POPXILATION FACTORS AND SELECTED POPULATION PROBLEMS 



373 



changes repeat themselves further on" (Cause, 

 1934, pp. 125-126).' 



We have already cited two illustrations 

 of predation in natural populations of 

 fishes. These are Russell's report on the 

 haddock (p. 322) and Huntsman's report 

 on the Atlantic salmon (p. 321). The for- 

 mer was concerned with a special, but ef- 

 fective, form of predation— the commercial 

 fishing activity of man. It was shown that 

 the haddock population (as indexed by 

 the commercial yield) dropped regularly 

 for several years before World War I, a 

 period when predation was intensive, in- 

 creased markedly during war years while 

 fishing was either suspended or reduced, 

 and again decUned sharply during postwar 

 years with the renewal of fishing effort. 

 This is an interesting case, for, better than 

 most, it suggests how destructive predation 

 pressure can be in the absence of checks 

 working against the predators. It will 

 also be recalled that Huntsman in his 

 studies on salmon pointed out that when 

 rainfall was low, the salmon parr were 

 more vulnerable to predation by local 

 birds; this predation resulted presumably 

 in a noticeable contraction of subsequent 

 salmon populations in the next few years. 

 Foerster and Ricker (1941) report a 

 most interesting study dealing with the 

 predation pressure exerted by several spe- 

 cies of fishes against a prey population of 

 young sockeye salmon. This study was con- 

 ducted at Cultus Lake in British Columbia, 

 Canada, an area where adequate ecologi- 

 cal and taxonomic records were obtainable 

 and field manipulations were feasible, 



Cultus Lake has a fish population of at 

 least ten species. Of these, four are im- 

 portant predators on the sockeye: squaw- 

 fish, trout, char, and coho salmon. The 

 investigators consistently removed these 

 predators from the lake, along with other 

 incidental forms, by gill nets, seines, bait 

 lines, and cage traps, and then eval- 

 uated the effect of this on the prey popu- 



" Winsor (1934) has certain pertinent reser- 

 vations about Cause's methods. He says: "In 

 a large part of his published work the number 

 of points on the growth curve is small; and in 

 many cases the scatter is so great as to make 

 any curve fitting highly arbitrary. In his work 

 with protozoa he seems ... to have used 

 populations which are probably too small to 

 expect even a quahtative agreement with 

 theory." 



lations. Gill-netting, the most efficient 

 method, was utilized from 1932 through 

 1938, and over this seven year period 

 10,602 squawfish, 2310 trout, 935 char, 

 and 730 coho salmon were caught. This 

 netting reduced those squawfish over 200 

 mm. in length and the char to about one- 

 tenth of their original abundance. The 

 trout populations were probably reduced 

 somewhat, though not so much as the other 

 two species. The effect on the coho salmon 

 was obscure. 



Along with this reduction of the preda- 

 tor populations, there went a correspond- 

 ing increase in survival of immature sock- 

 eye salmon, as indexed by the number of 

 migrant smolts counted each spring. This 

 survival rate of the "eyed-egg stages" in- 

 creased "to 8.98 per cent from a previous 

 average of 3.61 per cent; the survival rate 

 of free-swimming fry planted in the lake 

 increased to 13.04 per cent from a pre- 

 vious average of 4.16 per cent. The sur- 

 vival rate of eggs in female sockeye spawn- 

 ing naturally increased to 7.81 per cent 

 from a previous average of 1.78 per cent" 

 (p. 335). 



Foerster and Ricker attribute these in- 

 creases in prey populations directly to the 

 planned reduction of the predators. They 

 point out that two other types of ecologi- 

 cal factors could logically account for the 

 observed increase: namely, changes in 

 Cultus Lake other than those involved in 

 predation and/or an increase in food avail- 

 able to the sockeye; but after careful con- 

 sideration of the evidence these alternatives 

 are rejected. 



Up to this point predation has been 

 treated as a relatively uncompUcated 

 situation. In discussing predatory phenom- 

 ena in bird and mammal populations, ex- 

 amples can be presented that are also rel- 

 atively uncomplicated. It is necessary, 

 however, to point out that predation pat- 

 terns are frequently more complicated in 

 these groups, owing largely to the fact that 

 many of these prey populations are them- 

 selves highly organized and, when meet- 

 ing the pressure of predation, compensate 

 in some manner. Some illustrations of both 

 types follow. 



An instructive aspect of the predation 

 problem centers around the point that 

 predators are frequently euryphagous (p. 

 236) and prey upon what is available. Such 

 stenophagous or monophagous forms as ant 



