inshore fisheries. At the same time, chub fishermen are finding 

 proportionately fewer lampreys in the deeper waters. From Sep- 

 tember to December, tlie lampreys appear to concentrate in increas- 

 ing niorabers in the bays, shoreline indentations, and shoaler 

 waters in general. Trapnet fishermen of northern Lake Huron, 

 operating in 10 to $0 feet of water, notice this particularly in 

 October, November, and early December v/hen they suddenly begin 

 to talce many lainpreys and scarred fish among catches of suckers 

 and spavming whitef ish (virhitef ish may not be taken in November 

 by law but the statement applies to both species in general for 

 the period noted); they did not make such abundant catches of 

 lampreys and scarred fish during the summer months. This concen- 

 tration of lampreys apparently remains in the shoaler waters 

 through the ivinter months, for when fishing is resumed again in 

 late IJarch, great numbers of nearly-mature lampreys are found 

 congr-egated off the mouths and in the estuaries of the larger 

 rivers , 



This shift of distribution is accompanied by a general in- 

 crease in the size and maturity of the sea lampreys which is 

 described in the following section. In view of the estimate of 

 the length and character of the parasitic cycle ; also presented 

 subsequently, it appears that seasonal changes in the depth distri- 

 bution of the species takes the following pattern: initial dispersal 

 of recently-transformed adults migrating doirmstream in the fall, 

 winter, and early spring to all parts of the lake but primarily to 

 the deeper waters; these individuals shift generally to the inshore 

 waters during the late summer and concentrate in the bays and along 

 the shores during the fall and early tvinter; here they remain until 

 the following spring T±ien they congregate specifically off, and in, 

 the mouths of rivers; as the spring progresses, these sea lampreys, 

 now mature J move into the streams to spa^m and die. During the 

 winter and early spring of this latter period, a new group of re- 

 cently-transformed individuals has come downstream and dispersed 

 into the offshore waters of the lalce. 



Groirth of parasitic sea lampreys and estimated 



length of parasitic life 



From the time they enter the lalces as recently-transformed, 

 parasitic adults, it now appears that sea lampreys may spend as 

 few as 12 months or as many as 20 months in these bodies of water 

 before returning to streams to spa-ivn and die. Measured from the 

 mean date of doivnstream movement to the pealc of the upstream 

 (spaivning) migration, it is deduced that the mean length of the 

 parasitic phase is about 1? months; the majority of individuals, 

 hovrever, probably spend only lii to 1$ months in this stage since 

 the gi'eatest downstream migration occurs at the end of that period 

 of movement o 



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