Chinook Salmon Research Initiative
Unuk River Chinook Salmon

Unuk River Chinook Salmon Research

Overview

Unuk River watershed.
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The Unuk River is a glacial system located approximately 85 km northeast of Ketchikan, Alaska. Over the past couple of decades this river has supported runs of Chinook salmon averaging about 5,500 large (essentially 28” and greater in length) Chinook salmon. After leaving the freshwater as juveniles, these fish primarily rear in or near Southeast and this stock is caught incidentally in sport and commercial fisheries in northern and southern Southeast Alaska. Data from this project is used to estimate full parent year production estimates including details on harvest, exploitation rate, and marine survival.

Spawning abundance is germane to estimates of large (essentially 28” and greater) spawning fish determined using a mark-recapture study that has been conducted each year since 1997 (Hendrick 2008). Prior to 1997, aerial surveys were conducted on a consistent basis in 6 clear water tributaries. Nevertheless, estimating the annual escapements using mark-recapture methods is a substantial improvement over aerial surveys for population assessment and modeling (Jones III et al. 1998).

An escapement goal of 650–1,400 large fish was developed for this stock in 1997 (McPherson and Carlile 1997) based off of aerial surveys. After several years of mark-recapture study, this goal was updated and revised to 1,800 to 3,800 large spawning Chinook salmon 2008 (McPherson et al. 2008.)

Available information on this stock suggests that about 25% of the fish are harvested annually and the run is reasonably healthy with annual escapements meeting or exceeding the goal in all years except 2012 and 2013. This stock, like others in Alaska, has recently experienced a decline in productivity. Unuk River salmon research provides fishery managers the tools necessary to manage under the sustained yield principle.

Adult Spawning Abundance

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Unuk River set gillnet site.
Unuk River set gillnet site.
Unuk River spawning ground sampling areas.
Unuk River spawning ground sampling areas.

Coupled with aerial surveys, a mark-recapture study takes place annually to estimate the spawning abundance of Chinook salmon in the Unuk River. Since 1977, abundance has ranged between 956 and 10,541 large fish, with an average of 4,434 large fish. In the mark-recapture study, during June and July marks are applied to Chinook salmon in the lower river, and at the same time, fish are sampled for tags, age, sex, and length information. Then in late July and August, marks are looked for at 6 principal spawning areas and captured fish are also sampled for age, sex, and length information. Project operational plans are published annually to outline project methods, results, and data analyses, and are available on-line including the most recent plan which covers the 2013 mark-recapture project.

A set gillnet operated approximately 3km up from the mouth of the river is used to capture returning Unuk River Chinook salmon. In addition to sampling fish for age, sex, and length information, each fish is checked for the presence of adipose fins. Fish missing adipose fins are additionally sampled for the presence of coded wire tags (CWTs) which would have been applied to fish during their juvenile life stage (see Unuk River coded wire tag research - PDF 426 kB). Sampled fish are then tagged with an external spaghetti tag and, to mediate against tag loss, given two secondary marks and released. Fish are sampled and recaptured on the spawning grounds using rod and reel snagging gear, short sections of netting and dip nets or sampled as carcasses.

Information gathered from this work is published in the ADF&G Fishery Data Series and is complete through parent years 2005 (PDF 4,924 kB), and 2006 (PDF 2,814 kB).

Chinook salmon possessing a spaghetti tag, secondary mark and adipose fin either present or absent.
 

Juvenile Abundance

Minnow trapping of juvenile Chinook salmon in the Unuk River drainage.
Coded-wire-tagging of juvenile Chinook salmon in the Unuk River drainage.

The ADF&G, Sport Fish Division, has conducted studies to estimate the abundance of juvenile Chinook salmon in the Unuk River drainage since 1994. Juvenile Chinook salmon from the same parent year are marked with adipose fin clips and tagged with CWTs as fingerlings during the fall (September and October), and again the following spring as smolt before they outmigrate to sea in April and May. Tagging in the fall and spring greatly increases the total number of tagged fish leaving the Unuk River. On average, over 30,900 fingerlings and 9,400 smolt have been tagged each year and smolt abundance has ranged from 122,000 to 789,000 since 1994.

Fall fingerlings and spring smolt are captured primarily using minnow traps baited with disinfected salmon eggs. After juveniles are captured, they are transported back to camp and are anesthetized, marked, tagged, and held for 24 hours to assess retention of CWTs and ensure they are healthy prior to release back into the appropriate habitats.

Fall work begins in late September and spring work begins in early April and runs through early May. Trapping occurs exclusively in the main stem and side sloughs of the lower river.

Unuk River Chinook salmon rear at sea from one to five years and information accumulates annually on these parent year releases as returning adults are sampled. Information of the fraction of fish marked with adipose fin clips is used in combination with adult sampling information to estimate smolt abundance. In addition, the fraction of these fish possessing valid CWTs released in the Unuk River is used to estimate adult harvests in the various marine commercial and sport fisheries. The estimated harvest of a particular parent year is coupled with estimates of the parent year spawning abundance to reconstruct the complete return.

Data collected from CWT recoveries, when combined with adult spawning abundance estimates, allows for complete parent year run reconstruction, including marine harvests, smolt abundance, and marine survival of Unuk River Chinook. Project operational plans are published on-line annually to outline project methods, results, and data analysis, including the most recent plan which covers the 2013-2014 field season.

Unuk River Smolt abundance from 1994 to 2011.