CHINESE MITTEN CRAB — PANNING 373 



of damaged nets. When mitten crabs are caught as a by-catch in 

 dragnet fishing, not only is the fish injured and made useless for sale 

 by rubbing against the crab's toothed armor, but the nets wear out 

 much sooner and must be replaced more often. It is estimated that 

 the resulting cost of nets is tripled in many regions. When mitten 

 crabs come upon eel-basket pots, they swarm into them, attracted 

 perhaps by the scent, and as a result the eels do not go in, or if they 

 do, they are devoured. In central Germany large hoop nets with 

 which to catch the eels heading for the sea are often laid out behind 

 the sluice gate. The crabs migrating in the fall unfortunately take 

 the same route and are consequently caught in these hoop nets. When, 

 as happens in Havel, up to 500 kg of mitten crabs are caught in a 

 single night in a hoop net, the chances for eel fishing are impossible. 

 As eel fishing is the most important source of livelihood for the fisher- 

 men in many regions, it is very easy to understand that the fishermen 

 demand that adequate steps be taken to ward off their peril. 



MEANS OF CONTROL 



It is true that some predatory fishes and some aquatic birds and 

 waterfowl devour the mitten crabs, but this means of exterminating 

 them is altogether ineffective because of the crabs' enormous and 

 rapid increase. The possibility of preventing their reproduction by 

 catching the breeding swarms in the river mouths has been discussed, 

 but however logical the idea may seem, considerable difficulties stand 

 in the way of carrying it out. But more favorable opportunities 

 to control the crabs have presented themselves in the interior of 

 Germany. 



The mitten crabs travel on the bottom of the rivers, forcing their 

 way upstream where the current is strongest, and pile up below any 

 dam that temporarily stops them in their wandering. Advantage of 

 this opportunity is taken by the Weser dams in Bremen. Barrels 

 covered with wire netting or canvas are lowered with davits to the 

 bottom of the Weser. The crabs, jammed in against the dam, 

 crawl high up on the barrels, fall into them and are caught in this 

 way. In 1935 from January to May, 12,166 kg of mitten crabs 

 (3,444,680 specimens) were caught, the greatest amount at one time 

 being 407 kg (113,960 specimens) on April 15, 1935. In 1936, 12,786 

 kg (2,941,100 specimens) were taken. 



When crabs are jammed below a dam they try in many ways to 

 get by the obstacle. They crawl up on the walls and finally out on 

 the shore, so as to pass the dam by land. It was thus that it was 

 first learned what enormous masses of mitten crabs infest the German 

 rivers. During warm summer nights the shore region is black with 

 crabs; one cannot take a step without treading on them. In places 

 where a dam is close to a city, it happens occasionally that mitten 



