Greenland Halibut (reinhardtius Hippoglossoides) Spawn Annually but Successive Cohorts of Oocytes Develop over 2 Years, Complicating Correct Assessment of Maturity

Ovary development in Greenland halibut (Reinhardtius hippoglossoides) is complex, with several cohorts of developing oocytes present during vitellogenesis; this is unusual for a determinate spawner. There are also speculations that Greenland halibut are not capable of spawning every year. To investigate this possibility, ovaries from Greenland halibut caught throughout the year were examined histologically, and successive cohorts of oocytes were tracked through development. Results showed that the initial maturation of the ovaries from immature to spawning takes more than 1 year. The ovary initially develops as far as early vitellogenesis; however, the time scale for this is unclear. During the final year of development, the cohort of vitellogenic oocytes splits to form two cohorts; the larger cohort increases in size and is spawned in the coming spawning season. The smaller cohort also continues to develop, but at a much lower rate, in preparation for development for spawning in the following year. Within each month, there is a large range of oocyte sizes between fish; this leads to the extended spawning season that is known in many populations of this species. This complicates the assessment of maturity, and a more accurate microscopic maturity scale is proposed.