The migration was almost exclusively a nocturnal one, with the 

 exceptions appearing very late in the season and only in the larger 

 of the two streams. 



Variations in time and character of migration in different 

 streams of differing characteristics in the same zone . — The runs in 

 the following five streams of diverse character in Control Zone H-l 

 were selected for comparison: Ocqueoc and Trout u ivers and Green, 

 Milligan, and Grace Harbor Creeks (Pig. 6). Essentially, all runs in 

 a limited area such as this, regardless of the sire of the stream 

 entered, begin on about the same date. The first migrants entered 

 traps in these streams on the following dates: Ocqueoc River — May 12; 

 Trout River — *%y 3; Green Creek — April 2$; Milligan Creek — May 6; 

 Grace "Harbor Creek — May 9. The lateness of the first capture in the 

 Ocqueoc River is not normal for that river and is attributable to the 

 blocking action of the right-angle- type weir installed in the stream 

 (see Applegate, 195>0) . The early captures in Green Creek were due to 

 the location of the weir which was practically on the Lake Huron beach. 

 Earlier researches demonstrated that lampreys explore the mouths of 

 stre-ms for many nights orior to upstream movement until stream temper- 

 atures attain a satisfactory level for migration. Such exploring indi- 

 viduals evidently were captured in this trap. The weirs in the other 

 four streams are located seme distance upstream. 



Although water temperature is the greatest governing factor in 

 determining when and in what numbers upstream movement will occur, 

 the general character (Continuity) and duration of the runs can be 

 strongly influenced by the size of the stream and the stability of 

 its volume of flow. The Ocqueoc River, a moderately large stream with 

 the most stable volume of flow from season to season, has the most 

 prolonged and consistent run in the area. In 1950, this run continued 

 until August 30 ( ir ig.6). Green Creek, small, but with a stable volume 

 of flow throughout the year, attracted scattered migrants until August 

 15 but in far less numbers than the Ocqueoc River. During the period 

 of major movement, the run in Green Creek increases and decreases ab- 

 ruptly as in the other smaller streams of the area in quick response 

 to climatic changes. 



The Trout River and Milligan Creek, small streams with severe 

 daily and seasonal changes in flow, tend to decline to a trickle by 

 the end of June. This change is reflected in the sea lamprey runs which 

 cease at that time. Carp Creek, mentioned in previous sections, falls 

 in this category (Table 6). 



Very small tributaries, 2 to k feet wide, attract occasional mi- 

 grants where the flow of the stream crosses the lake beach in a narrow 

 concentrated channel. Crace Harbor Creek was typical of this type. 

 In another small tributary in the area, Cedar Creek, the discharge was 

 dispersed across the beach zone in a broad, shallow sheet. No lampreys 



25 



