HEARD: PROBABLE CASE OF STREAMBED OVERSEEDING 



z 100 



s 



g 1400 



500 1000 1500 



ACTUAL ECC DEPOSITION PER SQUARE METER OF SPAWNING GROUND 



Figure 3. — Production curves for pink salmon in three study 

 areas of Sashin Creek, 1961-67, showing relation between 

 number of eggs in the streambed at the end of spawning and 

 number of fry produced. Spawning did not occur in all areas in all 

 years. Location of the 1961 point on the abscissal axis is not 

 precise: success of spawning that year was based on subjective 

 estimates (see McNeil et al. 1964). Curves are fitted by eye. 



veloping initially at warmer temperatures had a 

 survival advantage over those developing initially 

 at cooler temperatures (Merrell 1962). In Sashin 

 Creek, spawning closely follows the time of entry 

 into the stream, so that early entry implies early 

 spawning. Although the 1967 spawners entered 

 Sashin Creek on the second earliest date on record, 

 freshwater survival fell well below the predicted 

 level (Figure 4). From the predicted freshwater 

 survival of about 18% shown in Figure 4, the 1967 

 spawners should have produced 8 million fry. 



On the basis of the relation of survival to time of 

 spawning, McNeil (1969) suggested the dome- 

 shaped freshwater production curve for Sashin 

 Creek (Figure 2) may be the result of large es- 

 capements, more superimposition, and replace- 

 ment of the more viable eggs from early spawning 

 by less viable eggs from later spawning. From 

 evidence available, there is little doubt that in 

 Sashin Creek, progeny from early spawners have 

 survival advantages over progeny from late 

 spawners. I suggest, however, that when escape- 

 ments are large, a point is reached where the 

 number of eggs in the streambed determines ulti- 

 mate survival, regardless of when spawning oc- 

 curs. 



The local climate and its resulting effects in the 

 stream are extremely variable and have a marked 

 influence on what actually constitutes an over- 

 seeded spawning bed at a specific time. Climate 

 influences efficiency of spawning through varia- 

 tions in streamflows, so that overseeding does not 

 occur at a fixed number of spawners. Similarly, 

 streambed overseeding results from a dynamic in- 

 teraction between the density of eggs in the 

 gravel, certain ecological characteristics that 

 define the fry production capability of the 

 streambed, and prevailing climatic influences on 

 the intragravel environment during the 6 to 8 mo 

 eggs and alevins are in the streambed. Overseed- 

 ing of streambed gravels will occur at lower egg 

 densities when climatic influences (rainfall, 

 stream discharge, ice cover, etc.) are more adverse 

 to the intragravel environment. 



Perhaps the most convincing evidence of 

 streambed overseeding in Sashin Creek is the sea- 

 sonal change in instantaneous (monthly) mortal- 

 ity coefficients of the 1963, 1965, and 1967 brood 



25 



Figure 4. — Relation of freshwater survival of pink 

 salmon in Sashin Creek to date when 5(W of 

 spawners had entered the stream. Data are from 

 period 1940 to 1967; escapements<l,000 adults are 

 excluded. The curve Y = 24.17 - 0.633X fitted by 

 least squares, where X = corresponds to 10 Au- 

 gust. Numbered points identify brood years (mod- 

 ified from McNeil 1968). 



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