FISHERIES 283 



to that derived from statistics, would enable us to determine with 

 a reasonable approach to accuracy the best methods, times, and 

 places for capturing such fish, securing on the one hand a profit- 

 able result, and on the other hand obviating wasteful depletion of 

 the natural supply. 



Food of Fishes. — Directly or indirectly the most important 

 source of fish food is found in "plankton ", i.e. the various floating 

 organisms which are found in vast numbers at or near the surface 

 of the sea (and of lakes). Minute plants, animalcules, small crus- 

 taceans (especially Copepods), various larvae, and floating eggs 

 (including those of fishes themselves) are among the more import- 

 ant constituents. Some valuable food-fishes, such as the herring, 

 feed solely on plankton. 



The amount of this food available bears a direct relation to the 

 fish-supply, and Professor Hensen of Kiel has devised ingenious 

 and elaborate methods of estimating it in a quantitative manner. 

 Details cannot be given here, but the following extracts from a 

 paper by J. Travis Jenkins (The Methods and Results of German 

 Plankton Investigations^ will serve to give some idea of the import- 

 ance of the matter: — "The plankton estimation methods of the 

 Germans, the credit for initiation of which is due to Hensen, difler 

 from and mark an advance upon the methods hitherto employed 

 in England, inasmuch as no attempt is made in the latter country 

 to arrive at a quantitative as distinguished from a qualitative 

 result. The questions that Hensen attempts to answer are — 

 (i) What does the sea contain at a given time in the shape of 

 living organisms in the plankton? and (2) How does this material 

 vary from season to season and from year to year? It may be 

 pointed out that the results obtained by the German investigators 

 are largely due to the liberal attitude taken by their Government 

 with regard to subsidizing scientific investigation of problems 

 connected with the sea-fisheries. It is to be hoped that the Irish 

 Sea may be subsequently investigated in like manner. A com- 

 parison with the results already obtained for the North and Baltic 

 Seas could not fail to be of interest and to yield important results." 

 Some of the most striking estimations were made on the number 

 of Fork -footed Crustaceans (Copepoda) in plankton, since it is 

 these which constitute the chief food of herrings, sprats, and their 

 allies. The following results were yielded by the method: — " For 

 a square mile of surface-water the annual consumption of Cope- 



