Homer Alaska - Seawatch

Story last updated at 8:24 PM on Wednesday, February 15, 2012

New website explains how salmon counted

The Alaska Department of Fish and Game is touting a new website that shows off the sonar and other technology used to help gauge salmon runs in-season and explains the various tools used to count how many of which kinds of salmon are swimming up Alaska's most important rivers.

The site features video of the newest technology, the DIDSON sonar, which stands for Dual frequency IDentification SONar, showing the difference between king and sockeye salmon in the Kenai River, seals chasing sockeye salmon, and a hooligan migration, among others.

Like other technology heavily relied upon by the fishing industry, such as GPS navigation, the DIDSON sonar was originally developed by the military, in order to detect divers and underwater mines. ADF&G began testing it as a fish counting tool in 2002, and claims it has considerably advanced sonar fish detection.

In a side-by-side study of sockeye salmon escapements on the Yentna River in the Susitna River drainage that compared the DIDSON and the older sonar technology, the Bendix, there was a stark difference between the two.

In 2006, the Bendix showed an escapement of 93,000 sockeye, while a weir count came up with 126,000. The DIDSON counted 160,000 sockeye.

The next year, the Bendix read 80,000, the DIDSON 130,000, the weir study 97,000 sockeye.

Those differences have raised escapement goals in some streams, not because ADF&G wants more fish to make it to the spawning grounds, but because it became obvious that more fish had most likely always been making it to the spawning grounds, they just were not all getting counted.

Although 11 of the 15 sonar sites ADF&G uses to count salmon now exclusively have DIDSON sonar, one in the Crescent River north of Anchorage still relies solely on the Bendix, the department also uses a third sonar technology, the split-beam sonar.

The split-beam sonar can cover longer distances, nearly 1,000 feet, compared to about 100 feet for the Bendix and 165 feet for the DIDSON, and is used in the broad Yukon River and one Kenai River site to count king salmon. Although it can detect those far-away fish, and can determine which direction they are going, it cannot determine their size, unlike the DIDSON, which is useful in telling king salmon from sockeyes.

However, size alone is not enough to determine species when there are five different varieties swimming up many rivers in Alaska, and even some king salmon are not as large as the bigger coho and chum salmon. Even the largest pink salmon ever recorded was 15 pounds, bigger than most sockeye.

To counter that problem, ADF&G uses a variety of methods, according to the new website, including weirs, fish wheels, counting towers, and mark-and-recapture studies to determine the proportion of species in the run.

ADF&G biologists have also been testing methods of using DIDSON sonar to identify salmon by tail beat patterns.

The website covers in detail how each of the sonars work, as well as all of the non-sonar methods, and can take visitors on a tour of each of the 15 sonar sites ADF&G operates, 4 of which are on the Kenai Peninsula. It also offers insights into escapement goals and terminology.

Find the website at http://alaskafisheriessonar.org/index.html.

A story by KUCB public radio in Unalaska indicates that the vicious ice conditions in the Bering Sea that have essentially shut down the opilio crab fishery could have a future impact on fish stocks, depending on how long it lasts and how quickly it melts.

The ice is now covering an area roughly the size of Texas. The thirty-year average is about a third less than that, covering an area more like California.

Part of the reason for the increase is a persistent low-pressure system in the Arctic. In other northern waters, the sea ice extent is shrinking. National Oceanic and Atmospheric Administration climate scientist Jim Overland says the Bering Sea frequently doesn't track with the rest of the Arctic. As average Arctic ice extents have gone down over the last 30 years, the Bering Sea has actually seen a slight increase. Overland says that's because despite climate change, the Bering Sea ice is mostly dictated by storms.

"The Bering Sea continues to do its own thing — this year and in the foreseeable future and that's based on how the North Pacific storm tracks behave," " he told KUCB.

Since 2006, the Bering Sea has been in a cold phase. But before that, from 2000 to 2005, the Bering Sea was at the other extreme, with record warm temperatures. According to Overland, these blocs of temperature don't represent any pattern, and they can have a huge impact on the ocean ecosystem.

Back in the 1970s there was a period of temperatures as cool as the current ones that Overland says caused a major reorganization of the Bering Sea ecosystem.

"The fact that pollock now dominates after the 1970s ... perhaps the loss of cod in the 1930s was also related to one of these five year events," he said.

But Overland is less sure what this period of cooler temperatures could mean for ocean ecosystems, especially give the stretch of warmer weather at the beginning of the millennium.

"If we had continued to have a whole decade of warm temperatures we might have precipitated some major extent in the ecosystem, but I think the fact that we've have four cold years, we've returned back to where we were in the 1980s and 90s."

Alaska Fisheries Science Center biologist Kerim Aydin agrees that the annual ice extent has a big impact on fish populations, but is cautious about this year's effects.

"It's not so much about how much the ice covers at this time of year, but how fast it melts back into the spring and summer."

Aydin says the biggest impact is on pollock, but also snow crab. He says consensus is emerging in the scientific community around the idea that colder springtime temperatures are better for pollock stocks because of how they affect plankton growth.

However, a recent study also showed that colder Bering Sea temperatures could be responsible for smaller runs of Bristol Bay salmon in the past two years.

The ice extent is expected to remain high for at least another month.

Find the whole story at kucb.org.

Cristy Fry has commercial fished out of Homer and King Cove since 1978. She can be reached at realist468@gmail.com.