in irrigation in this region is illustrated in Fig. 5. Much of the new irrigation in eastern and southern South Island is associated with the increase in dairy production and is used either to water grazing pasture or to sustain so-called dairy support; forage crops used for the production of dairy cattle feed such as haylage.
Worldwide dairy consumption has grown significantly since 1980, especially in developing countries such as India and China (Youfa & Shiru, 2008), and New Zealand is one of the major dairy producers taking advantage of this trend which is predicted to continue as standards of living rise in developing nations. Therefore, the economic incentive for further expansion in dairy production in New Zealand will continue for the foreseeable future despite declining precipitation in northern and eastern parts of the country. This will create significant pressures to increase water allocations and to build water storage capacity in other regions of New Zealand that are predicted to become drier with a warming climate, especially Northland which experienced one of the driest summers on record in 2009 to 2010. The Canterbury Strategic Water Study (Morgan et al., 2002) was initiated in 2000 following a severe drought in 1998, and it is likely that similar strategic planning will follow in Northland and other regions. Significant expansion of water storage capacity and surface water allocations in Northland, however, may adversely affect the native freshwater fish fauna which has the highest level of local species endemism after Canterbury and Otago.
Effects on fishes: climatic and socio-economic
Changes in the distributions of New Zealand's cool temperate freshwater fishes are likely to occur as a result of both climatic and anthropogenic (socio-economic) pressures. Glova (1992) and McDowall (1992) examined possible effects of climatic warming on New Zealand's freshwater fishes, although Glova (1992) mostly discussed effects on exotic species. McDowall (1992) proposed a variety of possible effects on native and exotic species ranging from the extinction of species with restricted ranges to adaptive behavioural or genetic change to accommodate a changing climate. Increases in mean annual temperature would be expected to push the ranges of at least some diadromous species further south, although the habitat suitability of some braided eastern rivers of the South Island may decline due to reduced average flows, resulting from lower rainfall and greater water abstraction, and more frequent or persistent river-mouth closures, due to a reduced frequency of floods, preventing access to the sea. Non-endemic species, such as shortfinned eel Anguilla australis Richardson and inanga Galaxias maculatus (Jenyns) in the north of New Zealand, are already close to their northern geographic limits when compared to their latitudinal range in other countries and may retreat southwards. Potentially greater effects, however, may arise from anthropogenic factors which have already imposed significant pressures on many diadromous species causing declines in some species including G. maculatus, and several of the endemic diadromous riffle-dwelling species such as torrentfish Cheimarrichthys fosteri Haast, redfinned bully Gobiomorphus huttoni (Ogilby) and bluegilled bully Gobiomorphus hubbsi (Stokell), which are particularly susceptible to reduced water flows, increased nutrient pollution and lowered dissolved oxygen (Allibone et al., in press). Overall key pressures identified in the decline in abundance of New Zealand's freshwater fish fauna are habitat destruction or deterioration, commercial harvest, pollution and interaction with invasive exotic species (Allibone et al., in press). These anthropogenic pressures, which show no decline in magnitude to date, combined with a warming climate are likely to greatly accelerate further declines in some species. Northern New Zealand also has many considerable problems with invasive, exotic, warm-tolerant fishes such as common carp Cyprinus carpio L., goldfish Carassius auratus (L.), rudd Scardinius erythrophthalmus (L.), brown bullhead catfish Ameiurus nebulosus (LeSueur), European perch Perca fluviatilis L. and Western mosquitofish Gambusia affinis (Baird & Girard) (Hicks et al., 2010) and New Zealand has been identified as a global hotspot for effects of exotic invasive fishes (Leprieur et al., 2008). A warming climate will probably increase both the geographic range and the intensity of ecological effects of these species, assisted by illegal human-mediated spread.
A drier northern climate will also increase pressure on locally endemic, non-diadromous, wetland-dwelling mudfish Neochanna spp., both as a result of natural (extended and more frequent drought periods) and anthropogenic (greater need for water and productive land) factors. Recent summer droughts in the Waikato province of the North Island resulted in several cases of dairy farmers conducting illegal wetland drainage because easy access was provided to land previously too water-logged to support earth-moving machinery and thereby directly threatening populations of black mudfish Neochanna diversus Stokell. The Canterbury mudfish Neochanna burrowsius (Phillipps) has also recently suffered considerable decline, with many populations lost due to drainage and agricultural conversion of remnant habitats (Allibone et al., in press).
Non-diadromous species with already highly restricted ranges in southern upland tributary streams have little or no ability to move, being effectively trapped between the upstream limits of permanent stream flow and the downstream pressures arising from higher water temperatures, nutrient pollution, and predatory exotic salmonids and large-bodied natives such as eels Anguilla spp. Moreover, isolation by the topography and geological activity of the local landscape has allowed parapatric divergence of these taxa, and any translocation of these threatened populations would probably lead to homogenization of the fauna.
Overall, socio-economic factors already implicated (habitat destruction or modification, water abstraction and agricultural pollution) in the decline of New Zealand's highly endemic freshwater fish fauna are likely to be exacerbated by a warming climate. Economic incentives to further develop and intensify dairy production to sustain increasing global demand and New Zealand's economic growth, and associated environmental effects will generate further pressure on an already stressed freshwater resource and a severely threatened endemic freshwater fish fauna.
I thank D. Hamilton for providing information on water allocation in Canterbury and M. Allan for assistance with GIS analysis and mapping. Thanks also to anonymous reviewers whose helpful comments significantly enhanced the manuscript.