Marine Fisheries Food Webs

Download 280.37 Kb.
Size280.37 Kb.
  1   2   3
Marine Fisheries Food Webs


Food Webs

Since the discovery of the astronomical number of microbes in the ocean, we now recognize two important, overlapping food webs in the ocean.

  1. The microbial food web discussed in that chapter. This web dominates the carbon, nitrogen, and other nutrient cycles of the earth system. (Microbes—bacteria, viruses.)

  2. The marine fisheries food web discussed here.

The two webs are coupled in many ways that are not yet understood. Microbes cycle nutrients, they produce other nutrients such as vitamins needed by primary producers discussed here, and they infect, sicken, and kill many organisms in the marine fisheries food web. We are not the only large animals that get viral and bacterial diseases.
Step 1: Phytoplankton: Primary Producers of the Marine Fisheries Web

The sunlit upper layers of the ocean, called the surface zone, are home to vast numbers of single-cell marine primary producers called phytoplankton. The term algae  is another catch-all term for primary producers with chlorophyll that formerly included many unrelated organisms, excluding land plants.

Phytoplankton are primary producers because they use energy from the sun to convert CO2 and nutrients into carbohydrates and other molecules used by life. Together, they account for about 95% of the primary productivity in the ocean and about half of all primary productivity on earth.

Phytoplankton are most common in cooler, mid-latitude zones with sufficient nutrients, especially nitrogen. Thus they are common in the north Atlantic and Pacific, and along coasts. They are much less common in the central regions of the ocean and in the southern hemisphere.

Step 2: Zooplankton

The phytoplankton are eaten by the smallest floating animals, the zooplankton . They range in size from single-celled organisms to larger multi-celled organisms. Small zooplankton are eaten by larger zooplankton. Zooplankton include

  1. Single-celled animals such as ciliates or amoeboids that never grow large.

  2. Copepods .

  3. Shrimp .

  4. Larval (baby) forms of barnacles, mollusks, fishes, and jellyfish , all of which grow to be much larger animals.

Example: Copepods

The copepods are a type of zooplankton. They are a class of crustaceans with over 7,500 species, most of which are marine. Copepods are small (only a few species over 1 mm) and extremely abundant, often dominating the plankton community. They form a link in the food web between the primary-producing phytoplankton and the plankton-feeding fish like Atlantic herring. Almost all fish found in temperate and polar waters rely at some point in their life cycle on copepods and other shrimp-like zooplankton (krill) as a food source.
Picture: Paraeuchaeta norvegica, a carnivorous copepod commonly found in fjords and North Atlantic waters.

Step 3: Small Predators

Zooplankton are eaten by small predators:

  1. Shrimp and krill .

  2. Immature stages of larger animals such as jellyfish and fish.

  3. Small fish such as sardines , menhaden , and herring .

Example: Clupeus harengus (Atlantic herring) is a small bait fish. It schools in coastal waters. It feeds on small planktonic copepods in the first year, thereafter mainly on copepods. Adults are about 30-35 cm in length, and they live about 20 years. They are eaten by many species of birds, fish, and marine mammals.


Download 280.37 Kb.

Share with your friends:
  1   2   3

The database is protected by copyright © 2024
send message

    Main page