Size Trends of Puget Sound Coho

(Oncorhynchus kisutch)


David Knutzen



This paper reviews the effectiveness of historical data in analyzing the size and return timing trends of Puget Sound Coho salmon (Oncorhynchus kisutch). Based on the historical (1940-1993) fecundity data the size of coho salmon is decreasing over time, while the fork lengths from coded wire tag data (1972-1993) show the trend to be less severe. The historical return and spawn time data shows the fish are returning later. These behavior are analyzed and discussed along with the effects of harvest.


The coho fishery is disproportionally harvesting the larger fish; thus, the size of the fish in the spawning population are smaller. This harvest is selecting faster growing individuals which results in the elimination of the historically large fish. The component of physical size with large variances in size are important to the future spawning population. These two traits increase the survivability, and are key components in the genetic integrity of the species. The coho has evolved under circumstances of over-abundance with large variations in physical size.


The coho have evolved to return at specific times. This was traditionally based on the seasons and hydrologic cycle. Currently, with added pressure from the fishing industry and changing watersheds, the coho have slightly changed their return timing. This change could be a factor the data gathering technique, too. The watersheds are influenced with faster and stronger freshets. The harvest is also taking the fish at various times which could be changing the timing of the fish.


It is important to remember that the watersheds and general environment is not at historical conditions. Increased environmental changes have had negative effects on the species, but along with these habitat effects, harvesting of the species is quickly changing the species, in a negative manner. Over-abundance and size variations of coho salmon spawning populations are being lost, and it is through changes in the harvest practices that the large body size and variability can be recovered.