As if the beauty of fly fishing in New Zealand weren’t enough, there’s also a natural phenomenon known as the mouse year that really has to be experienced to be believed.
Just picture this: close your eyes and imagine large trout gorging themselves on unfortunate mice under the cover of night, allowing them to reach astonishing sizes of ten pounds or more in a matter of weeks. Next, imagine catching one of these behemoth trout during daylight hours. Chances are, it will have strong shoulders, be the size of a woman’s thigh, and when you land the beast, it might snatch a few mice into your net – still stuffed from the night before.
The notion of a mouse year is almost mythical, but it does occur every three to five years in connection with the sowing cycle of New Zealand’s native beech trees. If you weren’t already aware of this phenomenon, you might have thought to yourself that you’ve been living under a rock and that you need to book a trip to New Zealand tomorrow to connect with your share of overweight trout. However, it is important to note that success in the mouse years depends on being in the right place at the right time and having the right attitude and expectations.
Mouse years require a combination of several natural factors to create the ideal scenario. First, beech trees must go through a “mast year” during which they go to seed. This happens approximately every three to five years depending on the region. Seeding years have occurred back-to-back in the past and have also been separated by as much as ten years. There has never been a way to truly predict these events, as temperature and climate play a large role in the timing of beech seed production. Consider it a natural way to keep fishermen guessing.
Now, since mice are opportunistic feeders, you can be sure that when the beech seeds start falling, they pounce on them like a pack of dogs on a three-legged cat. Not only do mice enjoy this sudden surplus of seeds, but they also feast on other creatures such as insects and caterpillars that multiply in response to the increased food supply. A well-fed mouse can give birth once a month and can produce up to ten offspring per litter. When you have a mammal that reaches sexual maturity in just 80 days, the surge in the rodent population in New Zealand during these “mouse years” can be downright terrifying. With a seemingly endless supply of food, mice are practically addictive. But, as with humans, the voracity and greed of mice can have deadly consequences.
As more and more mice compete and search for food, many of them will seek out new territories, often crossing rivers, streams, and lakes along the way. They are excellent swimmers, so crossing a river is not a problem. But sometimes there are so many mice that they just accidentally fall into the water – much to the delight of trout, which in evolution have looked at these animals as floating protein bars and do not hesitate to inhale them.
Since mice are mostly nocturnal, it is not surprising that most of these activities take place by moonlight. It is not uncommon for a trout to eat up to 13 mice in a single feeding session. It may seem too good to be true. And often it is – even a 5-kilogram trout can eat that many rodents in a night. But the long-term effect of such a diet for the average trout is that it gains three pounds or more per season. These trout often maintain this good condition for many years, but all of this can be lost during a major flood or after a drought. As with everything in fishing, timing is really crucial.
However, it’s important to note that we won’t know if this is going to be the right year for the mouse until the season has already started. It should also be noted that these double-digit fish do not come easy, and you will have to be prepared to use a number of techniques to convince these fish to eat, as some of them can act almost catatonic during the day after chewing all night. Not every river will have a “mouse fish” and not every mouse fish will eat a mouse pattern. In fact, most of them won’t.