The Lives of Food Fish - Sardines and Herring

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. There are several fish species known as “sardines”. They all have a similar appearance and are members of the herring family. Herring are also silver in color, but they are bigger than sardines. Sardines and herring are a nutritious food source and are rich in omega-3 fatty acids, which are believed to have several health benefits. They seem to improve memory and mood and may also help prevent cardiovascular disease and improve arthritis symptoms. Both sardines and herring live in large groups and have some interesting behaviors and ways to communicate with each other.

Sardines are eaten by marine animals and birds as well as by humans. They are rich in protein, omega-3 fatty acids, calcium (if the bones are eaten), vitamin D, vitamin B12 and selenium. They are sold fresh, frozen and canned. Canned sardines are healthiest when water-packed and unsalted. They 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 source and a good source of omega-3 fatty acids. Confusingly, young herring are sometimes known as sardines.

Sardines live in temperate and subtropical oceans. They may also be found in estuaries and there is a freshwater species in the Philippines. The “true” sardine, Sardinus pilchardus, is also known as the European Pilchard, or simply a pilchard, and as its name suggests is 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.

Every year between May and July - with a few exceptions - sardines migrate along the coast of South Africa in a huge group. Predators, including dolphins, sharks, seals, gannets, cormorants and humans, 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.

Pacific sardines have an attractive, elongated body which is silvery and iridescent. The upper surface is blue or green, depending on the angle at which it 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, and unlike most other bony fish sardines don't have a lateral line. The lateral line is a sense organ running along each side of a fish's body which detects vibrations in the water.

Sardines may reach a length of fourteen inches, but more usually are 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.

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. The 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 though it is one creature. A shoal may temporarily become a school and then go back to becoming a shoal again.

Sardines are filter feeders and eat plankton - 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 and pass over the gills and 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.

Pacific sardines reproduce multiple times in a breeding season. The male and female reproductive cells are released into shallow water, where they join. The fertilized eggs hatch in about three days.

The Pacific Herring, Clupea pallasii, may reach 18 inches in length, but most adults are around 12 inches long. Like Pacific sardines, Pacific herring have a blue-green upper surface, silver sides and a silver belly, and 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 eggs hatch into larvae after about two weeks, the exact time depending on the water temperature.

Herring release gas from their anal duct in a noisy stream of bubbles, which is used to communicate with other herring. Researcher have named these signals "Herring FRT's". FRT stands for "Fast, Repetitive Ticks". Both Atlantic and Pacific Herring make FRT’s.

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.

While many people think of sardines and herring simply as food, they are interesting fish. There is still much to be learned about their behavior and the ways in which they communicate with each other.