Length

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Fish length can be measured by standard, fork, or total length

Fish length can be measured by standard, fork, or total length

The length of a fish is often used for fish population assessments (e.g., length-age and weight-length relationships) and consequently are often used in recreational fishing regulations.  But, just as there is “more than one way to skin a cat,” there is more than one way to measure the length of a fish.  Below are three of the most commonly used metrics for measuring fish length:

  •  Standard length: A fish’s body length from the tip of its nose to end of its last vertebrae.  Standard length includes everything except the caudal fin.  This measure is used for most bony fish for which the last vertebrae is distinguishable.
  • Fork length: The length of a fish from the tip of its nose to the middle caudal fin rays.  This measure is best suited for fish that have forked caudal fins.
  • Total length: The length of a fish from the tip of its nose to the end of the longer lobe of its caudal fin.  This measure is primarily used for fish that have uneven caudal fin lobes, such as hagfish, lampreys, sharks, and rays.

For a comparison of the three length measurements, please review Kahn et al. 2004.

Finlets

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Finlets may reduce turbulence for high-efficiency swimmers

Finlets may reduce turbulence for high-efficiency swimmers

Finlets are highly specialized fins located on the dorsal and ventral sides of the body between the dorsal fin and/or the anal fin and the caudal fin.  They are only found on certain fish including those in the Scombrid family (mackerels, tunas, and bonitos), Scomberesocid family (sauries), snake mackerels (family Gempylidae), and Bichirs (family Polypteridae).  For Bichirs, the finlets are only on the dorsal side and they take the place of a traditional dorsal fin.  For Scombrids and Scomberesocids, finlets are small, rayless, non-retractable fins located on both the dorsal and ventral margins of the body.  Finlets in Scombrids have been evaluated for their contribution to locomotion because these fish are such high-performance swimmers.  Finlets may contribute to dampening of cross-flow turbulence around the caudal peduncle.

Finlets are found behind dorsal and anal fins

Finlets are found behind dorsal and anal fins

Anal fin

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Knifefish use their elongated anal fin for locomotion

Knifefish use their elongated anal fin for locomotion

The anal fin is found on the ventral side of fish, often, but not always, at the base of the anus.  Along with the dorsal fin, the main purpose of the anal fin is stabilize the fish and keep it from rolling in the water.  For knifefish (order Gymnotiformes), which have neither pelvic or dorsal fins, the anal fin has an additional purpose.  It is almost the entire length of their bodies.  Knifefish swim by rippling their anal fin while keeping the rest of their bodies rigid and straight.  As their primary form of locomotion, knifefish can swim backwards as easily as forward.

Snakeheads (family Channidae) also have an elongated anal fin.  Not native to the US, snakeheads were found in a pond in Maryland in 2002.  They are now permanently established in the Potomac River as an invasive species.  Snakeheads are food fish in their native range, so the introductions to the US were likely intentional.  Snakeheads are an ecological concern because they are top-level predators with no natural predators in US.  They also can survive on land for up to four days (as long as they are still wet) and can “walk” up to a quarter mile on wet land to find other habitable water bodies.

Snakeheads are invasive to the Potomac River

Snakeheads are invasive to the Potomac River

Pelvic fins

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The pelvic fins are paired fins found on the ventral (bottom) side of fish.  In teleost (bony) fishes, pelvic fins placement gives some indication of evolution.  For more basal groups, the pelvic fins are located at mid-body in the abdominal region.  For more derived fishes, the pelvic fins are located in a thoracic or even jugular (throat) position.

Male guppies have specialized pelvic fins

Male guppies have specialized pelvic fins

Some fish, such as Guppies (Poecilia reticulate) exhibit sexually dimorphic pelvic fins, where the males and females do not have the same fins.  The pelvic fin’s first and second rays are significantly shorter on the male Guppy than the Female guppy.  Research suggests that this specialization may assist with reproduction.

Other fish, like the Devil’s Hole Pupfish (Cyprinodon diabolis) lack pelvic fins entirely.  At less than one inch long, the Devil’s Hole Pupfish is the smallest of the desert pupfish species and one of the world’s rarest species (only 65 fish were counted in a Fall 2013 survey).  It is only found in one location on earth, Devil’s Hole, Nevada, and has evolved over thousands of years to survive in its harsh 93oC, low oxygen waters.

Devil's Hole Pupfish lack pelvic fins

Devil’s Hole Pupfish lack pelvic fins

Pectoral fins

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Pectoral fins are on the sides of the body

Pectoral fins are on the sides of the body

The pectoral fins are the two fins located on the sides of a fish (or marine mammal).  These fins are primarily responsible for control of directional movement, up and down or side to side.  Pectoral fins can come in all shapes and sizes which fill different functions for different fish.  The pectoral fins of a Coelacanth (Latimeria chalumnae) have a wide range of motion and are capable of “sculling” like the oar of a boat.  These specialized pectoral fins are useful for making small correction movements to maintain a Coelacanth’s position, hovering just off the ocean floor.  The pectoral fins for Bluefin Tuna (Thunnus thynnus) are retractable; they fit into slots so that when they are retracted, they are flush with the side of the fish.  For a fast-moving fish in open oceans where it doesn’t often have to change direction quickly, this feature is highly efficient – it reduces drag and saves energy.  Some flatfish, like the Hogchoker (Achirus fasciatus), lose their pectoral fins all together.  Their highly derived body shape and life history eliminates the traditional role for pectoral fins.

Coelacanths scull with their pectoral fins

Coelacanths scull with their pectoral fins

Dorsal fin

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The dorsal fin is a stabilizer fin

The dorsal fin is a stabilizer fin

The dorsal fin is located on the top of a fish (as well as some marine mammals).  Predominately, the fin is used to stabilize fish in the water and help direct the fish through turns and stops.  Some fish, like Striped Bass (Morone saxatilis), have two dorsal fins and others have very highly derived dorsal fins, such as the Ocean Sunfish (Mola mola) which uses its dorsal fin as a primary mode of locomotion.

The Ocean Sunfish has a highly modified dorsal fin

The Ocean Sunfish has a highly modified dorsal fin

The dorsal fin of sharks is one of the fins targeted in shark finning operations and destined for shark fin soup and traditional Chinese medicines.  Shark fin soup is a delicacy in the Chinese culture, served at special occasions, including weddings.  As China has become more affluent, the demand for luxury items, such as shark fin soup, has dramatically risen.

Dorsal fins of sharks are targeted for shark fin soup

Dorsal fins of sharks are targeted for shark fin soup

Caudal fin

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A caudal fin graced the Summer 2012 cover of Eddies magazine (fws.gov/eddies)

A caudal fin graced the Summer 2012 cover of Eddies magazine (fws.gov/eddies)

The caudal fin, or tail, of a fish is the only fin to be connected to the vertebral column.  It is the primary means of locomotion for most fish.  Unlike many marine mammals with tails that use an up-and-down motion, fish generally use a side-to-side thrust of their caudal fin for propulsion.  The shape of the caudal fin can be indicative of the style of motion for a fish.  For example, very fast swimming fish like tunas have lunate caudal fins for more efficient swimming while lie-in-wait predators like grouper have a strong, wide base of the tail, the caudal peduncle, for faster acceleration.

Caudal fin come in many shapes and sizes; form often follows function

Caudal fins come in many shapes and sizes; form often follows function

Adipose fin

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The adipose fin may serve as a "precaudal flow sensor"

The adipose fin may serve as a “precaudal flow sensor”

The adipose fin is a small fleshy fin found posterior to the dorsal fin and anterior of the caudal fin.  It is only found on few fish, including trout, salmon, and catfish.  When it was named, it was thought to hold fat, or adipose, tissue.  Once this hypothesis was disproved, fisheries biologists long considered this fin to be “non-functional.”  More recently, research suggests that the adipose fin may serve as a “precaudal flow sensor” to improve maneuverability in turbulent waters.  This new research may raise concerns because adipose fin clipping is commonly used to “tag” hatchery reared salmon to distinguish them from wild salmon for catch and release management purposes.

For more information on the adipose fin, please visit: http://thefisheriesblog.com/2013/05/28/the-adipose-fin-old-mysteries-with-new-answers/

Pelagic zone

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The pelagic zone is divided into layers based on light

The pelagic zone is divided into layers based on light

The pelagic zone is the region of a body of water (lake, river, or ocean) that is not associated with the bottom (see benthic zone) or shore (see littoral zone).  This habitat zone is truly a three dimensional habitat space.  Some fish that occupy the pelagic zone never encounter the bottom or shore throughout their lives.  Because the pelagic zone is a nutrient poor environment, large fish have two basic strategies to get meals – either swim long distances in search of nutrient-rich prey (like many oceanic sharks and tunas) or drift with currents and eat nutrient-poor prey (like the Ocean Sunfish Mola mola).  The pelagic zone is divided into zones based on light penetration from the surface:

  • Epipelagic (sunlight; 0-200m)
  • Mesopelagic (twilight; 200-1,000m)
  • Bathypelagic (no light; 1,000-4,000m)
  • Abyssopelagic (4,000m – ocean floor)
  • Hadopelagic (deep sea trenches)

Benthic zone

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The benthic zone is region of a body of water (lake, river, or ocean) that is near the bottom.  It includes the surface and some of the sub-surface layers of the sediment.  The sediment can sand, mud, rocks, coral, among other substances.  Benthos are organisms living in the benthic zone.  These organisms are predominately invertebrates, but the benthic community is very important to some fish species.  For example, the tripod fish (Bathypterois grallator) is a deep-sea benthic fish that uses modified fins and fin rays to stand on the ocean floor and wait for prey items to approach.  These fish can grow up to 3 feet long!

Tripod fish use the benthic zone to feed

Tripod fish use the benthic zone to feed