Standard weight in fish is the typical or expected weight at a given total length for a specific species of fish. Most standard weight equations are for freshwater fish species.
Weight-length curves are developed by weighing and measuring samples of fish from the population. Methods of obtaining such samples include creel surveys, or measurements of fish caught by commercial fishermen, recreational fishermen and/or by the researchers themselves. Some scientists use cast nets, trotlines, or other means to catch many individual fish at once for measurement. To determine a standard weight equation, several data sets or weight-length relationships representing a species across its range are used.
As fish grow in length, they increase in weight. The relationship between weight and length is not linear. The relationship between length (L) and weight (W) can be expressed as:
When the equation is for standard weight, the standard weight for a given length is written as Ws. The exponent b is close to 3.0 for most species. The coefficient a varies between species. If the exponent b is greater than three for a certain fish species, that species tends to become relatively fatter or have more girth as it grows longer. For largemouth bass, the value of b is 3.273. If the exponent b is less than three for a certain fish species, that species tends to be more streamlined. For burbot, the value of b is 2.898. [3] While the standard weight for a largemouth bass that is 500 mm long is about 2 kg, the standard weight for a burbot that is 500 mm long is only about 0.9 kg.
Standard weight curves are often based on the 75th percentile weight data rather than the average of all the data available. Murphy et al. (1991) suggested that it is preferable that standard weight equations represent the entire geographical range of a species, and that they be used for comparison purposes rather than management targets. [4] Practically, weight-length equations are often developed for sub-populations from specific geographic areas, but these are different from standard weight relationships.
Length measurements reported for fish may be of the fish's total length, fork length, or maximum standard length. For standard weight equations, the total length is used.
In some species, male and female fish have different standard weight curves. For example, Anderson and Neumann report different standard weight equations for male and female paddlefish. [5] Some researchers have also reported separate standard weight equations when a species has lentic (living in still water) and lotic (living in flowing water) populations. For example, separate standard weight equations have been published for lentic and lotic rainbow trout. [6]
Standard weight is used as a basis for comparison to assess the health of an individual or group of fish. Generally, fish that are heavier than the standard weight for their length are considered healthier, having more energy reserves for normal activities, growth and reproduction. [7]
Fish may weigh less than expected for their length for many reasons, and a scientist must consider more information before assigning a cause. One of the simplest reasons is lack of food/prey. Lack of prey in turn could be the result of overpopulation of the predator, for example, competition from another predator species, unsuitability of the environment for reproduction of the prey, or dying of the prey for some reason. A fish may also weigh less than expected due to a change in activity level or metabolism due to some environmental factor.
Standard weight equations, together with some measure of a fish's condition, can be used in aquaculture to measure the effectiveness of various feeding, temperature control, containment or other practices. The actual measure of a fish's condition using standard weight is done different ways.
The relative weight (Wr) of an individual fish is its actual weight divided by its standard weight, times 100%. [8] A fish of "normal" weight has a relative weight of 100 percent. The relative weight of a fish does not indicate its health on a continuous scale from 0 -100%, however. For example, Simpkins et al. found that juvenile rainbow trout with a condition index of less than 80% were at a high risk of dying. [9] Relative weight is one of several common measures of condition used in fisheries assessment and management. [10]
Fulton's condition factor, K, is another measure of an individual fish's health that uses standard weight. [11] Proposed by Fulton in 1904, it assumes that the standard weight of a fish is proportional to the cube of its length:
where W is the whole body wet weight in grams and L is the length in centimeters; the factor 100 is used to bring K close to a value of one.
Trout is the common name for a number of species of freshwater fish belonging to the genera Oncorhynchus, Salmo and Salvelinus, all of the subfamily Salmoninae of the family Salmonidae. The word trout is also used as part of the name of some non-salmonid fish such as Cynoscion nebulosus, the spotted seatrout or speckled trout.
Micropterus is a genus of freshwater fish in the sunfish family of order Perciformes. The species of this genus are known as the black bass.
The muskellunge(Esox masquinongy), also known as muskelunge, muscallonge, milliganong, or maskinonge, is a species of large, relatively uncommon freshwater fish native to North America. The muskellunge is the largest member of the pike family, Esocidae. The common name comes from the Ojibwa word maashkinoozhe, meaning "ugly pike", by way of French masque allongé, "elongated face." The French common name is masquinongé or maskinongé.
The largemouth bass is a carnivorous freshwater gamefish in the Centrarchidae (sunfish) family, a species of black bass native to eastern and central United States, adjacent southeastern Canada and northern Mexico, but widely introduced elsewhere. It is known by a variety of regional names, such as the widemouth bass, bigmouth bass, black bass, bucketmouth, largies, Potter's fish, Florida bass, Florida largemouth, green bass, Green trout, gilsdorf bass, Oswego bass, southern largemouth and (paradoxically) northern largemouth, LMB. The largemouth bass is the state fish of Georgia and Mississippi, and the state freshwater fish of Florida and Alabama.
The walleye, also called the yellow pike, is a freshwater perciform fish native to most of Canada and to the Northern United States. It is a North American close relative of the European zander, also known as the pikeperch. The walleye is sometimes called the yellow walleye to distinguish it from the blue walleye, which is a subspecies that was once found in the southern Ontario and Quebec regions, but is now presumed extinct. However, recent genetic analysis of a preserved (frozen) 'blue walleye' sample suggests that the blue and yellow walleye were simply phenotypes within the same species and do not merit separate taxonomic classification.
The northern pike, known simply as a pike in Britain, Ireland, most of Canada, and most parts of the United States, is a species of carnivorous fish of the genus Esox. They are typical of brackish and fresh waters of the Northern Hemisphere.
The burbot is the only gadiform (cod-like) freshwater fish. It is also known as bubbot, mariah, freshwater ling, the lawyer, coney-fish, lingcod, freshwater cusk, and eelpout. The species is closely related to the marine common ling and the cusk. It is the only member of the genus Lota. For some time of the year, the burbot lives under ice, and they require frigid temperatures to breed.
Angling is a method of fishing by means of an "angle". The hook is usually attached to a fishing line and the line is often attached to a fishing rod. Modern fishing rods are usually fitted with a fishing reel that functions as a mechanism for storing, retrieving and paying out the line. Tenkara fishing and cane pole fishing are two techniques that do not use a reel. The hook itself can be dressed with bait, but sometimes a lure, with hooks attached to it, is used in place of a hook and bait. A bite indicator such as a float, and a weight or sinker are sometimes used.
Population dynamics is the branch of life sciences that studies the size and age composition of populations as dynamical systems, and the biological and environmental processes driving them. Example scenarios are ageing populations, population growth, or population decline.
Broodstock, or broodfish, are a group of mature individuals used in aquaculture for breeding purposes. Broodstock can be a population of animals maintained in captivity as a source of replacement for, or enhancement of, seed and fry numbers. These are generally kept in ponds or tanks in which environmental conditions such as photoperiod, temperature and pH are controlled. Such populations often undergo conditioning to ensure maximum fry output. Broodstock can also be sourced from wild populations where they are harvested and held in maturation tanks before their seed is collected for grow-out to market size or the juveniles returned to the sea to supplement natural populations. This method, however, is subject to environmental conditions and can be unreliable seasonally, or annually. Broodstock management can improve seed quality and number through enhanced gonadal development and fecundity.
Cynoscion nebulosus, the spotted seatrout, also known as speckled trout, is a common estuarine fish found in the southern United States along coasts of Gulf of Mexico and the coastal Atlantic Ocean from Maryland to Florida. While most of these fish are caught on shallow, grassy flats, spotted seatrout reside in virtually any inshore waters, from the surf of outside islands to far up coastal rivers, where they often come for shelter during cold weather. Contrary to its name, the spotted seatrout is not a member of the trout family (Salmonidae), but of the drum family (Sciaenidae). It is popular for commercial and especially recreational fishing in coastal waters of the southeastern United States. Adults reach 19-32 inches in length and 3-15 pounds in weight.
Hippoglossus stenolepis, the Pacific halibut, is a species of righteye flounder. This very large species of flatfish is native to the North Pacific and is fished by commercial fisheries, sport fishers, and subsistence fishers.
The American gizzard shad, also known as the "mud shad," is a member of the herring family of fish, and is native to large swaths of fresh and brackish waters of the United States of America. The adult has a deep body, with a silvery-green coloration above fading to plain silver below. The gizzard shad commonly resides in freshwater lakes, reservoirs, rivers, and streams but can reside in brackish waters, as it does on the Atlantic coast of the United States. Their range is across most of the continental United States, although they typically go no further north than New York and no further west than New Mexico. They are large parts of many of the ecosystems they inhabit, and can drive changes in phyto- and zooplankton, thereby indirectly affecting other planktivorous fishes. The gizzard shad has been widely used as a food source for game fish, with varied success in management and effectiveness.
Eagle Lake is a lake at 5,098 ft (1,554 m) elevation in Lassen County approximately 15 mi (24 km) north of Susanville, California. An endorheic alkaline lake, it is the second largest natural lake entirely in the state of California, United States.
The brook stickleback is a small freshwater fish that is distributed across the US and Canada. It grows to a length of about 2 inches. It occupies the northern part of the eastern United States, as well as the southern half of Canada. Small populations are scattered throughout the Mississippi-Great Lakes basin extending to Colorado, New Mexico, Kentucky, Tennessee, etc., though some of these areas are not native to the species. This small fish inhabits clear, cool streams and lakes. They eat small invertebrates, algae, insect larvae, and occasionally their own eggs. They are also preyed upon by smallmouth bass and northern pike. Feeding time is usually dawn and sunset. The brook stickleback does have active competition mostly from minnows, but feeding times are different, along with diet. Spawning occurs in midsummer. Males secure a territory, build a nest, and mate with females. Males provide protection for the eggs, ward off predators, and usually die later in the season. This is considered an annual species. The nests are built out of aquatic grasses. Though the brook stickleback is not considered a threatened species, deforesting and changing waters are altering ecosystems of the species. Harvesting of trees around riparian environments is having a large effect of the stream ecosystem where the brook stickleback resides.
Fish stocking is the practice of raising fish in a hatchery and releasing them into a river, lake, or ocean to supplement existing populations or to create a population where none exists. Stocking may be done for the benefit of commercial, recreational, or tribal fishing, but may also be done to restore or increase a population of threatened or endangered fish in a body of water closed to fishing.
A fishery is an area with an associated fish or aquatic population which is harvested for its commercial or recreational value. Fisheries can be wild or farmed. Population dynamics describes the ways in which a given population grows and shrinks over time, as controlled by birth, death, and migration. It is the basis for understanding changing fishery patterns and issues such as habitat destruction, predation and optimal harvesting rates. The population dynamics of fisheries is used by fisheries scientists to determine sustainable yields.
The condition index in fish is a way to measure the overall health of a fish by comparing its weight with the typical weight of other fish of the same kind and of the same length. The condition index is its actual weight divided by its expected weight, times 100%. A fish of normal weight has a condition index of 100 percent. So if a tarpon, for example, has a condition index of 104 percent, that would mean it is above the normal weight for an average tarpon of that length. If a tarpon has a condition index of 92 percent, that would mean that it is thinner, or below the normal weight of other tarpon that length. The condition index depends on how much a fish is eating compared to the energy it has to spend to live, migrate, reproduce, and do its other activities. The condition index for fish is a simple measurement that can be used to provide important biological information that can then be used to make better management decisions.