Dear Kwang-seek Choi:
Courtship in many temperate and subtemperate seahorses is dominated by pouch displays. Cold-water species such as the Big Belly Seahorse (Hippocampus abdominalis) typically engage in a different type of pouch display known as “Ballooning.” This is a simple display in which they inflate their brood pouches to the fullest possible extent and parade around in front of the female in all their glory as though trying to impress her with the sheer dimensions of their pouches. The pumped up paramours perform proudly, putting on quite a show for the flirtatious fillies.
In your case, Kwang-seek Choi, I suspect that the continued swelling of the brood pouch, or nursery bag, as you call it, is such a courtship display, since Hippocampus abdominalis males typically deliver large broods of young rather than a single offspring.
But a pregnant male will sometimes release one or two newborns prematurely and then temporarily suspend the birthing process, only to deliver the rest of his brood a day or two later, so that’s also a possibility.
However, I don’t believe that your Big Belly Seahorse has excess gas bubbles building up in his “nursery bag” because that would result in positive buoyancy and he would be floating as a result.
Best of luck breeding your seahorses, Kwang-seek Choi.
Pete Giwojna, Ocean Rider Tech Support