Alaska Fisheries Sonar

Kenai (RM 14) River

Kenai River Mile 14 – King Salmon Sonar Site

Kenai River aerial map
The Kenai River Mile 14 king salmon sonar site - not to be confused with the historic Kenai River Mile 9 king salmon sonar site or the Kenai River Mile 19 sockeye salmon sonar site - is above tidal influence allowing for more complete sonar coverage of the river by the sonar beams.

Beginning in 2015, the Alaska Department of Fish and Game (ADF&G) discontinued operation of the Kenai River Mile 9 sonar project and will operate only one king salmon sonar project located at River Mile 14 Please see FAQ for answers to commonly asked questions concerning the transition from the historic site to the new site.

Move from River Mile 9 to River Mile 14 – a historical perspective

The sonar site located near River Mile 9 produced daily and annual estimates of king salmon passage from 1987 through 2014 using a combination of dual-beam, split-beam, and multi-beam (Dual Frequency Identification Sonar - DIDSON) technologies. Operation of the River Mile 9 site was discontinued following the 2014 season in favor of a new site located near River Mile 14.

The River Mile 9 site was originally chosen because the site, 1) was located below king salmon spawning grounds ensuring all king salmon passed the site prior to stopping to spawn, 2) was located close to the mouth of the river allowing fish to be detected early in the run, thus providing timely data for management purposes, and 3) had a near-perfect linear bottom profile required to detect fish in the middle of the river using dual (and later split-beam) sonar technology.

The site also had many disadvantages, primarily related to its location within tidal influence. Disadvantages included incomplete coverage of the cross section of the river due to tidal activity flooding the region behind the sonar transducers, milling fish behavior related to tidal flux, and physical risk to sonar gear by large debris carried by extreme tidal activity.

The transition from split-beam to DIDSON multi-beam technology between 2010 and 2012 opened the option of moving the sonar program upriver above tidal influence. In 2011, ADF&G began the process of identifying potential sites above tidal influence. The section of river at River Mile 14 exhibited a nearly ideal bottom profile for sonar deployment and was selected as the preferred site for further investigation. During the 2013 and 2014 seasons, ADF&G operated a full-scale experimental project at the River Mile 14 site using the newest generation of DIDSON technology referred to as Adaptive Resolution Imaging Sonar (ARIS), while continuing to operate the historical River Mile 9 site. Based on results of the 2013 and 2014 studies (PDF 376 kB), ADF&G chose to discontinue operations at River Mile 9 beginning in 2015 and to use sonar data collected at the River Mile 14 site to estimate daily and seasonal passage of king salmon into the Kenai River.

There are several advantages to moving to River Mile 14. One of the main benefits is the ability to achieve near bank-to-bank coverage of the river with sonar, which was not possible at the River Mile 9 site. Near complete coverage improves ADF&G’s ability to detect almost all fish migrating upriver past the site. There is also less potential for damage to the sonar gear and data loss at the River Mile 14 site because the narrow river width allows the gear to be deployed closer to shore. This combined with the lack of extreme tidal fluctuations reduces the chance of large debris dislodging the equipment deployed in the river. Finally, although some milling and holding activity occurs at the River Mile 14 site, the near complete sonar coverage across the river and the lack of slack water due to tidal fluctuations reduces the possibility that milling fish will pass upstream or downstream undetected by the sonar. Taking these factors into account, the move to the River Mile 14 site improves ADF&G's ability to more accurately estimate king salmon passage.

Estimating king salmon passage – the challenge

Estimating the abundance of Kenai River king salmon is a challenging task. Whereas most salmon swim close to shore where they can most easily be detected by sonar, king salmon are found at all ranges and commonly swim upstream in the deeper offshore regions of the river where it is more difficult to detect them with sonar. Estimating Kenai River king salmon abundance is further complicated by the need to separate them from sockeye and other salmon that are more abundant and migrate during the same time period. The Kenai River king salmon sonar project is the only project in the state that ADF&G uses sonar to distinguish fish by size.

In addition to sonar, ADF&G uses other tools to generate estimates and gauge king salmon run strength on the Kenai River. ADF&G also relies heavily on information from non-sonar tools including inriver gillnets and sport fish creel surveys.