We’re out of cod. Try the gelatinous zooplankton.

Very quietly, ocean ecosystems are showing signs of changing dramatically, and it’s not because of global warming.

Big fish like cod and tuna have been fished for centuries, but the increasing burden of recent decades may be too much.  In a study in the November issue of the Canadian Journal of Fisheries and Aquatic Sciences (CJFAS), two biologists showed a large population of Canadian cod is projected to essentially vanish within 40 years – even if fishing stops tomorrow.

The scary implication is that past overfishing – taking unsustainable levels of fish – can cause fish populations and the ecosystems in which they live to fundamentally change.  In the case of the Canadian cod, mortality rates are the critical factor causing the collapse.  The Canadian government heavily restricted or closed fishing for the cod populations in 1993, so populations should be increasing.  Instead, they have kept falling.

In 2002, David Conover and Stephan Munch published a study in the prestigious journal Science showing how harvesting only the largest fish in a population could produce smaller fish in the next generation.  The same idea may explain what’s happening to cod: past fishing has created intense selection pressure on smaller fish that mature earlier in life.

Not all cod populations show the same trend, so it would be premature to suggest Atlantic cod face imminent extinction.  But the trends in the population in CJFAS study are indicative of what can happen elsewhere: poor management and overharvest select for future generations that are smaller and less capable of recovering.  

And when cod disappear, other organisms fill the void.  This creates a different ecosystem, and one that may be less productive from our point of view.  There’s a real possibility fish like cod are being replaced by skates, rays, and smaller invertebrates in oceans worldwide.  

This is why fisheries policy matters.  It’s also a great example of a global environmental problem we can change, one day at a time.  Fish are only caught to feed our demand, so our purchases shape the future of the oceans.  In this sense, sustainable seafood is one of the wisest investments.  Get a handy guide to sustainable seafood from the Monterey Bay Aquarium.