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Old 24th February 2012, 08:54
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Default Counting Salmon using the Didson underwater sonar

Counting Salmon

Published by: Fisheries and Oceans Canada.

Like many biologists, George Cronkite has been with Fisheries and Oceans Canada for a long time; about 27 years. He first worked as a management biologist in the Yukon and for the past 16 years has been at the Pacific Biological Station in Nanaimo British Columbia, where he is currently the program head for Marine and Freshwater Acoustics.

A predominant focus for the Marine and Freshwater Acoustics group is methodology for counting salmon - adult salmon that return to rivers to spawn. Obtaining precise counts is critical and Mr. Cronkite’s group has focused on developing accurate counting methods.

There are three ways to count returning salmon: visually; through a method called mark-and-recapture; and using underwater sonar.

Visual counting can be the most accurate method, but it has its drawbacks. One cannot count fish at night, unless there are floodlights, which can sometimes disturb the fish. To count fish during the day or night, one must have clear water, and many rivers are only clear at certain times. For example, the Fraser River is never really clear in areas where fisheries officials must do their counts.

A third counting method is by DIDSON sonar. A DIDSON (Dual frequency IDentification SONar) is an acoustic camera which gives near video quality images for inspection and identification of objects underwater. When applied to fish biology it can be used to count fish either manually by the operator or in some cases by using motion detection software.

Mr. Cronkite explained: “The DIDSON system is aimed sideways across the river and we can count the salmon as they come up. Sometimes at these sites we need to build small weirs (a type of fish fence) to manipulate how the fish swim through the acoustic beam; this allows us to count them accurately. We also like to do experiments with these systems to fully understand how they operate and to determine if there might be any potential for miscounting. That’s why we do comparative studies. In clear water rivers we will use a DIDSON to count the fish, and at the same time we will visually count them. This allows us to check the DIDSON counts with a known accurate count.”

Due to the fact that weirs or fish fences actually work in very few rivers in BC, a lot of the population estimates, for example, of sockeye in the various rivers up in the interior, are done by mark-and-recapture programs. These counts are labour-intensive, requiring large crews operating over long periods of time on the river. They first mark the fish and then recapture them or look at the carcasses to obtain a population estimate from the tags.

Clearly, no system is perfect, and one system of counting does not immediately replace another. Researchers prefer some overlap between any two systems to compare counts. However, Mr. Cronkite notes, “The goal is to phase out mark-and-recapture where possible because it is labour-intensive and expensive.”

There are distinct advantages to acoustic monitoring. Mr. Cronkite stated: “Basically, you set up your acoustic system on the river and you aim it out so that you're satisfied you're covering the whole area where the fish are migrating. Then you look at those images and you count how many fish went by. Technically, you're counting every salmon that goes up the river. Unlike mark-and-recapture, the acoustic count is a count of something that you can see. At the moment, we usually do that manually, so the Tally Whacker (a hand-held mechanical counter) goes click, click, click, click. This method has proved effective up to many thousands of fish per hour.”

Another advantage of DIDSON is that researchers can leave it running 24 hours a day; go back each morning, collect and then analyze the data within a few hours, and have a count for the previous day. The DIDSON system is run by a computer and recorded data are backed up on an external hard drive. Researchers take the hard drive with them and then analyze the data later at their leisure. As it is a visual analysis of the data, it is easy to have non-acoustically trained people counting fish.

As to the accuracy of DIDSON compared to other systems tested, Mr. Cronkite notes the experience they have had at Qualark Creek, downstream of Yale, British Columbia. Mr. Cronkite stated: “The Qualark project is interesting. The river is about 150 metres wide there, but the current is extremely fast, so the fish are very bank-oriented. They're all within about 15 metres of the banks, so it ends up being a perfect setup for counting the fish. We started a counting project back in the '90s using a different type of sonar, a split-beam sonar, and developed the technology there to count salmon, the returning sockeye especially. But technically, it was a very challenging project. Switching to the DIDSON system made things much better. It's been much easier to get people to collect the data and count the fish. I also think it has increased our accuracy and precision.”

While DIDSON can provide pretty accurate numbers, it is not perfect. Its range is limited to about 40 metres (although biologists prefer to count fish within a 20m range) and when rivers are large and the fish you are trying to count are not near shore, its range limitation can be problematic. The DIDSON is also limited in its ability to distinguish species; a small chinook and a large sockeye, and they would appear the same in the image. Mr. Cronkite adds that sometimes there are behavioural differences between different species and one can usually tell the resident species from migrating sockeye because of the way they behave. “But”, he says, “Where we run into trouble is with different species of salmon migrating together, especially here on Vancouver Island where you might have coho, chinook and chum all migrating up the river together. That's where it can be troublesome.”

The Marine and Freshwater Acoustics group has been looking at other characteristics in the images, such as how fast the fish are moving their tails. This behaviour may relate to an individual species. Researchers think there is a possibility that the "tail-beat frequency" in pink salmon and sockeye might be different from each other. That part of the story remains a work in progress.

But the bottom line is this: When it comes to salmon counting, it remains critical to have accurate and precise estimates of returning populations, and being able to provide the best information to the managers so that the department can make decisions for the benefit of the fish. Says Mr. Cronkite, “That's the bottom line for me.”

And that’s the bottom line for the industry and all those who depend on an ecosystem that sustains the viability of west coast salmon. It all begins with the counting.
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