Some man-riade dasiis,, in rivers knoim to have sea lamprey runs, 

 apparently are effective barriers to further upstream movement 

 (Figure 6), In the light of present information we tentatively 

 consider certciin of these structures to be impassable for tivo 

 reasons: (1) the nature and/or structure of the dam and the manner 

 of passage of vfater over it, or through its povrer units, precludes 

 possibility of surmounting the dam, psid (2) as yet, no reporta of 

 the presence of sea lampreys have been verified above the barriers 

 (except the Cheboygan dam) . Dams at present thought to be insur- 

 mountable to lamprsys are located in the follcmng rivers s 

 Menominee, Manistique, Cheboygan, Au Sable, Ell-c, Boyne, Boardman, 

 Betsie, Manistee, LIuskegon, Kalamazoo, and St, Joseph (see Appendix 

 B for specific location and structural details). 



The greater portion of some of the major i^atersheds of I3.chigan 

 lie above these dams (Figure 3). In the Sto Joseph, Kalamazoo, 

 Muskegon, Lianistee, Manistique, Menominee, and Au Sable rivers, the 

 sea lajuprey is thus denied access to tremendous potential spasming 

 areas o This condition has doubtlessly restricted, to some degree, 

 the rate of increase of the species and most certainly limits the 

 total numbers vfhich Michigan watersheds might othenvise produce. 



It should be considered hereafter that any improvem.ent of the 

 apparently ineffectual fish ladders and chutes present on most of 

 the aforementioned dams "vvill materially aid the sea lampreys in 

 reaching new spav/ning grounds and increasing their total numbers. 



The Cheboygan, Elk River, Manistique, and Menominee dams are 

 all located very close to the mouths of their respective rivers. 

 Each year large spavming runs enter these rivers and are in evi- 

 dence below the dans throughout the migratory period. The fate of 

 these migrants is still unknomio It does not seem lil<ely that they 

 can spawn successfully in the deep estuaries prevalent below these 

 damso Assuming that they cannot spavni below such dams, tTfo alter- 

 natives are afforded: (1) they make their way to other accessible 

 streams nearly along the shore line? or (2) they remain in or near, 

 the estuary of the river "t\Mch they entered and ultimately die 

 without spaTimLngo The second condition might be more prevalent 

 than the first primarily because of a possible parallel in physi- 

 ology' and habits between the sea lamprey and the Pacific sa?ji\onE 

 which die without spa^vning when blocked from their spawning g-.'ouiids. 



IV. Sea lamprey spatvning runs 



Investigations of spawning runs of sea lampreys were made in 

 the field between April 1 and September 1, 19ii7, March 1$ and 

 September 1, 19U8^ and March 1$ and September 30, 19h9o I here 

 report first on the biological characteristics of, and factors 

 affecting, the spamiing runs of sea lampreys entering Carp Greek, 



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