DRAFT: Total Encounters and Mortality Estimates for Puget Sound Recreational Chinook Mark-Selective Fisheries Monitored using Baseline Sampling: 2017-2018

Category: Selective Fishing

Published: February 2020

Pages: 36

Author(s): Ty Garber


Based on agreements between the State of Washington and the Northwest Treaty tribes, the Washington Department of Fish and Wildlife (WDFW) has been conducting pilot1 recreational mark-selective Chinook fisheries (MSFs) in the marine catch areas of Puget Sound since 2003. The goal of these fisheries is to allow increased angling opportunities on hatchery-raised, marked (adipose fin-clipped) salmon while limiting impacts on unmarked (adipose fin intact) natural-origin stocks of conservation concern, particularly ESA-listed Puget Sound Chinook salmon (Chinook).

To monitor each fishery, WDFW’s Puget Sound Sampling Unit (PSSU) implements one of the four following sampling designs: i) Full Murthy Estimate Design, ii) Reduced Murthy Estimate Design, iii) Aerial-Access Design or iv) Baseline Sampling Design. The design selected depends on area and season considerations, the magnitude of the fishery and State-Tribal agreements made prior to the start of the fishing season. For a complete description of the methods associated with these sampling designs, see WDFW’s “Methods Report: Monitoring Mark-Selective Recreational Chinook Fisheries in the Marine Catch Areas of Puget Sound (Areas 5 through 13)” (WDFW 2012).

Except for Baseline Sampling, all above sampling designs are characterized as comprehensive, “intensive” monitoring programs and have been tailored to reliably estimate the critical parameters needed for evaluating mark-selective fisheries (WDFW 2012). In addition, PSSU has acquired and analyzed relevant data characterizing other aspects of the pilot MSFs, including descriptors of fishing effort, fishing success (catch [landed Chinook] per unit effort), recreational fishing methods, the length and age composition of encountered Chinook salmon, and the overall intensity of our sampling efforts. As such, the data collected through these comprehensive monitoring programs allow biologists to produce weekly or bi-weekly in-season estimates and to finalize post-season estimates of effort, catch, total encounters and fishery impacts in a timely manner.

Baseline Sampling, however, is a scaled-back monitoring program that is currently implemented in lower-magnitude MSFs as well as during year-round non-salmon focused sport fisheries throughout Puget Sound. Samplers collect data on salmon catch (retained and released) and effort via dockside sampling and angler interviews and obtain on-water encounter rate data from voluntary salmon trip reports (STR) submitted by private anglers. In contrast to the three comprehensive monitoring programs, the data collected through Baseline Sampling does not allow for in-season or immediate post-season estimates of effort, landed catch, total encounters, or fishery impacts.

Between 2011 and 2013, a framework was developed to estimate total Chinook salmon encounters for a given MSF relying only on Baseline Sampling data and Catch Record Card (CRC) estimates of Chinook harvest (WDFW and NWIFC 2013). Two methods were established to estimate total Chinook encounters. The first method, the “M2” approach, requires an estimate of legal-marked Chinook harvest and an estimate of the proportion of legal-marked Chinook (pLM) in the target population, derived from test fishing or STR data. Total encounters are then partitioned into size and mark-status groups using size/mark-status proportions from the test fishing or STR data. If the precision of total encounters estimates is too high due to small sample sizes in test fishing or STR data, the “M1” approach should be employed, which estimates total encounters using CRC-based Chinook harvest estimates and the ratio of encountered Chinook to retained Chinook observed during dockside sampling efforts. In the absence of sufficient test fishing or STR data, size/mark-status proportions are estimated using a “product method” where the mark-rate and legal proportion are estimated independently using dockside sampling data and combined to provide an estimate of the proportion for each group.

As of the date of this report, finalized CRC-based Chinook harvest estimates were available through the close of the 2018 summer season, which ran through September 2018. Contained in the following sections is a full analysis of fishery-total encounters and mortalities associated with the four Puget Sound Chinook MSFs that occurred and were sampled on a Baseline level only during the time period covered by this report; October 1, 2017 through September 30, 2018. For previous Baseline-sampled fisheries, see WDFW 2013c. The appropriate estimation method was selected by referring to the decision support schematic provided in WDFW and NWIFC (2013).

Estimates of the following parameters are presented below:
i) total Chinook harvested (by size [legal or sublegal] and mark-status [marked or unmarked] group),
ii) total Chinook released (by size and mark-status group),
iii) total Chinook mortalities (by size and mark-status group),
iv) comparisons of Chinook encounters and mortalities with pre-season expectations (based on Fishery Regulation Assessment Model [FRAM] predictions),
v) dockside sample rate for harvested Chinook, and
vi) total mortality of marked and unmarked double index tag (DIT) CWT stocks.

The beta regression was updated to include data from all intensively monitored fisheries through the 2019 summer season (Figure 1) having more than twenty encounters in the fishery’s STR or test-fishing program. Three parameters resulting from the beta regression were used to estimate the proportion of legal Chinook in the targeted population, based on the number of retained and released Chinook in the dockside sampling and interview data: the intercept (β0) = 1.055, slope (β1) = -1.058 and precision (Φ) = 13.85 (for further information see WDFW & NWIFC 2013).