Overfishing is a real problem around the world and it's mainly due to commercial fishing, which is responsible for 98% of the world’s catch. Spearfishing, together with other sport fishing activities, represent only 2% of the world’s fish landings, showing us how minimal the impact spearfishing has on the environment.
However, we can't ignore the damage that spearfishermen can do in small coastal areas, especially if it’s carried out by people without the necessary knowledge and preparation.
Spearfishermen should know the basics about the fish they target: Fish that grow fast, have a high reproductive capacity and tend to travel great distances are less susceptible to overfishing (wahoo, mahi mahi, etc). Other fish with a slow growth rate, that reproduce only once a year and tend to live in the same area, are much easier to get overfished (some groupers and other bottom fish). Just educate yourself and make good decisions on what to take.
Visit the International Union for the Conservation of Nature’s red list (www.iucnredlist.org) to find out about endangered species and different threat levels of fish.
Finally, stick to bag and size limits, each country will have different restrictions that apply to their local ecosystems. Many countries will have no regulations in place, it is your responsibility as a conscious hunter to do what is right, respecting endangered species, juvenile fish and limiting your numbers.
As spearfishermen, it's our responsibility to make sure the animals harvested are eaten and nothing gets wasted. Eat what you catch and don't shoot more than what you need. Respect.