Electrofishing

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Electrofishing is a survey technique that stuns fish in freshwater

Electrofishing is a survey technique that stuns fish in freshwater

Electrofishing is a survey tool used to estimate species composition, fish abundance, and fish density in freshwater systems using an electrical current.  An electroshocker creates a flow of current between an anode (i.e., ring on a backpack models) and cathode (i.e., rat tail on backpack models) that stuns fish that cross the electric field.  Stunning the fish makes them easier to capture and assess for research and monitoring purposes.  Backpack electroshockers are generally used for smaller, wadeable waterbodies; boat or raft mounted electroshockers are generally used for larger waterbodies.  While electrofishing can be a very effective technique for freshwater, electricity has poor conductivity in salt water.

For an example of boat electrofishing, check out this depletion electrofishing post on the Fisheries Blog!

Schreckstoff

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This piranha may look scary, but when it's scared, it will release Schreckstoff

This piranha may look scary, but when it’s scared, it will release Schreckstoff

Happy Halloween!

Schreckstoff, German for “scary stuff,” is a chemical compound, glycosaminoglycan chondroitin, that some fish species release when they are injured.  This “take-one -for-the-team” signal lets their conspecifics know that there is danger in the area.  But more than just an alarm signal, Schreckstoff also serves as an immune response for the injured individual, warding off parasites and pathogens such as molds, trematodes, and solar radiation.  Schreckstoff is utilized by fish superorder, Ostariophysi, such as minnows, catfishes, and characiformes (including piranha and tetras), and has also been documented in other species, such as salmonids.

For more information, please visit: The Fisheries Blog‘s post on “Scary Stuff!“

Meristics and Morphometrics

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Meristic and morphometric traits are used to identify fish

Meristic and morphometric traits are used to identify fish

A meristic is a countable trait, such as number of gill rakers or number of dorsal fin spines.  Morphometrics examines the size and shape using a measurable trait, such as standard length or wet weight, which can be gauged as a length, mass, angle or ratio of other measurements.  Meristic traits and morphometrics are often used to classify taxa, sometimes down to the species level or sub-species level.  In dichotomous keys, these counts and measurements can help identify a particular species of fish.  Prior to modern genetic techniques, meristics and morphometrics were the principal foundation for fish taxonomy and systematics.  Even today, meristics and morphometrics are commonly used for species identification and ground-truthing genetic analyses with phenotypic traits.

Habitat

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Fish out of water! Mudskippers have a very unique habitat

Fish out of water! Mudskippers have a very unique habitat

Habitat, is simply, the location where a fish lives.  It applies to any and all life stages.  It is where a fish survives, feeds, grows, and reproduces.  The habitat of a fish depends on the species, from wetlands, to rivers, to coral reefs, to lakes, and more.

Most fish do require well-oxygenated water.  But, there are a few exceptions in which fish have evolved to tolerate low oxygen conditions or even low water conditions.  Mudskippers (family Gobiidae; subfamily Oxudercinae), for example, are amphibious fish that are adapted to intertidal zones.  Some live in mud flat habitat that is only covered in water at high tides.

Habitat is where it’s at!

For more information, please visit:

Recruitment

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In fisheries, recruitment is the number of fish that survive to enter a fishery

In fisheries, recruitment is the number of fish that survive to enter a fishery

In fisheries, recruitment refers to the number of fish surviving to enter a fishery.  These fish have to pass through a number of life history stages (e.g., egg, larva, juvenile, etc.) before becoming vulnerable to fishing gear.

 

Understanding recruitment dynamics is a very complex process – dependent upon the spawning stock biomass and environmental factors.  In some species, recruitment is density dependent; in many of these cases, a larger number of spawners will produce fewer recruits per individual because of competition between larval fish, cannibalism by adults, and other factors.  In many species, 99% of mortality occurs at the egg stage.  This is just one of the many things that make it difficult to forecast population assessments for fish species.

Overfishing, overfished

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The terms overfishing and overfished are confusing because they address a similar subject.  The difference between the two terms is subtle but significant.

Status of overfishing and overfished species monitored by the National Oceanic and Atmospheric Administration (NOAA)

Status of overfishing and overfished species monitored by the National Oceanic and Atmospheric Administration (NOAA)

Overfishing refers to the current fishing rate which results in a higher harvest, or fishing mortality rate, than maximum sustainable yield (the maximum harvest level without negatively impacting the sustainability of the stock).  Overfishing is generally divided into two classes:

  • growth overfishing: where fish are harvested at a size smaller than would produce maximum yield per fish.
  • recruitment overfishing: where the reproductive capacity of a stock is diminished to a point where the spawning stock biomass is not sufficient to maintain the sustainability of a stock.

Overfished, on the other hand, is the state of a stock upon which overfishing has occurred.  The stock is no longer able to produce at a maximum sustainable yield.  It is important to note that a stock may be overfished, but overfishing may not be occurring.  Stocks that are overfished can be managed for fishing pressure that is low enough to allow the stock to rebuild to a level to support maximum sustainable yield.

For more information, please visit:

  • http://www.fishwatch.gov/features/overfishing_overfished_same_thing.htm
  • http://www.nmfs.noaa.gov/stories/2013/05/05_02_13status_of_stocks_2012.html

Teleost

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A Slingjaw Wrasse is an extreme example of a teleost's protrusible jaw

A Slingjaw Wrasse is an extreme example of a teleost’s protrusible jaw.

Teleosts are the most diverse group of fishes (over 26,500 extant species).  Over half of all living vertebrate species are teleosts.  Teleosts are characterized by a protrusible jaw (musculature gives them the ability to move their maxilla and premaxilla) and a symmetrical tail (their spine that ends at the caudal peduncle unlike, for example, sharks).  Teleosts are estimated to have evolved during the Triassic period. By the end of the Cretaceous, the fossil record shows that teleosts dominated both freshwater and marine habitats.

Caudal peduncle

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The caudal peduncle is the narrow region of a fish's body anterior of the caudal fin.

The caudal peduncle is the narrow region of a fish’s body anterior of the caudal fin

The caudal peduncle is the tapered region behind the dorsal and anal fins where the caudal fin attaches to the body.  The depth of the caudal peduncle, which is measured at its narrowest point, gives some indication of the power of a fish and the speed at which it can swim.  For example, ambush predators, like barracudas or gars, have a caudal peduncle that is not much narrower than their torpedo-shaped bodies.  They can lie-in-wait and then give a few powerful thrusts of their tail to surprise a prey fish.  Other very fast swimming, powerful fish, like tunas and mackerels, have a very narrow caudal peduncle.  They can even have keels, like those on a boat, to help support and stabilize the caudal fin and make swimming far and fast more efficient.

A narrow caudal peduncle is common in fast, far-ranging fish

A narrow caudal peduncle is common in fast, far-ranging fish

Operculum

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The operculum is a bony flap covering the gills of a teleost fish

The operculum is a bony flap covering the gills of a teleost fish

The operculum is a hard, plate-like, bony flap that covers the gills of a bony fish (superclass: Osteichthyes).  It protects the gills and also serves a role respiration.  Fish can acquire dissolved oxygen through pumping water over their gills by opening and closing their jaws and opercula.  The water is flushed from the fish’s mouth over the gills where blood inside capillaries is able to absorb the dissolved oxygen and out the body behind the opercula.

The posterior margin of the operculum is generally used in morphometrics to divide the head and body.  The operculum shape varies greatly from species to species.  Sunfish (family: Centrarchidae) are known to have particularly prominent opercula and a few species have common names include a reference to their “ear,” or operculum.

The Redear Sunfish is known for its conspicuous operculum

The Redear Sunfish is known for its conspicuous operculum

TED (Turtle Excluder Device)

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TEDs significantly reduce turtle bycatch

TEDs significantly reduce turtle bycatch

Turtle Excluder Devices (TEDs) are a specific type of bycatch reduction device (BRD) that are a modification to a trawl net to keep turtles, which are not targets of the fishery, from being caught in the net.  TEDs are generally composed of a set of bars over the trawl net.  While small target animals, such as shrimp, pass easily between the bars into the trawl net, larger animals, such as turtles or even sharks, can escape through a side opening in the net after they have hit the grid of bars.  TEDs were developed to eliminate turtle mortality from trawl fisheries.  Because turtles and other bycatch, or non-target catch, species often come from threatened or endangered populations, the fisheries had strict regulations on how much bycatch was allowed.   TEDs, by reducing bycatch, allows the shrimp trawls to operate more with more ecological and economic efficiency.

For more information, please visit: http://www.fisheries.noaa.gov/pr/species/turtles/teds.htm