A spirited debate on the effects of spearfishing was started in the POTW section. However the original purpose of that thread was to complain about the image and not about the effects and general discussion about spearfishing on shallow reefs. I've split the topic so that people can continue this discussion in the environment section.
Now in order to realize the actual impact of spearfishing in Spain we should probably do a math exercise:The number of spearos in Spain according to industry data I got few years ago is ~15 000. I remember the number because it's more or less the same with most of the Med Euro countries. I believe in Portugal they are less ~10000. And in Greece they are much more - maybe double the Spanish number. Even if those numbers vary, that's not of such big importance actually.
Let's assume all the spearos in Spain go spearfishing every weekend of the year - that's 104 days... Of course that's not true since it doesn't account for seasons, bad weather, and the fact that maybe half of the spearos are actually "holiday spearos" - they do not spearfish more than 20 days per year. But others spearfish more often, so for the sake of the calculation let's take the 104 days and the 15000 spearos.
Let's assume every spearo in Spain catches 3 kilos of fish every day of those weekends - that's 104 x 3 = 312 kilos a year. Now that's quite tough in the Med but the skilled guys may be able to do it. So we're assuming every Spanish spearo is actually a quite skilled one...
312 kilos x 15000 spearos = 4680 tonnes of annual spearfishing catch in Spain.
FAO statistics says that for 2004 Spain reported 803336 tonnes of catch (in Spain). Now that's the official number and doesn't account for bycatch, unreported catch and so on, but that's the only number I was able to find...
And if we compare the numbers it appears that the spearos are responsible for 0.58% of the catch in Spain.
Ivan
Your statistics are a little off. You are comparing catch statistics of giant schools of baitfish like sardine/anchovy with reef dwellers hunted by spearos. I would love to see spearos shoot sardines but they do not. 
There is plenty of anecdotal evidence of spearfishing heavily affecting local populations of big fish (normally the breeding mature adults), especially from former spearos as well as research, as is in the study in Waikiki on the effects of spearfishing as part of all fishing activities on the fish population on reefs in MPA. That study showed 25% of the spearos were responsible for 70% of the total catch of all non-commerical fishing practices.
Spearfishing competitions easily damage the number of big breeding fish in an area because they are looking for the biggest fish... and often not just one. In fact, anecdotal evidence from spearfishing ex-champions (the ones I spoke to are to me absolutely trustworthy) would prove otherwise since countries like Australia have had issues with species targetting by spearos and localized extinction because of it.
Even spearfishing clubs etc often encourage taking the 'intermediate' size fish, but how many guys obey that rule? Also, when you say size limits, how many spearos have shot a fish then go opps, under/over limit, dump it and catch another one?
The unfortunate truth is the environmentally conscious spearo or human beings are few and far between. The fact that we are discussing it means our lines of thought are similar. I cannot say that for the majority of people.
I know many spearos and don't hold anything against them. I wish there were some self-controllable measures but it is unrealistic. What seems to be barely working are MPAs where there is no fishing of any sort. Then those areas outside the MPAs can catch the overspillage.
But that would mean suffering for a few years without spearing. How many spearos would accept that? 
