Recreational fishing in the channel islands has resulted in tiny, undeveloped male sheephead and a lack of big females. Sheephead are naturally territorial; and unlike humans, they all start life as females. The largest fish in a territory becomes male, and enjoys the favors of the resident females. Sheephead have strong jaws and teeth, and are among the few fish that eat urchins, controlling their populations. When a diver enters the territory they swim up to investigate, usually passing a few times. They remind me of labrador retrievers: big, friendly, and not incredibly bright. Spearing a sheephead is as easy as spearing your neighbor's retriever; but it's legal, and a way to gratify our delight in destruction.
Because urchins eat kelp, sheephead are guardians of the kelp, and of the ecosystem it supports: rockfish, perch, octopi, bryozoans, and so on. Without sheephead and lobsters, urchins take over. The result is a desert full of starving urchins: the urchin barrens.
So many sheephead have been speared that the male in a territory is often hilariously tiny. His harem consists of even tinier females; small fish don't lay too many eggs, so sheephead are becoming scarce. The big ones in particular are gone. On my first visit to Catalina I remember two divers arguing on the boat back how big the sheephead used to be: "A big as me!" "Heck no, I remember ones as big as volkswagens."
There's no commercial fishing for sheephead, so this destruction was wrought entirely by recreational fishermen: including divers.
This article from the SB Independent describes sciences studies that document the destruction of sheephead:
http://www.independent.com/news/2009/may/13/california-sheephead-populations-dwindling/