FISHERY BULLETIN: VOL. 74, NO. 1 



Table 2. — Diet of 75 coho salmon fry emerging from two of four 

 simulated redds supplied with river water. 



Concurrent Changes in Photoresponse 



Photoresponse testing of fry denied the redd 

 experience began on day 1, 1 day before their 

 counterparts in the simulated redds began 

 emerging. Their photoresponse remained essen- 

 tially negative throughout the time period when, 

 normally, they would have emerged. Until the 

 eighth day of emergence (day 9), less than 3% of 

 the denied fry were seen in the light compart- 

 ments (Figure 6) and they remained strongly 

 photonegative although nearly 13% of their sibs 

 had emerged from the redds. By day 12, the col- 

 lective negative photoresponse had weakened 

 considerably, and nearly 15% of the denied fry 

 were recorded then in the light compartments. By 

 the 16th day of emergence, when 90% of their sibs 

 had emerged, the percent of the fry recorded in 

 the light compartments reached a plateau. From 

 day 17 onward, 20-30% of the fry were seen in the 

 light compartments (15 of 19 tests), but the re- 

 sponse was more variable during the last day of 



testing, two of the tests (LC and DC) providing 

 heterogeneous data. Interaction stemming from 

 territorial behavior was the most likely source of 

 variability, the light compartments being sporad- 

 ically defended by single fry attempting to drive 

 the others away. 



Despite a decidedly negative photoresponse 

 during the first 10 tests (Figure 6, Table 3), in 8 of 

 these tests more fry held in darkness between 

 tests were recorded in the light compartments 

 than were those exposed to illumination between 

 tests (P<0.01). Because there was no significant 

 difference attributable to light history in sub- 

 sequent tests, novelty due to limited light experi- 

 ence may have stimulated exploratory behavior 

 during testing in fry held in darkness between 

 tests. When tested on days 15 and 17, the control 



DAY OF EMERGENCE 



Figure 6. — Change in photoresponse of coho salmon fry 

 held in baskets between tests. The histogram depicts the 

 concurrent rate of emergence of 584 sibling fry from the four 

 simulated redds. 



TABLE 3. — Fry sightings in the light compartment of each of four choice 

 boxes containing 10 fry during photoresponse tests, expressed as a percen- 

 tage of possible sightings (400/test). Bracketed values are standard errors. 



172 



