REPRODUCTION-MOLTING CYCLE OF NORTHERN SHRIMP PANDALUS EOUS (CARIDEA, PANDALIDAE) IN THE NORTHWESTERN JAPAN SEA

Abstract

The data on northern shrimp Pandalus eous from the northwestern Japan Sea are presented, as the bioanalysis results for 204,502 ind. in trawl catches of research and fishing vessels collected in 2004–2018. The data were combined in the common database that allowed to consider annual time-series of biological parameters with 10-day resolution and to reveal long-term cycles of these parameters and determine their periods and phases. Dynamics of the mass molting was clarified for certain years. Duration of reproductive cycle for the northern shrimp females in the Japan Sea is determined as 2 years. In the first year of this cycle, the gonads develop in intersexes and females (group I) that is finished with their pre-spawning molting and spawning. In the second year of the cycle, the females (group II) hatch the eggs on pleopods, release the larvae, and then molt, too. Appearance of the hairs for egg attaching to pleopods in the process of molting is considered usually as a sign of the females transverse from the group I to the group II. In the studied population, the females of both groups were presented in the catches in approximately equal proportion throughout the year. Molting of both groups occurred simultaneously, with the peaks in early-middle May and finish by the end of June. New reproductive cycle for one of the two groups started in July. The gonads of group I and the eggs of group II developed intensively from November to March-April. Hatching of the larvae proceeded actively in March-April, with the peak in early April, spawning — in April-May, with the peak in early May. Hence, in the scale of population, the time from the gonads appearance to completion of their development and spawning for 50 % of the group I individuals is 7 months and the time of gestation for the group II is 11 months. Sex change in males began in August-October, the transitional stage between the peaks of molting lasted about 8 months: from the summer-autumn molting of males to the pre-spawning molting of intersexes — during this time they change their secondary sexual characteristics. In total, five mass molts of northern shrimp in the northwestern Japan Sea are observed annually: i) winter molting of males, with the peak in middle February; ii) pre-spawning molting, with the peak in early May; iii) molting of females after hatching of larvae, with the peak in middle May; iv) summer-autumn molting of males with partial sex change, with the peak in early September; and v) autumn molting of females, with the peak in early November. All molts have similar dynamics: each of them lasts about 3 months with the intensive molting during 1.0–1.5 months, at the peak of molt the portion of molting shrimp in catches increases to 55–71 % of the respective group. Generalized scheme of reproduction-molting cycle of northern shrimp in the northwestern Japan Sea is proposed.