Blue cod waterslide

Here at DOC we go the extra mile for the species we protect, and sometimes that means creating a bespoke blue cod waterslide.

Blue cod

Recently one of our marine scientists headed out to Long Island – Kokomohua Marine Reserve to survey the blue cod there. Part of a collaborative study funded by MPI, she and the NIWA crew spent just over two days ‘potting’ for blue cod and taking measurements from the fish they caught. You can read about the trip in more detail in this post, but we’re here to talk about one piece of gear on the boat in particular – the waterslide.

While we hope that the fish might have enjoyed shooting down the slippery slide, the purpose of the contraption wasn’t fish enjoyment, it was fish safety. See, NIWA’s vessel the R.V. Ikatere is quite big. This means that if Kirsten and the crew were to just throw the fish off the side they’d have a long way to fall, and when you’re blue cod size, that’s a pretty big belly flop!

NIWA’s R.V. Ikatere. Photo: Dave Allen

Falling from a height like that might harm the fish, and that’s exactly what we don’t want. Long Island – Kokomohua is a marine reserve, which means that we need to do no harm to any of the native plants or animals inside. Getting an idea of how many blue cod are in the reserves and how big they are is important, because it lets us better understand the value of marine reserves for blue cod, but it must be done in a way that doesn’t harm them.

So the crew needed to create something that would transfer the fish safely from the boat back to the ocean. This is where some good old kiwi ingenuity came in. The team got to work with some tape and pieces of PVC pipe. The pipe was light and flexible, which was important as the slide would need to be lifted out of the water while the boat was on the move. It took some testing to find the best waterslide design, as the team wanted to ensure the measuring set-up and slide were positioned in the best possible way for fast, smooth processing of the fish. Once the set-up was sorted, there was one last thing to do – add water. As you can imagine, if you’re wet and your slide is dry it’s going to hurt, so the team rigged up a hose that sent a constant stream of saltwater down the slide. Et voilà! The waterslide was complete.

Now NIWA scientist Mike Page could grab a fish.

Measure it.

And send it down the waterslide.

So it could swim to safety.

And that’s just what the cod did. More than 900 fish were surveyed, and we had a 99.8% survival rate thanks to both the waterslide and skilled crew.

The survey method’s success goes beyond this study, and will change the way we’re able to survey blue cod in the future. Because the survival rate of fish was so high, we can be confident that the potting method used, combined with the waterslide, is a safe way to survey blue cod. This is vital for any work we do in marine reserves, as we aim to to cause the least possible harm to anything living in the reserves.

Having a method that we know is effective and safe means that we are able to survey more fish in our reserves, which will help us to better understand the value of marine reserves for blue cod and ultimately contribute to a better understanding of the health of our marine ecosystems.