Absolutely not an expert here and would love to be corrected but my poorly informed personal impression is that the whole idea of a sustainable amount of fishing and whether all species even have a number that is safe to harvest (especially given that no species exists in isolation), let alone what that number is, is on fairly shaky grounds scientifically. Meaning that a lot of this is guesswork based on too little information and "doing something is better than doing nothing" compromises with the fishing industry. If a species population has appeared to be stable for x number of years with y amount of fishing, does that mean that it's sustainable? Indefinitely? Given other changes in ecosystem and environment?
colordrops|3 years ago