Where Do Mature Grayling Go in the Gulkana?

An ADF&G staff member surgically threads a telemetry tag antenna through the body of a grayling, while another staff person hydrates the fish’s gills. Photo courtesy of ADF&G.

By Allison Sayer

This February, WISE hosted Alaska Fish and Game (ADF&G) biologist Corey Schwanke to learn about his research on Arctic grayling in the Gulkana. Schwanke shared that while thousands are harvested annually, fishery managers still did not have a lot of basic information about their local habits and distribution. 

In particular, prior researchers had difficulty determining where the largest fish went through the seasons. Through a partnership between ADF&G and the Bureau of Land Management (BLM), Schwanke led a series of studies to better understand mature grayling distribution in the Gulkana system. 

Grayling in the Gulkana River travel all over the watershed to spawn, summer, or overwinter. They are found in high densities in the Gulkana and are “easy to catch.” For one study, Schwanke and a small team landed about 2,500 fish over the course of a few days with rod and reel. 

In the Gulkana watershed, fish over 11 inches long are considered “mature.” Although grayling can be long lived in colder waters, most Gulkana grayling are three or four years old. Gulkana grayling over seven years old are rare. Schwanke believes this has to do with the warmer water in the Gulkana compared with some other areas in the state. 

Schwanke discussed three studies in his talk. The first was a telemetry study conducted in 2017 to examine seasonal distribution of mature fish and search for spawning areas. Schwanke and a team surgically inserted telemetry tags into 126 fish throughout the mainstem of the Gulkana and then searched for the tags through the seasons. 

Schwanke found some predictable results, and some surprises. 

Many tagged fish clustered in a deep spot on the main stem above the Sourdough Creek outlet for the winter. Others grouped below the Sourdough Creek outlet. A few scattered along other portions of the mainstem. There was a cluster near Stump Creek, later revealed to be of greater importance to local grayling than had been known. One group was scattered along the west fork of the Gulkana, which surprised Schwanke because he said “there’s usually not hardly anything going on in the west fork.”

Schwanke searched for tags again in late May, the peak of spawning season. Clusters of tags were found in and near Sourdough Creek as expected. Fish were scattered throughout the main stem, and one had swum all the way up to Dickey Lake. Fish also spawned in the west fork of the Gulkana, which was not expected. The biggest surprise was a large concentration of spawning fish in Stump Creek and above and below its confluence. 

Hungry Hollow Creek. Photo courtesy of ADF&G.

Poplar Grove Creek is a known spawning area, but only one tag was recorded there during the spawning season in this study. Schwanke thinks there would have been more if the researchers had tagged more fish lower in the watershed. There are also some years where the fish can not get up Poplar Grove Creek and go elsewhere. 

In the heat of midsummer, tagged fish were observed scattered along the mainstem, up several streams, and even above Paxson Lake. Tagged fish were not observed in the west fork of the Gulkana. 

Schwanke noticed one cluster of fish - thirty percent of the surviving tagged fish - in Hungry Hollow creek in midsummer. He decided his next study would investigate Hungry Hollow Creek more. In 2019, his team performed a mark recapture study and collected stream temperature data. 

Schwanke learned Hungry Hollow creek is very popular with mature greyling in the hot part of summer. In six kilometers of the relatively small and shallow stream, he estimated a density of 4,800 fish above 11 inches long per kilometer. The estimated population of mature fish was 29,000 in that little stretch. 

Temperature data showed Hungry Hollow Creek was substantially colder than other measured parts of the watershed. Water temperature was recorded 24 hours a day by data loggers in Hungry Hollow Creek and several other points through the study area. Hungry Hollow Creek was shown to be substantially colder than the other parts of the watershed that were measured. 

Figure courtesy of ADF&G.

The Paxson Lake outlet and the lower west fork were above 70 degrees Fahrenheit for many afternoons in June and July, and sometimes above 75 degrees. Upper Hungry Hollow Creek was never found to be above 60 degrees during the study period, and was often colder. Schwanke shared that 58 degrees or colder is healthy for grayling. 

The summer of 2019 was an exceptionally hot and dry summer. Schwanke surmised the extent to which grayling would concentrate in a small, cold refuge such as Hungry Hollow Creek could fluctuate with variable seasons. However, he shared that he has visited Hungry Hollow Creek in subsequent years and observed a high density of grayling in midsummer. 

In 2020, Schwanke decided to start at Hungry Hollow Creek in the summer and see where the grayling went from there. He observed an exodus from Hungry Hollow Creek in late summer and into the fall. 

There were some issues with survival and technical issues with tags in the 2020 study. However, Schwanke observed a group of tagged fish swam about 80 miles to the west fork within six weeks of tagging, to be joined by others for the winter. Some other fish scattered to interesting locations such as the north side of Paxson Lake. 

Grayling tagged in Hungry Hollow Creek were shown to overwinter and spawn throughout the drainage. Schwanke said that, despite low sample sizes, this confirms the importance of Hungry Hollow Creek to the population. The following summer, over half of the surviving fish were located back in Hungry Hollow Creek. 

If you would like to view Schwanke’s hour-long talk, visit https://www.wise-edu.org/science-lecture-series.html.