Fishery bulletin of the Fish and Wildlife Service - NOAA

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SALMON KUNS OF THE COLTJMBIA RIVER IN 1938 135 Data on the blueback run are presented in modified form in tables 20 and 21. As previously stated (p. 114), two methods have been applied in changing the poundage records to numbers of fish; (1) assuming an average weight of 3 pounds throughout the season in all zones, and (2) assuming an average of 3 pounds throughout the SEPT. THOUSANDS FIGDEK в.—Dominant elements in the 1938 bluebaek salmon run, by weeks. season below Bonneville and of "2% pounds throughout the season above Bonneville. The first method gives the figures of table 20 and the second those of table 21. It is known that the fish caught above Bonneville are smaller than those caught below on account of the selective effect of the gill nets which provide a large portion of the catch below Bonneville, while the catch above Bonneville is made largely by means of dip nets which are not selective. These conversion figures are based on data secured from Harlan B. Holmes, of the Fish and Wildlife Service,
136 FISHEKY BULLETIN OP THE FISH AND WILDLIFE SEBVICE The general features of the run are much simpler than in the case of the chinook salmon just considered. There is a single, well defined peak formed by fish that enter the lower part of the river late in June and early in July. The first fish of this species to appear in the records were in the Bonneville count for the second week in May, and a few were counted past the dam during the next 4 weeks. It was not until the week ending June 18, however, that bluebacks began to show up in large numbers at Bonneville. From that tune on for the next 6 or 7 weeks bluebacks were a very important element in the Bonneville count, but after the first of August their numbers dwindled rapidly although a few were recorded as late as the second week in November (table 7). It is to be noted that bluebacks did not appear in the catch of corresponding weeks as early as they were recorded in the Bonneville count, nor as late (tables 20 and 21). This is probably due in part to the use of small-meshed nets especially adapted for catching bluebacks while this species is most abundant; and also in part to inaccurate identification hi the Bonneville count. Evidence has been given above to show that during the height of the blueback run there is a tendency to mistake the smaller chinooks (grilse) for bluebacks. It seems not unreasonable to suppose that the same error may also be made while bluebacks are scarce (or even entirely absent) which would account for the very long "tails" to the time-frequency curve given hi figure 6, since these tails are formed almost entirely from the fish recorded in the Bonneville count (tables 20 and 21). There are no complications due to spawning below Bonneville because in all probability all of the fish of this species spawn in streams tributary to lakes far above the upper limits of commercial fishing. The bluebacks of the Columbia undoubtedly represent a number of races, populations, or stocks, each breeding in its "home" lake basin; but so far as the immediate problems are concerned they act from the mouth of the Columbia to Celilo as a unit run. Above Celilo not much is known of the representative races; the available data consist chiefly of the counts made at the Rock Island Dam. In preparing the modified tables for the blueback run the same rate of migration up the river has been assumed as for chinooks; i. e., that fish that were in Zones 1 and 2 in the first week would be found in Zones 3 to 5 the second week, at Bonneville and between Bonneville and Celilo during the third week, and at Rock Island the fifth week. The data in tables 20 and 21 and figure 6 show that this assumption is well justified, since the conspicuous peaks are made to coincide almost perfectly. These data have been analyzed by applying methods similar to those used in the study of the chinook salmon. From the totals given in table 20 it may be seen that, for the entire season, the ratio of the estimated number of fish taken in the commercial fishery to the estimated escapement is approximately 3.32:1. In other words, as shown by this estimate, over 3 fish are caught to 1 that escapes, passes through the commercial fishing area, and becomes available on the spawning grounds for perpetuating the run. The Rock Island count was 17,123. Multiplying this by the ratio of catch to escapement gives 56,800 as an estimate of the number of fish caught out of the populations normally spawning in the Columbia River above Rock Island. Reconverting this to pounds by multiplying by 3 gives a total of 170,000 pounds caught that may be attributed to the runs spawning above Rock Island. These figures are based upon a consideration of the catch and escapement for the entire spring season up to and including August 25, and the total Rock Island count.
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UNITED STATES DEPARTMENT OF THE INT
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ABSTRACT The results of an investig
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