Ocean Predators, Fish Feeding and the Marine Food Chain

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marine food chain
As most every school child knows the food chain is key to the marine ecosystem. Large fish like tuna and sharks feed on medium size fish like cod. They feed on smaller fish like whiting, which in turn feed on even smaller fish like herring. The herring in turn feed on krill and copepods and they in turn feed on “zooplankton” (animal plankton) and they feed on “phytoplankton” (plant plankton) such as diatoms.

Any disruption of the food chain can affect members of the entire system. If the supply of diatoms is reduced there is less food for the copepods and if their numbers are reduced there is less for fish further up the chain. If the supply of cod is reduced there may be a an explosion of whiting which could also reduce the number of herring. Pollutants or toxins that are taken in by one link of the chain are passed on up.

According to Animal Diversity Web (ADW): Fish are essential components of most ecosystems in which they occur. While many Fish prey on each other, they can also have significant impacts on nearly all other animals in their habitats. Zooplanktivorous fishes, for instance, select for specific types and sizes of zooplankton when they feed, thus influencing the type and quantity of zooplankton, and, by extension, phytoplankton present in surface waters (zooplankton consume algae; together they are simply termed plankton). [Source: Nicholas White, Animal Diversity Web (ADW) /=]

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Fishbase fishbase.se ; Encyclopedia of Life eol.org ; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; Woods Hole Oceanographic Institute whoi.edu ; Cousteau Society cousteau.org ; Monterey Bay Aquarium montereybayaquarium.org ; MarineBio marinebio.org/oceans/creatures

Food and Eating Behavior of Fish

In terms of feeding habits, researchers broadly classify fish as herbivores (animals that primarily eat plants or plants parts), carnivores (animals that mainly eat meat or animal parts), omnivores (animals that eat a variety of things, including plants and animals), planktivores (mainly eat plankton), zooplanktivores (animals that primarily eat zooplankton), scavengers and detrivores (animals that feeds on dead organic material, especially plant detritus). There are some sanguivores (mainly consumes blood) and molluscivores (mainly eat mollusks).[Source: Nicholas White, Animal Diversity Web (ADW) /=]

According to Animal Diversity Web (ADW): There is considerable nuance within each of these categories because many fish are opportunistic feeders — they tend to consume whatever is around, especially when food is scarce.

20110307-NOAA fish blackrockfish_100.jpg
black rockfish
Herbivorous fishes posses specialized organs, such as extended guts, pharyngeal mills and gizzards, that allow them to exploit various reef plants and algae. Some of the most successful freshwater families (such as minnows , catfishes , cichlids), and most abundant coral reef families (such as halfbeaks , parrotfishes , blennies , surgeonfishes , rabbitfishes), include many species of herbivorous fishes. Several groups of herbivorous coral reef species defend territories or form feeding shoals (freshwater cichlids have many of the same behaviors). Some parrotfishes and surgeonfishes utilize shoals to overwhelm the defenses of territorial (defend an area within the home range), species, thus gaining access to areas with higher concentrations of plant material. (Bertelson and Pietsch, 1998; Ferraris, 1998; Helfman, et al., 1997; Moyle and Cech, 2004; Parrish, 1998) /=\

Zooplanktivores (animals that primarily eat zooplankton), which feed on small crustaceans like water fleas and copepods floating in the water column (termed zooplankton), abound in oceans throughout the world. Groups such as silversides , herrings and anchovies often congregate in feeding shoals numbering in the millions. Smaller shoals of zooplanktivores (animals that primarily eat zooplankton), such as rabbitfishes and the juvenile forms of many other reef species, are also found hovering above and around coral reefs. The characteristic features of zooplanktivorous fishes are small size, streamlined and compressed bodies, forked tails, few teeth, and a protrusible mouth that forms a circle when open. When patches of zooplankton are particularly high, many pelagic (living in the open ocean, far from land) zooplanktivores (animals that primarily eat zooplankton) keep their mouths agape, and when patches are low they pick animals out individually (the latter are also termed suction feeders)./=\

Predatory Fish

According to Animal Diversity Web (ADW): Primary feeding habits are often associated with body form, mouth type and digestive apparatus, as well as teeth. For instance, gars , pike-characids , pike , needlefish , pike killifish and barracuda represent a diverse range of taxa, yet they all have elongate (long and narrow) bodies, long snouts, and sharp teeth with the fins placed toward the back of the body; this is the design of a fast-start predator, which often lurks motionless in the water column, slightly camouflaged and ready to lunge quickly at unsuspecting prey. [Source: Nicholas White, Animal Diversity Web (ADW) /=]

These fishes are not made for sustained speed and maneuverability, whereas tunas and billfishes (suborder Scombroidei), with their rounded and highly tapered bodies, are streamlined pelagic (living in the open ocean, far from land) chasers capable of very high speeds over long periods. These two fishes are termed ram feeders. Other predators avoid the extra energy expenditure of chasing prey, and instead wait passively, depending largely on good vision, explosive thrust and large mouths capable of forming strong vacuums and effectively inhaling prey (the latter method is termed suction feeding). These sit-in-wait predators are often completely hidden with elaborate camouflage or by burying themselves beneath sediment with only the eyes exposed. Fishes of this type include many scorpionfishes , flatheads , hawkfishes , sea basses , stonefishes , stargazers, flatfishes , frogfishes, and lizardfishes. /=\

Several groups of fish have quite peculiar methods of capturing prey. Deepsea anglerfishes , among many others in the Stomiiformes and Lophiiformes orders, have developed a luminous bait to attract prey in the deep, dark waters they inhabit. Turbid habitats are home to many fishes that utilize electroreception to find prey, and some predators (such as knifefishes and the electric eel) use intense electrical shocks of as much as 350 volts to stun prey before consuming them. Archerfishes exploit a food source that is unavailable to most other fishes: terrestrial insects in overlying vegetation. By shooting jets or bullets of water, and correcting for light refraction, archerfishes knock insects down to the water surface and quickly consume them. Finally, some boxfishes and triggerfishes use an equally novel technique for capturing prey. Both groups expel jets of water from their mouths to uncover buried animals, while triggerfishes use jets and their snouts to flip over and consume otherwise inedible prey, such as spiny sea urchins./=\

Predators That Have Success Capturing Schooling Fish

predators size up a fish school

Predators that have developed successful strategies to capture schooling fish include swordfish and sailfish which swim headlong into a school slashing around randomly. Any fish that is injured or killed are gobbled up.

Schooling predators such as jacks hunt at twilight like a pack of wolves trying to bring down a caribou. Working in teams and individuals, the fish try to break up schools of prey into small groups and separate victims a few at time and corral them against the surface, where they are easy targets. Sometimes individual jacks dash into the center of a school of prey in an effort to scare individuals and distract them enough to be separated from the school so they can be picked off.

Sailfish catch fish not be spearing them with their bill but rather by slapping them with their bills to stun them and then gobbling them up. Large groups often cooperate like wolves pursuing caribous to bring their prey under control, swatting, corralling the prey. Individuals take turns making rapid fire strike. Reports of sailfish stabbing one another are rare. [Source: Paul Nicklen, National Geographic, September 2008]

Describing sailfish on the hunt Jennifer Holland wrote in National Geographic, “Adorned for the hunt, with fin raised and changeable colors flashing, a sailfish in the Gulf of Mexico circles a ball of sardines, preparing to strike....More than a hundred sailfish keep tabs on an elephant-size school of sardines of Mexico’s Isla Mujeres, The big fish...drive the prey up from deeper waters for easier feeding near the sunlit surface...Wielding its bony bill, the predator slices through a sardine school to isolate a smaller cluster — more controllable as the prey zigzag to elude capture, Sardines seek safety in numbers, moving as one.”

“Males and females alike circle the prey, pushing the school into tighter formation, and taking a few bites in turn. Each forward rush is punctuated by a startling flare of the dorsal fin, which more than doubles the hunter’s profile.”

Once the sardines are contained and in a position the sailfish want them to be, Holland wrote, “The predators shoot in from all sides, popping open fins and flashing iridescent colors as they get up close...Once a ball is under control, the sailfish take turns shooting through it, heads whipping side to side as they use their bills to bat sardines with remarkable precision. Pursuers tem nab stunned fish before they can escape. Whittle down to it last bloody stragglers, the ball spins in a slow vortex, prey exhausted and no longer in perfect concert., Typically sailfish will consumer every last one.”

Warmer Ocean Temperatures Making Predators More Ravenous?

Research published in Science in June 2022 by a Smithsonian-lead team suggests that ocean predators are possibly going to be even more ravenous as a result of climate change. Cassidy Ward wrote in SYFY: Previous research indicated a relationship between the levels of predation and the temperature of the water, revealing that predators in the tropics are more active and consume more prey animals than their counterparts in colder parts of the world. That research, however, didn’t take into account how the increased predation might impact prey populations, particularly as global ocean temperatures rise. [Source: Cassidy Ward, SYFY, June 20, 2022]

Scientists gathered data from 36 partner sites in the Atlantic and Pacific oceans, ranging as far north as Alaska and as far south as the tip of South America, covering a total of 115 degrees of latitude. To get a baseline of predatory activity, they dropped dried squid attached to stakes — lovingly referred to as squid pops — at different sites and left them to be eaten. After an hour, they went back to collect the remains, if any existed. Comparing the rate of consumption at different water temperatures confirmed that prey consumption was more intense in warmer waters, with predation ceasing almost entirely at temperatures below 69 Fahrenheit.

In another experiment involving live prey, scientists placed protective cages over populations of stationary invertebrates like sea squirts, and allowed them to colonize plastic panels for a few months. Later, they removed the cages from half of the populations, giving local predators access. Those populations which were unprotected were devoured by predators in warmer waters. Meanwhile, in colder regions there was little observable difference between colonies which were protected and ones which weren’t.

How Fish Avoid Predators

According to Animal Diversity Web (ADW): Fish generally avoid predators in two ways, through behavioral adaptation and physical structures, such as spines, camouflage and scents. Usually, several behavioral and structural tactics are integrated because it is advantageous for fishes to break the predation cycle (1-4) in as many places as possible, and the earlier the better. For instance, 1) the primary goal of most fish is to avoid detection, or avoid being exposed during certain times of the day. If detected, 2) a fish might try to hide very quickly, blend in with the surroundings, or school; 3) if the fish is about to be attacked then it must try to deflect the attack, and if attack is unavoidable 4) the fish will try to avoid being handled and possibly escape. Therefore, many fishes avoid even the chance of attack through particular cycles of activity, shading (or lighting, see below) and camouflage, mimicking, and warning coloration. [Source: Nicholas White, Animal Diversity Web (ADW) /=]

For example, fishes usually avoid dusk because predators often take advantage of quickly changing light conditions that make it difficult for prey to see predators. (Species that feed at dusk are termed crepuscular (active at dawn and dusk), and include jackssnappers , tarpon , cornetfishes and groupers). Most fish feed during daylight hours (diurnal (active mainly during the daytime)), when they can see predators. Zooplanktivores (animals that primarily eat zooplankton), cleaner fishes, and many herbivores (animals that primarily eat plants or plants parts) are abundant and conspicuous by day but hide within the reef at night.

Several wrasses and parrotfishes even secrete a foul-smelling mucous tent or bury themselves in the sediment for protection. Shoaling, which is common among many groups (found in sticklebacks , bluegills , gobies and many others), provides many benefits as a daytime defense. Some predators actually mistake shoals for large fish and avoid attacking. Also, when shoals detect predators they form a tight, polarized group, or school, that is able make synchronous motions. Attacking predators may find it difficult to isolate individuals as the school morphs around them, and some groups (snappers , goatfishes , butterflyfishes , damselfishes , etc.) even mob the predator, nipping and displaying, to thwart an attack. /=\

Marine food chain and the oceanic carbon cycle

Benthic (living on or near the bottom of the sea) fish also utilize numerous methods of camouflage (for both hunting and predator avoidance). A common and elaborate method in tropical seas is mimicking the background of the habitat (protective resemblance), which involves variable color patterns as well as peculiar growths of the skin that may resemble pieces of dead vegetation, corals , and a variety of bottom types (such as flatfishes). There are numerous examples of this type of crypticity, from sargassumfishes and leafy seadragons that mimic the seaweed among which they hover, to clingfishes , shrimp fishes and cardinalfishes that have black stripes resembling the sea urchins they use for cover. Another method of camouflage is to look and behave like something inedible, but remain conspicuous. Juvenile sweetlips and batfishes mimic certain types of flatworms and nudibranchs that have toxins in their skin and associated bright coloration, making possible predators wary. /=\

Bold or bright coloration in fish (termed aposematic) usually means that the species posses a structural or chemical defense, such as poisonous spines, or toxic chemicals in the skin and internal organs. Surgeonfishes and lionfishes , for instance, have bold coloration to match scalpel-like and poisonous spines, respectively. Aposematic fishes also advertise their inedibility by moving slowly, instead of darting away when predators are present. However, displays of aggression back up this behavior. When disturbed, weevers erect a dark-colored and highly venomous dorsal spine, while pufferfishes , also poisonous, puff up into a ball of spikes. /=\

Bioluminescence, Camouflage and Avoiding Predators

Because many larger species of zooplankton and other invertebrates come out at night, several groups have developed nighttime feeding patterns (nocturnal (active at night),) and associated defense mechanisms. Many of these groups, including flashlight fishes , ponyfishes , pineapple fishes and some cardinalfishes , have luminescent organs. While luminescence is likely used for communication (shoaling and mating) and catching prey (via luminescent eyes, which can be turned on and off (!), and baits), several species use luminescence for defense. [Source: Nicholas White, Animal Diversity Web (ADW) /=]

Rows of lights along the bottom of the body make these fishes indistinguishable to benthic (living on or near the bottom of the sea) (living at the bottom) predators because they match the intensity of moonlight or dim sunlight shining down. This peculiar method of invisibility is similar to countershading, which is common in several other pelagic (living in the open ocean, far from land) ray-finned fishes (as well as sharks and rays).

Countershaded fishes are graded in color from dark on top to light on bottom, rendering them invisible from nearly any angle because their coloring is opposite that of downwelling light; the light reflected is equivalent to the background (as above). Two other methods by which pelagic (living in the open ocean, far from land) fishes remain invisible are by having a shiny coating (mirror-sided), as in anchovies , minnows , smelts , herrings and silversides ; or by having transparent bodies, like glassfishes , African glass catfishes and Asian glass catfishes. /=\

Ocean at Night

Amy McKeever wrote in National Geographic: In the dark — whether it’s the Sargasso Sea of the North Atlantic or the tropical waters off Indonesia’s Raja Ampat archipelago divers and marine biologists see things they don’t see in the day. Black-water diving is “the equivalent of a marine ‘Sorcerer’s Apprentice,’” National Geographic photographer David Doubilet says. “All strange things that are dancing around at night.”[Source: Amy McKeever, National Geographic, September 9, 2021]

When night falls on the open sea, there’s a world to explore as zooplankton swim up from the depths to feed. Many of these small organisms are still larvae. “It’s the nursery of the ocean,” says Doubilet. Doubilet and marine biologist Jennifer Hayes capture rare images of creatures in their larval forms and observe the clever ways the animals survive the night. “It’s all at the mercy of the current,” Hayes says. “You’re just moving with [the animals], lucky to encounter them.”

A pelagic squid releases a cloud of ink before vanishing into the depths of Indonesian waters. A juvenile jack hides behind a jellyfish — driving it like a motorboat. As the jellyfish provides protection from predators, the juvenile fish may feed on parasites that have latched onto its host. Many night sea encounters are not immediately identifiable; photographs are circulated within an active community of scientists and divers who work together to learn about this unique ecosystem. To travel safely through the night, larval carangid hop a ride on the back of a moon jelly. Some animals attempt to camouflage themselves for protection — such as this pipefish, which is pretending to be the stick it’s carrying through the night sea in Anilao, Philippines. A juvenile trevally hides itself inside a jellyfish to escape the notice of predators near Moalboal, Philippines.

Zooplankton — such as a jellyfish and a larval lionfish surrounded by tiny, shrimplike amphipods — often swim from the deep toward the surface at night to feed. The sea butterfly is a free-swimming snail that can be as small as a grain of sand whose foot has evolved into wing-like lobes that flap to propel it through the water. A juvenile African pompano, or threadfin trevally, swims through the Verde Island Passage, a major shipping lane in the Philippines. Its streaming filaments resemble the tentacles of a jellyfish — a possible advantage for evading predators that patrol the night sea.

Relations Between Fish and Other Living Things

A variety of terrestrial vertebrates, such as mammals , amphibians ,, reptiles, , and many marine and freshwater birds depend on Fish as a primary source of food. Piscivorous Fish compete with many of the organisms above and in some cases are involved in symbiotic relationships with them. A simultaneous competitive and commensal (one benefits and the other is unaffected) relationship is found between bluefish and common terns. These two species interact at a critical period of the terns’ feeding cycle, just after mating when there are chicks to feed. At this time, bluefish migrate to feed on anchovies, concentrating and driving them up in the water column, where terns can catch sight of the anchovies (commensalism). However, bluefish reduce anchovies’ populations considerably, and terns that breed after the bluefish migration are usually unsuccessful (competition). [Source: Nicholas White, Animal Diversity Web (ADW) /=]

There are numerous other examples of symbiosis, mutualism, commensalism and parasitism between Fish and other groups. For example, gobies share burrows with several shrimp-like crustaceans (mutualism) or live among sponges and corals (commensalism). Cardinalfishes and pearlfishes live inside large gastropods and mollusks , respectively (inquilism-sheltering inside living invertebrates). Recently, researchers have begun to appreciate the importance of fish in linking terrestrial and aquatic ecosystems. This is especially true of anadromous species, which grow primarily in the sea but return to aquatic areas before they, spreading nutrients from the ocean up and down rivers. During rainy periods in tropical watersheds, ray-finned fish forage in flooded areas, consuming seeds and dispersing them throughout the floodplain. /=\

Several groups of invertebrates (mostly marine), such as cone shells , crabs , anemones , squids and siphonophores (colonies of organisms, e.g. man-o-war), also regularly consume various ray-finned fish. There is even some unlikely predators like dinoflagellates , that can cause large fish kills, known as “red tides”. Some dinoflagellates consume the scales of the dead fish as they sink. Fish also have significant impacts on a variety of plant species. The trophic cascade example (above) illustrated an indirect connection between microscopic plants (phytoplankton) and fish, but fish also excrete soluble nutrients into the water, such as phosphorus. Phosphorus is essential for phytoplankton growth, and fish secretions may provide significant amounts of nutrients in some lakes. A more direct connection is simply the consumption of numerous plant species (see Food Habits). Finally, fish may significantly alter the geological dynamics of their habitats. Many ray-finned fish build nests or burrows (such as several minnows , trout and salmon and tilefishes), while others break down substrates, such as dead coral, into sand (such as parrotfishes , wrasses , surgeonfishes , triggerfishes and pufferfishes). /=\

monitoring food chain export processes in the ocean with remote sensing

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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