Forage fish

Forage fish are fish which are preyed on by larger predators for food. Predators include other larger fish, seabirds and marine mammals. Forage fish are usually abundant and often swim in large schools. They are an important link in the marine food web because they transfer energy between primary and secondary producers, such as plankton, to the top predators.

Forage fish occupy the central positions in ocean and lake food webs. The ocean primary producers, mainly phytoplankton, produce food energy from the sun and are the raw fuel for ocean food webs. Forage fish transfer this energy by ingesting the plankton and becoming food themselves for top predators such as large fish, seabirds, sharks and whales.

Their schooling behaviour and superabundance make them ideal food sources for top predators. Without forage fish, ocean food webs would collapse. The word forage comes from the 14th century Old French fourrage, meaning fodder or straw.

In the oceans

Typical ocean forage fish are small, silvery schooling oily fish such as herring, anchovies, menhaden and butterfish, and other small, schooling baitfish like capelin, smelts, sand lance, halfbeaks, pollock and juvenile rockfish. Herrings are a preeminant 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".

Ocean forage fish

	File:6742 aquaimages.jpg
Anchovies	Caribbean reef squid	Menhaden
Sardines	Shrimp	Northern krill

Forage fish are the dominant converters of the enormous ocean production of zooplankton, utilizing the biomass of copepods, mysids and krill in the pelagic zone. They are on the other side a central prey item for higher trophic levels. The dominance of forage fish may be related to their outstanding way of living 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 of 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 (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.

An ecological pyramid.

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 a 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. These

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. The larger producers, such as seagrasses and seaweeds, are generally confined to the littoral zone and adjacent shallow waters, where they can attach to the underlying substrate but still be within the photic zone. The majority of 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. When present in high enough numbers, they can appear as a green discoloration of the water. Since they increase their biomass mostly through photosynthesis they live in the sun-lit surface layer (euphotic zone) of the sea.

Pytoplankton form the base of the ocean foodchain

Phytoplankton	Dinoflagellate	Diatoms

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. Although usually microscopic, some species of diatoms can reach up to 2 millimetres in length.

The second trophic level (primary consumers) is occupied by zooplankton which feed off the pytoplankton. Together with the phytoplankton, they form the base of the food pyramid that supports most of the world's great fishing areas. 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 pytoplankton 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.

Zooplankton form the second level in the ocean food chain

Segmented worm	Tiny shrimp-like crustaceans	Juvenile planktonic squid

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 the sea and nearly every freshwater habitat. They constitute the biggest source of protein in the oceans, 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 (Greek planktos = wanderer or drifter). By definition, organisms classified as plankton are unable to resist ocean currents; they cannot swim against the ambient current and control their position. In ocean environments, the first two tropic 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).

What forage fish eat

External image
Forage fish and the food web

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. Adult herring 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 along with their mouths open, filtering the plankton from the water as it passes through their gills.

Ocean halfbeaks are omnivores feeding on algae, marine plants such as seagrasses, plankton, invertebrates such as pteropods and crustaceans; and smaller fishes. Some tropical species have been observed to feed on animals during the day and plants at night, while other species alternate between carnivory in the summer and herbivory in the winter. 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.

Ocean predators of forage fish

File:Tuna.jpg	File:G. cuvier.jpg
Tuna	Shark	Striped bass
File:Albatros ceja negra - paso drake - noviembre 2005.jpg		File:Hawaiian Monk Seal, photo in water.jpg
Albatross	Dolphin	Hawaiian monk seal

Schooling

Underwater video loop of a school of herrings migratiing at high speed to their spawning grounds in the Baltic Sea.

Forage fish compensate for their small size by forming huge schools. These immense gatherings fuel the ocean food web. Most forage fish are pelagic, meaning they form their schools in open water rather than on the bottom (benthic) or near the bottom (demersal). 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 inspired by them to make migrations that can span thousands of miles. 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 up to 4 cubic kilometers in size, containing an estimated 4 billion fish. These schools traverse the open oceans. A school of herring in general has a very precise arrangement thus allowing the school to maintain a relatively constant cruising speed. Schools that are made up of an individual stock generally travel in a triangular pattern between their spawning grounds e.g. Southern Norway, their feeding grounds (Iceland) and also their nursery grounds (Northern Norway). Such wide triangular journeys are probably important because herring feast efficiently on their own offspring. A school of herring can react very quickly to evade predators; they have excellent hearing. Around scuba divers or predators like killer whales, can form a vacuole (fountain effect). The phenomenon of schooling is however, far from understood, especially the implications on swimming and feeding-energetics. Many hypotheses have been put forward to explain the function of schooling, such as predator confusion, reduced risk of being found, better orientation, and synchronized hunting. However, schooling can also have some disadvantages such as: oxygen- and food-depletion, excretion buildup in the breathing media. The school-array probably gives advantages in energy saving although this is a highly controversial and much debated field.

Schools of herring can on calm days sometimes be detected at the surface from more than a mile away by the little waves they form, or from a few meters at night when they trigger the bioluminescence of surrounding plankton ("firing"). All underwater recordings show herring constantly cruising with high speeds up to 108 cm per second, and much higher escape speeds.

School of blacksmiths being cleaned by parasite eating fish

They are fragile fish, and because of their collective schooling behaviour they are on display in very few aquaria worldwide, this despite their natural abundance in the ocean. Even with the best facilities that these aquaria can offer they appear slim and slow compared to a quivering school in the wild.

Blacksmith fish live in loose schools. They have a symbiotic relationship with the parasite eating senorita fish. When they encounter senorita fish, they stop and form a tight ball and hang upside down (pictured), each fish waiting it's turn to be cleaned. The senorita fish pick dead tissues and external parasites, like parasitic copecods and isocods, from the skin of other fishes.

Copepod

Slow motion video loop of a juvenile herring feeding on copepods.

Hunting for copepods

Copepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic (drifting in sea waters), but more are benthic (living on the ocean floor). Copepods are typically 1 millimetre (0.039 in) to 2 millimetres (0.079 in) long, with a teardrop shaped body and large antennae. Although like other crustaceans they have an armoured exoskeleton, they are so small that in most species this armour, and the entire body, is almost totally transparent.

They are usually the dominant members of the zooplankton. Some scientists say they form the largest animal biomass on earth. They compete for this title with Antarctic krill. Because of their smaller size and relatively faster growth rates, however, and because they are more evenly distributed throughout more of the world's oceans, copepods almost certainly contribute far more to the secondary productivity of the world's oceans than krill, and perhaps than all other groups of organisms together.

Copepods are very alert and evasive and can jump with extreme speed over a few millimeters. Herrings are pelagic feeders - their prey consists of a wide spectrum of pytoplankton and zooplankton, amongst which copepods are a dominant prey.

Young herring usually capture copepods by hunting them individually (particulate feeding) (Kils 1992).

The (half speed) video loop at the left shows a juvenile herring feeding on copepods - the fish approach from below and catch each copepod individually. In the middle of the image a copepod escapes successfully to the left. The opercula 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, as in microlayers, at fronts or directly below the surface, herring ram forwards with wide open mouth and far expanded opercula over several feet, then closing and cleaning the gill rakers for a few milliseconds (filter feeding). In the photo on the right, herring ram feeding on a school of copepods. All fish have the opercula wide open all at the same time (the red gills are visible) and the mouth wide open (click to enlarge). The fish swim in a grid with a distance of the jump length of their prey, as indicated in the animation below.

In the animation, juvenile herring hunt for the copepods in synchronization: The copepods can sense with their antennae the pressure-wave of the approaching herring and react with a fast escape jump. The length of the jump is quite similar. The fish arrange in a grid of this characteristic jump length. Copepods can dart about 80 times before they tire out. It takes 60 milliseconds to spread out the antennae again, and this time slot is utilized often by the herring to snap finally a copepod. A single juvenile herring would never be able to catch a large copepod.

Capelin migration

Migration of Icelandic capelin

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 from great deal of krill and other crustaceans. Capelin move inshore in large schools to spawn and migrate in spring and summer to feed in the plankton-rich oceanic area between Iceland, Greenland, and Jan Mayen. Capelin distribution and migration is linked with ocean currents and water masses. Around Iceland maturing capelin usually undertake extensive northward feeding migrations in spring and summer and the return migration takes place in September to November. The spawning migration starts from north of Iceland in December to 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.

The sardine run

The 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.

External images
Dolphins herd sardines
Gannets "divebomb" sardines

Gannet

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 mi) long, 1.5 kilometers (1 mi) wide and 30 meters (100 ft) 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.

When threatened, sardines instinctively group together and create massive "bait balls". Bait balls can be up to 20 meters (70ft) 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 (100 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 mature and develop into juvenile fish. Once strong enough, they aggregate into dense shoals and migrate southwards, returning to the Agulhas banks in order to restart the cycle.

Commercial fishing

History

Medieval herring fishing in Scania (published 1555).

Herring has been a known staple food source since 3000 B.C. In Roman times, anchovies were the base for the fermented fish sauce called garum that was a staple of cuisine and an item of long-distance commerce produced in industrial quantities.

Fishing for sardela or sardina (Sardina pilchardus) on the Croatian Adriatic coasts of Dalmatia and Istria is an ongoing activity tracing its roots back thousands of years. The region was part of the Roman Empire, then largely a Venetian dominion, and has always been sustained through fishing mainly sardines. All along the coast there are many towns that promote the age-old practice of fishing by lateen sail boats for tourism and on festival occasions. Today this tradition is also continued by many producers.

Pilchard fishing and processing was a thriving industry in Cornwall from around 1750 to around 1880, after which it went into an almost terminal decline. However, as of 2007, stocks are improving (q.v. River Cottage: Gone Fishing 22/11/08). The industry has featured in numerous works of art, particularly by Stanhope Forbes and other Newlyn School artists.

Contemporary

Historically, commercial fisheries focused more on high value ocean predators such as cod, rockfish, swordfish and tuna, rather than the more humble forage fish. As technology advanced, fisheries became so efficient at locating and removing predator fish that many of their stocks collapsed. To compensate, the industry set their sights on species lower in the food chain.

Traditionally, forage fish were more difficult to fish profitably, and were a small part of the global marine fisheries. But modern industrial fishing technologies have enabled the removal of increasing quantities. Industrial-scale forage fish fisheries need large scale landings of fish to return profits. They are dominated by a small number of corporate fishing and processing companies.

Purse seine boats encircling a school of menhaden

Commercial herring catch

Forage fish populations are very vulnerable when faced with modern fishing equipment. Because they swim near the surface in tight schools, they are relatively easy to locate under the surface with sophisticated electronic fishfinders and from above with spotter planes. Once located, they are scooped out of the water using highly efficient nets, such as purse seines, which can remove most of the school.

Spawning patterns in forage fish are highly predictable. Some fisheries use knowledge of these patterns to harvest the forage species as they come together to spawn, removing the fish before they have actually spawned. Fishing during spawning periods or at other times when forage fish amass in large numbers can also be a blow to predators. Many predators, such as whales, tuna and sharks, have evolved to migrate long distances to specific sites for feeding and breeding. Their survival hinges on their finding these forage schools at their feeding grounds. The great ocean predators find that, no matter how they are adapted for speed, size, endurance or stealth, they are on the losing side when faced with the machinery of contemporary industrial fishing.

Forage fish fisheries are the largest in the world. The Peruvian anchoveta fishery is now the biggest in the world (10.7 million tonnes in 2004). Seven of the top ten fisheries (by weight) target forage fish. The total world catch of herrings, sardines and anchovies alone in 2005 was 22.4 million tonnes, 24 percent of the total world catch.

The Alaskan pollock fishery in the Bering Sea is the largest single species food fish fishery in the world. More than 3 million tons of Alaska pollock are caught each year in the North Pacific from Alaska to northern Japan. The Alaskan pollock is said to be "the largest remaining source of palatable fish in the world.". However, the biomass of pollock has declined in recent years, perhaps spelling trouble for both the Bering Sea ecosystem and the commercial fishery it supports. Acoustic surveys by NOAA indicate that the 2008 pollock population is almost 50 percent lower than last year's survey levels. This decline has led some scientists to worry that Alaska pollock could be about to repeat the kind of collapse experienced by Atlantic cod, which could have negative consequences for the world food supply and the entire Bering Sea ecosystem. Halibut, salmon, endangered Steller sea lions, fur seals, and humpback whales all eat pollock and rely on healthy populations to sustain themselves.

Use as animal feed

Altogether, forage fish account for 37 percent (31.5 million tonnes) of all fish taken from the world's oceans each year, and 90 percent of that catch is processed into fishmeal and fish oil. In 2002, 46 percent of fishmeal and fish oil was used as feed for fish farming, 24 percent for pig feed, and 22 percent for poultry feed. The pigs and poultry alone consume more than six times the seafood eaten by the U.S. market.

Over 30 million tonnes of forage fish are taken from the sea each year and processed into 6.4 million metric tons of [[fishmeal and 0.9 million metric tons of fish oil. It takes many kilograms of fish to produce just one kilogram of fishmeal or fish oil.

According to Turchini and De Silva (2008), another 2.48 million tonnes of the annual forage fish catch is consumed by the global cat food industry each year. In Australia, pet cats eat 13.7 kilograms of fish a year compared to the 11 kilograms eaten by the average Australian. The pet food industry was increasingly marketing premium and super-premium products, when different raw materials, such as theby-products of the fish filleting industry, could be used instead.

Environmental issues

Despite the large scale extraction of forage fish, few management plans exist for guiding their sustainable harvesting, and not enough is known about the role of forage fish in marine ecosystem and how fishing effects them and the predators that feed on them.

The National Coalition for Marine Conservation (NCMC) is asking fishery managers to put "Forage First!". NCMC’s Forage First! Campaign was launched with the publication of the report, Taking the Bait: Are America’s Fisheries Out-competing Predators for their Prey? The report encourages fishery managers to protect predator-prey relationships as a first step toward an ecosystem-based approach to fishery management. However this campaign is confined to U.S. fisheries.

A recent (2008) study titled "Forage Fish: From Ecosystems to Markets" by fisheries scientist Daniel Pauly and colleagues is a product of a nine-year Sea Around Us Project. The study says that forage fish are a precious food resource for humans which is being squandered and that a serious overfishing crisis in the oceans is being disregarded. Forage fish are highly nutritious and well-suited for direct human consumption, and the authors urge that other foods be used to feed farmed animals.

"It defies reason to drain the ocean of small, wild fishes that could be directly consumed by people in order to produce a lesser quantity of farmed fish," said Ellen K. Pikitch. "Skyrocketing pressure on small wild fishes may be putting entire marine food webs at great risk."

According to Joshua Reichert, managing director of the Pew Environment Group, "Human beings are not the only, or necessarily, the most important consumer of these fish. Whatever people take out of the sea needs to be carefully calibrated to ensure that sufficient fish are left to sustain populations of other fish, seabirds and marine mammals which all play a major role in the healthy functioning of the world's oceans."

The research team from the Sea Around Us Project say that alternative feeds from soy and other land-based crops would be better suited for feeding livestock. But, fish-meal and fish oil are popular because forage fish are easy to catch in large numbers and are relatively inexpensive. Along with the obvious point that these fish would be better utilized if consumed directly by humans the authors are concerned that too little is known about what role forage fish play in the ocean ecosystem and this fishery "may be putting entire marine food webs at great risk."

"We must find a better way to manage forage fisheries before we cause irreversible damage to the broader ocean environment which depends on them as a food source," said Joshua Reichert, managing director of the Pew Environment Group. "Human beings are not the only, or necessarily, the most important consumer of these fish. Whatever people take out of the sea needs to be carefully calibrated to ensure that sufficient fish are left to sustain populations of other fish, seabirds and marine mammals which all play a major role in the healthy functioning of the world's oceans."

This fall the Institute for Ocean Conservation Science at Stony Brook University will launch the Lenfest Forage Fish Task Force, a team of scientists and policy experts from around the world that will address this escalating environmental dilemma of inadequately controlled forage fisheries. Task force members will by 2010 develop scientific approaches to sustainably manage forage fisheries using ecosystem based fisheries management, which emphasizes the interconnectedness of species and habitats and breaks from traditional species-based management.

In lakes and rivers

Forage fish also inhabit freshwater habitats, such as lakes and rivers, where they serve as food for larger freshwater predators. Usually smaller than 15 centimetres (6 in) in length, these small bait fish make up most of the fish found in lakes and rivers. The minnow family alone, consisting of minnows, chubs, shiners and daces, consists of more than fifty species. Other freshwater forage fish include suckers, killifish, shad, bony fish as well as fish of the sunfish family, excluding black basses and crappie, and smaller species of the carp family. There are also anadromous forage fish, such as eulachon.

Freshwater forage fish

Golden shiner	Killifish	Southern redbelly dace
		File:Alosa fallax.png
Chinese minnow	Swarm of carp	Twaite shad

Within any fresh or saltwater ecosystem, there will always be both desirable and undesirable fishes, and this varies from country to country, and often from region to region within a country. Sport fishermen divide freshwater predators of forage fish into those:

• which have a good fighting ability and are good to eat, called sport (or game) fish.
• the other less desirable fish, called rough fish in North America and coarse fish in Britain

Rough or coarse fish usually refers to fish that are not commonly eaten, not sought after for sporting reasons, or have become an invasive species that reduce the populations of desirable fish. They compete for forage fish with the more popular sport fish. They are often regarded as a nuisance, and are not usually protected by game laws. Forage fish generally are not considered rough or coarse fish because of their usefulness as bait.

The term rough fish is used by U.S. state agencies and anglers to describe undesirable predator fish. In North America, anglers fish for salmon, trout, bass, pike, catfish, walleye and muskellunge. The smallest fish are called panfish, because they can fit in a normal cooking pan. Examples are crappies, perch, rock bass, bluegill and sunfish.

The term coarse fish originated in the United Kingdom in the early 19th century. Prior to that time, recreational fishing was a sport of the gentry, who angled for salmon and trout which they called "game fish". Other fish were disdained as "coarse fish". These days, "game fish" refers to Salmonids (other than grayling) – that is, salmon, trout and char. Coarse fish are made up mostly of gars, lampreys and the larger species of carp. Coarse fish are no longer distained, indeed, fishing for coarse fish has become a popular pastime.

Freshwater predators of forage fish

Brook trout	Black crappie	Macquarie perch
Rainbow trout	Pink salmon	Channel catfish

External links

• Sanders, Michael (1995) Impacts of predator-prey relationships on harvesting strategies and management] FAO.
• Daniel Pauley at Fish Forever - Youtube: Pauley talks about his "epiphany" about directly eating anchovies.