08 WATERLINE
DECEMBER 2012
Showing whitebait the ropes Throwing fish a line is being given a new meaning by Researchers from Waikato Regional Council and the Bay of Plenty Polytechnic, who are using mussel farming ropes to help fish through culverts. Hundreds of thousands of fish traverse New Zealand’s streams and rivers each year to reach their breeding grounds. The advent of culverts is making this journey increasingly difficult. Current culvert designs create problems for fish. Many are often raised above the waterline so fish are unable to reach the pipe, or the angle means the speed of the water flowing through the pipe is difficult to swim against. The researchers found that mussel spat ropes help fish both into and through the passage. The ‘stringy’ texture of the ropes means that fish can hide amongst the strands and fibres, allowing them to rest on their way upstream. “The first study was to find out whether
fish, which are good climbers, can get up and into the culverts. For the second study we focused on trying to get other fish with less ability to climb through the culverts,” says Environmental Management academic staff member at the Polytechnic, Dr Jono Tonkin.
Trout, inanga - a common whitebait species, and paratya, a freshwater shrimp, were shown the ropes. Staging a culvert flow in a lab, students
used 350mm diameter pipes three metres and six metres long - similar to the standard size used under New Zealand roads. The pipes were angled at varying degrees from lying flat to three degrees. The average angle of a culvert in the real world is around one and a half degrees. By running water through the pipes, some with and some without ropes, the researchers compared the fish’s success rates at swimming against the current. “The fish always performed better with the ropes,” says Jono. “And generally, they did okay on the low slopes and short lengths. On the higher slopes and longer pipes they struggled – sometimes none made it through.” “The results have been really positive for smaller species as well as larger fish such as trout, each showing a greater success rate in reaching the other side of pipes.” It proves fish have a greater success swimming through culverts with ropes, providing a relatively inexpensive solution for a worrying problem.