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What a new study revealed about how some Chinook salmon migrate

Researchers say data supported calls to restore the lower Green-Duwamish River to prepare a better habitat for Chinook salmon.

KING COUNTY, Wash. — Scientists with the King County Department of Natural Resources and Parks recently completed a study examining how Chinook salmon use habitats in the lower Green-Duwamish River as they migrate. They placed small transponders inside young, wild and hatchery Chinook salmon to document their movements as they passed a specialized floating antenna near Tukwila.

"The biggest finding was yes, the fish are actually staying here and we documented juvenile Chinook staying in the Green for two months, which is huge," said King County Water and Land Resources Division Fisheries Biologist Chris Gregersen. "This is something we've never known before."

KING 5 spoke with Gregersen and with King County Lower Green Basin Steward Katie Beaver about what they found and what it indicates about the potential impacts of restoration in the area.

What was the purpose behind this study?

Gregersen: The need arose first. We had a need to collect data in the lower Green for how fish were using it, and we've tried for years but most of the traditional ways we have of looking for fish just give you a quick snapshot. You can find that fish and you know they were there for that time and that habitat. You don't know how long they're there, you don't know if they're going to survive their journey, and you don't know where they're going. So we needed something to tell us how long those fish are staying there and give us a bigger picture of life in the Green River. We tried a lot of different ways and recently some new technology became available to us. This tagging system is unique because when you insert a tag into these fish, it gives you a unique number for each one. So as the fish moves through the river, it becomes detected again by an antenna further downstream, and it gives you that unique identification number. So then you know the story of that fish from when you released it to when you detected it again. 

So the reason we needed this information was because the lower Green for a long time is highly developed. A lot of historical habitat is gone. So there's been a big question of whether salmon actually use it or not. So we need to know this because if we're going to spend millions of dollars on salmon recovery, we need to make sure the fish are actually going to use the habitat we create. So for a long time, we have wondered if fish are actually using it but we've never actually been sure. We've been investing money in this restoration so as soon as we had this opportunity to use this technology, we wanted to find out if we could answer that question. 

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Did you find answers to those questions and what did the data show?

Gregersen: Three of the big findings from this study were, first of all, the new technology we used performed great. So these really small tags you could fit into juvenile salmon – smaller than 2 inches – they worked great. The new antenna we used was a floating barge platform. We actually anchored it from a bridge and it was able to operate under any flow conditions and shed debris, whereas the older, more traditional antennas are prone to breakage from debris. The first finding was yes, this technology was awesome for this study. 

The second finding – and the biggest one – was yes, the fish are actually staying here and we documented juvenile Chinook staying in the Green for two months, which is huge. This is something we've never known before. So we actually found the smaller fish were staying longer. The larger fish ended up moving through pretty quickly, and it was those really small ones that were staying for the longest time. That's important because these fish have different habitat needs as they grow. So the smaller fish have different habitat requirements from the bigger fish. So knowing which fish are actually staying in the lower Green helps us tailor how we build our restoration projects and provide that kind of habitat.

Finally, the third big finding from this is that a lot of our tagged fish actually went missing. We had a good idea for how the barge worked because we tagged fish and released them just upstream as kind of a test to see how well the technology worked and how efficient the barge was at actually detecting these fish. So comparing those results to how many fish we detected that were released further upstream, we noticed a big difference. So there's definitely a concern we're losing a lot of fish in this area of the river, and it's something we're going to follow up on in the future.

Beaver: Chris' research really justifies the work I do and has provided a lot of justification for our continued application for our grant funds and trying to drive and accelerate the pace of restoration in the lower Green. Before having this data, we all suspected it was important to restore this urban basin but to have data that really backs up what the juvenile fish are doing really adds a lot of legitimacy to the strategy we're pursuing in the lower Green. I think it also compels us to think more creatively about the limited space we do have in this urbanized basin and fight for all those remaining opportunities that exist to improve the habitat for these little fish we know are using it.

What can you now use this information to do and what happens next in the process?

Beaver: A big part of it is, we already have these salmon recovery plans and let that kind of guide the work we do and the facts of the projects are in the plan. But I think this data that Chris has generated helps us make a strong, compelling case for funders that we should be focusing on funding projects in the lower Green basin versus in the more rural, wild parts of the river. In the past you could make a compelling argument that we know the fish are using that habitat. With the data we had of the lower Green it was, "Hmm, is that the best use of the resources?" This drives us to say, "Yes, this lower basin is a good use of our resources and it will help us design projects that these little juvenile fish will use." It also relates to Chris' prior work that these fish also really love these little tributaries in the lower Green, so it's focusing on new data showing best conditions to support these juvenile fish.

Gregersen: Restoration is really expensive. It's hard to get the money for it. There's way more work that needs to be done than money to pay for it so making sure we spend it in the best way possible is one of our primary goals. So this helps us tailor our salmon habitat plan so we're making sure whatever money we do get is spent in the best way possible. On a bigger scale than that, this is also showing a lot of these urban watersheds that often get written off as poor habitat or not worth restoring. This information is really showing that, yes, indeed, the fish are using them and they are important to these fish. So it's something that will give us positive results if we invest time and money into them.

Beaver: And I think from a cost-benefit perspective, land in the urban part of the watersheds is so expensive. These projects are always going to get more expensive as you get to urban reaches of the river. If we build it, they'll come.

What does restoration entail?

Beaver: We have multiple strategies and we're talking about a really constrained river right now that has levies on both sides of the river for most of it. We're talking about setting back those floods, enlarging the floodplain, creating more room for the water to flow, more kind of edge habitat for fish to use. We're talking about creating side channels to create an exit ramp off that superhighway of the river for fish to go off and eat and rear and get refuge during high flow events. We're talking about investing in the right buffers on this river in places that are quite devoid of trees and improving that habitat. Even in this really urban part of the river, since we know the fish need it.

Why are these fish so important to our area?

Gregersen: These Puget Sound Chinook salmon are listed as threatened and these have been a keystone species for our region. The region they built around the salmon resources we once had. We know they're important. They're important for tribes, they're important for wildlife and as food for orcas. It's something that is a major part of our ecosystem and it's something that affects everything going on in the river, in Puget Sound. It's something we need to take care of and it will take care of the rest of our ecosystem. 

What else is important to understand here?

Gregersen: A lot of times when we think of salmon habitat, we picture these wild places in the forest with clear flowing water and it's often overlooked in places that we are every day. It has been eye-opening through this work to actually see the places where salmon are trying to live, and it's right before our eyes. The lower Green, the Duwamish, just right in Seattle – these are all places overlooked every day but they are important places for salmon and for us to take care of.

Beaver: These urban basins – which we know are important for salmon – they're also home to many people and we know a healthy river for salmon is also providing a lot of benefits to the surrounding community. Through healthy forests, cleaner air, cleaner water, a healthier ecosystem for those people to live. So we think there's a lot of co-benefits for both our local community and having healthy, thriving wildlife populations.