New Zealand Trout Adventures

Reinvestigating Searun Brown Trout

We are truly blessed that in New Zealand we are able to fly fish for sea run brown trout. New Zealand’s backcountry rivers are being loved to death by anglers and the time is upon us to spread the load. Our lowland rivers and estuary’s all hold quality trout fishing and in certain areas, Sea run trout.

The South Island has an abundance of rivers that Searun browns are available to the flyfisher. In some rivers the only trout in attendance are in fact sea run. These rivers are often small and unstable in the middle and upper reaches but have a population of sea run fish that frequent the lower reaches in spring in conjunction with the Whitebait runs. These fish often disappear once the easy food source has vanished.

As a fly fishing guide I am often asked about searun brown trout. The most common question is about their anadromous behaviors. It is my feeling these fish are not anadromous in a true sense. I believe the Brown trout spend time in and just outside the river mouths during spring and early summer. They are seldom encountered more than 500 meters away from river mouths. They seem to follow food such as whitebait and small mullet into the rivers as the tide rises.

My thoughts on how these Brown trout become sea run trout stem back to the beginning of their lives as fry in the headwater spawning streams. Brown trout are naturally territorial and find it difficult to coexist with each other. This is very evident in our rivers where pools may only be inhabited by one or two trout. These trout are usually apart within the pool occupying different feeding lies. As the fry develop and grow so does their need for space. To find this space they must move downstream. As the fingerlings move downstream looking for a niche of their own they end up fighting other fish from their year group. It is as always the weaker of the fish that end up missing out on these prime pieces of real estate. Those fish just keep moving down river until inevitably they end up in the estuary. It is here that the trout adapt a foraging feeding behavior to obtain food. There is a large variety and abundance of food available to them in the tidal reaches of New Zealand’s rivers. I have found Shrimps, Crabs, Mullet, Whitebait, Snails and even Flounders in the stomach contents of Sea run trout I have eaten.The fish put on weight quickly in this food rich environment.

The appearance is often different to the fish who have remained upstream. They take on a silver sheen and the blue on the gill plates is far more prominent. The spots along their flanks are large and have an irregular shape. When caught and handled some of the small scales come off on your hands or the net.

In their third year in early summer the urge to spawn kicks in, as it does for all trout. The estuarine based fish begin to move upriver on their quest for the headwater spawning streams. After some time upriver the fish begin to take on the appearance of the resident river fish. The colors begin to become rich along the lower flanks and their backs take on a brown or green hue. The spots still remain large and the scales still flake. It is very often that these fish are encountered by fishermen in the upper river in late Summer and Fall. The searun fish are able to spawn with the regular river trout as genetically they are identical. A lot of the time these Searun fish are dominant on the spawning gravel due to their superior condition from the high protein diet.

Once the trout have spawned I believe it is the fish who have come upstream from the river mouths return to that area. These fish know about the abundance of food available there. This theory is backed by the observation that from August onward there appears to be an amount of post spawning fish in the 3 to 5 pound class. The majority of the fish are still maiden fish of around 2lb but are always in great condition.

Far more research needs to be done into the habits on New Zealand’s sea run trout. It is important to know as much as we can about the movements of these fish when considering potential hydro schemes and other water usage consent applications. In some fisheries our headwater trophies are supplemented by searun trout from the lower river. Estuaries provide our upper river with nursery areas for young fish to develop. These lowland rivers and estuaries need protecting from unnecessary agricultural development. Hopefully the research to prove the recruitment value of searun trout to our upper rivers can happen sooner rather than later.

Anton Donaldson


6 CommentsLeave a comment

  • Anton – this was really useful. My son and I are visiting the very north of the South Island in late Jan to mid Feb in 2020. Hopefully we can stumble across some of them.

  • Just when do they start coming in to our ri er mouths , I was watching a good fish the other nite feeding , could of been Karwai I got sum nice ones last year but wasnt sure how early it was cherrs

    • I find they increase in numbers from august onwards. I saw a good number of kahawai two days ago just off the river mouth. Good luck out there!

  • Anton I agree your not alone in that greater research needs to happen worldwide, I’m sure that some of the remotest rivers in New Zealand have been populated by sea run browns, where I’m from Wales seatrout or as we call them sewin fishing is a religion and it wetfly tactics at night, think that its a well underrated resource you have over there,
    P. S enjoyed catching kingfish you live in a great part of the world