Food Habits

From NOAA’s NMFS Ecology of the Northeast Continental Shelf Toward an Ecosystem Approach to Fisheries Management, courtesy of Michael Fogarty.

Cod preying on forage fish
Cod preying on forage fish

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Predator-prey interactions are an essential component of ecosystem structure and function. The flow of energy through an ecosystem depends on the interaction between predators and their prey. Preserving a balance between these ecosystem components is therefore essential.

The food web involving cod on the Northeast continental shelf
The food web involving cod on the Northeast continental shelf. Cod prey on a wide variety of benthic and pelagic animals, including many that are commercially important*

Who Eats Whom?

Looking at the diets of fish reveals a complex web of interactions among many parts of the system. Even examining a small part of the food web illustrates the large number of linkages possible. For fish, which grow in size over a thousandfold over their lifetime, the progression in the food items they consume is remarkable. As they grow, their diets shift dramatically, so that over the lifespan, a large network of interactions develops. For example, cod begin feeding on zooplankton as larvae and then as juveniles feed on an assortment of larger zooplankton species as well as benthic animals. As adults, they feed on these food items but also become increasingly dependent on fish and squid in their diet. Among these prey are many commercially important species including hakes, herring, and mackerel among others. We see that throughout the lifespan of cod, connections are forged with the planktonic and benthic ecosystems both, highlighting the need to understand the system as a whole as we consider the factors affecting cod.

Why it Matters – It’s a Fish Eat Fish World

Recognition of the importance of predator-prey interactions among exploited populations will require tradeoffs in management strategies in an ecosystem context. Depending on the strength of the predator-prey interaction, management actions that affect the predator may have indirect effects on the prey and vice versa. Therefore ecosystem-based fishery management will require an additional set of considerations in establishing objectives for management. For species linked by predator-prey interactions, it will not be possible to have all at high levels of abundance.

Forage Fish: The Herring Example

Fish that are consumed by a broad spectrum of predators are called "forage" fish. Natural predators such as other fish, marine mammals and seabirds often eat more forage fish than humans catch. For forage species such as Atlantic herring, the amount consumed by predators is now substantially higher than the harvest itself. The amount of herring consumed by natural predators has increased as the abundance of this species has increased. Many predators are opportunistic and will feed on the most abundant prey items they encounter. In recent years, marine mammals and fish consumed roughly equal amounts of herring, with far lower consumption by seabirds and apex predators such as tunas and billfish. We have also seen the relative importance of different predator groups change with time. For example, in the early 1990s, the dominant natural predators of herring were other fish, accounting for 70% of the natural predation and consuming nearly three times the herring eaten by marine mammals.

Fish and marine mammals consumption chart
Fish and marine mammals consume the highest proportions of sea herring with lesser amounts taken by apex predators (sharks and billfish) and seabirds

The amount of herring consumed by herring predators
Atlantic herring are important prey for a large number of fish, marine mammal, and seabird species. The amount of herring consumed by these predators has increased as the abundance of herring has increased and it is now larger than the amount taken by the commercial fishery

Because forage species such as herring are so important as prey, consideration of ecosystem approaches to fishery management will entail an evaluation of the food requirements of these predators to maintain a balanced ecosystem.

*Cod illustration courtesy of GLOBEC International Program Office, Artist Glynn Gorick (Glynn@Gorick.co.uk)