608 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE, 



Figure 22. — Regressions of the percentage of lake trout 

 possessing sea lamprey wounds on body length of lake 

 trout for 1949, 1950, and 1951. Regression formulas 



1949, K= 88.7628+ 18.8330 AT-0.8337 AT 2 + 0.0132 X 3 



1950, Y= 28.7041-11.1589 X+1.1443 X 2 -0.0252 X 3 



1951, Y= 16.2757 -6.3353 X+0.6322 A 2 -0.0128 X 3 . 



further discussion is based entirely on the wounds, 

 except to mention that annual variations in at- 

 tacks on trout are evident also in the incidence of 

 scarring. 



Annual differences in the rate of lamprey para- 

 sitism show most clearly in the average number 

 of lamprey wounds borne by lake trout. The 

 values listed in column 5 of tables 42, 43, and 44 

 have been smoothed by a moving average of 3 

 and plotted in figure 23. It is readily apparent 

 that the average number of wounds per trout de- 

 creased steadily during the period 1949-51. 

 Trout of almost all sizes showed this trend. 



Another measure of the intensity of lamprey 

 parasitism is the percentage of lake trout possess- 

 ing lamprey wounds (column 4 in tables 42, 43, 

 and 44; fig. 22). The percentages of trout bear- 



ing lamprey wounds are 60, 46, and 38 for the 

 years 1949-51. Each year the percentage of 

 wounded trout exhibited a marked decrease ; how- 

 ever, the yearly differences in this relation are less 

 pronounced than are those of the number of 

 wounds because of the high incidence of parasitism 

 among the large trout in all years. 



The most plausible explanation of the decline 

 in sea lamprey parasitism upon lake trout from 

 the high incidence in 1949 to the low in 1951 is 

 that it resulted primarily from the decline in 

 abundance of lampreys. This view is supported 

 by the data in table 45 in which are listed the 

 estimated numbers of sea lampreys, mean num- 

 bers of lamprey wounds per lake trout, and mini- 

 mum lengths of lake trout bearing lamprey wounds 

 for each year from 1949 through 1951. 



Since the number of lampreys varied widely 

 from one year to another, it is reasonable to as- 

 sume that the ratio of lampreys to lake trout was 

 principally dependent on the fluctuation in lam- 

 prey abundance. An estimate of the number of 

 sea lampreys in the lake during a particular year 

 can be determined from the number of spawning 

 migrants. Except for a brief period in early 

 spring, only one year class of parasitic-phase sea 

 lampreys is present in the lake. Their abundance 

 can be closely estimated by determining the num- 

 ber of migrants that enter Cayuga Inlet for spawn- 

 ing the following spring. Thus, the number of 

 sea lampreys in Cayuga Lake during 1950 and 

 1951 was determined from the number of sea lam- 

 preys in the spawning migration in 1951 and 1952, 

 respectively. 



All three measures of lamprey parasitism indi- 

 cate more intensive feeding in years when lam- 

 preys were most abundant, and less feeding when 

 lampreys were fewer. As shown in table 45, the 

 average number of lamprey wounds per trout 

 decreased as the number of lampreys decreased. 

 Also, the percentage of lake trout bearing sea 

 lamprey wounds decreased as the number of lam- 

 preys decreased. The small size of the shortest 

 trout bearing lamprey wounds when lampreys 

 were most numerous further indicates more inten- 

 sive feeding when lampreys are abundant. 



Incidence of Sea Lamprey Parasitism in Various Parts 

 of Cayuga Lake 



Lake trout taken in July and August 1949-51 

 from five sections of Cayuga Lake differed consid- 



