THE DENSITY OF ANIMALS 

 ON THE OCEAN FLOOR 



By R. Sparck 



The usual method of exploring the ocean bed by trawling and dredging 

 fails to provide a correct picture of the density of bottom animals. Of 

 course, the trawl or dredge will bring up more specimens of a species if 

 there are many than if there are few, but gear which is dragged, moving 

 over the bottom more or less unevenly, cannot give a measurement in terms of 

 number and weight per square metre. To get this we must use other gear. 



Such gear was employed, nearly 50 years ago, by the Danish fisheries 

 biologist C. G. J. Petersen, for measuring the amount of fish food on 

 the sea bed. For many years small plaice have been transplanted from the 

 western Limfjord, in Denmark, which is overpopulated with plaice, to 

 the inner broads of the same fiord, where there is an abundance of food 

 but where plaice never go of their own accord in any large numbers. 

 The desire to get a fairly exact measurement of the number of living 

 animals per square metre of bottom led Dr. Petersen to design a special 

 sampler, known internationally as the Petersen grab. This consists of a pair 

 of heavy steel jaws which will close tightly over a given area of bed. 

 When the gear is lowered the jaws are open, but as it strikes the bottom 

 and the cable slackens the jaws close, the grab sinking slightly into the 

 bottom under its own weight. By this means a sample is obtained of 

 one-tenth or one-fifth of a square metre of bottom, according to the size 

 of grab used. When it has been hauled up on deck, the sample passes 

 through a system of sieves which remove the sediment, leaving animals, 

 stones, shells, and the rest behind. The animals can then be identified, 

 counted, and weighed. Given a sufficient number of samples and fairly 

 even distribution, it is possible to calculate the number and weight of 

 the various species per square metre of sea bed. 



Rather extensive samplings have been made in recent years with this 

 grab, especially in Danish waters, off the Faroes, Iceland, and Greenland, 

 in northern Russian waters, and in the Adriatic. Most of them, however, 

 have been in shallow water and only a few have been at depths beyond 

 100 metres — a few also at a little over 1,000 metres. They have shown 

 that only a comparatively small number of species occur in these waters 

 in large quantities per square metre, and, furthermore, that the same 



