Resident Chinook in Alaska?

 

Lake Creek Chinook captured angling (Scott Slater)

Lake Creek Chinook captured angling (Scott Slater)

See the paper here, or e-mail me for a PDF.

A few years ago, we learned about a fishing lodge on Lake Creek, a tributary of the Yentna River, which drains into Cook Inlet, Alaska. For years the guides and clients at the lodge had observed a curious phenomenon; small, male Chinook salmon feeding on salmon eggs weeks or months before larger Chinook arrived to spawn. Anglers are in general keen natural historians, and they recognized that the Chinook they were seeing may have been non-anadromous, which would be essentially undocumented in a natural system with ocean access.

There were a few reasons why the anglers thought these might be resident fish:

  • They caught them really early in the season. Weeks before any large anadromous fish were in the river, these fish could be found.
  • They were all males, and some were quite small (< 300 mm fork length or ~11.5″), which seemed smaller than the jacks they were expecting to see.
  • They were oddly colored; many were a deep olive green with few individuals looking like bright, recent arrivals from the ocean.
  • They were aggressive feeders. Anglers regularly caught these little Chinook while fishing for rainbows. Scott has even caught them on dead drifted nymphs and even mouse patterns!

Tom Quinn and I collaborated with Scott Slater, a guide at the lodge, to initiate a small research program to investigate a potentially unique Chinook life history. Scott began collecting some basic body size data, and archived otoliths and muscle tissue. The otoliths were used to determine if the fish had ever been to sea by analyzing the ratio of strontium to calcium at different points along a transect from the core (juvenile) to the edge (recent). Sr/Ca is generally high in the ocean, and low in most fresh waters (the Yentna is no exception). Similarly, we analyzed the stable isotopes of carbon and nitrogen in the issues, as these can also be indicative of life in marine or fresh water.

Along the way, Scott also kept track of what was in the gut of these fish. The fact that these fish were readily taking egg patterns while people were trying to angle for rainbow trout was one of the first indicators that they might be resident fish. Scott found lots of invertebrates in the guts which is curious for semelparous salmon returning from sea.

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Inverts found in the gut of a small Lake Creek Chinook. (Scott Slater)

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More invertebrates in another small Chinook gut. (Scott Slater)

So, what did we find?

  • Stable isotopes indicated a pretty strong marine signal. We compared these to large, clearly anadromous, fish, and found that the little guys were actually more enriched. This is probably not an indication that the small fish were feeding at a higher trophic level than the large fish, but rather, an indication of a more coastal distribution of a fish that is only at sea for a year or less. Still, it is possible that as egg eaters, they could have a marine stable isotope signature without going to sea.
  • The otolith chemistry showed clear extended residence in waters with elevated Sr/Ca. In this system the Sr/Ca is very low in fresh water, so they would have to go to sea to pick up a signature like this. But, the signature was pretty variable, and might indicate a marine residence that was coastal and possibly estuarine.

Jack Chinook often return earlier than older age classes, but Lake Creek appears to have a very robust population of them, and they come back very early. How important their feeding is to their ecology remains to be seen. In addition to the unusual consumption patterns, we would like to know where these fish go at sea.

A few more photos of these little kings:

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Lake Creek jack Chinook (Scott Slater)

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Lake Creek Jack Chinook (Scott Slater)

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Scott Slater with a slightly larger Chinook in Lake Creek