ON THE ABUNDANCE OF SOME COMMON MARINE ANIMALS. 247 



Spolia Runiana. — IV. Notes on the Abundance of some Common Marino 

 Animals and a preliminary Quantitative Survey of their Occurrence. 

 By W. A. Herdman, D.Sc, LL.D.. For.See.R.S., F.L.S., Professor 

 of Oceanography in the University ot Liverpool. 



( With 8 Text-figures.) 

 [Read 11th December, 1919.] 



For many years I have taken any opportunities that offered, both on dredg- 

 ing expeditions in the ' Runa' and also when tramping the sea-shores of 

 various parts of the world, of making observations on the abundance and 

 mode of occurrence of the common marine animals and plants. Two years 

 ago I brought before the Societj^ some statistics and conclusions as to the 

 occurrence and distribution of the commoner planktonic organisms, Diatoms 

 and Copepoda, throughout the year in the Irish Sea *, with the object of 

 showing that the supply of such food-matters to our plankton-eating fishes 

 depends upon a surprisingly small number of species which are present in 

 enormous abundance. Some half-dozen kinds of Diatoms and about the 

 same number of Gopepoda are the all-important organisms upon which our 

 fate depends so far as concerns our food from the sea. I desire now to nlace 

 on record some notes as to the occurrence of a few of the commoner fixed or 

 sedentary bottom-living animals of the sea-shore or shallow-water — animals 

 which are also, like the plankton, of great practical importance in nature as 

 the chief food of some of our most valuable fishes. 



Our knowledge of the number of animals living in different regions of the 

 sea is for the most part relative only. We know that one haul of the dredo-e 

 is larger than another, or that one locality seems richer than another, but we 

 have very little information as to the actual numbers of any kind of animal 

 per square foot or per acre in the sea |« Some years ago Hensen attempted 

 to estimate the number of food-fishes in the North Sea from the number of 

 their eggs caught in a comparatively small series of hauls of the tow-net, but 

 the data were probably quite insufficient ami the conclusions may be 

 erroneous. It is an interesting speculation to which we cannot attach any 

 economic importance. 



All biologists must agree that to determine even approximately the number 

 of individuals of any particular species living in a known area is a contri- 

 bution to knowledge which may be of great economic value in the case of 

 the edible fishes ; but it may be doubted whether Hensen's methods, even 



* "Spolia Runiana — 111.,'' Journ. Linn. Snc, Zool. xxxiv. p. 95 (1918). 



+ Professor Mcintosh, in his book ' The Resources of the Sea ' (1899), gives a vivid picture 

 of the abundance of life of all lrinds in the sen, but does not venture upon any numerical 

 estimates. 



19* 



