2501 SW First Avenue, Suite 230, Portland, OR 97201-4752
Phone: (503) 230-4099     Fax: (503) 230-7559


Date:      April 29, 1991

To:         SMP and FTOT sites in the Snake River
              (Idaho traps, Lower Granite, Little Goose and Lower Monumental dams) 

From:     Michele DeHart 

RE:        Separation of fall chinook from yearling spring and summer Chinook for 1991 

        At the pre-season FTOT meeting on march 28, Tom Berggren (FPC) expressed the importance of making a distinction between subyearling fall chinook and yearling spring and summer chinook arriving at monitoring sites in 1991. He provided a tentative length threshold for separating age 0 and age 1 chinook at that time, and indicated that the use of length frequency data during the current year would be used to modify the size thresholds. He also stated that we would be sending out information at a later date on visual characteristics* that are helpful in distinguishing spring and fall chinook.

        Enclosed is a booklet with an illustration and set of photographs (courtesy of Lynette Hawkes and staff at NMFS Rufus Office) that will be useful for purposes of distinguishing fall chinook this summer. These characteristics are currently used by NMFS at Columbia River monitoring sites. It appears that these characteristics rather than length thresholds will be the main discriminating factor between fall chinook and spring and summer chinook (characteristics for spring and summer chinook are assumed to be the same).

        Plots of length data from 1990 collections at Lower Granite Dam during June and July show a unimodal distribution. Few fish < 100 mm were present in the samples during June. It is expected that most of those fish were fall chinook. Length data at the Snake River trap in 1989 show a definite bimodal distribution with one mode below 100 mm, probably wild fall chinook (shown in plot), and a larger mode above 100 mm which included yearling chinook and age-0 spring chinook from Lookingglass hatchery (not shown in plot). The < 100mm subyearling chinook mode shows a gradual increase of about 5 - 10 mm every 10 days at the trap. It is likely that wild fall chinook grow substantially before they arrive at Lower Granite Dam depending on river temperatures and number of days of travel through the reservoir. Therefore, substantial overlap in size between wild spring and summer chinook that are still passing Lower Granite Dam in June and July with fall chinook may occur. The following length thresholds are provided only as a guide to the anticipated upper size threshold for wild fall chinook. It is unlikely that very many wild fall chinook will be observed above these length thresholds.

Date Range


1990 Avg 

Below Threshold


Sample #



May 1 - 31

<100 mm




June 1 - 10

<110 mm



25 % 

June 11 - 20

<115 mm



26 % 

June 21 - 30

<120 mm



28 %

July 1 - 10

<125 mm



56 %

July 11 - 20

<130 mm



55 %

July 21 - 31

<140 mm



86 %

August 1 - 31





        Additionally, scale samples are requested to be taken once per week at the Idaho traps (or purse seine or alternate gear) collections and at Lower Granite Dam during June through August. Scales will be taken on up to 50 fish that are smaller than the above length thresholds and on all fish determined by visual characteristics as a fall chinook that are larger than the above length thresholds. Details on which agency will be contracted to age the scales still needs to be worked out with BPA.

        Additional information on length of wild fall chinook at Lower Granite Dam may come out of the proposed PIT tagging of fall chinook under the USFWS contract in 1991. By using the PIT tag slide gate to divert PIT tagged chinook to the existing PIT tag system sample tank, lengths on these PIT tagged fall chinook and any other PIT tagged wild chinook arriving at Lower Granite Dam would be possible.  If this operation is undertaken in 1991, it would begin after the first PIT tagged wild fall chinook began to arrive at the projects, during the June thorough August period. At the present time, no decision has been made as to whether or not PIT tagged chinook may be handled this summer at Lower Granite Dam so that length data can be obtained.

cc: FPAC