Forage fish, also called prey fish or bait fish, are small pelagic fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Typical ocean forage fish feed near the base of the food chain on plankton, often by filter feeding. They include particularly fishes of the family Clupeidae (herrings, sardines, shad, hilsa, menhaden, anchovies and sprats), but also other small fish, including halfbeaks, silversides, smelt such as capelin, and the goldband fusiliers pictured on the right.

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 migrateacross open oceans. The shoals are concentrated fuel resources for the great marine predators. The predators are keenly focused on the shoals, acutely aware of their numbers and whereabouts, and make migrations themselves that can span thousands of miles to connect, or stay connected, with them.

The ocean primary producers, mainly contained in plankton, produce food energy from the sun 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 central positions in ocean and lake food webs.

The fishing industry catches forage fish primarily for feeding to farmed animals. Some fisheries scientists are expressing concern that this will affect the populations of predator fish that depend on them.

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".[5]

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.

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 (Greek trophē = food). 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 productivitychanges at each trophic level in an ecosystem. The first or bottom level is occupied by primary producers or autotrophs (Greek autos = self and trophe = food). 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 (euphotic zone) 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.[6] Diatoms are usually microscopic, although some species can reach up to 2 millimetres in length.

The second trophic level (primary consumers) 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 (recently hatched fish). 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 can't 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 seaand 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 (Greek planktos = wanderer or drifter). 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 (Greek phyton = plant) and the consumers, who eat the phytoplankton, are the zooplankton (Greek zoon = animal).

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 (recently hatched fish). 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.[9] 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.

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.

These sometimes immense gatherings fuel the ocean food web. Most forage fish are pelagic fish, which means they form their schools in open water, and not on the bottom (benthic fish) or near the bottom (benthopelagic fish). They are short-lived, and go mostly unnoticed by humans, apart from an occasional support role in a documentary about a great ocean predator. While we may not pay them much attention, the great marine predators are keenly focused on them, acutely aware of their numbers and whereabouts, and make migrations that can span thousands of miles to connect with them. After all, forage fish are their food.

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.[13] 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.

📷Copepod📷Slow motion video loop of a juvenile herring feeding on copepods

Copepods are a group of small crustaceans found in ocean and freshwater habitats. Many species are planktonic (drifting in the ocean water), while others are benthic (living on the sea floor). Copepods are typically one millimetre (0.04 in) to two millimetres (0.08 in) long, with a teardrop shaped body. Like other crustaceans they have an armoured exoskeleton, but they are so small that this armour, and the entire body, is usually transparent.

Copepods are usually the dominant zooplankton. Some scientists say they form the largest animal biomass on the planet. The other contender is the Antarctic krill. But copepods are smaller than krill, with faster growth rates, and they are more evenly distributed throughout the oceans. This means copepods almost certainly contribute more secondary production to the world's oceans than krill, and perhaps more than all other groups of marine organisms together. They are a major item on the forage fish menu.

Copepods are very alert and evasive. They have large antennae. When they spread their antennae they can sense the pressure wave from an approaching fish and jump with great speed over a few centimeters.

Herrings are pelagic feeders. Their prey consists of a wide spectrum of phytoplankton and zooplankton, amongst which copepods are the dominant prey. Young herring usually capture small copepods by hunting them individually— they approach them from below. The (half speed) video loop at the left shows a juvenile herring feeding on copepods. In the middle of the image a copepod escapes successfully to the left. The opercula (hard bony flaps covering the gills) are spread wide open to compensate the pressure wave which would alert the copepod to trigger a jump.

📷Herring ram feeding on a school of copepods📷Juvenile herring hunt for the very alert and evasive copepods in synchronization

If prey concentrations reach very high levels, the herrings adopt a method called "ram feeding". They swim with their mouth wide open and their opercula fully expanded. Every several feet, they close and clean their gill rakers for a few milliseconds (filter feeding). In the photo on the right, herring ram feed on a school of copepods. The fish all open their mouths and opercula wide at the same time (the red gills are visible—click to enlarge). The fish swim in a grid where the distance between them is the same as the jump length of their prey, as indicated in the animation below.

In the animation, juvenile herring hunt the copepods in synchronization: The copepods sense with their antennae the pressure-wave of an approaching herring and react with a fast escape jump. The length of the jump is fairly constant. The fish align themselves in a grid with this characteristic jump length. A copepod can dart about 80 times before it tires out. After a jump, it takes it 60 milliseconds to spread its antennae again, and this time delay becomes its undoing, as the almost endless stream of herrings allows a herring to eventually snap the copepod. A single juvenile herring could never catch a large copepod.

📷Coastal upwellings can provide plankton rich feeding grounds for forage fish.📷Migration of Icelandic capelinSee also: Fish migration

Forage fish often make great migrations between their spawning, feeding and nursery grounds. Schools of a particular stock usually travel in a triangle between these grounds. For example, one stock of herrings have their spawning ground in southern Norway, their feeding ground in Iceland, and their nursery ground in northern Norway. Wide triangular journeys such as these may be important because forage fish, when feeding, cannot distinguish their own offspring.

Fertile feeding grounds for forage fish are provided by ocean upwellings. Oceanic gyres are large-scale ocean currents caused by the Coriolis effect. Wind-driven surface currents interact with these gyres and the underwater topography, such as seamounts and the edge of continental shelves, to produce downwellings and upwellings.These can transport nutrients which plankton thrive on. The result can be rich feeding grounds attractive to the plankton feeding forage fish. In turn, the forage fish themselves become a feeding ground for larger predator fish. Most upwellings are coastal, and many of them support some of the most productive fisheries in the world. Regions of notable upwelling include coastal Peru, Chile, Arabian Sea, western South Africa, eastern New Zealand and the California coast.

Capelin are a forage fish of the smelt family found in the Atlantic and Arctic oceans. In summer, they graze on dense swarms of plankton at the edge of the ice shelf. Larger capelin also eat krill and other crustaceans. The capelin move inshore in large schools to spawn and migrate in spring and summer to feed in plankton rich areas between Iceland, Greenland, and Jan Mayen. The migration is affected by ocean currents. Around Iceland maturing capelin make large northward feeding migrations in spring and summer. The return migration takes place in September to November. The spawning migration starts north of Iceland in December or January.

The diagram on the right shows the main spawning grounds and larval drift routes. Capelin on the way to feeding grounds is coloured green, capelin on the way back is blue, and the breeding grounds are red. In a paper published in 2009, researchers from Iceland recount their application of an interacting particle model to the capelin stock around Iceland, successfully predicting the spawning migration route for 2008

Schooling forage fish are subject to constant attacks by predators. An example is the attacks that take place during the African sardine run. The African sardine run is a spectacular migration by millions of silvery sardines along the southern coastline of Africa. In terms of biomass, the sardine run could rival East Africa's great wildebeest migration

Sardines have a short life-cycle, living only two or three years. Adult sardines, about two years old, mass on the Agulhas Bank where they spawn during spring and summer, releasing tens of thousands of eggs into the water. The adult sardines then make their way in hundreds of shoals towards the sub-tropical waters of the Indian Ocean. A larger shoal might be 7 kilometers (4.3 miles) long, 1.5 kilometers (0.93 miles) wide and 30 meters (98 feet) deep. Huge numbers of sharks, dolphins, tuna, sailfish, Cape fur seals and even killer whales congregate and follow the shoals, creating a feeding frenzy along the coastline.[18]

When threatened, sardines instinctively group together and create massive bait balls. Bait balls can be up to 20 meters (66 feet) in diameter. They are short lived, seldom lasting longer than 20 minutes. As many as 18,000 dolphins, behaving like sheepdogs, round the sardines into these bait balls, or herd them to shallow water (corralling) where they are easier to catch. Once rounded up, the dolphins and other predators take turns plowing through the bait balls, gorging on the fish as they sweep through. Seabirds also attack them from above, flocks of gannets, cormorants, terns and gulls. Some of these seabirds plummet from heights of 30 metres (98 feet), plunging through the water leaving vapour-like trails behind like fighter planes.

The eggs, left behind at the Agulhas Banks, drift northwest with the current into waters off the west coast, where the larvae develop into juvenile fish. When they are old enough, they aggregate into dense shoals and migrate southwards, returning to the Agulhas banks in order to restart the cycle.

Depending on how you define "forage fish", you could argue that they do increase their body size: If you define forage fish as those species that feed on plankton, the biggest "forage fish" out there, then would be the whale shark! Of course the term, forage fish, suggests that these are fish small enough that they are availabe as "forage" for a broad range of other species--fish too large to serve as such are, then, no longer "forage fish".

Mohammed Ali Makki Alrubaee's excellent response, points out that that forage fish compensate for their small size in other ways (eg schooling).

Yes agreed but I wanted to know why small size forage fishes not grow too much despite leaving in years

I know they are the source of food for other fish species but why they do restricted their own body size

Thanks Dr Mohammed Ali for details but still I am in search of my answer why do they could not extend their size

Now I understand your question better...I'm not sure if there's a solid answer out there (we may not know), but here are a few points to consider: Fish are often described as having indeterminate growth (ie rate of growth may change with age, but growth basically continues indefinitely), but my understanding is that this is not universal--some fishes do have determinate growth. Dutta (1994 Gerontology 40(2-4):97-112; not sure to what extent I trust this) states that short-lived spp from warm regions have determinate growth; long-lived spp from cooler regions have indeterminate growth. A simple (simplistic?) answer to your question may be that forage fish (ie small, schooling species) have determinate growth. We know that there are multiple endogenous and exogenous factors that regulate growth (genetics, endocrine, temperature, food availability, competition, size of environment, etc; see McDowall 1994 Ecol FW Fish doi:10.1111/j.1600-0633.1994.tb00108.x). Because forage fish depend on schooling for predator avoidance, it's likely to be adaptive that these spp grow quickly to near some max size and then spend those growth resources on reproduction instead, in part because distinctly larger individuals may be more conspicuous in a school. Sorry for the rather chaotic answer; hope this gets you started!

Hello Imtiaz,

Forage fish increase in their body size but if you compare them with carp and others like carp then it becomes like your version in the form of question you asked.