The eggs (roe) of a kosher fish are kosher, and the eggs of a non-kosher fish are not kosher. Since capelin is a kosher fish, its eggs (masago) would be acceptable. But how can you know with absolute certainty that the product in your package is indeed the roe of a kosher fish, and that nothing unkosher has been added? You truly cannot, especially since the eggs of some non-kosher fish are also referred to as masago. As such, only roe with kosher certification is acceptable.