Figure 1

Collection system for fertilized eggs of chub mackerel. (a) Results of fertilized egg collection during the natural spawning season of 2015. The blue column shows the number of fertilized eggs collected each day. The bar graph shows the fluctuation of sea water temperature during the period. (b) Environmental conditions for artificial maturation. (c) Ovarian section of females artificially matured by environmental control (top) and in-vitro bioassay results with human chorionic gonadotropin addition (bottom). Ovarian samples were fixed in Bouin’s solution overnight, dehydrated in a series of ethanol solutions (up to 100%), embedded in paraffin, sectioned at 6 μm using an HM 355S microtome (Thermo Fisher Scientific, Waltham, MA, USA), stained with hematoxylin, and counterstained with eosin. The shredded ovary pieces were transferred to L15 medium (Thermo Fisher Scientific) containing 100 IU of human chorionic gonadotropin (ASKA Pharmaceutical, Tokyo, Japan) and cultured at 18.5 °C for 24 h. Red arrows indicate eggs with germinal vesicle breakdown. (d) The number of fertilized eggs collected when spawning was induced in the winter of 2016. Early egg collection was performed by adjusting the environment. (e) Spawning time of chub mackerel under natural (upper bar) or artificially reversed day-night photoperiod (lower bar). Black and yellow bars represent night and day times, respectively. (f) Time at which fertilized egg was obtained, with day and night of the spawning tank reversed.