The Lives of Food Fish - Pacific Sardines and Herring
Sardines and Herring
Sardines and herring are very nutritious and popular fish. They are both valuable foods. It's interesting to study these fish while they are alive and in their natural environment, however. They live in large groups and have some unusual behaviours.
The words "sardine" and "herring" apply to a variety of fish species that belong to the family Clupeidae. Sardines are small, silvery fish that are named after the island of Sardinia in the Mediterranean, which was once an important area for the sardine fishery. Herring are also silver in colour, but they are bigger than sardines. In this article I focus on the Pacific sardine and the Pacific herring.
Sardines and herring are rich in omega-3 fatty acids, which are thought to have important health benefits. They seem to improve memory and mood and may also help prevent cardiovascular disease and improve arthritis symptoms.
Nutrients in Sardines and Herring
Sardines are eaten by marine animals and birds as well as by humans. They are an excellent source of protein, omega-3 fatty acids, calcium (if the bones are eaten), vitamin D, vitamin B12 and selenium. The fish are sold fresh, frozen and canned. Canned sardines are healthiest when water-packed and unsalted. Sardines are very low in mercury, an environmental poison that enters water and contaminates the bodies of fish.
Herring are also a nutritious, low-mercury food and a good source of omega-3 fatty acids. Confusingly, young herring are sometimes known as sardines.
Distribution of Pacific Sardines
Sardines generally live in temperate and subtropical oceans but may also be found in estuaries. There is a freshwater species in the Philippines. The “true” sardine (Sardinus pilchardus) is also known as the European pilchard. As its name suggests, it's found in ocean waters around Europe.
The Pacific Sardine (Sardinops sagax) is found along the coasts of countries bordering the Pacific Ocean. It's found on the west coast of Central and North America from Baja California to Alaska, although its exact location depends on the time of year. Sardines migrate to find a suitable water temperature as the year progresses. The sardines off the coast of Peru and Chile are the same species as the Central and North American sardine, but a different subspecies. Pacific sardines can also be found off the coast of South Africa.
Pacific sardines have an attractive, elongated body that is silvery and iridescent. The upper surface is blue or green, depending on the subspecies and the angle at which a fish is viewed, while the sides and belly are silver. The fish has a row of dark spots along its side, only one dorsal fin on the top of its body and a strongly forked tail. There are no scales on the head.
Unlike most other bony fish, sardines lack a lateral line. The lateral line is a series of sense organs running along each side of a fish's body. The sense organs detect low-frequency vibrations in the water.
Sardines may reach a length of fourteen inches, but are usually about nine inches long when they are adults. They can live as long as fourteen years, but the majority of the population is younger than six years of age.
A Shoal of Sardines in South Africa
The Annual South African Sardine Run
Every year between May and July - with a few exceptions - sardines migrate along the coast of South Africa in a huge group. Eager predators include dolphins, sharks, seals, gannets, cormorants and humans. They all converge for a fantastic feast as the sardines travel northwards.
The gigantic sardine group is known as a shoal and may be several miles long. Dolphins herd the sardines into dense, dark balls of fish which make it easy for the animals to catch their prey. Sharks also herd the sardines. Sometimes the sardines are driven on to the shore where eager humans grab as many fish as they can. A dive festival is held in conjunction with the sardine run. The run is a very exciting event for both predators and observers.
The scientific name of the fish involved in the South African sardine run varies. Many sources list its name as Sardinops sagax ocellatus, making it a subspecies of the Pacific sardine. Some sources give its name as Sardinops ocellatus, however, making it a different species from the Pacific sardine.
A Group of Sardines - A Shoal and a School
Pacific sardine groups may contain millions of fish. Fish come together for protection from predators. An individual fish is less likely to be eaten when traveling in a group than when traveling as an individual.
A group of fish is known as a shoal or school. The two words are often used interchangeably, but some fish researchers use the word "shoal" to refer to a social group of fish and "school" for a shoal in which all the fish's movements are coordinated. The fish in a school move in a synchronized fashion, with the fish suddenly changing their swimming direction in the same way at the same time. The school acts as thought it's one creature. A shoal may temporarily become a school and then go back to becoming a shoal again.
The Life of a Pacific Sardine
Sardines are filter feeders and eat plankton, which consists of tiny plants and animals moved by water currents. The sardines open their mouths as they swim through the plankton. Plankton and sea water enter the fish's mouth, pass over its gills and then flow back into the water through the opening under the gill cover. As the water flows through the gills, structures on the gills called gill rakers trap the plankton. The gill rakers direct the plankton into the esophagus, which then transports the food to the stomach.
The main breeding area for the Pacific sardines in North America is located off the coast of southern California. The fish reproduce multiple times in a breeding season. Fertilization is external. The eggs and sperm are released into shallow water, where they join. If they aren't eaten by predators, the fertilized eggs hatch in about three days.
The Pacific Herring (Clupea pallasii) may reach eighteen inches in length. Most adults are around twelve inches long, however. Like Pacific sardines, Pacific herring have a blue-green upper surface, silver sides and a silver belly, and they have no lateral line. Also like Pacific sardines, herring travel in large groups and are filter feeders. Herring have a protruding lower jaw and eat mainly zooplankton (tiny animals). They spend the day in deep water and move close to the surface at night to feed.
The females lay their eggs in shallow water. The eggs stick to subtidal and intertidal vegetation, where they are fertilized by the male. The male releases his sperm in milt, a milky white liquid that colours the water. The eggs hatch into larvae after about two weeks, the exact time depending on the water temperature. A Pacific herring may live for as long as eight years if it isn't caught by a predator.
The Pacific herring performs a behaviour known as "flashing". As it turns suddenly while it's swimming, its silvery, reflective body produces what looks like a flash of light. The effect is especially noticeable when multiple fish in a school flash at once.
A School of Pacific Herring
Pacific Herring in the Environment
Pacific herring play an important role in their ecosystem and have been very important in the lives of indigenous people. Many marine mammals, fish and birds feed on the herring and/or their eggs. Even łand animals such as bears visit beaches to eat herring eggs from seaweed that washes ashore. Herring spawning is time for a feast for many animals.
As a food, herring are energy-rich and nutritious. They have a short shelf life, however. Humans eat them when they are fresh, frozen, dried, smoked, salted, pickled or canned. The fish are also used to produce a meal and an oil.
In the past, herring played a dominant role in the life and culture of the First Nations groups along the Pacific Coast of North America. These groups still catch herring, as do commercial fisheries. The herring population is not as large as it once was, however. The fish are abundant in some areas but are reduced in number in other parts of their range.
Herring are so important in the food chain that a collapse in the fish stocks could be disasterous. Collapses have occurred periodically in recent times, probably due to overfishing by commercial fisheries, although there may be other contributing factors. There are other signs that some populations are in trouble, including slow return from a collapse and smaller fish size.
The Pacific Herring - A Foundation Species
Herring release gas from their anal duct in a noisy stream of bubbles. This process is used to communicate with other herring. Researchers have named the signals "Herring FRTs". FRT stands for "Fast, Repetitive Ticks". Both Atlantic and Pacific Herring make FRTs.
The bubbles are released at night when many herring are in the same area. Tests with captive herring show that the sound is produced whether or not the herring have recently fed. This suggests that the gas bubbles are not simply a by-product of food digestion. In addition, the frequency of the bubble release doesn't increase when the scent of a shark is placed into the tank holding the herring, so the bubble production doesn't seem to be a response to fear.
Researchers say that herring gulp air from the water surface. The air enters the swim bladder of the fish and is then released through an opening located near the anus. Herring have good hearing. Their sounds are thought to be used for communication between the fish, enabling them to come together in the dark and maintain contact with each other. Herring and sardines can hear higher pitched sounds than most other fish. Most marine predators can't hear the herring sounds. Dolphins and whales can, however.
The Importance of Sardines and Herring
Sardines and herring are interesting fish that play an important role in their environment. If they disappear, many animals will be adversely affected. Human collection of the fish needs to be a sustainable industry, both for our benefit and for the benefit of animals.
There is still much to be learned about the behaviour of sardines and herring and the ways in which they communicate with each other. There are probably many more interesting discoveries to be made. Though they are an excellent source of nutrients, it's sad that the fish are often thought of as only a food.
© 2011 Linda Crampton