Te Wharau Stream's native fish survive major flood

After heavy rainfall left parts of Banks Peninsula badly flooded in February, biodiversity experts feared the worst for native critters and fish in Te Wharau Stream, which flows through Te Ahu Pātiki and Orton Bradley Park.

On 20 January, shortly before the flood, Banks Peninsula Conservation Trust staff tested environmental DNA (eDNA), genetic material shed by organisms into their surroundings, at two locations along the stream and found a wide range of fish, invertebrates and birds in and around it.

The flooding event on 16 February created slips, gouged out the stream banks and washed a huge amount of debris downstream. Locals and ecologists thought that a lot of the species recently found living in the stream would be dead or washed away.

When eDNA was tested again at the same two sites after the flood on 19 March, the findings showed that, against all odds, a number of the stream's precious residents likely survived, swam upstream or were able to find refuge during the flood event. Whaka-Ora Healthy Harbour programme manager Sarah Wilson said, “beyond all our wildest dreams, many critters survived the deluge.”

What the eDNA revealed

Across the two rounds of eDNA testing, nine species of native fish were detected before the flood, and eight were detected after it.

Environment Canterbury biodiversity advisor Martin Rutledge said, “the latest eDNA analysis shows, while some macroinvertebrate species are absent post-flood, there is still a good diversity of macroinvertebrate species such as mayfly and caddisfly, which are basic food for the no doubt hungry native fish.”

“The range of macroinvertebrate species found post-flood also reflects good water quality – a positive sign given the horrendous amount of sediment that washed down the stream in the flood.”