A couple decades ago the concept of overfishing became more well-known, and the industry was forced to change. The reason that the industry reacted also has to do with the fact that a true collapse of the fish population would also decimate the entire industry - they have an interest in keeping it stable.
If 99% of a fish population goes away, it would take decades for the population to recover to normal levels. During those years, there would be no profit.
The true reason why the fish population is down 70% is due to the fact that every year the fishing industry takes out as many fish as possible without causing a lasting depression of fish population.
If all fishing would be stopped right now, the global fish population would recover to pre-human levels within probably 20 or 30 years.
Here's what someone from the Australian fishing industriy has to say about this:
"We have learned from the ’60s to ’90s era and today most large fisheries are well managed (some in Asia/Africa are not) and either recovering or operating at maximum economic yield. You cannot catch fish to extinction – they will always recover if you stop fishing. It’s the recovery time that is the issue – and the time to when you can recommence harvesting, sustainably."
Of course these people are heavily biased, and the industry is rotten to the core and full of corruption, but it's hard to argue against this self-evident logic.