The numbers of larval lampreys that moved down- 

 stteam in each season between the summer of 1948 

 and that of 1958 are presented in table 9 where the 

 catches are summarized by semimonthly periods. 

 Downstream movement of larvae is similar to that of 

 the recently transformed sea lampreys in that it is 

 largely associated with flood conditions in the water- 

 shed. The greatest activity is in late March, April, 

 and May during the "spring breakup . " Rising or sus- 

 tained high water in almost any other season is nearly 

 always accompanied by some downstream drift of 

 larval lampreys. 



LENGTH AND WEIGHT OF MIGRANT 

 LAMPREYS 



Periodic samples of recently transformed sea lam- 

 preys were measured for total length to the nearest 

 millimeter in all seasons, except those of 1950-51 

 and 1951-52. These collections normally consisted 

 of all individuals captured in the trap on alternate 

 days. Occasionally, this schedule became irregular 

 due to adverse collecting conditions. Random sam- 

 ples of migrants were also weighed individually on a 

 dietary scale to the nearest 0. 1 gram. Length- 

 frequency distributions for transformed sea lampreys 

 captured in each of 8 migratory seasons are presented 

 in table 10 and the minimum, average, and maxi- 

 mum weights of samples of these migrants are pre- 

 sented in table 11. 



Among all sea lampreys measured (15, 110 

 individuals), the smallest was 95 millimeters and the 

 largest was 243 millimeters in total length. The av- 

 erage lengths of migrants in the runs of each year 

 varied from 143. 6 to 151.4 millimeters. The weights 

 of the sea lampreys ranged from 1. 8 to 9. 8 grams; 

 the average weight of samples varied from 3. 6 to 5. 1 

 grams. 



The average length of migrants increased steadily 

 between the 1948-49 and 1954-55 seasons (table 10). 

 This change is attributed to reduced competition among 

 the residual stocks of larvae since the Carp Lake River 

 dam and trap blocked all, or nearly all, adults from 

 spawning in the watershed during these years. Begin- 

 ning with the 1955-56 season, the size of migrants de- 

 clined. An appreciable escapement of spawning -run 

 adults above the trap is believed to have taken place 

 about this time. The smaller size of recently trans- 

 formed individuals in the last 3 years of study may 



reflect increased competition with new year classes 

 of larvae which were added to the stock. 



The size of the recently transformed sea lam- 

 preys increases progressively during each annual peri- 

 od of downstream movement. The autumn migrants 

 are composed of the smaller individuals. Average 

 size increases during the winter and is greatest during 

 the late winter and early spring when downstream 

 movement is at its peak. This change of length is 

 reflected to some extent in the mean lengths of the 

 daily samples as given in tables 3 through 8. It is 

 illustrated better by the average lengths of all mi- 

 grants taken in each month of a typical migratory 

 season as presented in table 12. Further evidence 

 of seasonal differences of size may be had by com- 

 paring the weights of autumn and spring samples of 

 transformed lampreys as given in table 11. 



DURATION OF LARVAL LIFE 



A primary objective of the extended operation 

 of the Carp Lake River dam and inclined -screen trap 

 was to determine the length of larval life of the sea 

 lamprey. It was estimated by Apple gate (1950) 

 that this phase of the life history lasted 5 years during 

 which 4 years of growth were followed by a 1-year 

 "rest period. " Presumably, the dam, after its final 

 reconstruction in 1949, blocked all spawning-tun 

 adults from further migration upstream and prevented 

 the addition of further year classes of larvae to the 

 stock above the barrier. Had the block been perfect, 

 the cessation of downstream runs would have permit- 

 ted calculation of the maximum duration of larval 

 life based on the last year when spawning in the river 

 was known to have occurred. 



No spawning above the dam is known to have 

 taken place during the early years of operation. 

 There is evidence, however, that after 1951 sea 

 lampreys may have spawned in the watershed. Stud- 

 ies by Staufferl/ of the resident larval populations 

 above the dam suggest sporadic recruitment. 



1/ Stauffer, Thomas M. , "An investigation of the con- 

 tinued capture of downstream -migrating recently 

 transformed sea lampreys in the downstream trap of 

 a weir on Carp Lake River, Emmet County. 1955. " 

 Michigan Institute for Fisheries Research, Report 

 No. 1474 (May 4, 1956), Typewr. , 16 p. 



