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Discussion

This analysis of New Zealand fish community data at 22,500 river and stream sites showed a significant decline in fish IBI scores nationally over the past 37 years. This represents a significant decline in indigenous freshwater biodiversity in these waterways. This decline indicates that freshwater ecosystem condition has also declined nationally over this time, particularly over the last decade.

Strong relationships between fish biotic integrity scores and land-cover type were revealed using the River Environment Classifications (Fig. 3). The term biotic integrity is based on the concept that to function, an ecosystem must have all its component parts, thus any loss of parts is effectively lost integrity . Consequently, the difference shown in IBI scores within different land uses reveals the link between integrity and ecosystem function. Fish IBI scores were significantly higher at indigenous/scrub vegetation sites, and lower at urban, pasture and exotic forest sites. This strong association between fish IBI and land use shows the influence degradation of terrestrial systems has on freshwater ecosystems.

The very low scores for tussock land cover are likely to be the result of the fact that these sites are generally a long way from the sea and are at high elevation. The majority of native fish are migratory, thus these sites generally have low diversity or cannot be reached by fish. This is supported by the low mean and median scores (median = 0, mean = 17.98; Table 1) and the fact that 1453 or 51% of the tussock sites had no fish species present, so scored an IBI score of 0.

The low scores for exotic forest were not expected as the general consensus is that exotic forest is not as big an impact on streams as pasture or urban land use. While growing, the forests do provide protection for streams. However, the clear felling harvest system commonly used in New Zealand forests means that post-harvest impacts are extensive. In Figure 9 there is an obvious dip in IBI scores for exotic forest sites in the 1990s. This is likely to reflect the large amount of harvesting that took place in New Zealand, following an intense period of planting in the 1960s.

Indigenous forest sites revealed a significant increasing trend with a peak in the 1990s (Fig. 5). This could be related to improving conditions in the lower reaches of rivers with removal of or improvements in point source discharges, allowing increased fish access to the headwaters. This increase in scores at indigenous forest sites could also be related to improvements in sampling efficacy over time, the focussing of sampling efforts into more remote areas and/or an increase in the use of spotlighting as a survey tool over time. There may also have been a trend over time towards less or more intensive sampling or change in area sampled. However, any of these changes should not be specific to any one land-use class so do not unduly influence the results presented here.

Because of a lack of consistent detail in the database on sampling intensity and fish abundance, all data used in this analysis were necessarily reduced to presence/absence. This restriction means that all results are inherently conservative. This is because any species within a fish community/population will show a gradual decline before local extirpation even with relatively sudden environmental impacts. Thus, for a reduction in IBI score, fish species must become extinct at that reach. Accordingly, the observed changes exposed in this report reveal the endpoints of longterm cumulative changes to fish communities.

To take advantage of fish as a bioassessment resource that can track change in ecosystem status nationally over time, this analysis has highlighted the need to have a national set of regularly sampled sites. To keep the valuable history already in existence, a set of sites at reaches with a record of quality sampling events in the past should be added to systematically. This could be done regionally and combined into a national long-term monitoring dataset.

This initial analysis of the NZFFDB has revealed that, while there is considerable information available, any future analysis would be much improved by consistently sampled long term single site data as well as further investigation into the different sampling methods. Furthermore, analysis of the trends in relation to biological parameters such as the migratory status of species and whether or not they are part of a recreational or commercial fishery could add important information. The conservation status of New Zealand freshwater fish species is being undertaken at present by the Department of Conservation and this process could be improved by targeted species specific analysis of the NZFFDB.

 

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