Source: Environment Canterbury Regional Council
Thinking outside the box
The old top-hung flood gate that was in place at the creek, almost totally blocked fish passage at some points of the tide cycle.
To get past at high tide, fish had to be able to dive down under a small gap along the bottom of the gate.
We needed an innovative solution.
In 2019, our engineers found that the flood gate needed replacing due to leaks – and the opportunity arrived to try something new.
Working together, engineering, science and biodiversity staff came up with the “fish letterbox”.
Taranaki Creek floodgate fish passage letterbox
The team built a vertical slot into the new flood gate to act exactly like a letterbox gap. Only a small amount of water would be able to get through when the main gate was closed, and fish could use the letterbox to swim up or downstream. If flooding is a concern, the letterbox could be manually shut.
Biodiversity advisor Chloe Armour said the process was very much an experiment, especially in trying to find the right size and placement of the slot.
“We were trying to have the maximum impact for migrating fish and the minimum impact on water levels,” she said.
Did the experiment work?
The proof is in the pudding, or in this case, the net.
To monitor the effectiveness of the letterbox, a fyke net was cut down to size and posted through the upstream side of the floodgate to catch migrating fish heading upstream. It was left out for 24 hours or two tide cycles.
The results from 24 hours of traffic through the fish letterbox slot:
| Species | Count | Length range in mm |
|---|---|---|
| Shortfin eel (Anguilla australis) | 20 | 500-950 |
| Whitebait | 367 | 49-57 |
| Īnanga (Galaxias maculatus) | 8 | 73-119 |
| Giant bully (Gobiomorphus gobioides) | 2 | 94-174 |
| Unidentified bully (Gobiomorphus) | 3 | 40-94 |
| Estuarine tripplefin (Forsterygion nigripenne) | 2 | 66-103 |
The experiment was a success.
“These results showed that the letterbox was a popular upstream pathway for a wide variety of fish species,” Chloe said.
“It’s a good indicator that it’s made a positive impact for the fish that live in the estuary and this creek.”
What we learned
As with any experiment, our engineers and biodiversity staff have taken away learnings that they’ll put in place next time, and there are still lots of questions to answer.
“For future designs, we’ll build in a deeper letterbox slot, stretching down to lower tidal levels to make the gap available to fish for longer periods of time,” said Chloe.
Staff will also make changes to the monitoring, separating day and night sampling so they can see which species are travelling through when.
“We have learned a lot from this, and we plan to keep picking up these opportunities to improve fish passage as structures get upgraded on our land,” Chloe said.
“We also hope this project inspires other public and private landowners to look at ways to support fish passage as they upgrade their own in-stream infrastructure.”
Find out more about managing instream structures on your land.
Putting the net through the letterbox for monitoring
Environment Canterbury © 2024
Retrieved: 9:53am, Thu 04 Jul 2024
https://www.ecan.govt.nz/get-involved/news-and-events/2024/fish-letterbox-a-gamechanger-for-fish-passage-in-local-waterway/
