322 



POPULATIONS 



ing in the rivers. This mortality in the 

 young salmon is reflected in later years in 

 the size of catch of older forms. On this 

 point Huntsman says: 



"Study of the river life has revealed the 

 chief mortality of the larger parr as due to 

 fish-eating birds (belted kingfisher and Amer- 

 ican merganser), which nest and rear their 

 young along the salmon streams. These birds 

 are largely unable to secure food when the 

 streams are swollen and murky, as in rainy 

 weather. I have accordingly explored the pos- 

 sibiUty of dry summers being responsible for 

 increased mortality of the large parr (of the 



For our present purposes this study fur- 

 nishes an example of fluctuations in a nat- 

 ural population and advances a partial ex- 

 planation of the observed changes in abun- 

 dance. It is to be noted that the curve is, 

 in general, of the sharp peak and valley va- 

 riety somewhat reminiscent of that shown 

 for Gnathoceros (Fig. 113), although more 

 exaggerated. 



Another illustration of fluctuation in fish 

 populations, reported by Russell (1942), 

 concerns, among others, the haddock, a bot- 

 tom fish caught commercially in Icelandic 

 waters. Russell reports the total fishing yield 



o 



50,000 



^ 40.000 



Q i 30,000 

 _J O 



y ^ 20,000 



>: 10,000 

 ffi 



200 



1905 1910 1915 1920 1925 1930 1935 



— 1 — r-r-| — I I I I I I I I I I I — [—1 — rx"' — t"" — i | i i i i — n — i — r— r 



>- 

 CD 



■z. 



UJ 

 Q 



-o 



100 



I I I I I I 



I I I I I I I I I I I 1 



H — I — HH- 



I I I I I I I I 



1905 1910 



WAR 



I I I I I I I I I I I I I I I I I I I I I 1 I I I I 

 1915 1920 1925 1930 1935 



HADDOCK FROM ICELANDIC GROUNDS 



Fig. 116. Haddock density and yield from Icelandic fishing grounds. (From Russell.) 



smolt year-classes related to the times of salmon 

 scarcity) by restricting their habitat and ex- 

 posing them to attack by birds. The last 

 scarcity affected chiefly the 1926 and 1927 

 smolt year-classes and in correspondence with 

 this the summers of 1923 to 1926 prove to have 

 been dry, as shown by both rainfall and river 

 discharge records. Pronounced scarcity of 

 salmon is thus seen to follow with the proper 

 interval, a succession of dry summers" (pp. 

 13-14). 



Figure 115 shows a fair correlation be- 

 tween the number of salmon and the sum- 

 mer rainfall. Huntsman stresses that the 

 years of particular scarcity were preceded 

 "at the proper intervals" by periods of four 

 successive dry summers. 



of this species by fishermen of all participat- 

 ing countries from 1905 through 1935. The 

 density of the population is estimated also 

 by computing the "landing per day's ab- 

 sence" from port. This author makes the 

 following points which the reader can 

 confirm for himself from Figure 116: 

 ( 1 ) Fishing depleted the population before 

 the first World War. (2) The stock regener- 

 ated when fishing was interrupted by hos- 

 tilities. (3) After the war the catch re- 

 mained high until 1926 because the had- 

 dock had replenished themselves somewhat 

 and because of increased fishing effort. (4) 

 In 1926 an improvement in fishing gear 

 faoihtated eflBciency of catch. (5) After 



