Brown trout vary greatly in size, shape, and colouring. River-dwelling fish are brownish-grey, with red and brown spots on the back and sides, and average 1 –2 lb in weight. They are usually non-migratory, being more or less stationary, spawning in suitable gravels near their feeding grounds. Large river-dwelling fish, up to 8 lb, may be taken from the headwaters of rivers. Some brown trout are migratory, living in estuaries or the sea and moving upstream in the autumn or winter to spawn. Fresh-run fish from saline water are silvery in appearance, but once in freshwater soon darken, resembling river-dwelling fish in colour. Adult brown trout of this migratory type are larger than river-dwelling fish, commonly reaching 6 lb or more. The young fish resemble the river variety in colour, until at the end of their first year when they turn silvery and migrate down stream in small shoals.
The small river trout feed on the larvae of aquatic insects, while the larger estuary and lake-dwelling fish live on small fish (bullies and smelts). The spawning habits of brown trout are similar to those of rainbow trout.
Lake-dwelling brown trout are also migratory, moving into tributaries to spawn. The number of fish in a lake is often controlled by limited spawning facilities in the adjacent streams, but with the abundant food available in the lake, the adults are able to grow to a considerable size. Lake browns average 6–8 lb and tend to become yellowish in colour.
by Lawrence James Paul, B.SC., Fisheries Division, Marine Department, Wellington.