A while back we talked to Tom, a ranger in Akaroa. He’s part of the Mahaanui DOC Office which had developed a partnership with MPI and NIWA to survey the marine reserves on Banks Peninsula. The survey looked at blue cod numbers in the two marine reserves, with the study allowing DOC to look at the effect those reserves are having on the local blue cod populations.
Today we follow Kirsten, one of our marine scientists, in the second part of this collaborative study funded by MPI. She’s heading to the Marlborough Sounds on the NIWA research vessel Ikatere to ‘pot’ for blue cod in the Long Island – Kokomohua Marine Reserve at the top of the South Island.
Like Tom explained in the last post, blue cod potting is a method of fishing for cod using a large mesh ‘pot’ that fish swim into and then struggle to leave. Do you remember using a baitcatcher to fish for cockabullies off the pier when you were little? It’s like a big version of one of those.
So Kirsten and the crew loaded the R.V. Ikatere with cod pots and headed out to Long Island, whose surrounding area has been a marine reserve since 1993. Popular with snorkellers, the Long Island – Kokomohua Marine Reserve has an abundance of blue cod that show little fear of humans. They have been known to regularly bite divers’ fingers, and anything else that looks tasty!
As the boat reached its first stop, the expert skipper flicked on his depth sounder to locate the band of rocky reef and rubble around the island where the crew would lower the first pot. Nine pots baited with paua guts were lowered 100m apart before it was time to double back to pull up the first one.
Once the cod pot had been in the water for an hour it was pulled back onto the boat so that the fish inside could be measured and recorded before being safely returned to the sea. It’s important for this to run smoothly so that the fish don’t become stressed. Luckily the NIWA crew had already been potting in the Marlborough Sounds for 4 weeks surveying the rest of the area for MPI so they ran like a well-oiled machine, with each fish being out of the water for only seconds before the measurements were taken and it was returned to the ocean down a special chute.
It took just over two days to cover the entire reserve, and in that time the cod pots were lowered 72 times, catching 911 blue cod. Surprisingly few other types of fish where caught in the pots, with only 1 scarlet wrasse, 1 red moki and 2 leather jackets caught, and immediately returned to the sea.
The data collected on this study will be useful for both DOC and MPI. We will gain data that will help us to understand the value of marine reserves for blue cod, while MPI gains a glimpse into the benefits of marine reserves to the wider fishery and insight into what blue cod populations look like when they are not fished.
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could you please tell me what the mortality rates are with cod potting