system
Given the following post, respond as if you're commenting on Reddit:
human
How do scientists know that 1 Billion crab went missing ?

If they are tracking them that accurately it seems like fishing then would be pretty easy, if they’re trying to trap them and just not finding any it could just be bad luck. 

Canceling the crab season is a big deal so they must know this with some certainty. What methods do they use to get this information?
gpt
There are a lot of ways you can estimate the population of a species. In fisheries we usually do surveys exactly the same way every year to get a general idea of the population size and trends, and mark recapture studies. If you tag 100 Cod and then next year you catch 100 Cod and three are tagged, that would suggest that you tagged about 3% of the population. Actually it's a lot more complicated because you have to correct for things like the tag causing mortality but that's the gist.

The big one though is catch per unit effort. You track the number of boats fishing in a certain way, how many hours they spend fishing, and how much they catch. If it takes three days at sea for a guy with a fishing rod to catch a Cod one year, and the next year it takes six, the population was probably cut in half.

Obviously 100 fish or one guy with a rod is a tiny, tiny number compared to the amount of data that actually gets collected- it's usually tens of thousands of tags or data from every fishing boat in a fishery. Even that ends up being a tiny sample of a commercially harvested species numbers though, so often fisheries management ends up depending on fairly imprecise estimates of population sizes. It's far better than managing with no data at all though, and a drop of 90% is large enough that they're almost certainly right that the fishery is imploding. ☹️