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Trout heads give up browns' spawning secrets

Trout heads give up browns' spawning secrets

What clues to the origins of the Waikato’s brown trout can you gather from a fish head…or more precisely, the small bones in a fish’s head?

The answer, according to Fish & Game Auckland-Waikato, is plenty.

Fish & Game officers have completed the first phase of a study aimed at tracing the origins of brown trout in the Waikato fishery.

Auckland-Waikato fisheries manager Dr Adam Daniel explains that researchers can use the ear bones of fish, called an otolith, to estimate age and also track their life history from the geochemical signals preserved in the otolith.

The otolith is formed as consecutive layers of calcium carbonate are laid around a core that forms soon after a fish hatches, so it is possible to cut through an otolith, revealing the layers like growth rings of a tree. Each ring contains a fingerprint of the water chemistry that matches the water the fish swam in when the layer was formed. So if you can extract the information, you come up with a sort of ‘fish's diary,’ from hatching through to when the fish was captured.

Dr Daniel says the first phase of their study involved collecting more than 70 heads from adult brown trout. A local eel fisher Bob Clark, the organisers of the Kilwell Lower Waikato River Trout Contest and a handful of keen anglers donated heads for the project. “We are very grateful to them all for their help in this work,” he says.

The adult brown trout, some over 50cm long, had come from a wide area, stretching from Tuakau to the base of the Karapiro Dam.

The study also involved catching juvenile browns by electrofishing (electric shock).

Dr Daniel says to determine where the brown trout that inhabit the lower Waikato River are spawning, the chemical signatures from the ear bones of the adults, will be matched up with the signatures of otoliths from juvenile brown trout caught in tributaries of the Waipa River – and water samples.

“We won’t be able to get it down to an exact tributary, but we should be able to get it down to at least a catchment or basin.

“For example, fish from Pirongia should have a different signal to those we get out of south Waikato streams, like the upper Waipa sites.”

Dr Daniel says the benefits will come in finding out which streams are actually producing the large numbers of brown trout found in the lower Waikato fishery. “We know we have a pretty healthy fishery there but we don’t know where those fish are coming from.”

Collecting the juvenile trout has involved a huge amount of foot slogging. So far more than 14 different tributaries have been electrofished with little success, but ironically, a dozen juvenile browns were pulled out of a “mud puddle” next to the Mangatutu River which rises in the Pureora Forest.

Dr Daniel says that once their collecting has been completed, and samples have been analysed by Cawthron Institute scientists, the information can be applied “almost immediately.”

What we need to know, he explains, is which are the “priority rivers” for brown trout spawning, which can be applied to fisheries management – for example, decisions on season lengths, enforcement and environmental protection.

This knowledge can be used in Fish & Game submissions on RMA-related issues like water takes or discharges from sewage plants. “The information will be very helpful when prioritising fencing and planting projects.”

Dr Daniel says: “It will help us better utilize our resources because we simply can’t monitor all 20,000-plus kilometres of trout streams we have in the Auckland-Waikato Region every year.”


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