Undersøkelser over torsken i Oslofjorden

Abstract

"The Cod Population of the Oslofjord".

The hydrographical conditions of the fjord are mentioned. Owing to the sills at Drøbak and Nesodden the water masses of the inner fjord are often insufficiently aerated (Fig. 1 shows a plan of the fjord). In addition to my own material for the years 1939-51, consisting of spring - and autumn - samples, results from investigations carried out by Ruud 1936-38 and by several investigators at the Flødevig Sea Fish Hatchery are taken into consideration. The distribution of the egg stages is quite different in the inner and outer fjord. In the inner fjord - north of Drøbak - few eggs in advanced stages are found. The number of young fish of the 0-group is also very low in this part of the fjord. (Fig. 3). As possible explanations to this phenomenon the effect of bacteria has been discussed (Fig. 2). The strength of the year-classes varies. Very abundant are the 1938- and 1945-, especially poor the 1940-, 1941-, 1946- and 1947-yearclasses (Fig. 4). These differences are brought foreward already when the fish are at an age of half a year (see fig. 3). The cod in the inner Oslofjord is characterized by a low number of vertebrae, otoliths with many secondary zones and very stationary habits. Annual mortality averages 80 %, a value considerably higher than for cod from other localities in Southern Norway. More than 60 % of the fish are 1½ and 2 years old, mainly due to the low size limit - 30 cm from 1947. Males attain maturity earlier than the females (fig. 5). At an age of 2 years about 30% of the fish have ripe sex products. My values for the length of 1-4 year old cod 1939-51 are 2-4 cm lower than the corresponding values for the years 1936-38 (Fig. 6). This is a consequence of the extremely slow growth of the rich 1945-yearclass. The yield of the fishery has decreased alarmingly during the last 25 years (Fig. 7). The peaks of the curve are due to the influence of the 1945-, 1948- and 1949-year-classes. To obtain a higher yield the following possibilities have been discussed: I. Raise of the size limit to 35 cm. 2. Control with dumping of sewage etc. in the fjord. 3. Release of fry.