Most of the fish used for breeding are 3-year-olds, along with a smaller number of 4-year-olds which are breeding for the first time. Trout stop growing once they reach maturity and spawn, so by selecting older maiden trout for breeding the hatchery improves the chance of their offspring growing bigger.
Previous spawners are often rejected because they have poorer quality eggs and these can be picked by signs such as the ragged edges on their fins where they've been digging redds (nests) in the gravel, said Fish & Game officer Lloyd Gledhill,
This year the largest fish appear to be around the 65cm to 68cm mark, so fish which are over 62cm fit the selection category for the time being.
"Later in the season we might change the selection size depending on how the run is going. You never quite know how many fish might come in, and once they have been through the trap it's too late to get more.
"If we don't get eggs now, there'll be no fish to release next year so the pressure in on" said Mr Gledhill.
After selecting for size, it comes down to a "judgement call" on the look of the fish. Mr Gledhill said you got fish that were different shapes including those which were "all head and shoulders" and tail off to the back.
"I like a fish that's a much more even shape all the way through. We look for robust, healthy fit fish."
Fish & Game staff aim to end up with 135,000 viable fish, which means stripping an estimated 250 thousand eggs from the "ripe" hens. Every spawning season they "cross" a total of about 90 males with the same number of females, 180 fish in total.
"That gives us a good margin - some eggs die and others die at hatching so that gives a good number of viable fry at the end. It also ensures we get good genetic variety in the fish being released to the lakes."
How are the eggs extracted? The fish are firstly sedated in a fish bin with a synthetic clove oil added, then a small gauge tube is inserted into the fishes belly cavity, and compressed air used to gently push the eggs out.
"The sperm activates as soon as it hits the water - so you've got about 20 seconds for the sperm to get to the egg and fertilise it."
The eggs are taken inside the hatchery and placed into trays with running water piped over them, to begin the process of growing first into first "eye ova," and then into fry.
When they're big enough, the young fish are loaded into the 'live trout truck' and carted off for one of the many releases into North Island lakes.
