sometimes i see goldfish or koi, but i guess it can happen to all species, they get this massive gut, way bigger than just from carrying eggs. and its some disease whereby the kidneys or some organ fill with liquid. so the mass in the stomach does not look evenly spread out but clumped up in one area, like its from one organ being swollen rather than the length of the roe. more pictures would be needed to show whats going on.