Sea lanprey spaTminr runs per se 

 (1) Tir.e limits and ^"eneral character of the runs 



Sea lamprey runs in the northern Lake Huron tributaries studies 

 began as early as April 9 (19U9) and as late as April 19 (I9I4.7) and 

 varied mth the climatic conditions in any given year (Figures 1^-13 )« 

 For this reason, calendar dates can only give approjcimate predictions 

 of "when the runs vail occur. As subsequent data on the factors af- 

 fecting these migrations ivill indicate, "water temperature is the best 

 guide as to I'shen migratory action. ty vdll begin as "trell as to fluctua- 

 tions in its in-bensity once it has started » 



Prior to the beginning of the run, sea lanpreys congregate off 

 the mouths of the streams. Before any enter a "watercourse, they 

 may appear for a number of nights on the allu"vlal fan off the mou"th 

 of a stream (Figure lij.) and then drop back into the lake each day 

 rather than enter "tlie stream. This action is evidently induced by 

 temperature differences betiveen the lake and the stream, the latter 

 being colder than the lalce iThen this beha"vior T/as studied. 



Lligratory activity during the early "weeks of the run is spora- 

 dic (Figures 1^-1? )• Peak migratory activity generally lasts for a 

 35- to 50-day period, occurring sometime between April 2$ and June 

 15 "with the date of greatest activity being in early or mid-Iiay. 

 Generally, bet"ween Q$ and 99 percent of the run enters the creek 

 during this period. 



Even at its peak, migratory activity is erratic, particularly 

 in a small stream, and reflects varying climatic conditions (Figures 

 15-18). Cold, Tret spring seasons have a depressing effect on the 

 runs, and movement into a stream on any given day in such a season 

 is never very great. A natural corollary of this is that the up- 

 stream movement of the bulk of the migrants is extended o"ver a 

 greater period of time. The runs in vrarraer spring seasons are 

 characterized by large, sporadic influxes of migrants on particu- 

 larly vrsarm nights Tfhich may sometimes be of spectacular proportions. 

 In "waim seasons, the period -vrhen the buUc of the migrants enter a 

 stream is thus reduced. 



The runs maj terminate in small streams between July 6 and 13 

 depending largely on "water levels in the stream. In larger streams, 

 such as the Ocqueoc River, "r;here an appreciable volume of water is 

 discharged in the summer months, the last migrants may enter the 

 river as late as Septem.ber 30 (Figure 18), It may be added, however, 

 that migratory activity after J"aly 1 is generally limited to a few 

 scattered indi-viduals . 



-3k- 



