Undersøkelser i Oslofjorden 1936-1940 - Egg og yngel av vårgytende fiskearter

In recent years the Oslo fjord has been made the subject of a more intense study as regards the hydrographical and biological conditions. The fjord is 55 naut. miles long, and is divided into several basins by narrows and submerged barriers. (Fig. 6 and 7). The entrance to the fjord from the Skagerack is comparatively open. In the outer part fresh water from the rivers, the Drammenselv and the Glomma, affects the salinity of the surface layers especially during the spring. The inner part is affected by sewage water from Oslo. Owing to the submerged bariers and the stability of the water the intermediate and lower layers inside the Drøbak sound are often stagnant and their content of oxygen is low (fig. 7). The seasonal temperature variations are also small (fig. 8. Ramtongrunnen). The salinity near the surface varies greatly according to the rainfall and the direction of the wind. A southerly wind keeps the fresh water inside the fjord, a northerly wind carries it out. It is an integral part of the present investigations to ascertain the amount of fish eggs and larvae in the fjord. The report gives the results of this part of the work up to the spring of 1940, when operations had to be discontinued owing to the war. In 9 localities (see fig. 6) the hydrographical conditions were examined and ascertained. As a rule 4 horizontal hauls of 10 mins. duration at deptht of 0, 10, 20 and 30 m below the surface were carried out with the egg net (1 m in diameter), and 2 hauls of 20 minutes at 10 and 30 m with the young-fish net (2 m in diameter). These operations were performed in the course of three cruises each spring. In the summer and autumn three cruises were made for the purpose of examining the hydrographical conditions, The following results were ascertained. The quantaties of spawn show that spawning must be intense in the outer fjord from the beginning of March. In the inner fjord spawning is generally less intense until April, when there is an abundance, especially of sprat spawn. It would seem, however, that the eggs do not develop at a normal rate in the inner fjord, the percentage of eggs with advanced embryos being extremely low inside Drøbak, but normal outside. With regard to the eggs of cod and haddock see fig. 25. Table 15 on page 60 shows the total number of fish larvae & young. The number accords with the occurrences of eggs at later stages, being high in the outer and low in the inner fjord, except during the month of May. The poor results of the natural hatching of eggs in the inner fjord during early spring are probably due to the effects of sewage water from Oslo. In late spring the higher temperature and increased intensity of light may have a natural purifying effect on the sea water. As has been pointed out by Wiborg (1940), crustacean plankton also develop very late in the inner fjord. This naturally provides poorer nourishment for the early hatched larvae. From table 16 it will be seen that in 1936 we caught less than 25 % of the amount of larvae taken in the subsequent years. In 1937 there was an abundance of sprat larvae, in 1938 of cod larvae, and in 1939 of herring and haddock larvae. No attempt has been made to indicate the cause or causes of this variation, as we lack the material for estimating the amount of spawners of the different species. A special aspect which is dealt with is the effect of releasing artificially hatched cod fry into the inner fjord. This was done only in one year - 1938, but it is the intention to continue the experiment. Not much can be said on the basis of the results of a single year, owing to the great variation in the natural breeding.