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

Bycatch

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1280px-Shrimp_bycatch

Shrimp fisheries have historically had very high bycatch

Bycatch is non-target catch within any fishery.  Bycatch can be either different species from the target of the fishery or individuals from the target species, generally juveniles, which are not of a marketable size.  Most bycatch is tossed back because it is illegal to possess or it has no commercial value.  In certain fisheries, however, a certain amount of marketable bycatch can be kept to sell.  Including bycatch reduction devices (BRDs) that have been specifically developed for some fisheries can have significant impacts on the amount of bycatch and/or bycatch survival.  In cases where fisheries have bycatch quotas, the use of BRDs, such as Turtle Excluder Devices (TEDs), can often allow fisheries to operate longer because the quotas are not exceeded quickly.

Because bycatch is difficult to quantify, it makes it very difficult to assess the population status of bycaught species.  Overfishing for these species is sometimes not recognized until the population is severely overfished.  Including bycatch in stock assessment techniques is helping to improve sustainability of these species.

FAD (Fish Aggregating Device)

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Sophisticated fish aggregating devices can include their own moorings

Sophisticated fish aggregating devices can include their own moorings

Fish Aggregating Devices (FADs) are man-made objects that are used to attract fish and facilitate their capture.  FADs can range from crude wooden rafts tied to navigational buoys to sophisticated anchored systems.

These devices are very effective at attracting fish that favor submerged objects.  Prey fish use them for shelter and predatory fish follow the prey fish.  As a result, whole fish communities can develop around them and make them reliable fishing grounds.  Pelagic fish, such as tunas, billfish, dolphin fish, sardines, and sharks, have all been known to frequent FADs.  While they are predominately used to increase fishing productivity, FADs have also been used to support research on fish behavior and used within fisheries management strategies.

A crude wooden raft can still serve to attract fish

A crude wooden raft can still serve to attract fish

 

Ganoid scales

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Diamond-shaped ganoid scales form an interlocking protective armor for fish.

Diamond-shaped ganoid scales form an interlocking protective armor for fish

Ganoid scales are dimond-shaped scales found in lower order fishes such as the bichirs (Polypteridae), Bowfin (Amia calva), paddlefishes (Polyodontidae), gars (Lepisosteidae), and sturgeons (Acipenseridae).  Unlike ctenoid or cycloid scales, ganoid scales are comprised of bone.  They have a bony basal layer, a layer of dentin (also found in human teeth), and an outer layer of ganoine which is the inorganic bone salt for which these scales are named.  These scales interlock with peg-and-socket joints which make them quite inflexible, compared with ctenoid or cycloid scales, but very protective.

Spotted Gar have ganoid scales

Spotted Gar have ganoid scales

Ctenoid scales

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Ctenoid scales are characterized by small teeth on their posterior margin

Ctenoid scales are characterized by small teeth on their posterior margin

Ctenoid scales are scales with comb-like edge found in higher order teleost fishes, such as perch and sunfish.  Cteni are the tiny teeth on the posterior margin of the scale.  Similar to cycloid scales, they are overlapping which allows for greater flexibility in movement than other types of scales such as ganoid scales.  The surface layer of the scale is comprised of calcium-based salts and the inner layer is predominately collagen.  As a fish grows, its scales grow, adding concentric layers, similar to tree rings.  For certain species, these rings can be counted to estimate the age of a fish.