CHINESE MITTEN CRAB PANNING 369 



April and decrease temporarily during the end of May and early June, 

 probably as a result of a shedding which takes place about this time. 

 The migration is at its height in June and July and falls off during 

 August. The swarms that penetrate into the Havel are even larger. 

 About 100,000 kg of crabs were caught there in similar traps in 1936 

 (pi. 4, fig. 1). And even as far up in the Saole as Calbe 42,500 kg 

 were caught in 1936. The migration decreases considerably higher 

 up in the Elbe beyond the mouth of the Saole, inasmuch as the 

 greater mass has branched off into suitable feeding grounds. In 

 single instances only do they penetrate as far as through Saxony into 

 Czechoslovakia. The mitten crabs, as we see, indeed, accomplish 

 extraordinary wandering feats. The distance which the tiny larvae 

 travel from the sea up to the vicinity of Hamburg is about 100 km, 

 the distance along the Elbe from Hamburg to Doemitz about 120 km, 

 from Hamburg to Garz on the Havel 220 km, and from Hamburg to 

 Calbe on the Saole 350 km. 



I have ascertained through marking tests the rate of speed at which 

 the young crabs travel upstream against the current and in doing this 

 I have marked the backs of 13,000 mitten crabs with good ship's 

 paint of different colors during series of experiments in the Weser, 

 the Elbe, the Havel, and the Saole. The crabs were placed in lots 

 of 1,000 specimens at certain distances apart below the traps and the 

 time was noted when the marked crabs again landed in the traps from 

 which they had been taken previously. The evolution of these 

 13 experiments showed that the small animals, migrating from the 

 tidal region in the lower course of the Weser, traveled a distance of 

 1 to 1.5 km daily and that the larger ones, moving against the current 

 in mid Elbe, a distance of 2 to 3 km daily. In their upstream mi- 

 gration, they reach the Havel during the first summer and the Saole 

 during the second summer. 



When the mitten crabs have reached their full growth in the interior 

 of Germany, they leave and wander back to the sea to reproduce. 

 A shedding of the shell precedes this downstream migration so the 

 crabs start out with thin, light shells and subtle elastic synovial 

 membranes. The migration commences everywhere simultaneously 

 in August and is in all certainty induced by incipient growth of the 

 sex glands, inasmuch as the sex organs begin to develop in these 

 migrating crabs during their wandering through mid Elbe. The 

 migration downstream decreases slowly in mid Elbe during October 

 after its height in September, whereas the migration continues in its 

 lower course into November. The rate of speed at which these crabs 

 travel has also been ascertained through experiments with color 

 markings (pi. 4, fig. 2). In the course of four experiments, 1,600 

 crabs were marked and set adrift in the Havel and the Weser. Of the 

 crabs which were captured, the three which traveled the longest dis- 



