Om dypvannsreken ved Spitsbergen

Abstract

The life history of the deep sea prawn, Pandalus borealis, in Spitsbergen waters is traced on the basis af samples collected mainly in 1938 and 1939. In measuring the prawn a different method than that used by previous authors has been employed, as instead of the total length, the length of the carapace without the rostrum has been measured (see fig. 2). The measurement is taken in millimeters from the base of the eye to the posterior edge of the carapace. For comparison of the measurements thus obtained with the results of previous authors stating the total length of the prawn, the length of the carapace is multiplied by the factor 5,3 which is the average of nunmerous control measurements. The adult females carrying eggs are fouild as early as in July, but the main spawning of the Spitsbergen prawn takes place in August- September. The females with hatching eggs are found during the following May-June, and the egg-bearing period is estimated to last about 9 months. The smallest bottom stages of the prawn have been caught in the shrimp trawl at the end of June and beginning of July, and they measured averagely about 39 mm. These small individuals cannot have been hatched the same summer, and they are therefore considered to be 1 year old. Next summer, when the prawn is 2 years old, it has reached an average total length of about 62 mm. When the prawn is 3 years old it is on the average about 85 mm long in the summer and it matures as a functioning male the same autumn. The Spitsbergen prawn thus reaches maturity as male at the same size as the prawn in southern Norwegian waters, but this latter is by then only 1 year old. When the Spitshergen prawn is 4 years old it has reached an average total length of about 103 mm and it participates in the spawning as male for the second time. The following spring and summer the male prawn goes through the transitional stages, and in autumn they reach the maturity of females and become ovigerous. The Spitsbergen prawn can also spawn before all the transitional stages are completed. Thus ovigerous females in 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 first time, and they have then reached an average total length of about 117 mm, i. e. the same size as prawns in the corresponding stage of maturity in southern Norwegian waters, but the Spitsbergen prawn is by then 3 years older than the southern prawn. In the Spitsbergen prawn there is no apparent difference in size between the transition individuals and the first time spawners. In fig. 6 are represented the size 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 of May and beginning of June the ovigerous prawns can still be found with hatching eggs, or the hatching is just completed. The major part of these females does not seem to become ovigerous again the same autumn, but enters apparantly a period of sexual rest. During the summer, after moulting, they can be sorted out in the catches as females with undeveloped ovaries. These prawns are by then 6 years old, and have an average total length of about 123 mm. These old female prawns apparently become sexually mature again during the following winter and spring, and they are in the summer found as large females with ripening ovaries. They should thus spawn for the second time when they are 7 years old after 1 year of sexual rest. These older prawns also seem to become ovigerous later in the autumn than do the first time spawners (fig. 7 C). The rate of growth of the Spitsbergen prawn compared with the growth of the prawn in southern Norwegian waters is illustrated in fig. 9.