FISHERY BULLETIN: VOL. 74, NO. 4 



Figure 1. -Geographic 

 range of coho salmon in 

 North Pacific Ocean and 

 Bering Sea. Dashed line in- 

 dicates coastline where coho 

 salmon enter streams for 

 spawning. 



In our studies at Sashin Creek, southeastern 

 Alaska, we have attempted to determine impor- 

 tant aspects of the life histories of populations of 

 the coho salmon near the center of its range 

 (southeastern Alaska). We have compared our 

 findings with life histories of populations in other 

 parts of the range and have emphasized 1) char- 

 acteristics of adult spawners (including time of 

 stream entry and time of spawning, longevity on 

 the spawning grounds, age structure, and fecun- 

 dity), 2) survival of eggs and alevins in the gravel, 

 and 3) survival and growth of juveniles up to the 

 time of seaward migration. 



STUDY AREAS 



Sashin Creek empties into Chatham Strait in 

 the inner bay of Little Port Walter on the south- 

 eastern shore of Baranof Island (Figure 2). The 

 stream originates in Sashin Lake about 3 km from 

 tidewater and drains about 10 km^ of forested 

 watershed-mostly western hemlock. Tsuga het- 

 erophylla, and Sitka spruce, Picea sitchensis (U.S. 

 Geological Survey 1972). 



The discharge pattern of Sashin Creek is 

 governed by seasonal rainfall and the rate of 

 melting of accumulated snow. For the 10-yr period 

 1963-72, annual precipitation at Little Port Walter 

 averaged about 587 cm (231 inches).-^ Although 



^This average was computed from data from volumes 49-58 of 

 the U.S. Weather Bureau s "Climatological Data, Alaska, Annual 

 Summary." However, because precipitation for August 1967 (vol. 

 53) was reported incorrectly as 6.99 inches, we used the figure 

 from the original records at Little Port Walter of 19.08 inches for 

 August 1967 in computing the 10-yr average precipitation. 



Sashin Lake intercepts part of the runoff and 

 tends to even out flows in Sashin Creek, discharge 

 varies from less than 0.3 mVs in midwinter to as 

 much as 34 m-^/s after heavy rains in September 

 and October. 



Salmon have access to the 1,100 m of stream 

 between the weir at the upper limits of salt water 

 and a high waterfall upstream. Coho salmon rarely 

 spawn in the 160 m of stream immediately below 

 the waterfall or in the intertidal stream channel; 

 both areas have a steep gradient and coarse 

 bottom material. 



The spawning ground is divided into three areas 

 (upper, middle, and lower) which have different 

 physical characteristics but in total contain about 

 13,000 m- of spawning gravels (Table 1). The upper 

 area contains about 25% of the stream's suitable 

 spawning gravels and is characterized by a steep 

 gradient (relative to the other sections) and coarse 

 bottom materials. The middle area has about 30% 

 of the spawning gravel and an intermediate 

 gradient with a higher proportion of smaller 

 gravel and fines. The lower area is the largest and 

 contains about 45% of the spawning gravel; it has a 

 low gradient and a high proportion of fines in the 

 bottom materials. 



Rearing areas of juvenile coho salmon include 

 the three spawning areas plus pools, backwaters, 

 and to a limited extent, the 160-m section of 

 stream in the canyon immediately downstream 

 from the waterfall. In our investigation of juvenile 

 coho salmon, the three ecologically distinct study 

 areas were maintained. An additional 3,473 m^ 

 were included in the study areas to incorporate 



898 



