Marine Fisheries Food Webs

At the top of the marine food web are the large predators:

1. 
   Jellyfish and cephalopods  (squid  and octopus).
   
2. 
   Large fish such as sharks, tuna, and mackerel.
   
3. 
   Marine mammals including seals , walruses, dolphins , and some species of whales (some eat fish, others eat zooplankton directly).
   
4. 
   Birds such as pelicans , albatross , penguins.
   
5. 
   People, the dominant top predator.
   

Phytoplankton, small zooplankton, larger zooplankton such as jellyfish, larger animals including bait fish and squid , and top predators such as tuna, all interact in a marine food web . Each species eats and is eaten by several other species. Big fish eat little fish; that’s how the food cycle works. Of course, there’s more to it than that. The interactions in a food web are far more complex than the interactions in a food chain. Furthermore, the branching structure of food webs leads to fewer top predators compared with the numbers of top predators in a food chain. Food chains are much rarer than food webs in marine ecosystems.

Cod used to be abundant throughout the Atlantic Ocean, but they have been severely overfished. Their numbers are especially low on the East Coast of Canada, even though the fishery there was closed more than a decade ago. Scientists have found that the removal of cod and other large fish changed the entire structure of the food web from top to bottom:

1. 
   The population of small fishes and large invertebrates, including northern snow crab and northern shrimp increased markedly.
   
2. 
   The population of large plant-eating zooplankton (> 2 mm) decreased markedly.
   
3. 
   Phytoplankton increased markedly.
   
4. 
   Seal populations are increasing exponentially.
   
5. 
   The economic value of the crab and shrimp fisheries now exceeds the earlier value of the cod fishery.
   
6. 
   Actions to restore the cod fishery have failed despite a nearly complete shutdown of cod fishing.
   
7. 
   Cod stocks in other areas north of 44 degrees North have also failed to recover, while cod stocks in areas south of 44 degrees North have started to recover.
   

The changes in marine ecosystems due to overfishing is often called fishing down the marine food web. As top predators are removed by fishing, fishers target smaller fish lower in the food web, reducing their numbers.