Large numbers of dead fish washing up on the shores of Osoyoos Lake may be a bit of a shock for tourists, but it’s not causing a great deal of concern among those monitoring the largest run of sockeye salmon the lake has seen since 1938.
Jeanne Berryman was staying at Walnut Grove Resort when she spotted some of the dead fish.
“This morning a friend and I were walking along the east side of Osoyoos Lake and saw a very disturbing site, hundreds of dead fish floating along the shoreline of the lake,” she said, describing them as looking lake trout of about four pounds in size. They were, however, sockeye salmon, which have also been washing up at the provincial campground at Haynes Point.
“They have also observed dead sockeye washing up on the beach there,” said Howie Wright, a fisheries biologist with the Okanagan Nation Alliance. The dead fish, he said, are the result of an oxygen/temperature squeeze, a condition that usually happens in September in the shallow South Okanagan lake.
The sockeye have been flooding into Osoyoos Lake in the tens of thousands since early July, and waiting there before beginning the final stage of their journey to the spawning grounds.
“Right now, they are waiting and sitting in the lake, or migrating into the north basin. They’re waiting for October, when the temperatures get better in the Okanagan River,” said Wright. The band of suitable water for the sockeye is narrow, he said, only about two to three metres.
That’s for both the juveniles, in their year of maturation before they make the journey out to sea, and the adults heading back to the spawning ground. Osoyoos Lake is broken up into three natural sections, and Wright said the dead fish are washing ashore in the shallower south and central basins, not the deeper north basin.
Wright said the water temperature is high, 20 C or higher, going down 40 or 50 feet. And below that level, the amount of dissolved oxygen in the water isn’t high enough to sustain the sockeye.
“Essentially, they get squeezed out. Unlike the north basin, the squeeze for the south and central basin has happened already,” said Wright. The stressful conditions means that some of the sockeye don’t survive and others become more susceptible to secondary factors, like parasites.
“In those lower return years, you may see one or two wash up. But on the larger return years, you will see much more wash up on the beach,” said Wright, adding that they are constantly studying the run to gain a better understanding. “Because of the size of the return, we are seeing more en-route mortality. In terms of how much, the scale or the magnitude, that is yet to be determined.”
Overall, Wright said the 2012 run is meeting early forecasts, which predicted more than 200,000 sockeye coming into Osoyoos Lake, though there has been a larger harvest of the fish south of the border than expected.
“Right through the whole Columbia river, there have been fisheries occurring,” he said. “Above Wells Dam, there have been a couple of other fisheries in the U.S. before they make it to Canada.”