Author: abby

Invasive species

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Lionfish are non-native and invasive to the Caribbean Sea.

Lionfish are non-native and invasive to the Caribbean Sea.

A fish species is considered invasive if its position in an ecosystem negatively impacts other species.  Generally, these are non-native species which have been introduced into a region by humans (either intentionally or unintentionally) with detrimental consequence to resident fish and other aquatic organisms.  However, some argue that a native species can also be considered invasive if humans have altered its natural ecosystem to the point where the species can throw the whole system into disequilibrium.

For further information, please check out a related post on The Fisheries Blog – Blurred lines: Can climate change-induced range expansion qualify a species as invasive?

Non-native species

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Sea lamprey are non-native to the Laurentian Great Lakes.

Sea lamprey are non-native to the Laurentian Great Lakes (photo credit: Great Lakes Fishery Commission).

A fish species is considered non-native to a location if it does not occur naturally there and only is present as a result of direct or unintentional human introduction.  This does not necessarily mean that these fish cannot thrive in their non-native habitat.  In fact, some fish, such as Sea Lamprey (Petromyzon marinus), can be in decline in their native range but can be considered invasive and even imperil native species in their non-native locations.

Native species

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Lake Trout are a native species in the Great Lakes but not lakes in the western U.S.

Lake Trout are a native species in the Great Lakes but not lakes in the western U.S.

A fish species is considered native to a location if it occurs naturally there.  It may have evolved in that region or dispersed and become established there without human assistance.  For example, Lake Trout (Salvelinus namaycush) is native to some locations in North America but is considered non-native in other locations where it has been introduced by people.

Holomictic

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Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Dimictic lakes commonly have turnover in spring and fall with stratification in summer and winter (Nature Times).

Holomictic references the most common type of lake which turns over at least once per year (as opposed to meromictic lakes which are constantly stratified).  This mixing is an important process for maintaining fish and aquatic communities by distributing nutrients and oxygen throughout the lake before stratification occurs again.  There are four categories of holomictic lakes:

  • Oligomictic: mixing is irregular,
  • Monomictic: turnover occurs once a year (most common in polar areas)
  • Dimictic: turnover occurs twice a year (most common in temperate areas), and
  • Polymictic: frequent turnover (most common in tropical areas).

Meromictic

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The Black Sea is the largest meromictic lake (NASA).

The Black Sea is the largest meromictic lake (NASA).

Meromictic references a type of lake which is constantly stratified.  The surface and bottom waters do not ever mix.  In most cases, the bottom layer has very low oxygen levels, where few fish and other organisms can live, restricting them to the surface layer.  Meromictic lakes are uncommon (most lakes are holomictic and turnover at least once per year) and may be formed because they are:

  1. small and unusually deep, or
  2. the lower layer is denser and more saline than the surface layer.

Homocercal

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The rose fish has an even-lobed homocercal tail.

The Rose Fish has an even-lobed homocercal tail.

A homocercal tail is a caudal fin composed of two lobes of equal proportion.  Homocercal tails are the most common caudal fin type in fish but can come in many symmetrical shapes.  A homocercal tail is contrasted with a heterocercal tail which has unequal lobes.

Heterocercal

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The heterocercal tail of a shark is a caudal fin with unequal lobes.

The heterocercal tail of a shark is a caudal fin with unequal lobes.

A heterocercal tail is a caudal fin composed of two asymmetrical lobes.  Often, such as the case in many sharks, the vertebral column passes through the upper lobe, making it the larger of the two lobes.  A heterocercal tail is contrasted with a homocercal tail which has equal lobes.

Lotic

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Darters have evolved to survive in fast moving lotic systems.

Darters have evolved to survive in fast moving lotic systems.

Lotic refers to freshwater ecosystems involving flowing water, such as a river, stream, brook or creek.  Certain fish species are lotic specialists and have evolved to live in higher flow water, such as darters which prefer to live in swift-moving riffles.

Lentic

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Cichlids have radiated in the lotic environment of Lake Victoria.

Cichlids have radiated in the lentic environment of Lake Victoria.

Lentic describes freshwater ecosystems characterized by still water and low flow, such as a lake or pond.  Certain fish species have evolved to become lentic specialists, such as the high diversity of cichlid species found in Lake Victoria.

Scute

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Sturgeon have multiple rows of protective bony scutes.

Sturgeon have multiple rows of protective scutes.

A scute is an external bony plate on the surface of a fish.  Scutes serve a protective function, acting as a body armor for fish against environmental abrasions and even predation. In some fishes, such as Alewife (Alosa pseudoharengus), scutes are a row of scales modified into sharp, protective plates.  In other fishes, like Shovelnose Sturgeon (Scaphirhynchus platorynchus), scutes serve a homologous function but are derived from ossified deposits in the dermis (essentially bone over skin).