harvesting fertilized eggs from Tetraodontidae species?
For DNA extraction, you would need only a few eggs. PCR would then give you all the copies of your target gene(s). A quick Google Scholar search on Takifugu fecundity revealed a paper by Yang and Chen (2008). They found that T. obscurus produced an average of 320.8 oocytes mg$^{-1}$ ovary wet weight. In comparison, T. ocellatus produced 125.2 oocytes mg$^{-1}$ ovary wet weight. Takifugu rupripes and other species are likely in a similar range, which is plenty of tissue for DNA. Kusakabe (1962) published on the fecundity of T. rubripes but the paper is in Japanese and is not readily available to me. 

Beware: T. rubripes ovaries are highly toxic. Why not use other tissue instead?