Where do the salmon go when they leave our rivers is a good question. Gavin James of NIWA talks about what they are eating while at sea in this article:
It is reasonable to assume that they are going to follow the most readily accessible food supply. But they don’t like warm water which is a large part of the reason salmon have not become established in the North Island.
According to New Zealand King Salmon chief executive Grant Rosewarne, warm sea temperatures at the company’s Waihinau Bay farm, in Pelorus Sound, had contributed to the deaths of a large number of salmon. Rosewarne would not say for commercial reasons how many salmon had died, or how many fish were at the farm, but said the mortality rate was a “multimillion-dollar problem to solve”. Water temperatures at the Waihinau Bay farm had stayed above 18 degrees Celsius for three months, Rosewarne said. “I don’t think we’ve ever had it quite as bad as this year.” King salmon cannot regulate their body temperature. They function best when water temperatures are between 12C and 17C.
So I’m picking they won’t be headed very far north once they leave the Rakaia and Waimakariri Rivers. Some years sea water temperatures are warmer than others. Temperatures in the deep ocean are colder and more constant than surface temperatures. So salmon may go deeper when the surface water is too warm. However they also have to eat while at sea so will have to move to where the food supply is in the water column.
Sea temperatures in Canterbury have been warmer in the last few years. A snapper was caught recently at Birdlings Flat, and yellowtail kingfish have been taken this summer in Otago Harbour. There have also been a lot more kahawai around the Canterbury coastline. These species are generally associated with warmer waters further north.