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THERE is scarcely any subject which at the present time lies more in the dark than the question of nature's economy with respect to the maintenance of the stock of food fishes in the open sea.

Although this question is of the greatest actual interest to the nations who live on the coasts of the North Sea, the Baltic, and the North Atlantic, the laws which regulate the propagation, the growth, and the nutrition of sea fishes are still undelineated by science, and among the authorities who have the care of the fishery interest of the different countries there seems to prevail an opinion that it would be hopeless to try to find out scientific principles whereupon to base a rational fishing industry or international regulations for the sea fisheries of the North Sea area.

It is scarcely thought worth while to investigate such questions as, for instance, why the oyster-beds of England, as well as of Norway and Sweden, are gradually growing sterile ; why the lobster catch steadily diminishes in quantity and quality in many districts, or why the herring fishery in the North Sea and the cod fishery on the banks of Iceland, Spitzbergen, and Lofoten are liable to extreme variations from one year to another. All efforts are directed upon the practical measures to increase the catch or improve the fishing methods and enlarge the market by facilitating the transport of fish, without any regard to the question whether the stock of food fishes and the natural supply of fish food existing in the North Sea area will bear the demand upon the resources of nature's economy by the augmented catch. Twenty years ago we could have been excused in neglecting this question, as we could then still retain the delusion that the store of food fishes in the North Sea and adjacent parts of the ocean is practically unlimited, and that the fish caught by fishermen is insignificant compared with the numbers of useful fishes devoured by prey fishes, &c.

At present, after the recent improvements in fishing methods, and after the introduction of steamboats in the fishing industry and the enormous increase, especially of the German and English fleet, of steam trawlers, matters look otherwise, and there is plenty of experience to warrant the common opinion that the North Sea is subject to overfishing, and that some precautions must be taken against it.

Necessary as such restrictions upon the sea fishery may appear, they will, however, be impossible to realise by international agreement, so long as it is not known when and where protection against overfishing is needed.

The fact is, that we know too little to venture upon legislative experiments, which afterwards may prove to be inefficacious. Nor is it in the least probable that the nations who are economically interested in the North Sea fishery will agree to international legislative restrictions upon their fishery industry without full and strong reasons being produced for the necessity of such rules. The facts upon which an international legislation about sea fisheries must be based can only be elucidated by careful scientific investigation. The way of scientific research may seem long and difficult to traverse, but we must remember that it is the only one which leads to certain results, and it is now no longer an untrodden path, since the late investigations of British, German, and Scandinavian biologists and hydrographers already represent a fair advance in our knowledge of the ocean and of oceanic life.

The Swedish Government recently issued an invitation to the Foreign Offices of Great Britain and other North Sea powers to take into consideration the desirability of a systematic international investigation of the North Atlantic and North Sea in the interests of the fisheries.

This scheme is founded on the opinion that all fish-life in the North Atlantic, and especially the presence of the migratory fishes, depends upon the great currents in the upper layers of the sea, and the variations of the presence in these layers of the food required by the fishes, viz. 'plankton,' or organisms of animal or vegetable origin, floating in the water. A knowledge of these currents, and of the quality and quantity of the food which they contain, is necessary in order to determine the legislation required for the creation of a rational organisation of the fisheries.

In consequence, a research ought to be made concerning the existing conditions, and the currents during all seasons, in the upper layers, between the surface and a depth of about 400 to 500 fathoms, as well as concerning the nature and the quantity of the 'plankton' to be found in these layers. The best way to make the research would be to divide the field between the participating countries, in order that each country might establish a system of observations over a certain area of the adjoining sea. For Sweden, it would, for instance, be most convenient to explore the middle parts of the Baltic and the Skagerak.

The necessary expenditure for a research of this kind would be comparatively small for each country. The research is less difficult when the intention is not to explore the deepest, but only the upper layers; the methods actually used for the observations are also less complicated than previously, and, as a result of the experience already acquired, it is now possible to avoid useless work by concentrating the observations upon essential points. Almost all the countries on the North Sea have now—with the exception of Swedenestablished on their coasts scientific stations and institutes to make observations with regard to fishing interests.

Private associations are also found everywhere for this same purpose, and if all these institutions, which are now working without any common plan, could be persuaded to devote part of their work to a great common purpose, a great part of the object pursued could thereby be attained. The time for the research ought to be extended to five years. The observations ought to continue during all seasons, and as a convenient date to begin, the 1st of May 1899 or 1900 might be selected.

The initiative step thus taken by Sweden is moreover founded on experience that international regulations of fishery questions can be founded upon scientific research. I here allude to the recently signed convention between Sweden and Denmark in regard to the plaice fishery in the Kattegat.

The Kattegat is physically as well as biologically the best investigated sea area existent, thanks to the systematical and scientific research which has been made there during the last sixteen years by the Danish biological station under the leadership of Dr. C. G. Joh. Petersen.

A short account of the method which has been adopted in this survey will follow here.

In the course of the years 1883–1889 the physical conditions, as well as the distribution of the invertebrates, constituting the bottom fauna of the entire Kattegat had been investigated and mapped out by a commission of specialists with Dr. Petersen as their leader, on board the Danish ss. Hauch.

The following chart of the Kattegat will serve to orientate the reader as to the situation of the fishing-grounds of the northern part

of this area.

The most important places for the plaice fishery are the submarine plateaus surrounding the Danish islands, Laesoe and Anholt.

The bottom there, consisting partly of sand and partly of blue mud, contains abundant food for plaices and other flat fishes, who feed upon the shells of Mactra, Solen, Abra, &c., living on the bottom.

Dr. Petersen invented a special method for estimating the quantity of fish food contained in a certain area of the bottom. A

"Report of the Danish Biological Station to the Home Department VII. 1897.


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closer investigation showed that the amount of such animals as constitute the nourishment of flat fishes on the fishing grounds in the Limfjord was, although abundant, by no means inexhaustible.

In fact, the actual result of the fishery, which in the Limfjord and in the Kattegat is carried on with highly effectual fishing gear, the plaice-seine, corroborated the conclusions of the biological research, viz. that the fishing-grounds of the Kattegat were already subject to over-fishing, in a high degree, and that a rapid decrease in the size of the flat fishes has been the result.

The next question was to decide what particular restrictions must be introduced in order to maintain the stock of really valuable adult plaices in the Kattegat undiminished, with the least possible disturbance to the fishery. How this was achieved will be best understood from Dr. Petersen's own description.

It has been ascertained that the stock of plaices in the Kattegat consists of four classes of fishes, different in size and age, which we will denote by the group numbers I., II., III., IV.

Group 1. consist of the youngest fry (the fry of the year) which, after it has been hatched from the drifting (pelagic') eggs, during the first half-year of its life, lives close to the shore in water of less than two metres depth. The dotted line on chart shows the limit of the coast waters inhabited by these young flat fishes.

If it was permitted to catch small flat fishes within these limits, the consequence might be that the entire plaice fishery of the Kattegat would be destroyed.

In the course of one year the fishes of Group I. acquire a size of 5-6 centimetres.

Group II. In the next year these fishes, which we now denote as Group II., emigrate to somewhat deeper waters (4-8 metres) and grow to the size of 8–16 centimetres. They are still too small to be useful as food fishes, but in the course of the third year they appear as :

Group III., which is for a great part subject to catch, in so far that the largest members of this class, which have attained the size of 25 centimetres, hitherto have been considered as marketable in Denmark. On account of the energetic effect of the plaice-seine relatively few fishes of Group III. are spared until they reach the fourth year, when the plaices get mature, and have a much higher value in the market.

Group IV., the adult plaices, are consequently few in number at present, in comparison to their occurrence twenty years ago. Before the over-fishing by means of the plaice-seine had commenced the individuals belonging to this group were of much larger size (i.e. attained a greater age).

At that time there existed in the Kattegat many plaices up to 50–60 centimetres in length, which were called 'Hanser' by the fishing population. This class of plaices is now wholly extinct in the Kattegat.

The growth of the plaices in the Kattegat has been studied partly by counting and measuring the separate individuals of each catch, partly by labelling the living fishes with numbered labels.

If the plaices caught in the Kattegat by means of the plaiceseine with small meshes are counted and measured, the individuals will be found to group about four average values corresponding to the groups I., II., III., IV. here mentioned, which constitute the maxima of the curve in the diagram overleaf.

The full drawn line denotes the dimension of Group IV. at the the present time, and the dotted line the supposed extent of Group IV. twenty years ago, before the introduction of the plaice-seine.

We infer from this that the cause of the decrease of the catch with regard to the size of captured fishes must be ascribed to overfishing by means of the plaice-seine and also by other fishing implements, but in a smaller degree).

The detrimental effect of this fishing gear is not due to the catch of larvæ or the youngest class of plaices, which is as abundantly represented as ever, but to the overfishing of the plaices belonging to Group III. and the undersized individuals of Group IV., which, although considered to be marketable fishes, have not attained

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