Kingsley Owen, of Broad Bay School, asks :-

How far at sea have salmon sold in New Zealand shops travelled?

Martin Unwin, a fisheries biologist with NIWA, responded.

Pacific salmon are native to the North Pacific Ocean, occurring in Japan, Russia, Canada, and the United States. Ocean migration varies widely among species and populations. Some Californian stocks stay within a few hundred km of the coast, but fish from other US populations roam as far as Japan.

Salmon from California’s Sacramento River were introduced to New Zealand from 1901 to 1907. All modern-day New Zealand populations are descended from these ancestors. The main stocks are confined to Canterbury and Otago, although smaller runs also occur on the West Coast.

Our knowledge of salmon migrations in New Zealand waters relies heavily on fish taken, as a by-catch, by commercial fishers off the South Island east coast. Most fish are caught within 100 km of the coast, in waters less than 200 m deep. This region is bounded to the east by low-nutrient sub-Antarctic water, and to the north by subtropical water too warm for salmon to thrive. As with their Californian cousins, it seems New Zealand salmon do not travel far from home.

Salmon can complete their life cycle without going to sea. New Zealand has several freshwater populations, in lakes such as Coleridge and Hawea, where hydroelectric structures block their seaward migration route. Freshwater salmon farms, like those in the upper Waitaki canals, exploit this ability to grow fish for the market.

Most farmed salmon sold in New Zealand are grown in sea cages. These fish spend their adult lives at sea, but remain captive throughout and do not migrate. If wild salmon are like high country merinos, left to fend for themselves and forage wherever they can find feed, farmed salmon are like sheep raised in a grassy lowland paddock.