Forage fish
Forage fish, also called prey fish or bait fish, are small pelagic fish that feed on planktons and other small aquatic organisms. They are in turn preyed upon by various predators including larger fish, seabirds and marine mammals, this making them keystone species in their aquatic ecosystems.
The typical ocean forage fish feed at the lower trophic level of the food chain, often by filter feeding. They include particularly fishes of the order Clupeiformes, but also other small fish, including halfbeaks, silversides, smelt such as capelin and goldband fusiliers.
Forage fish compensate for their small size by forming schools. Some swim in synchronised grids with their mouths open so they can efficiently filter plankton. These schools can become immense shoals which move along coastlines and migrate across open oceans. The shoals are concentrated energy resources for great marine predators. The predators keenly focus on the shoals, acutely aware of their numbers and whereabouts, and make migrations themselves that can span thousands of miles to pursue them.
The ocean's primary producers, mainly contained in phytoplanktons, produce food energy and biomass photosynthetically from sunlight and are the raw fuel for the ocean food webs. Forage fish transfer this energy by eating the plankton and becoming food themselves for the top predators. In this way, forage fish occupy the lower-central positions in ocean and lake food webs.
The fishing industry sometimes catch forage fish for commercial purposes, but primarily for use as feeder fish to raise more valuable piscivorous animals. Some fisheries scientists are expressing concern that this will affect the populations of predator fish that depend on them. The practice of harvesting forage fish can also be a sign of overfishing as fish stocks of larger species in the fisheries are depleted thus forcing the industry to start fishing down the food web.
In the oceans
Typical ocean forage fish are small, silvery schooling oily fish such as herring, anchovies and menhaden, and other small, schooling baitfish like capelin, smelts, sand lance, halfbeaks, pollock, butterfish and juvenile rockfish. Herrings are a preeminent forage fish, often marketed as sardines or pilchards.The term "forage fish" is a term used in fisheries, and is applied also to forage species that are not true fish but play a significant role as prey for predators. Thus invertebrates such as squid and shrimp are also referred to as "forage fish". Even the tiny shrimp-like creatures called krill, small enough to be eaten by other forage fish, yet large enough to eat the same zooplankton as forage fish, are often classified as "forage fish".
Forage fish utilise the biomass of copepods, mysids and krill in the pelagic zone to become the dominant converters of the enormous ocean production of zooplankton. They are, in turn, central prey items for higher trophic levels. Forage fish may have achieved their dominance because of the way they live in huge, and often extremely fast cruising schools.
Though forage fish are abundant, there are relatively few species. There are more species of primary producers and apex predators in the ocean than there are forage fish.
Ocean food webs
Forage fish occupy central positions in the ocean food webs. The position that a fish occupies in a food web is called its trophic level. The organisms it eats are at a lower trophic level, and the organisms that eat it are at a higher trophic level. Forage fish occupy middle levels in the food web, serving as a dominant prey to higher level fish, seabirds and mammals.Ecological pyramids are graphical representations, along the lines of the diagram at the right, which show how biomass or productivity changes at each trophic level in an ecosystem. The first or bottom level is occupied by primary producers or autotrophs. These are the names given to organisms that do not feed on other organisms, but produce biomass from inorganic compounds, mostly by a process of photosynthesis.
In oceans, most primary production is performed by algae. This is a contrast to land, where most primary production is performed by vascular plants. Algae ranges from single floating cells to attached seaweeds, while vascular plants are represented in the ocean by groups such as the seagrasses. Larger producers, such as seagrasses and seaweeds, are mostly confined to the littoral zone and shallow waters, where they attach to the underlying substrate and are still within the photic zone. Most primary production in the ocean is performed by microscopic organisms, the phytoplankton.
Thus, in ocean environments, the first bottom trophic level is occupied principally by phytoplankton, microscopic drifting organisms, mostly one-celled algae, that float in the sea. Most phytoplankton are too small to be seen individually with the unaided eye. They can appear as a green discoloration of the water when they are present in high enough numbers. Since they increase their biomass mostly through photosynthesis they live in the sun-lit surface layer of the sea.
The most important groups of phytoplankton include the diatoms and dinoflagellates. Diatoms are especially important in oceans, where they are estimated to contribute up to 45% of the total ocean's primary production. Diatoms are usually microscopic, although some species can reach up to in length.
The second trophic level is occupied by zooplankton which feed off the phytoplankton. Together with the phytoplankton, they form the base of the food pyramid that supports most of the world's great fishing grounds. Zooplankton are tiny animals found with the phytoplankton in oceanic surface waters, and include tiny crustaceans, and fish larvae and fry. Most zooplankton are filter feeders, and they use appendages to strain the phytoplankton in the water. Some larger zooplankton also feed on smaller zooplankton. Some zooplankton can jump about a bit to avoid predators, but they cannot really swim. Like phytoplankton, they float with the currents, tides and winds instead. Zooplanktons can reproduce rapidly, their populations can increase up to thirty percent a day under favourable conditions. Many live short and productive lives and reach maturity quickly.
Particularly important groups of zooplankton are the copepods and krill. These are not shown in the images above, but are discussed in more detail later. Copepods are a group of small crustaceans found in ocean and freshwater habitats. They are the biggest source of protein in the sea, and are important prey for forage fish. Krill constitute the next biggest source of protein. Krill are particularly large predator zooplankton which feed on smaller zooplankton. This means they really belong to the third trophic level, secondary consumers, along with the forage fish.
Together, phytoplankton and zooplankton make up most of the plankton in the sea. Plankton is the term applied to any small drifting organisms that float in the sea. By definition, organisms classified as plankton are unable to swim against ocean currents; they cannot resist the ambient current and control their position. In ocean environments, the first two trophic levels are occupied mainly by plankton. Plankton are divided into producers and consumers. The producers are the phytoplankton and the consumers, who eat the phytoplankton, are the zooplankton.
Diet
Forage fish feed on plankton. When they are eaten by larger predators, they transfer this energy from the bottom of the food chain to the top and in this way are the central link between trophic levels.Forage fish are usually filter feeders, meaning that they feed by straining suspended matter and food particles from water. They usually travel in large, slow moving, tightly packed schools with their mouths open. They are typically omnivorous. Their diet is usually based primarily on zooplankton, although, since they are omnivorous, they also take in some phytoplankton.
Young forage fish, such as herring, mostly feed on phytoplankton and as they mature they start to consume larger organisms. Older herrings feed on zooplankton, tiny animals that are found in oceanic surface waters, and fish larvae and fry. Copepods and other tiny crustaceans are common zooplankton eaten by forage fish. During daylight, many forage fish stay in the safety of deep water, feeding at the surface only at night when there is less chance of predation. They swim with their mouths open, filtering plankton from the water as it passes through their gills.
Ocean halfbeaks are omnivores which feed on algae, plankton, marine plants like seagrass, invertebrates like pteropods, and crustaceans and smaller fishes. Some tropical species feed on animals during the day and plants at night, while others alternate summer carnivory with winter herbivory. They are in turn eaten by billfish, mackerel, and sharks.
Predators
Forage fish are the food that sustains larger predators above them in the ocean food chain. The superabundance they present in their schools make them ideal food sources for top predator fish such as tuna, striped bass, cod, salmon, barracuda and swordfish, as well as sharks, whales, dolphins, porpoises, seals, sea lions, and seabirds.Schooling
Forage fish are small, short-lived species that often form large schools. These groups play an important role in the marine food web by transferring energy from plankton to larger organisms. Most forage fish are pelagic fish, meaning they live in the open sea rather than the sea floor. Due to their high abundance, they act as a primary food source for many predators, prompting these predators to migrate long distances to locate and feed on forage fish populations.Herring are among the most spectacular schooling fish. They aggregate together in huge numbers. Schools have been measured at over four cubic kilometres in size, containing about four billion fish. These schools move along coastlines and traverse the open oceans. Herring schools in general have very precise arrangements which allow the school to maintain relatively constant cruising speeds. Herrings have excellent hearing, and their schools react very fast to a predator. The herrings keep a certain distance from a moving scuba diver or cruising predator like a killer whale, forming a vacuole which can look like a doughnut from a spotter plane. The intricacies of schooling is far from fully understood, especially the swimming and feeding energetics. Many hypotheses to explain the function of schooling have been suggested, such as better orientation, synchronized hunting, predator confusion and reduced risk of being found. Schooling also has disadvantages, such as excretion buildup in the breathing media and oxygen and food depletion. The way the fish array in the school probably gives energy saving advantages, though this is controversial.
On calm days, schools of herring can be detected at the surface a mile away by little waves they form, or from several meters at night when they trigger bioluminescence in surrounding plankton. Underwater recordings show herring constantly cruising at high speeds up to 108 cm per second, with much higher escape speeds.
They are fragile fish, and because of their adaptation to schooling behaviour they are rarely displayed in aquaria. Even with the best facilities aquaria can offer they become sluggish compared to their quivering energy in wild schools.