PEARCY and FISHER: MIGRATIONS OF COHO SALMON 



were swimming to the north during this sampHng 

 period. Maturing fish over 300 mm FL (age .1 

 echo) showed the opposite trend. 



Miller et al. (1983) made several hundred 

 paired purse seine sets open to the south and 

 north during three cruises off the northern 

 Oregon-southern Washington coasts in 1980. 

 During their May-June cruise, they caught 76% 

 of the juvenile coho salmon, 80% of the chinook 

 salmon, and almost all the steelhead trout in sets 

 open to the south, indicating northward move- 

 ment. We note a positive relation between the 

 proportion of juvenile salmon caught in south- 

 facing sets in their three cruises and strength of 

 upwelling during these cruises (mean daily 

 Bakun indices of 52, 39, and 19 m^ s"! 100 m'^ 

 coastline at 45°N, 125°W in May, July, and Au- 

 gust, respectively (Mason and Bakun 1986)), sug- 

 gesting that surface currents to the south result- 

 ing from Ekman transport may be cues for 

 orientation of salmon smolts. 



Hartt (1980) and Hartt and Dell (1986) found 

 that 83% of the combined species of juvenile 

 salmonids caught in 19 paired purse sets along 

 the coast from Cape Flattery, WA to Yakutat, AK 

 were caught in sets held open to the southeast and 

 only 17% in sets open to the northwest and north. 

 They concluded that juvenile salmonids tended to 

 migrate in a northwest direction along the coast 

 during July-September. 



Of the 100 juvenile coho salmon (135-315 mm 

 FL) caught in the gill nets set in an east-west 

 direction off the Oregon coast in July 1985, 90 

 coho were caught as they approached the south- 

 ern face of the gill net (heading north) and 10 in 

 the northern face (heading south). Jaenicke et al. 

 (1984) reported that 63% of the juvenile coho 

 caught in a surface gill net fished off southeastern 

 Alaska in July moved north at night, but only 6% 

 moved to the north during the day. 



Available data indicate that most juvenile coho 

 salmon caught off Oregon and southern Washing- 

 ton, as well as juveniles farther to the north, are 

 predominantely swimming in a northerly direc- 

 tion during summer months. 



Depth Distribution 



One-half of the juvenile coho salmon caught in 

 gill nets set off the Oregon Coast in 1985 were in 

 the upper 2 m of the surface gill net (Table 3). 

 Catches in the surface net exceeded those in the 

 subsurface net, except for the last set that fished 

 during daylight hours, indicating that juvenile 



coho salmon were most common in the upper 4 m 

 of the water column. 



Other information on the vertical distribution 

 of maturing coho and other species of salmon 

 caught in gill nets or with longlines in oceanic 

 waters also indicates that they usually swim near 

 the surface, between and 20 m (Manzer 1964; 

 Godfrey 1965; Godfrey et al. 1975). Machidori 

 (1966), for example, fished gill nets from the sur- 

 face to 50 m and caught 79% of the coho salmon in 

 the upper 10 m of the gill net. Although catches in 

 gill nets at different depths may be biased by ver- 

 tical differences in avoidance reactions to the net 

 or swimming speeds (Hartt 1975), acoustical 

 methods have also shown that salmon are usually 

 distributed near the surface (Susuki and Sonoda 

 1972; Lord et al. 1976). We conclude that most 

 juvenile coho salmon in coastal waters and ma- 

 turing coho in oceanic waters reside at depths 

 above 20 m, the minimum depth that our purse 

 seine fished. We recognize, however, that matur- 

 ing coho and other species of salmon may feed in 

 deeper water. Some salmon (including coho 

 salmon) caught in surface gill nets in the oceanic 

 waters of the Gulf of Alaska contained prey in 

 their stomachs characteristic of mesopelagic 

 depths (200-1,000 m), suggesting that some indi- 

 viduals may feed well below the thermocline 

 (Pearcy et al. in press). 



Table 3. — Catches of juvenile coho salmon in four gill net sets 

 in 50 m lengths of 29, 33, 37, and 42 mm mesh at different 

 depths and times 24-25 July 1985, each set adjusted to 10-h 

 fishing duration. 



North-South Trends in Catch per Set 

 and Sizes of Juvenile Coho 



Variations in the average catches and sizes of 

 juvenile coho salmon in purse seine sets in differ- 

 ent regions of the Oregon-Washington coast dur- 

 ing the summer provide indirect evidence for 

 north-south coastal movements. Histograms 

 showing average catches per set for 10 mm size 



177 



