Changes since the turn of the century in the fish fauna and the fisheries of the Oslofjord

Abstract

1. The material for this study is drawn from two sources, (a) investigations of the fish fauna in the inner Oslofjord between 1897 and 1967, (b) fish landing statistics available since 1872 at the Oslo fish market. 2. The investigations of the fish fauna reveal that 4 species of fish - 2 sharks and 2 arctic bullheads - have disappeared from the deep waters of the Oslofjord where they were known to exist in 1897. We assume that unfavourable conditions, great changes in temperature or lack of oxygen in the stagnant deep water to which these species belonged have been injurious to their propagation and survival. 3. The annual fluctuations in the landed quantities of fish are great, but nevertheless there exists a marked decreasing trend since about 1930; total landings in recent years are less than 1/10 of what they were 30 or 40 years ago. 4. The fluctuations in the landings of cod, herring and mackerel are studied in some detail. Three periods (of 20 to 22 years' duration) from 1872 to 1932 show increasing average landings of cod: 44, 57, and 74 tons respectively; in the period 1933 to 1955 landings decreased to an average of 16 tons per year. The effect of fluctuating year classes is discussed. 5. The landings of herring and mackerel were very good between 1911 and 1919 or 1920 with a maximum of 1500 tons of herring and 800 tons of mackerel. In the following years there was a trend of decrease with a few years of moderately good landings. Mature mackerels are immigrating into the fjord for feeding and spawning, while mature herrings, to a major extent, are supposed to belong to a local stock. The highest landings of both species are due to extremely great catches of young fish, and the success of the fishery in any one year, therefore, dependent of a successful spawning and hatching one ort two years before. 6. In conclusion, the attention is drawn to the fact that there are reasons to believe that the profuse growth of bacteria in the polluted waters seems to be injurious to the hatching of fish eggs and to the survival of fish larvae (Dannevig 1945, Oppenheimer 1955). © 1968 Biologischen Anstalt Helgoland.