Wherefore its Shifting of Grounds. 235 



Thames estuary habitat every day in the year. They may be 

 scattered about or many together in droves ; the shrimps being 

 either small or of moderate size, according to locality and other 

 circumstances. Withal there is among them what may be termed 

 a partial local to and fro movement of the family groups, 

 impelled by several contingencies. For instance, seeing that the 

 crustacean section of their prey roam about, there is a necessity 

 to follow them. Then the various species of worms and shell- 

 fish multiply at different periods and places intermittingly, so 

 shrimp visits to this or that bed in the course of things result 

 periodically. Sexual impulse, again, doubtless brings shrimps 

 together, though special spawning grounds are quite unknown. 

 There is an absolute necessity at times to shift ground and 

 avoid a crowd of enemies always hovering in pursuit. The 

 weather influences we have already mentioned. Besides, tidal 

 currents have a potent effect. With powerful spring tides the 

 shrimps seem driven from their usual haunts ; during neaps 

 they cling more to the main estuarine channels. 



Again, we know of an instance where spratters at work 

 with stow-iiet in early winter opposite the Garrison Point, 

 Shecrness, in fourteen fathoms water, have taken a shoal of 

 shrimps in their net. Whitebaiters, also, on the Essex shore 

 below Southend pier have had similar experience. It shows 

 that shrimps on occasions do leave their bottom haunts and 

 travel about in mid- water. Whether this is of the nature of a 

 limited migration, or, as we suspect, sexual congregations, we 

 can only hazard a guess. 



To sum up, everything known points to the inference that 

 the brown shrimp remains pretty much within estuarine limits. 

 Its local shifting about though, other things being equal, is com- 

 parable, and indeed is analogous to the longer distance journeys 

 of the pink shrimp. Notwithstanding, Grangon vulgaris is not 

 migratory in the sense of the anadromous fishes (e.g. salmon 

 tribe), which ascend rivers solely to deposit their eggs, and 

 then return to the deep sea to recuperate. 



