with time, emphasizes the danger of basing migratory rates on short periods 

 particuJarly immediately after release of drills, and coincides with the limited 

 rate of migration of drill populations reported by others . 



Additional detailed studies using marked drills on bottoms covered with 

 varying concentrations of shell with oysters of varying ages ; and with varying 

 mixtures of shell and oysters, must be carried out before the complete story on 

 migration can be presented Such explorations should be carefully related to 

 temperature, salinity, current, velocity^ turbidity, type of sediment,, depth of 

 water including mtertidal areas, degree of concentration of shell and of oysters, 

 available food organisms other than oysters, if on barren bottom the proximity of 

 food organisms size of the drills, the stage in the reproductive cycle of Urosalpinx , 

 ?;nd time of release 



Spawning Migration 



A. number of observations demonstrate that a portion of the drills normally 

 occupying subtidal bottoms in the winter migrate inshore onto favorable mtertidal 

 bottoms to spawn early in the summer., Gibbs (pers, com) for many years has 

 reported the absence of Uro salpinx in the intertidal zone in the winter and their 

 presence 'here in the summer and has supposed that this is explained by migration, 

 Orion (1930) states that in England there is an inshore migration in the. spring and 

 the species spawns heavily in shallow water. Cole (1942) adds that although this 

 spawning is evident in the mtertidal zones of the River Biackwater, very large 

 aumbei s also spawn at all depths, Mistakidis (1951) reports a similar inshore 

 migration in the Rivers Crough and Roach, England, during the spawning season 



Cole (1942) by simultaneous sampling of large number s of drills in the 

 intertidal zone and in deepej water in the River Biackwater in June and at monthly 



■ '■' " ai ■-. thereafter in the mtertidal zone alone., has shown that the first drills to 

 < ome ashore in June are mainly four, year old females and five and six year old 



The younger and older drills remain in deeper water . By July, three year 

 old females increase enormously inshore and form the dominant group and thus 

 appear to spawn about a mouth after the older drills . Older and one and two year 

 o-d drills also appear but in lesser numbers. Among males the tendency is similar 

 and as the season advances the younger age groups appear inshore in increasing 

 numbers; bur one and four year old males are never as numerous inshore as in 

 deeper water The largest numbers of drills were collected in June and July and 

 by August many had already returned to deeper water. Cole's data also show that 

 out ol a total of 1, 288 male and female drills collected in the River Biackwater in 

 1939 and L941, 814 were females and only 474 were males . Whether it is a general 



ule that more females than males participate in the inshore migration cannot be 



98 



