176 A CENSUS OF THE SEA [PART II 



must not forget that there are a considerable number of fishes, like 

 the herring and mackerel, which inhabit the upper layers of the sea. 



Very many observations point to the conclusion that the density 

 of life, both fishes and invertebrates, is greater on the sea bottom 

 in close proximity to the land than in deeper water further out to 

 sea. It is in such inshore waters that one finds the greatest wealth 

 of animal life, and I may give, as an instance, some figures relating 

 to the density of fishes and invertebrates on a comparatively small 

 part of the sea bottom just outside the estuary of the river Mersey 

 in Lancashire. The results of several hundreds of hauls with a. 

 shrimp trawl made during the years 1893 — 1899 on this ground 

 shew^ that the average number of fishes of all kinds captured per 

 haul of the net was about 5000, of shrimps 4500, and of other in- 

 vertebrates about 2500. Thus the total number of animals captured 

 per haul was about 12,000, and very often much larger catches 

 than this were made. The length of the average haul of the trawl 

 net was about two miles, and the width of the mouth of the net 

 was twenty-one feet ; thus in each drag an area of about 21,000 

 square metres was swept, and one animal was captured for every 

 two square metres of sea bottom. Now we may be fairly sure that 

 not more than one-fifth of the larger animals present on the bottom 

 was actually captured by the net ; and we may also be sure that 

 a considerable fi-action of the total population of the sea bottom 

 consisted of small animals, above microscopic dimensions of course, 

 either resident on the floor of the sea, or burrowing in the sand and 

 mud, which were too small to be capable of capture by a net having 

 the mesh of a shrimp trawl. I think it quite probable that there 

 were on this part of the sea bottom not less than twenty, and not 

 more than two hundred animals varying in size from an amphipod 

 (J inch long) to a plaice (eight to ten inches long) on every square 

 metre of bottom. 



Of course the variability of density of life on the sea bottom in 

 inshore waters is so great that these numbers are often vastly 

 exceeded. Thus part of a mussel bed may have a population of 

 16,000 molluscs to every square foot (say 1000 sq. cm.) and at 

 times, when such a mussel bed is being devastated by star-fishes, 

 the sea bottom may literally be carpeted by these animals. Some- 



1 Ann. Rept. Lancashire Sea-Fish. Lahy. Liverpool, 1900, p. 39, 1901. 



