By Jenny Neyman
The black computer screen lit up with blueish flashes moving across the window. It looked like a maternity ultrasound, but the images weren’t depicting the developing limbs of a baby. Clear as day — or clear as night with a high-powered flashlight — the display showed fish swimming by.
“When we went from split beam to DIDSON it was like turning a flashlight on underwater because you could ‘see’ so much more. You could make out what was going on so it was more than just these funny squiggly lines,” said Jim Miller, Kenai chinook sonar project biologist with the Alaska Department of Fish and Game.
The difference in the king salmon sonar program in the Kenai River 10 years ago to today isn’t quite “I-was-blind-but-now-I-see” biblically dramatic, but the advancement is revelatory.
When Miller started in Alaska’s salmon-counting sonar program in 1992 on the Nushagak River, Bendix sonar was the technology of the day. The interface spat out data on a paper tape and displayed the echoes bounced off the fish on a tiny oscilloscope screen.
“You could see the blips on the oscilloscope as the fish went by and that’s all you had — a blip on an oscilloscope and a tickertape,” Miller said.
On the Kenai, king counting used to be done with split-beam sonar. The echoes from the low-frequency sound waves appeared on a computer screen as a series of dots in patterns called fish traces. Technicians would count the fish traces to determine the number of fish passing by, and use the pattern of echoes to determine whether it was a larger fish — a king salmon — or something smaller, such as a sockeye.
“And then split beam, you had an echogram where you could see squiggles of fish going through. They were just squiggles, but you get the ‘S’ shape to them so they look like a fish swimming through,” Miller said.
While S-shaped squiggles were an improvement, split-beam left a lot to be desired. It was difficult to differentiate kings from other fish, and to tell fish apart when swimming next to each other.
The sonar location, too, was a challenge. At River Mile 8.6, it was close enough to the river mouth that water levels were tidally influenced. At higher water fish could swim behind the transducers and be missed. It was assumed that kings stuck to the deeper water midriver while sockeye preferred to hug the shore, but biologists eventually realized that wasn’t always the case at that site, with water levels and currents being variable.
Plus, the site was continually plagued by debris.
“On the outgoing tide was the possibility of damaging your gear because of the big trees. At the end of last season we actually had a huge tree come down and hook onto this sonar and pull it downstream about a half mile and snap the cable. The only reason we were able to retrieve it is because the guy on shift saw it going down and so he jumped in the boat and followed it,” Miller said.
A new era dawned in 2002 with DIDSON technology. The dual-frequency identification sonar emits high-frequency sound waves and displays the bounced-back data as videolike images. The imagery is so clear biologists can “see” fish swimming by — and river otters, and seals and whatever else happens to float through the beams.
“We have the occasional, ‘What the heck is that?’” Miller said, bringing up a clip of a circular object with an appendage of some sort floating through the sonar. “That’s bizarre junk of some kind. We thought maybe a tire, or a bucket, maybe? Normally buckets have a rounded handle on them. Whatever it is it’s circular, but it’s got something hanging out back here so I don’t know.”
DIDSON allows the sonar technicians to pause, play back and save the images for later analysis, more-easily tell fish apart even when jumbled together, and even measure a fish on the screen.
“We can stop it, zoom in on it and then just using mouse clicks measure the contour of the fish from snout to tail. That gives us a length, so that’s a 90-cm fish there, so definitely a king,” Miller said, demonstrating with fish images onscreen. “And we can do the same thing here — it’s a 67-cm, that’s a sockeye-sized fish there.”
DIDSON was tested from 2002 to 2009 and replaced split-beam in the Kenai in 2010. The king sonar program improved yet another level when it moved upriver in 2013 to a new site at River Mile 14. The river channel is narrower there, there’s less debris to damage the equipment, water levels are more constant without tidal influence and the current is more steady, so kings do tend to stay in the middle of the channel with sockeye closer to shore.
But when kings occasionally do stray from midriver, the sonar is more likely to see them at the new site. Even with an island breaking up the channel, the sonar techs are able to insonify the entire river, deploying a transducer on the channel behind the island and another four on the main channel.
An upgraded version of the technology, called ARIS, came out in 2012, and has added even more functionality to the post-processing software. All in all, Miller said that this summer has been the best in the Kenai king sonar program’s history.
“We’re very happy with the way things have gone this summer, and the equipment,” he said. “… It’s definitely leaps and bounds from where it was when I first started out in ’92 and here in the Kenai from 2000 to now, both in our ability to more accurately estimate fish passage, as well as to be able to see what’s going on in the river with the video-quality images. It’s awesome.”
That’s not to say further improvement isn’t possible, of course. There is still the occasional downed tree that snags the gear, which is mostly just an inconvenience these days as the sonars take readings in such a way as to produce backup data if one transducer goes out.
And the technology can’t yet differentiate a small king from a sockeye. Given that there are less big kings returning to the Kenai than there used to be, especially in the early run, that’s increasingly problematic. It’s dealt with by a test net, where a sample of salmon are caught and counted, and the proportion of kings to other salmon is applied to the sonar results to produce what’s called a mixture model estimate.
The downside to DIDSON and the newer ARIS is that, being high-frequency sonar, they have a limited range, to 35 meters. That’s fine to span the 90-meter width of the river at Mile 14 with two transducers covering the near-shore area off each bank and two covering the offshore area. But there is a loss of resolution toward the end of the range.
“You get really good resolution at close range — even the rocks and stuff. And then at far range it’s a little bit less,” Miller said. “It’s more pixilated when you get offshore like this. Someone referred to it as HD vs. regular.”
Who knows what the next advancement in technology will bring. In the meantime, Miller counts his blessing that the Kenai sonar program has come as far as it has.
“I know that I feel a lot more confident in what we’re seeing now than what I did back then. I remember coming to the project and looking at those echograms and saying, ‘How can you tell if that’s a king or a sockeye?’ You could look at the target strength (a way to differentiate kings from sockeye with split-beam sonar) and the target strength, as we found out, was just so variable it wasn’t doing what it was supposed to do, vs. now where you can actually see the size of the fish swimming through.
“I just think it helps build people’s confidence when they learn about the project. I think the more people learn about it, the more confident they’ll be in what we’re putting out for estimates.”