Marine trophic index: Chatham Rise

  • Image, Marine trophic index: Chatham Rise.

    The marine trophic index (MTI) measures the changing abundance and diversity of demersal fish species (living and feeding on or near the seabed) in fishery catches. The Chatham Rise has more than 180 species of fish. It is one of the most productive areas of our exclusive economic zone and an important fishing ground. We calculate the Chatham Rise MTI to assess the change in marine ecosystems resulting from fishing and climate variability.

    We classified Marine trophic index Chatham Rise as a case study.

    Key findings

    Between 1992 and 2014, the MTI of the Chatham Rise demersal fish community showed no long-term change.

    • The mean value of the Chatham Rise MTI was 3.87 between 1992 and 2014. This indicates a predatory fish community for the area.
    • Hoki dominate the fish biomass on the Chatham Rise, and comprised 22–64 percent of the total estimated biomass for the region between 1992 and 2014.
    • Changes in hoki biomass have a strong influence on the Chatham Rise MTI. They account for one-third of the MTI change.
    Note: The horizontal line represents the mean of the data from 1992 to 2014.

    Definition and methodology

    Changes in the marine trophic index (MTI) are based on research trawl survey data.

    The Convention on Biological Diversity (2004) recommended using MTIs to assess change in marine ecosystems. MTIs are widely used internationally to assess marine ecosystem change arising from fishing and climate variability.

    A species’ trophic level corresponds with its position on the food chain. Primary producers, such as phytoplankton, have a trophic level of 1 and are the base of the food chain. Herbivores and detritivores (feeding on dead organic matter) have a level of 2; omnivores generally have levels between 2 and 3; and predators have levels greater than 3. Apex predators generally have trophic levels of about 5. A decrease in an MTI usually means a decrease in the number of large predatory fish.

    For each year from 1992 to 2014, the biomass of 183 fish species was estimated using scientific trawl survey data. This study followed the usual procedure for calculating MTI, using a single value of the trophic level for each species (Pauly et al, 1998; Pauly & Watson, 2005; Tuck et al, 2009).

    The trophic levels of 49 fish species on the Chatham Rise were determined using stable isotope measurements. On average, these species represent 96.9 percent of the total fish biomass in the 200–800m depth range of the Chatham Rise. When no stable isotope data were available, the trophic level dataset developed by Tuck et al (2009) was used. For more information about the methodology, see Pinkerton (2015).

    Data quality

     Topic Classification   Relevance Accuracy 
     Marine ecosystems and habitats  Case study

     relevance-partial
    Partial

     accuracy-high
    High

     See Data quality information for more detail.

    References

    Pauly, D, Christensen, V, Dalsgaard, J, Froese, R, & Torres, F (1998). Fishing down marine food webs. Science, 279(5352), 860–863. Available from www.sciencemag.org.

    Pauly, D & Watson, R (2005). Background and interpretation of the ‘marine trophic index’ as a measure of biodiversity. Philosophical Transactions of the Royal Society B 360, 415–423. Available from http://rstb.royalsocietypublishing.

    Pinkerton, M (2015). Marine trophic index: Based on research trawl surveys of the Chatham Rise, 1992–2014. Prepared for the Ministry for the Environment. Wellington: NIWA.

    Secretariat of the Convention on Biological Diversity (2004). Convention on Biological Diversity COP 7 decision VII/30, Annex I Provisional indicators for assessing progress towards the 2010 biodiversity targets. Available from www.cbd.int.

    Tuck, I, Cole, R, & Devine, JA (2009). Ecosystem indicators for New Zealand fisheries. New Zealand Aquatic Environment and Biodiversity Report No. 42. 180. Available from http://fs.fish.govt.nz.

    Archived pages

    See Marine trophic index: Chatham Rise (archived October 2016).

    Updated 27 October 2016

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