Rainbow Trout

They are the same species, Oncorhynchus mykiss, living two different lives. A rainbow trout that stays in fresh water its whole life keeps the classic look, an olive back, silvery sides, black spots, and a rosy pink band along the flank. A steelhead is a rainbow trout whose young migrate down to the ocean, spend one to four years feeding and growing in the sea, and then return to fresh water to spawn; at sea it becomes chrome-silver and much larger, often several times the weight of a stream resident, and the pink band fades, returning once it is back in the river. The two forms can be born to the same parents, and offspring of a resident may go to sea while offspring of a steelhead may stay put.

Because it has been deliberately spread almost everywhere and, in many of those places, it eats or outcompetes the local fish and amphibians. In New Zealand and Australia introduced rainbow and brown trout have driven declines in native galaxiid fish and in frogs. In the mountain lakes of the western United States and the Andes, stocked trout eat the tadpoles and larvae of native frogs and salamanders that evolved in fishless water, contributing to steep amphibian declines. Rainbow trout also hybridise with and genetically swamp closely related native trout such as the westslope and Yellowstone cutthroat. The species is listed among the world's hundred worst invasive alien species for these reasons.

It is generally rated as one of the better fish choices. Rainbow trout is farmed in freshwater raceways, ponds, and tanks, and increasingly in recirculating systems, which have less impact than open ocean net pens, and it converts feed to flesh efficiently. The main concerns are the sourcing of the feed, which contains fishmeal and fish oil, and local water pollution from farm effluent, both of which well-run operations manage. Trout raised in the United States, and steelhead labelled as farmed, typically score well on seafood guides. Wild steelhead, by contrast, includes threatened and endangered runs and is best avoided unless the source is clearly a healthy population.

A stream rainbow trout is often only 25 to 40 centimetres and half a kilogram, limited by a short growing season and modest food. In a big productive lake, resident rainbows commonly reach 2 to 5 kilograms. Steelhead, having fed in the ocean, are larger still, typically 2 to 5 kilograms and sometimes much more. The all-tackle world record, a fish from Lake Diefenbaker in Canada in 2009, weighed about 21.8 kilograms, though it was a farm-escaped fish selected for fast growth. In their native streams, a wild rainbow of a kilogram is a good fish.