368 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 8 



In the late fall the crabs return in large masses to deep water for 

 winter rest, causing such crowding in the narrow river-channels that 

 the young ones, then in their second year, are not strong enough to 

 fight for a place for winter rest there and are, therefore, forced to 

 move on. 



Consequently, as the catches by the dam in the Weser by Bremen 

 show, the young animals begin to move on upstream in the beginning 

 of winter but only sparsely during the cold weather in January and 

 February. As soon as it gets a little warmer in March, these crabs, 

 not yet 2 years old, commence to migrate upstream in such huge masses 

 that more than 30,000 of them are caught and destroyed daily in 

 Bremen in traps specially constructed for this purpose. The con- 

 gestion is lifted when the crabs again swarm out into the shallow 

 regions with the advent of warmer weather in the beginning of May. 



These swarms, which once migrated from the tidal regions, are now 



IV V VI VII VIII 



Figure 3.— Catch of mitten crabs at Doemitz, Germany, during April, May, June, July, and August, 1937 



forced to wander farther on because rivers that have been converted 

 into navigation arteries do not hold sufficient nutriment for them. It 

 is a wandering without a goal. Wherever a canal, or a rivulet, 

 empties into the river some of the crabs always leave the large swarms 

 to move into it. But in the Elbe, from Hamburg on, they have very 

 few opportunities to branch off into suitable feeding grounds. The 

 crabs heading upstream are, therefore, forced to remain for a long 

 time in huge swarms. Only the Elde near Doemitz (in Mecklenburg), 

 the Havel and the Saole can accommodate these swarms and conse- 

 quently receive heavy visitations of the crabs. In these three rivers 

 the migration begins in April about the time when it already has 

 reached or passed its height in the tidal region, and continues into 

 August. In Doemitz (in Mecklenburg) 44,400 kg of mitten crabs 

 moving upstream were caught in specially constructed traps by the 

 dam in the Elde rivulet in 1936, and 34,925 kg in 1937. As the curve 

 (fig. 3) shows, the migration and the catch commence suddenly in 



