FISHERY BULLETIN: VOL. 76, NO. 3 



sible,^ but partial counts indicated that by March 

 the densities of planarians in some parts of Sashin 

 Creek commonly reached several thousand per 

 square meter. A similar seasonal increase in 

 streambed populations of planarians concomitant 

 with the seasonal occurrence of sockeye salmon, O. 

 nerka, embryos has been noted elsewhere.^ 



In Sashin Creek there is little doubt that high 

 planarian populations are related to the presence 

 of salmon eggs and alevins, because planarians 

 are scarce in streambed gravels above the impass- 

 able falls where salmon do not spawn. However, 

 the precise role of these organisms in the ecology of 

 spawning beds is unknown. To learn something 

 about the role of planarians, I conducted tests with 

 various combinations of planarians and live and 

 dead salmon eggs and alevins in experimental 

 containers. In these tests planarians did not prey 

 on and were not toxic to live embryos, nor did they 

 feed on dead eggs unless the chorion was broken 

 and the egg contents exposed. 



EVIDENCE OF OVERSEEDING 



In assessing the probability of streambed over- 

 seeding in Sashin Creek in the 1967 brood year, it 

 is most useful to compare fry production in 1967- 

 68 with production in other years. Since 1940, 



^When large numbers of planarians are excavated with the 

 hydraulic sampler, many elongate their bodies and pass through 

 the meshes of the collecting net. 



6W. L. Hartman, W. R. Heard, and C. W. Strickland. 1962. Red 

 salmon studies at Brooks Lake biological field station, 1961. 

 Unpubl. manuscr. on file, NWAFC Auke Bay Lab. NMFS, 

 NOAA, P.O. Box 155, Auke Bay. Alaska. 



production has varied from 50 fry to almost 6 mil- 

 lion fry; corresponding parent escapements varied 

 from 8 to 92 ,085 ( Table 1 ) . Only three escapements 

 exceeded that of 1967, and only one of these ( 1940) 

 produced more fry (about 0.4 million more) (Table 

 1). When the numbers of fry are plotted against 

 potential egg deposition, a dome-shaped fry pro- 

 duction curve is derived for Sashin Creek (McNeil 

 1969). The relative position of fry production for 

 the 1967 brood year falls near the descending limb 

 of the curve; fry production from the 1941 and 

 1942 brood years indicates a continuing decrease 

 in fry production as escapements increased (Fig- 

 ure 2). 



Data collected since 1961 on the density of eggs 

 in the three study areas at the end of spawning 

 provide a means of more precisely defining the fry 

 production potential of the stream. Plotting fry 

 production as a function of actual egg deposition 

 for each area produces curves that suggest the 

 potential maximum fry production in the upper 

 and middle areas is around 500 fry/m^ and the 

 potential in the lower area is about half that 

 number (Figure 3). In 1967 the actual density of 

 eggs deposited considerably exceeded twice the 

 theoretical maximum fry production in all three 

 areas and the fry production was considerably 

 below the maximum; it appears that overseeding 

 occurred in 1967. 



Until 1967 the timing of entry of adults into 

 Sashin Creek had usually been an accurate indi- 

 cator of the freshwater survival of progeny (Mer- 

 rell 1962; McNeil 1968; Ellis 1969). The presumed 

 biological basis for the correlation between early 

 spawning and high survival was that embryos de- 



500 



Figure 2. — Production of pink salmon in 

 Sashin Creek, 1940-67. The shaded area 

 in the lower left is shown on a larger scale 

 in the upper right corner. Nine genera- 

 tions of the even-year line 1946-62 were 

 excluded because fry productions were all 

 <l/m2. Adult escapement in these years 

 was correspondingly low. The curve (mod- 

 ified from McNeil 1969) is fitted by eye. 



1000 2000 3000 1000 5000 6000 7000 



POTENTIAL ECC DEPOSITION PER SQUARE METER OF SPAWNING GROUND 



578 



