Categories: Nelson-Marlborough

Radio Tagging Wild Trout in the Wairau River, Marlborough

Radio Tagging Wild Trout in the Wairau River, Marlborough by Ben Grady The phone rang one night during I the…

Radio Tagging Wild Trout in the Wairau River, Marlborough

by Ben Grady

The author with a butter coloured brown trout taken from the lower Wairau River.

The phone rang one night during I the week. On the other end a familiar voice said: ” Gidday Ben, it’s Pete Boyes here, any chance of giving us a hand tagging trout with the radio transmitters this weekend?”

“Not a problem,” I said, as he gave me more details. Any excuse to go fly fishing is a good one, so how could I refuse to help in a scientific study. I hoped my wife Jo would see my way of thinking. As always she was very supportive!

Having read articles in past magazines on tagging exercises, I decided I couldn’t tum this opportunity down.

Over the next three months of tagging, I managed to participate on three outings. If the day was on a weekend, and if the river conditions would allow, I was there with bells on! Each trip I managed to catch a suitable size trout to be tagged and released. A small contribution, but one I will always remember. Clayton Nicholl and Peter Boyes caught the majority of the trout, with a little bit of friendly rivalry helping to keep the fish numbers up.

Since one of the earlier radio tagging days, a very energetic fish that deserves special mention, has already travelled 41kms upstream. I always thought of trout being more territorial and staying close to home. Obviously, in the bigger rivers, they are quite capable of moving long distances throughout the river system.

Having completed radio transmitter trout tagging firsthand, I can now share a brief summary and informative photos of my experience. During this radio transmitter tagging exercise, the Nelson/Marlborough Fish and Game Council, Marlborough Electric, Cawthron Institute and members of the Marlborough Fresh Water Anglers Club were all involved.

A transmitter being implanted.

Marlborough Electric already operates a hydro scheme on the Branch River and may look at the feasibility of another scheme in the upper Wairau. Nelson-Marlborough Fish and Game Council expressed interest in a radio tagging study for obtaining information on trout movements in the Wairau River, because of their concern with the possible impact of these schemes on the trout fishery. Radio tracking has become a basic, and widely used tool in fisheries research, with the availability of waterproof, miniature, radio transmitters proving to be a useful method in several New Zealand studies.

The study in the Wairau was commissioned by Marlborough Electric and run by Cawthron Institute, with joint participation with Fish and Game staff and volunteers (anglers).

Fifty-eight trout have been caught by local expert anglers from the Marlborough/Nelson region. The majority of the trout (all Browns) have been caught with small weighted nymphs, sizes 12-14, Dark Olive Antron to name a few.

In the upper reaches, Black Stonefly and Buller Caddis patterns tied on 10-12 long shank hooks have been successful as well. A handful of Trout were also taken on dry flys, Deer Hair Humpys and Royal Wulffs.

The tagging exercise was employed in three sections of the Wairau River, Lower, Middle and Upper reaches, with twenty Trout being caught in each. The size of the Trout to be used were ideally four-pound plus, as the transmitter and battery are nearly as big as a matchbox.

When the Trout was landed it was placed in a specially designed holding bag and the Cawthron Surgical team were called upon handheld radio phones and given the anglers location.

Once in the holding bags, the Trout were quite happy as grommets at the end of the bags let a continuous flow of aerated water through their gills. The Cawthron staff, along with Fish and Game staff who know the river inside out, were usually only half an hours four-wheel driving away, depending how many trout were caught at once.

After the trout were anaesthetized they were placed upside down in a foam rubber cot. An incision thirty millimetres long was made in the gut cavity, where the transmitter was carefully placed in-situ with an aerial one hundred millimetres long protruded out towards the tail. Three or four stitches later and the trout was returned to the holding bag. Once fully revived it was then gently released back to the safety of the river. The operation was quick, only taking a few minutes, while freshwater and anaesthetic was continually hosed through the gills and over the trout’s body. The most difficult part of the whole exercise was the low river conditions, with Marlborough in the middle of its worst drought this century.

The river flow was down, the water temperature was up, 17-22 degrees and most days a dry windy nor’wester blew from mid-morning straight downstream, which didn’t help a novice fly fisherman like me. It was frustrating, to say the least.

All the tags are colour coded for easy identification and have different radio frequencies, which can be picked up one kilometre away on land and from a distance of ten kilometres from the air.

Only one tagged trout has been re-caught so far with the aerial and bead colour, the weight and the location being recorded, and information being sent to one of the participants in the programme listed at the beginning of this article.

In a brief summary, fifty-eight trout were tagged throughout the Wairau River. The heaviest fish was 3.4 kg and the longest fish was 640 mm. The average weight of the trout tagged was 2.24 kg and 562 mm long. From the first tracking results from the air, most of the trout have travelled 1 to 2 km from where they were released, a couple of them have moved 15 to 20 km upstream, and one courageous trout was located 41 km upstream. Only two trout have not been located since tagging. The return information is indicating good post-surgery survival so far, and some impressive early movement.

Many thanks to everyone involved. We are looking forward to more results in the future.

Clayton Nicholl about to release a 6 1/4 lb brown and Wayne Stronach from Marlborough Electric.

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