Brook Stream Fish get DNA testing

12/05/2021 10:26am

Nelson City Council's freshwater science team is using a technique known as eDNA testing to investigate the diversity of aquatic species at different sites in the Brook Stream, to help with understanding how barriers to fish passage are affecting stream life.

Fish and other aquatic life shed cellular material into their environment, called environmental DNA (eDNA).  Larvae, faecal matter and scales are all examples of the kind of particles shed into the stream, which can then be extracted by filtering the water through a fine membrane. The particles are then tested for DNA.


Water has been sampled for eDNA at five sites along the Brook Stream to detect the presence or absence of fish species. Sampling sites were chosen above and below known barriers to fish passage giving an accurate and efficient way of finding out what creatures are living in specific stream locations.


The diversity of fish species is greatest near the estuary and naturally falls away the further you go from the sea, because the stream environment changes as fish move upstream and they require specific adaptations to survive.


Some species – like the native eels, bully species, banded kōkopu and kōaro - have successfully adapted to climbing up cascades and waterfalls so can live higher up in the catchments.

eDNA testing enables us to compare what species we would expect to live in any part of the stream, and what is actually found to be there. We can then look at why they are absent, which could be because of fish passage barriers and/or available fish habitat.  


The eDNA fish survey is part of a larger collaborative programme of work between Council’s Environmental Science and Engineering teams to trial fish passage improvements to the Brook Stream concrete channel and wider catchment whilst maintaining flood protection.