strong evidence to support these contentions is foimd in an 

 examination of the size composition of collections of parasitic 

 sea lampreys taken from the Great Lakes in different months of the 

 year in relation to the size composition and periods of migration 

 of recently-transformed individuals doimstream and sexually mature 

 sea lampreys (lipstream). Empirical data collected in 19h7 , 19U8, 

 and 19h9 were combined upon a scale of all. or part, of three 

 successive^ hypothetical years for the purposes of estimating the 

 duration of the parasitic phase (Figure 65). 



As sho-vvTij spaiwiing runs of mature^ adult sea lampreys gen- 

 erally entered streams from April 1 to June 30 of each year; the 

 peaks of these migrations occuri'ed about x,he end of tlay. Indivi- 

 duals composing these spasming runs ranged from 11.0 inches to 

 23„I|. inches ard averaged about 17.1 inches in total length. Dur- 

 ing the early part of this period^ two classes of individuals were 

 present in the lakes: (1) nearly-mature sea lampreys which were 

 about to enter streams to spawn and corresponded to spawning run 

 individuals in size, and, (2) sexually immature sea lampreys which 

 a-'^eraged between 7 and 8 inches in total length and among iniiich 

 none were as large as those in the preceding category (Figure 65^ 

 Table 6). After the peak of spawning run acti-'/ity, when nearly 

 all mature lampreys had entered streams, only the smaller size 

 group of immature individuals was found in the lalces. 



Tlie size com.position of this latter group increased constantly 

 throughout the summer and fall. Their average length was approx- 

 ijnately 8 inches in June, 10 inches in July, 12 l/2 inches in 

 August, 13 1/2 inches in September, 16 inches in October^, 17 inches 

 in November., and slightly over 17 inches in. December (Table 6). 

 Among all of these parasitic specimens, a comparable development 

 of the gonad (degree of maturity implied) was found in all indivi- 

 doals collected on any particular date or within any short period 

 of time. 



Data collected by means of weirs and traps demonstrated that 

 recently-transformed sea lampreys came dovmstream and entered the 

 Is-kes from about October 15 until April 1$; a lesser pealc of do-vm- 

 stream movement occurred in mid-November and a greater one at the 

 end of March in the following spring. These individuals ranged 

 from 3,8 inches to 7.6 inches in total length iirith a mean of 5.7 

 inches. During this period when the arjiual contribution to the 

 parasitic; population was entering the lakes, the older adults al- 

 ready present had attained a size distribution comparable tci that 

 of the sexually mature migrants which entered streams in the fol- 

 lowing spring. 



D-uring the winter and early spring months these two groups, 

 distinct in both size and degree of maturity, were present and 

 iihowed little grcwbh. With the advent of a new spring season., 

 the largest size group (mature specimens) was present briefly in 



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