Climate change or not, an environmental shift did have a role in the decline in cod and increase in shrimp and cod stocks.  Starting in the 80s, the waters around Newfoundland cooled a bit which made the habitat more hospitable to shrimp and crab and less so for cod.  Now that trend seems to be reversing and shrimp and crab stocks are in decline while cod stocks are  recovering, at least in the more northerly areas.
http://news.nationalpost.com/news/canada/the-cod-are-coming-back-to-newfoundland-and-theyre-eating-the-shrimp-that-had-taken-over