I recently listened to this podcast from the OU Kashrut Division on the kashrut of Bowfin eggs in particular and of fish eggs in general, and I learned something interesting about the kashrut of fish eggs:
Eggs from a kosher fish are not considered kosher if they are black-colored, perhaps even if they are dyed black.
The reason for this, if I understand correctly, is that it was determined by Talmudic or post-Talmudic authorities that all red fish eggs come from kosher fish, so if we only allow red fish eggs, we can prevent anyone from eating eggs that they think come from a kosher fish, but don't.
I'm wondering why we employ such a restrictive heuristic, entirely excluding the eggs of various kosher fish species just because the eggs aren't obviously kosher by visual inspection, instead of allowing some other reliable demonstration (e.g. the testimony of a God-fearing Jew) of their provenance. Why don't we treat fish eggs similarly to how we treat:
Fish fillets, whose fish of origin is difficult to determine once they've been butchered, so we just make sure to buy them from a kosher fish butcher?
Kosher beef, which is physically indistinguishable from non-kosher beef (before the former is salted, or if the latter happens to have been salted for some reason), so we require that it came from a reputable slaughterer and that it was guarded along the way?
Milk, whose animal of origin can (in theory) be difficult to determine, so we require that its provenance is verified all the way from the source, either by Jewish supervision or, at least according to many, certain governments' supervision?