Dictionary of Ichthyology  

fishing down the food web = the shift in fishes sought commercially from long-lived, high trophic level, piscivorous bottom fish toward short-lived, low trophic level invertebrates and planktivorous pelagic fish. At first catches increase, but then become stagnating or declining. The removal of fish and invertebrates from lower trophic levels removes the food higher trophic level fishes need to reproduce and maintain their populations.

North Sea perspective from Wikimedia Commons: http://en.wikipedia.org/wiki/File:Fishing_down_the_food_web.jpg

Canadian Museum of Nature