124 



Part III. — Eighteenth Annual Report 



very possibly simply in consequence of the crabs being nearly all fished 

 up. The explanation given by the fishermen is that the crabs " hole 

 up," or " do not creep." The fact of the double migration of the crab is 

 not accepted by all the fishermen. They are of the opinion that there 

 are certain " bodies " of crabs restricted to deep water and other bodies 

 to shallow water. And since during the winter they get no crabs in- 

 shore, the reason is that the inshore crabs " hole up " in winter ; and 

 since in summer no crabs are got off in deep water, it is owing to their 

 retiral to their burrows, from which they cannot be tempted by the 

 ordinary baits. A generally accepted belief is that crabs will not 

 " creep " all the year round. These are merely attempts to account for 

 the phenomena of migration. Another difficulty, which gives rise to 

 the view that there are varieties of crabs restricted to special areas, is 

 due to the fact that the appearance of the crab changes with the season. 

 As the shell of the crab grows older it darkens, becomes yellow under- 

 neath ; and its general appearance is affected by the ectozoa which attach 

 themselves to it. 



What, then, are the causes of the migrations of the crab ? The 

 reason for the migrations may be sought for in two directions — (1) the 

 influence of temperature ; (2) the necessity for food. The time in the 

 crab's life when temperature will most probably affect it is (1) during the 

 incubation of the egg ; (2) during the process of casting. From the 

 appended Table of temperatures of the bottom water of the sea at Dunbar, 

 taken in June, July, and August, 1899, by a fisherman, it is seen that 

 with one insignificant exception the water at 6 to 8 \ fathoms was much 

 warmer than the water offshore in 2 1 to 25 fathoms. During that period 

 the eggs hatch, and as we know that warmth has the effect of hastening 

 the development of the ovum, we may conclude that in the case of the 

 berried crabs the migration shoreward aids the hatching of the eggs. 

 Berried crabs extrude their eggs offshore in winter, when the bottom 

 water is warmer than that near the shore ; their presence inshore in 

 summer pre-supposes a migration thither. Secondly, there is every 

 ground for holding that the soft crabs cast in the shallow inshore water ; 

 the warmth possibly influences favourably this process. 



Temperature of Water at Bottom. 



Date. 



Three Miles from Land. 



Half Mile from Land. 



Just outside Harbour. 





Tempera- 

 ture. 



Depth. 



Time. 



Tempera- 

 ture. 



Depth. 



Time. 



Tempera- 

 ture. 



Depth. 



Time. 



1899. 

 June 8 



Degrees. 

 0. 

 7-5 



Fathoms. 

 25 



12.45 p.m 



Degrees. 

 O. 

 8 



Fathoms. 

 13 



1.30 p.m. 



Degrees. 

 O. 

 9 



Fathoms. 



2.10 p.m. 



» 15 



8 



25 



4.30 „ 



9 



12 



2.45 „ 



11 





2.0 „ 



„ 23 



8 



25| 



1.40 „ 



9 



13 



11.50 a.m. 



10 



3 



11.30 a.m. 



„ 30 



8-5 



25 



5.30 „ 



9-5 



12£ 



3.15 p.m. 



11-5 



6* 



2.30 p.m. 



July 6 



8*5 



25 



5.15 „ 



10 



13 



3.10 „ 



12 



6* 



2.45 „ 



„ 14 



9-9 (10) 



22 



12 noon. 



11-3 



13 



1.5 „ 



11-9 



7 



i.is „ 



» 21 



9-5 



24 



4.30 p.m 



10 



18 



5.15 „ 









Aug. 2 



11 



23 



6.30 „ 



12-5 



12£ 



7.0 „ 



13 



8 



7.10 „ 



»• 11 



13 



21 



7.0 „ 



]2-5 



13 



7.30 „ 



12-8 





7.45 „ 



18 



13 



22 



8.0 „ 



14-5 



14i 



8.30 „ 



15 



n 



8.45 „ 



„ 25 



13 



22 



7.15 „ 



15 



14 



8.0 „ 



15-5 





8.20 „ 



Se . 25 



11-9 



25 



12.15 



11-6 



10 



3.0 „ 









