Trends in freshwater fish

  • Image, Freshwater plants and animals.

    Freshwater fish are an important component of freshwater ecosystems and have high cultural, commercial, recreational, and intrinsic biodiversity value. The presence of a fish species at a site can be affected by changes in catchment land cover, land use, in-stream habitat, fish passage (routes for moving up and down waterways), pests, and contaminants. We report on national trends for the abundance of 10 freshwater fish species (nine native and two exotic) and koura (freshwater crayfish) over the 39-year period 1977 ̶ 2015.

    We classified Trends in freshwater fish as a case study.

    Key findings

    Of 11 freshwater species, six declined in abundance and two improved, over the 39 years from 1977 ̶ 2015. 

    • There was not enough data on three freshwater species to confidently determine a trend (redfin bully, torrentfish, and kōura /freshwater crayfish).
    • Four native (longfin eel, kōaro, Canterbury galaxias, and common bully) and two exotic (rainbow trout and brown trout) fish species have declined in abundance over the past 39 years.
    • Both species with improving trends were native fish (upland bully and shortfin eel).
    • For each of the eight fish species with a confident trend direction, the trends improved or declined by less than 0.5 percent a year.

    Table 1 

     Trends in the abundance of 11 freshwater species, 1977–2015
     Species Trend direction

    Trend magnitude
    % change/year (95% CI)

     Upland bully  Image, Increasing trend, improving state.  0.35 (+/-0.09)
     Shortfin eel  Image, Increasing trend, improving state.  0.18 (+/- 0.01)
     Longfin eel  Image, Decreasing trend, declining state.  0.09 (+/-0.08)
     Brown trout (exotic)  Image, Decreasing trend, declining state.  0.44 (+/-0.04)
     Rainbow trout (exotic)  Image, Decreasing trend, declining state.  0.21 (+/-0.05)
     Canterbury galaxias  Image, Decreasing trend, declining state.  0.36 (+/- 0.09)
     Common bully  Image, Decreasing trend, declining state.  0.22 (+/- 0.02)
     Koaro  Image, Decreasing trend, declining state.  0.05 (+/- 0.02)
     Koura (freshwater crayfish)  Image, Indeterminate trend.  
     Redfin bully  Image, Indeterminate trend.  
     Torrentfish  Image, Indeterminate trend.  
     Note: CI is confidence interval.
     Source: NIWA

    Figure 1

    Figure 2

    Definition and methodology

    NIWA analysed the trends in abundance of freshwater fish using data from the New Zealand Freshwater Fish Database (NZFFD). Anyone can enter information in the NZFFD, which means there are differences in sampling methods across sites and time.

    To reduce the impact sampling differences have on trends in fish species’ abundance over time, NIWA standardised the NZFFD for 11 species (with sufficient data) using generalised linear models (Crow et al, 2016). These models estimate the probability of species occurrence for each year of the 1977–2015 period.

    NIWA used a weighted Sen Slope Estimator (WSSE) to simplify the complex variability in trends over time into straight lines. The WSSE approach means NIWA put greater ‘weight’ on years for which there was greater confidence in the estimate of the probability of the presence of a species. We used these results to display trends in abundance over time.

    Data quality 

    Topic Classification   Relevance Accuracy 
     Freshwater species, taonga species, and genetic diversity  Case study

     Image, Direct relevance.


    Image, Medium accuracy.


    See Data quality information for more detail.


    Crow, S, Snelder, T, Jellyman, P, Greenwood, M, Booker, D, & Dunn, A (2016). Temporal trends in the relative abundance of New Zealand freshwater fishes: Analysis of New Zealand Freshwater Fish Database records. Prepared for the Ministry for the Environment. NIWA Client Report No CHC2016-049. Retrieved from

    Archived pages

    See Freshwater fish (archived April 2017)

    Published 27 April 2017

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