Fun fact: For all intents and purposes, the emergence of the oil and gas industry shares some of the credit for whales not going extinct during that time. With the sudden availability of waxes and oils, blubber was no longer as needed for candlemaking.

During the whale industry's peak though, they didnt necessarily have the technology to keep track of global whale populations. As such, the only way I think they could notice that populations were decreasing would be if they noticed they were finding fewer whales - which they could have attributed to any number of things outside of decreasing populations.