Of course they didn't create it on their own, BUT in response to overfishing, these mid-sized predators (i.e dogfish, cownose rays etc..) fill the niche left by the lack of Cod; however the way I understand it, it is a negative feedback, i.e. the lack of cod and others increased the population of dogfish which is now creating a situation where cod can't compete w/ the increased dogfish....
this is based on a talk and papers I read by a guy named 'RAM' (Ransom A Myers I think) who was a Canadian fisheries biologist (I think I heard he passed away)
but what do I know.. I'm a geologist, not a fisheries biologist or a sailor/emt/ski bum



