Puget Sound Chinook Mark-Selective Fishery Sampling Methods

The Puget Sound Sampling Unit uses three different sampling designs to intensively monitor mark-selective Chinook fisheries (MSFs) in Puget Sound marine areas. These include the Full Murthy Design, Reduced Murthy Design and the Aerial Access Design (Figure 1). The sampling design used depends on area and season considerations as well as State-Tribal agreements made pre-season (Table 1).

The Full Murthy Design is the most intensive sampling design and is used to monitor the Areas 9 and 10 summer MSFs. Here, sampling occurs on 2 days during the Monday-Thursday time period and every Friday, Saturday and Sunday. The 2 days of sampling from the Monday-Thursday period are averaged and multiplied by 4 to create an estimate of catch for all four days. Then this estimate is added to the Friday, Saturday and Sunday estimates to provide a total estimate for the week.

The basic idea for the daily catch estimate is that we select and sample 2 ramps or exit locations on every sample day, count all the Chinook that are caught on a given day for those ramps, and expand those counts for the areas we do not sample. We conduct on-the-water interviews approximately weekly to determine where anglers will be leaving the fishery and determine how many people will exit the fishery at our sampled locations. This tells us how many Chinook came through the locations we sampled and what percentage of the anglers exited the fishery at the sampled locations. Although the mathematical formula that we use is a bit more complex, an approximation of the catch can be simplified as the number of Chinook observed at the sampled locations divided by the percentage of anglers leaving the fishery through the sampled locations.

We estimate total Chinook encounters (landed and released fish) based on the assumption that all legal-marked fish that are encountered are kept. This is done by dividing the estimate of total landed Chinook by the proportion of legal-marked fish encountered in the test fishery or reported in voluntary trip reports (VTRs).

However, this estimate is incorrect because anglers do not keep every legal-marked fish that is encountered. To correct for bias in this estimate due to intentional and unintentional releases of legal-marked fish (approximately 13% based on past studies_¹), the initial estimate of total encounters is divided by 0.87.

Finally, the estimate of fishery total Chinook encounters can be apportioned into the four size/mark-status groups (legal-marked, legal-unmarked, sublegal-marked, sublegal-unmarked) by multiplying the total encounters estimate by the proportion in which each group is found in the fish population, which is estimated using information from the test fishing or VTR reports.