794 
BULLETIN OF THE BUREAU OF FISHERIES 
TRENDS OF ABUNDANCE 
The indices from traps and seines represent the relative availability of coho 
salmon to the particular type of gear and for the particular area in which that gear 
operated. The trap indices are for three individual areas, whereas the purse-seine 
index is necessarily based on catches made throughout the entire Puget Sound region. 
In order to show more 
clearly the trend of abun- 
dance of the species, the in- 
dices were smoothed once 
by threes. Since some 98 
percent of these fish ma- 
ture at 3 years of age, such 
smoothing also minimizes 
the effect of any predomi- 
nate age-cycle. To facili- 
tate comparison of the 
curves, it was necessary to 
reduce the indices to the 
same general range of vari- 
ation, therefore each curve 
was proportionally reduced 
or increased so that the 
sum of the index figures for 
the years 1912-33 equalled 
100.00. These smoothed 
indices are shown graph- 
ically in figure 27. 
In the three sections of 
the figure the indices from 
traps north of Sandy Point, 
from Rosario Strait, and 
from Admiralty Inlet are 
compared with the seine 
index. The same general 
trend is apparent in all the 
indices. 
A general high level of 
abundance is indicated 
prior to 1910, but the continued increase thereafter in numbers of all types of gear 
was accompanied by a decrease in abundance of the species. During the post-war 
depression, the considerable decrease in fishing effort throughout the region resulted in a 
general rise in abundance; this increase was quickly terminated, however, when fishing 
once more became profitable and the trend of abundance has declined from that point. 
The indices correspond fairly well throughout, and since the post-war years the 
seine index is very similar to that for Admiralty Inlet traps. This is a direct corollary 
of the more intense purse-seine fishery in the southern district during these years, 
ms 
06 
oq 
15 18 
YEAR 
24 
27 
30 
33 
Figure 27.—Trends of abundance of coho salmon. Indices calculated from trap catches 
in three different areas are compared to the index calculated from purse-seine catches 
taken from the entire Puget Sound region. A considerable decrease in abundance has 
taken place since the early years of the fishery. 
