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In Search of Salmon — Eavesdropping in the Juan de Fuca Strait

It’s 5 am as we head out onto the outermost pier, where our small research vessel awaits. Fog clings to the surface of the water as harbour seals play in the marina’s kelp beds below. Our team from UBC led by Dr. Mei Sato is on a mission to assess the availability of food for southern resident killer whales foraging in Juan de Fuca Strait. Mildly exhausted from our early-morning wake-up, the 6 of us cram into a boat, squeezing ourselves in amongst scientific equipment worth more than our cumulative annual salaries.

Today we’re comparing the sizes of salmon we catch with the acoustic signals we see on the most expensive instrument on our boat – a special underwater sonar that will hopefully provide UBC’s Marine Mammal Research Unit with unprecedented levels of detail.

Chief scientist Dr. Mei Sato describes how it works: “[The sonar] uses four frequencies. The lower frequencies only detect large animals like salmon, while the higher frequencies pick up both large and small animals – a range that spans everything from salmon to zooplankton. We can subtract the different size classes of animals recorded by the different frequencies to separate fish from zooplankton.”

Commercial and sport fishermen have been using consumer-grade sonars to find fish and their bait for decades, but the accuracy of our sonar system is considerably higher. The sonar is so accurate in fact, that it is possible to discern between different fish species. In order to test the accuracy, however, we have to catch and measure the fish that show up on the reading to see if our prediction was right.

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