Still, if you could control the number of does in the herd, you would have quite a good grip on fawn production. This is obvious. In some species, from cats to rats, pregnancy rates and litter sizes are more variable. But we can still get a fair idea of the number of young by counting the number of adults. This even works with some marine animals.
Grey seal cows, for example, only have one pup per year each.
Blackfish probably have one every two years. And most female sharks carry their young until they are born live, usually as a litter of a dozen or so pups. If you could count all the blue shark cows now, you could calculate roughly how many pups would be born next season, just as you could with coyotes or wolves. But obvious as all this seems, it is not true of the vast majority of marine species. Codfish are a classic example.