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Alaska Fisheries Sonar

Alaska Fisheries Sonar
Kenai River King Salmon Sonar Transitions

In an effort to better assess early and late run king salmon abundance on the Kenai River, in 2015 the Alaska Department of Fish and Game began using sonar estimates from a site at River Mile 14 instead of sonar estimates from a site at River Mile 9.

In 2013 and 2014, the department operated sonars at both River Mile 14 and River Mile 9 as part of a research project to determine if the king salmon runs could be assessed by sonar at River Mile 14. Findings from the research project showed that the River Mile 14 sonar provided king salmon abundance assessments that were equivalent, on average, to the expanded estimates used for in-season management of the fishery from the sonar located at River Mile 9. The new location allows sonar technology to span nearly the entire width of the river, which increases the accuracy of assessments. In 2015 the River Mile 9 sonar was discontinued and king salmon abundance estimates were assessed at the River Mile 14 site.

While the abundance of large king salmon (greater than 34 inches in length) can be assessed directly by the sonar at River Mile 14, test netting projects at River Mile 9 are still crucial to assessment at the new site. Assessing the abundance of all king salmon (regardless of size) requires sonar data be supplemented by additional information from the test netting project.

Sonar operations at River Mile 14 begin on May 16 and achievement of the existing escapement goals (5,300-9,000 early run; 15,000-30,000 late run) will continue to be the management objective for the king salmon fishery.

The Next Phase for the River King Salmon Sonar Transition

Presently estimates of king salmon passage by Adaptive Resolution Imaging Sonar (ARIS) are made by using two sources of information. The first source is an estimate of king salmon greater than about 34 inches in total length, passing the sonar. Nearly all large fish counted by sonar are king salmon, so this count is very accurate and is considered a direct estimate of fish greater than 34 inches in total length passing the sonar. This estimate is expanded to the hour from the amount of time the sonar is operating during each hour of each day.

The second source of information is estimated from the department’s inriver netting project and is designed to estimate the passage of small king salmon, those less than about 34 inches in length. Although abundance of large king salmon can be directly assessed by sonar alone, assessing the abundance of all king salmon (regardless of size) requires sonar data supplemented by additional information from the inriver netting project. The netting data provide the size information necessary to estimate the number of king salmon that are too small (less than 34 inches) to be distinguished from sockeye, coho, and pink salmon. Such estimates are produced by fitting statistical mixture models to sonar and netting data. During netting fish captured are measured and the ratio of small to large king salmon passing the sonar is estimated. Since the count of large king salmon is estimated directly by sonar and ratio of small to large king salmon is estimated from netting, this ratio is applied to fish passage estimates to determine the proportion of all species of small fish counted by the sonar that are small king salmon. Together these sources provide data to estimate the total passage of all sizes of king salmon.

Historically the mixture model estimates relied on king salmon size composition data from mid-river netting. Over recent years the netting program evolved to also incorporate data from nearshore netting. Findings indicated that small king salmon were likely under-represented in the mid-river netting program leading to under-estimation of the total numbers of small king salmon.

Managing stocks inseason for an escapement goal based upon large fish instead of king salmon of all sizes would require only the direct estimates of large king salmon passage. Bias in passage estimates associated with small fish would not be an inseason management issue because daily mixture model estimates to incorporate small king salmon would not be required. However, passage estimates of all king salmon will be available for ongoing assessment of stock productivity from future escapement goal analysis as well as for management.

The department has been assessing the feasibility of transitioning to management of the Kenai River king salmon based on daily estimates of large fish. Direct estimates of king salmon ≥75 cm as measured by the sonar are more accurate and timely, and can be produced without netting data. While selective size management has been used successfully in other king salmon fisheries in Alaska and has many advantages, the department needs to determine an appropriate escapement goal based on large fish to recommend to the Alaska Board of Fish, and needs time to work with the Board of Fish and public to ensure potential biological and fishery management regulation impacts have been considered and addressed.

Frequently Asked Questions

Q1: What guides the management of the Kenai River early and late king salmon runs?

A1: ADF&G manages the Kenai River king salmon runs in accordance with the Kenai River and Kasilof River Early-run King Salmon Conservation Management Plan and the Kenai River Late-Run King Salmon Management Plan. Both were established through the Board of Fisheries process.

Q2: How does ADF&G measure king salmon escapement on the Kenai River?

A2: ADF&G uses the best available technology to measure escapement on the Kenai River. King salmon escapement measurements on the Kenai River are based on ARIS sonar fish counts at River Mile 14 and inriver test netting data collected at the River Mile 9, and subtracting inriver fishing mortality (i.e., harvest and release mortality) upstream of this site.

Q3: How do the king salmon counts at the Kenai River Mile 14 sonar site compare to the counts at the River Mile 9 sonar site?

A3: Figure 1 below is our assessment of king salmon passage using the RM 9 and the RM 14 sonars. The RM 9 estimates use the expansion factors of 1.55 for early run and 1.28 for late run. Both sonar sites used data from the test netting program at River Mile 9. Abundance of large king salmon can be directly assessed by the sonar alone, however assessing the abundance of all king salmon (regardless of size) requires sonar data be supplemented by additional information from the test gillnetting project. The netting data provide the size information necessary to estimate the number of king salmon that are too small to be distinguished from sockeye, coho, and pink salmon.

Figure 1.- Daily abundance estimates of Kenai River Chinook salmon as measured by sonar sites at river miles 9 and 14, 2013 and 2014.

Q4: Does ADF&G use the same expansion factor at the River Mile 14 sonar site?

A4: No. The expansion factor will not be used because the sonar arrays at River Mile 14 span nearly the entire river.

Q5: Does the Kenai River king salmon assessment at River Mile 14 include the test netting program?

A5: Yes the test netting program will continue at River Mile 9. Abundance of large king salmon can be directly assessed by the sonar alone, however assessing the abundance of all king salmon (regardless of size) requires sonar data be supplemented by additional information from the test netting project. The netting data provide the size information necessary to estimate the number of king salmon that are too small to be distinguished from sockeye, coho, and pink salmon. ADF&G is also analyzing data to possibly develop a large fish escapement goal for the early and late run of Kenai River king salmon as an alternative approach.

Q6: Does the abundance of small fish affect the escapement estimates or inseason management decisions on the Kenai River king salmon runs?

A6: Yes, but not very much. “Small” fish, primarily those fish that spent only one year in salt water (1-ocean) and are less than 20 inches in size (mid-eye to fork) were observed in the past. Although their abundance was likely not accurately assessed, other sources of data besides sonar and netting indicate they have been a small fraction of the total run. These fish do not have any appreciable effect upon escapement estimation or inseason management. ADF&G plans to improve our assessment of “small” fish in the future to better understand their contribution to escapements and stock productivity. ADF&G is also analyzing data to possibly develop a large fish escapement goal as an alternative approach.

Q7: Does the sonar detect fish that are less than 20 inches in size?

A7: Yes, the sonar detects all sizes of fish but the sonar is not able to distinguish species of the same size. Both the formerly used split-beam and the newer imaging sonar (i.e., DIDSON and ARIS) detect fish, even pink salmon, regardless of size. Nearly all large fish counted by the sonar are king salmon, so this count is very accurate. To count small king salmon we use additional information from our netting program to determine the proportion of all species of small fish counted by the sonar that are king salmon. ADF&G is also analyzing data to possibly develop a large fish escapement goal as an alternative approach.

Q8: Does the historical mid-point or halfway point of early and late king salmon run at River Mile 14 occur on the same date as River Mile 9 or is it later because the sonar is 5 miles further upstream?

A8: The mid-point is not the same as the historical mid-point estimated from data collected at River Mile 9. The median date of early-run passage was 4 days earlier at River Mile 14 than at River Mile 9 in 2013, but no different in 2014 (Figure 2 and 3). Median date of late-run passage was 4 days later at River Mile 14 than at River Mile in 2013, and 6 days later in 2014 (Figure 2). Radio-telemetry results were consistent with these findings: Chinook salmon radio-tagged at River Mile began to exhibit less consistent upstream migration in late July.

Findings show that information for inseason management based on the River Mile 14 sonar will be delayed as fish transit the 5 additional miles upstream before abundance is estimated. This is especially true during the late run, when migration seems to slow or pause between the sites. The run timing midpoint based on mean run timing at the River Mile 9 sonar is July 21. At the River Mile 14 site the run timing midpoint does not occur until July 28, and only 34% of the run has passed the River Mile 14 site by July 21. During 2015 the mid-point of the king salmon runs at river mile 14 was earlier than those mid-points during 2013 and 2014. The mid-point for the early-run was June 10 and the mid-point for the late-run was July 23 in 2015.

Figure 2. Early run Kenai River king salmon cumulative run timing as measured by sonar sites at River Mile 9 and River Mile 14, 2013 and 2014.

Figure 3. Late run Kenai River king salmon cumulative run timing as measured by sonar sites at River Mile 9 and River Mile 14, 2013 and 2014.

Q9: How does ADF&G keep track of the sport harvest occurring downstream of River Mile 14?

A9: The late run inriver creel survey in 2015 estimated that 2,073 (53% of the inriver river sport harvest were taken below the river mile 14 sonar. No harvest occurred during the 2015 early run because the fishery was closed by EO. The sport harvest will be estimated inseason by creel survey above and below River Mile 14. This data will provide information necessary to accurately assess the inriver run abundance because of the harvest that occurs downstream of River Mile 14.

Q10: Do king salmon spawn downstream of River Mile 14?

A10: Yes. A primary advantage of the transition to River Mile 14 was to move the sonar site upstream to an area of the river above tidal influence to increase accuracy of the final king salmon escapement estimate. Telemetry projects estimated that 4.2% (2013) and 5.4% (2014) of radio-tagged late run king salmon completed migration and spawned downstream of River Mile 14. Spawning occurs after the sport fishery closes on July 31 from mid-August through September. The inseason assessment accounts for spawning fish downstream of River Mile 14.