The Propagation of the Common Food Fishes on the Norwegian Skager Rack Coast, with Notes on the Hydrography
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The Skager Rack coast of Norway is characterized by narrow coastal banks, a well developed system of skerries and small fjords. Intensive fishing is carried on, mackerel, herring, cod and some other Gadidoid fish, etc., being of special importance. Fishing statistics from years back reveal the occurrence of great fluctuations in the herring fisheries. Mackerel fishing has increased while the output of the cod fisheries is undoubtedly declining. There is no doubt that the fluctuation in the herring fisheries is, in a high degree, caused by the variation in hydrographic conditions. The increase in the mackerel fisheries on the other hand is obviously a result of the high standard of development of motor craft and fishing gear while the decline in output of the cod fisheries is attributed to fishing on too intensive a scale, as fishing experiments tend to show. It is commonly assumed that there may be great variations in the result of the spawning of the different species. In order to comprehend the fluctuation in the output of the fisheries it is therefore of importance to study the conditions under which fish eggs and larvae develope in the different years. Investigations having a bearing on this point have previously been undertaken on this coast by KNUT DAHL in 1904 and 1905 (6) and by me in 1917 (8) and 1924 (3). DAHL found great quantities of fish eggs and larvae in the fjords and advanced the suggestion of possible great interchanges between the fjords and the open sea of fish eggs and larvae by means of currents. My investigations in 1917 showed that the content of oxygen in the fjord at the intermediate and deep water layers was much lower than out at sea. It was not reasonable therefore to assume that great interchanges of water masses took place as a matter of course, and the collection of fish eggs and larvae demonstrated some difference between skerries and open sea both as regards species and comparative numbers. In the skerries there was a fauna of eggs and larvae differing from that of the fjord and the sea just a few miles outside. The investigations in 1924 demonstrated the effect of a cold period in the middle of the spawning season of the spring spawners. The eggs disappeared from the waters. The influence of the cold water, however, was more restricted to the open sea. In the fjords there were reservoirs of water of a sufficiently high temperature. When the ice melted here a good quantity of eggs was found to be present, and when the temperature of the sea outside the coast and in the skerries increased fish eggs and larvae were again found, but only in small numbers. A tentative suggestion was made to the effect that a cold period in the middle of the spawning process was detrimental to the eggs spawned and the larvae hatched and it was assumed that the best spawning and hatching results would be obtained if the temperature of the sea during the early winter prevented spawning. When the temperature subsequently rose conditions would be good for hatching and development of the larvae. In all years since 1917 regular hydrographic observations have been made, both near the coast and in the fjords. Outside the coast both salinity and temperature in the surface and intermediate layers vary greatly according to the meteorological conditions obtaining during the year. Easterly winds in winter and spring bring comparatively fresh and cold water near the coast. Spawning will not, therefore, take place and the spawners will seek warmer water layers. Westerly winds bring higher temperatures and salinities in their train, conditions well-suited for the spawning, hatching and development of the brood. The last mentioned process, however, is entirely dependent on the food available. Directly and indirectly the larvae feed on the phytoplancton and as the latter is very dependent on the light it is of great importance that hatching of the larvae should not take place until reproduction of the phytoplancton has commenced. In the fjords the investigations have shown that the variations in the hydrography of the deeper layers are insignificant. Except in special circumstances, between which there may be intervals of many years, no renewal of the waters takes place here. For this reason the quantity of oxygen dissolved in the bottom layers will be consumed by the life processes of animals and plants and by the oxydizing of organic matter. Thus for a time the bottom layers may be unfit for animal life. The intermediate layers of the fjords are also very stationary and the quantity of oxygen here may vary in quite a different manner to that of the water outside. Here, however, the quantities are never so low as to render the waters unsuitable for animal life. The material dealt with in this paper was collected in 1934, 1935, 1936 and 1937 - from the waters outside the coast, the skerries and the fjords near Arendal, Kristiansand and Kragerø. The results arrived at in 1917 and 1924 are referred to. The figs. illustrating the occurrences of the different species show that the eggs and larvae of cod are found most abundantly in the fjord, the skerries and close outside the coast. The highest numbers of larvae caught in the Topdals fjord (11.-12. May 1934) and Kragerø Fjord (14.-16. May 1934) occurred, however, as already mentioned, some weeks after millions of cod fry had been liberated in these fjords. Haddock larvae are most numerous some miles further from the coast. Sprat larvae occur both in the fjords and in the open sea. Spawning appears to commence about one month earlier in the fjords than outside the coast. Herring larvae are most abundant in the skerries. Mackerel larvae occur almost exclusively outside the coast. Few hauls have been made in the fjords, however, during the mackerel spawning period. The results already attained in 1917 (DANNEVIG 8) , namely, that each area of water, fjords, skerries and open sea, has its own special communities of fish eggs and larvae, are confirmed by the present investigations. Cod and herring are coastal forms both as to spawning and larvae. Haddock larvae are most prominent further out. This, undoubtedly, accords with the more oceanic character of the haddock as compared with the more littoral cod on this coast. The sprat and mackerel are pelagic forms. The former spawns both in the fjords and the open sea. A study of the detailed tables brings to light other differences between the fjord and the skerries. A number of species only occur in the open sea or in the skerries. In the most isolated waters investigated, namely, those of the Kils Fjord, only a few species are present. The special hydrographic conditions in the fjords will only permit a restricted number of species to live and propagate in the fjord. Such a distribution cannot take place if the waters in question interchange to a great extent and when a few species do occur in nearly all areas investigated they must be regarded as representatives of species with a wide spawning area.
SeriesFiskeridirektoratets skrifter, Serie Havundersøkelser
vol 6 no 3