Why Trout Grow Big – Especially in New Zealand

Fly fishing for supersized New Zealand trout is a favored activity for both locals and international tourists – because we grow trout big here!  Here’s why. 

Trout grow big as the result of four combined conditions – food supply, water temperature, water quality, and angling pressure.  Big trout are the result of abundant insect life, clear rivers and lakes, a temperate climate (meaning waters stay cool all year round), plus low angling pressure. 

In New Zealand, these conditions are common which means rainbow trout are often 3-4 lb in weight, but in larger rivers and lakes where food is abundant they may reach 10 lb or more. Large brown trout may get to more 15 lbs, with any fish over 10 lbs considered a trophy.

Here’s how each of the factors plays out in growing big trout, in New Zealand and elsewhere.

The foods that grow big trout

Trout that have access to a steady source of food to grow consistently.  And that means the right water and climate conditions where there is a high level of insect and aquatic life.  

There are 1,000s of different insects that live in and on the water at all stages of their life cycle.  These include nymphs, larvae, midges, mayflies, and stoneflies to name a few.  

As these insects develop through their life cycle, trout will feed on them as float downstream in the current, or when they are hatching and spend time near or on the water surface. 

For trout to grow big they require a steady source of insect life until they reach a certain size, but once they are over 12 inches (30 cms) – then their core diet moves to small fish, worms, shrimp, and larger insects such as cicadas. 

The larger trout also reside in the most productive locations, which gives them greater access to the best food sources. 

When the food source is right, then they have the opportunity to become trophy trout. 

And here in New Zealand, they are even known to eat mice which makes them huge and fat very quickly. 

Temperatures role in growing big trout

Warmer water contains less oxygen than colder water. As water temperature rises, the amount of dissolved oxygen in the water decreases. Likewise, as the water temperature decreases, the amount of dissolved oxygen increases. 

Fish require an optimum level of oxygen to thrive.  So as the temperature rises and dissolved oxygen decreases, fish begin to experience stress. 

These stresses set in well before the water temperature reaches lethal limits. For example, rainbow trout are said to be able to survive in temperatures up to and exceeding 77°F (24°C), but stop growing at 73°F (23° C). 

All three major trout species (brook, brown and rainbow) begin to experience some level of stress at around 68°F (20°C), 

Fluctuations in temperature affect feeding levels, and can turn off their appetite altogether. 

So as well the effect of temperature on the insect life, it also affects trout behaviour as their metabolism fluctuates.

Trout metabolism is at its highest levels, when water temperatures reach up to 68° (20°). But when the water gets below about 50° (10°C) cold then their metabolism slows down and with that growth slows.  

Does New Zealand have a hot or cold climate?

Well neither.  While summer temperatures can cause our rivers to dry up and get too hot for hungry trout, our winters are not extreme either.  

Many of our rivers start their life in the mountains, amongst winter snow where most of the snow falls, but most of their journey is across land that seldom is covered by snow.

Therefore our river temperatures mean trout will continue to grow at a steady rate, during nine months of the year.  (As opposed to about 3 months in the United Kingdom.)

New Zealand Climate

September – NovemberDecember – FebruaryMarch – MayJune – August
(61 – 66˚F)16 – 19˚C(68 – 77˚F)20 – 25˚C (62 – 70˚F)17 – 21˚C (53 – 61˚F)12 – 16˚C 
Average seasonal temperatures in New Zealand

River conditions and water quality

River clarity matters for trout growth because trout primarily use their eyes to locate their food.

If the water is too cloudy or milky then fish reduce feeding efforts.  This is often the case when waterways are stirred up by rain which is when trout stop feeding.

However when the water clears fish resume feeding which makes it a great time to go fishing.

When the water is not clear, trout have to abandon drift feeding in favor of active searching – which is energetically more expensive. 

In other words, water clarity reduces foraging efficiency with the result that trout spend more time (and energy) foraging in order to meet their food requirements. This results in slower growth rates. 

brown trout
Some of New Zealand’s biggest, trophy trout are found in exceptionally clear conditions. This allows them to maintain a position where they can spot their prey from a distance, then move effortlessly to capture the nymph as it floats past in the current.  This way they use minimal energy.

Here’s my Easy 7-Step Complete Guide to Cleaning Your Wading Boots. Do your bit to prevent the movement of invasive aquatic pests.

The impact of angling pressure on trout size

Lower angling pressure means trout get to grow bigger.  This is because fewer fish are being taken.

While New Zealand has a population of 5 million people, there are less than 100,000 annual fishing licenses issued each year. 

(Though there are other categories of licenses issued, eg. to international visitors, and those who only wish to buy a license for a day or a week rather than a full season.)

And for a small country, New Zealand has a lot of rivers, more than 110,000 miles (180,000 km) of mapped rivers fed by high rainfalls in the mountains. Unlike other countries, where you must pay to fish rivers, once you have purchased your fishing license (for NZ$133) you have access to 1,000s of fishing access locations for free.  

Most of our rivers are available as public/crown land, though there are some parts that don’t attract many anglers because they are in rough terrain and therefore difficult to access. 

Rivers as entities in their own right
Interestingly, there are unique circumstances now governing at least one New Zealand river.  

On 30 August 2012 agreement was reached that entitled the Whanganui River to a legal identity, a first in the world, with the river being given an individual identity "with all the corresponding rights, duties and liabilities of a legal person" which can be likened to a family trust, or company, or incorporated society." 

The river is represented by two officials, one from Māori and the other from the government.  This provides the river and it's wildlife with unique protections.  

‘Trout, after all, like their rivers the same way we do—refreshingly cool, crystal-clear and well shaded with trees.’