2 



MARINE AND FISHERIES 



6-7 EDWARD VII., A. 1907 



Many of our important food-fishes, such as those of the herring and mackerel 

 families, are known as plankton feeders, for their gill-arches are provided with a sifting 

 apparatus which enables them to sift out from the water which they are breathing, the 

 minute organisms it contains, and the young stages of all fish pass through a phas© 

 when they are dependent on the same kind of nourishment. Without a glance at the 

 catch of a tow-net it seems incredible that fish of any size should be dependent on such 

 inconspicuous food, but sometimes at the height of the summer a careful inspection of 

 the water itself betrays its richness in life. In our inland lakes, e.g., the * blossoming' 

 or ' flowering ' of the lake in August, when the water is full of minute green points, 

 is a phenomenon which often attracts attention and is only a temporary exaggeration 

 of a permanent condition. The astounding rate at which these minute creatures re- 

 produce themselves, is one of the noteworthy facts about them. 



Although there are various methods of reproduction, one of the commonest is that 

 of division into two after they have grown to their typical size. Maupas has calculated 

 that if a little Infusorian, not as big as the head of a pin, continued to reproduce at its 

 ordinary rate of division — five times a day — it would, at the end of a month, form a 

 mass of protoplasm a million times as big as the sun ! It is obvious that the rate of 

 consumption of such creatures by larger forms must be very high to keep down the 

 population to the normal relations in which we find them, and of course the rate of 

 reproduction of the minute plants is dependent on the amount of the carbon, nitrogen, 

 and other elements of their food available in the sea water. 



But it must be remembered that these minute plants are constantly being devoured 

 by animals, some little bigger than themselves, others much larger, hence no one species 

 ever gets the opportunity of monopolizing the ocean. 



Another noteworthy circumstance is that our northern waters appear to be richer 

 in plankton vegetation than those nearer the equator, richer at least, in the mere 

 quantity of vegetable matter, not in beauty or variety of form, for the tropical species 

 are certainly more varied, and in many cases more beautiful than the northern ones. 

 To this wealth in microscopic organisms of our waters we owe the circumstance that 

 we are able to supply warmer climes with the surplus of our fish production. The 

 reason of this greater richness is not apparent; Brandt has suggested that it may be 

 due to a deficiency of nitrogen in warmer waters owing to the more favourable con- 

 ditions for the growth of denitrifying bacteria. 



Before giving a detailed description of the minute life of the ocean, a few remarks 

 as to its general character will be appropriate. The simple plants which constitute 

 the bulk of the marine vegetation are frequently Peridinia (Plate L), single cells of 

 odd shape usually furnished with a decorated shell, and swimming actively by means 

 of two long lash-like ' fiagella.' Some of these Peridinia it is improper to describe as 

 plants, for they seem to be destitute of chlorophyll and therefore obliged to depend 

 upon preformed living matter for their food. 



Another group abundantly represented in the open water is that of the Diatoms . 

 (Plate II.). These have always a resistant siliceous shell, and do not swim actively 

 like the foregoing. Both of these groups of plants, however, require to live in the . 

 stratum of water penetrated by sunlight, and they do this either by their own exer- 

 tions, but usually owing to the presence of organs which render floating easy, such as 

 long delicate spines or the like, or again, to the presence of fat or oil which diminishes 

 ihe specific gravity of the cells. 



The chlorophyll in the Peridinia and Diatoms is often masked by other colouring 

 matters usually of a brownish hue, but there are also unicellular plants of a pure green 

 chlorophyll like some of those represented in Plate III., while in addition to these 

 there occur many extremely minute forms of various colours, but in shape approaching 

 that of the Ohrysomonad, Fig 11, so small as to elude the meshes of the fabric gen- 

 erally employed. The mesh of the latter is usually %oo of an inch on the side, but 

 many little creatures actively swimming by means of lash-like prolongations of their 



