Om dypvannsreken ved Spitsbergen

The life history of the deep sea prawn, Pandalus borealis, in Spitsbergenwaters is traced on the basis af samples collected mainly in1938 and 1939. In measuring the prawn a different method than thatused by previous authors has been employed, as instead of the totallength, the length of the carapace without the rostrum has been measured(see fig. 2). The measurement is taken in millimeters from the baseof the eye to the posterior edge of the carapace. For comparison ofthe measurements thus obtained with the results of previous authorsstating the total length of the prawn, the length of the carapace ismultiplied by the factor 5,3 which is the average of nunmerous controlmeasurements.The adult females carrying eggs are fouild as early as in July, butthe main spawning of the Spitsbergen prawn takes place in August-September. The females with hatching eggs are found during thefollowing May-June, and the egg-bearing period is estimated to lastabout 9 months.The smallest bottom stages of the prawn have been caught in theshrimp trawl at the end of June and beginning of July, and they measuredaveragely about 39 mm. These small individuals cannot have beenhatched the same summer, and they are therefore considered to be1 year old. Next summer, when the prawn is 2 years old, it has reachedan average total length of about 62 mm. When the prawn is 3 yearsold it is on the average about 85 mm long in the summer and it maturesas a functioning male the same autumn. The Spitsbergen prawn thusreaches maturity as male at the same size as the prawn in southernNorwegian waters, but this latter is by then only 1 year old. Whenthe Spitshergen prawn is 4 years old it has reached an average totallength of about 103 mm and it participates in the spawning as malefor the second time.The following spring and summer the male prawn goes throughthe transitional stages, and in autumn they reach the maturity of femalesand become ovigerous. The Spitsbergen prawn can also spawnbefore all the transitional stages are completed. Thus ovigerous femalesin transitional stage C and D (fig. 4) are not infrequent in the catches.The Spitsbergen prawns are 5 years old when they spawn for the firsttime, and they have then reached an average total length of about117 mm, i. e. the same size as prawns in the corresponding stage ofmaturity in southern Norwegian waters, but the Spitsbergen prawn isby then 3 years older than the southern prawn. In the Spitsbergenprawn there is no apparent difference in size between the transitionindividuals and the first time spawners. In fig. 6 are represented thesize of the various age grtiops of the prawn up to first time spawners.The female prawns carry their eggs all winter, and at the end ofMay and beginning of June the ovigerous prawns can still be foundwith hatching eggs, or the hatching is just completed. The major partof these females does not seem to become ovigerous again the sameautumn, but enters apparantly a period of sexual rest. During thesummer, after moulting, they can be sorted out in the catches as femaleswith undeveloped ovaries. These prawns are by then 6 years old, andhave an average total length of about 123 mm. These old female prawnsapparently become sexually mature again during the following winterand spring, and they are in the summer found as large females withripening ovaries. They should thus spawn for the second time whenthey are 7 years old after 1 year of sexual rest. These older prawnsalso seem to become ovigerous later in the autumn than do the firsttime spawners (fig. 7 C).The rate of growth of the Spitsbergen prawn compared with thegrowth of the prawn in southern Norwegian waters is illustrated infig. 9.