Gill net

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Gill net (image credit: Michigan Sea Grant).

A gill net is a simple type of fishing gear that is staked to a substrate to create a wall of mesh.  Gill nets are often made of a material, such as monofilament nylon, which is practically invisible to fish attempting to swim by.  As the name implies, fish are often caught by the gills because these nets are designed for the head of a fish to fit through but not its body.

Management strategies to reduce bycatch in gill nets include regulating the mesh size so that the net openings are large enough, for example, for undersized fish to swim through unharmed but for legal-sized fish to be harvested.

For an example of how this fishing gear is used, please visit “Know Your Nets” from Michigan Sea Grant.

Tubercles

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Breeding tubercles (photo credit: G. Engman and P. Cooney).

Tubercles are skin nodules made of keratin, the same materials as hair, hooves, and fingernails.  They are present on species representing at least 15 families of fishes, including even pet goldfish.  In many species, tubercles are found only on males during the breeding season and are shed shortly there after.  They are often called breeding tubercles for that reason.  The main functions for tubercles include species recognition, fighting and defense of spawning territory or nests, and stimulation of breeding females.

For more information, please see “Breeding tubercles and contact organs in fishes: their occurrence, structure, and significance” (Wiley and Collette 1970) and the related Fisheries Blog post: “Top 10 Weirdest Things Found on a Fish’s Head.”

BRD

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A shrimp net with a mesh-panel, bycatch reduction device (BRD) and grill-type, turtle excluder device (TED; photo credit: NOAA).

BRD is an acronym for bycatch reduction device.  Bycatch is non-target catch within any fishery.

For certain fisheries, these modifications to the gear (i.e., BRDs) can help lower the amount of bycatch.  For example, a turtle excluder device (TED) is a specific type of BRD designed to help turtles escape trawl nets.  Especially in cases where fisheries have bycatch quotas, the use of BRDs can often allow fisheries to operate longer, and more profitably, because the quotas are not exceeded quickly.

BRDs are generally designed with the cooperation of the fishery – they can be continually modified and improved to help ensure their effectiveness without compromising the productivity of the fishery – which is in the best interests of the industry and conservation, alike.

Population dynamics

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Fish population dynamics (image credit: Katherine Nicholson, slideplayer.com)

Fish population dynamics is the study of change in a fish population over time.  Fundamentally, a population size after some time interval equates to the population size before that interval plus births (i.e., recruitment) and immigration and minus mortality and emigration.  Many fish populations follow a logistic pattern of density-dependent growth.  Beginning with a population size where space and food are not limiting, the population grows rapidly in an exponential pattern; at a certain population density, population growth slows and stabilizes at a given carrying capacity.

Density-dependent factors governing fish population dynamics include competition, predation, disease, and parasitism.  Fish population dynamics can also be driven by density-independent abiotic factors such as temperature, dissolved oxygen, and water chemistry.

In fisheries, these basic population dynamics principles can be applied to harvest strategies to estimate maximum sustainable yield (MSY) with the goal to prevent overfishing.

Exclusive Economic Zone (EEZ)

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Exclusive economic zone of the United States (image credit: NOAA).

An Exclusive Economic Zone (EEZ), as defined by the United Nations Convention on the Law of the Sea, is a 200 nautical mile jurisdiction granted to coastal nations for “sovereign rights for the purpose of exploring and exploiting, conserving and managing the natural resources, whether living or non-living, of the waters superjacent to the seabed and of the seabed and its subsoil, and with regard to other activities for the economic exploitation and exploration of the zone.”  Within an EEZ, a country claims exclusive rights to manage fisheries and prevent overfishing.  While the outer 200 mile boundary determines a country’s sovereign limits, within the U.S., most coastal states claim an inner three nautical mile zone from the shore as state waters (some places claim nine nautical miles).  For fisheries, such as defined under the Magnuson-Stevens Fishery Conservation and Management Act, management in the inner zone belongs to the states and the remainder of the EEZ is under purview of the federal government (e.g., the National Oceanic and Atmospheric Administration).

Maxium sustainable yield (MSY)

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Optimizing fishing effort (Diagram credit: http://ib.bioninja.com.au).

Maximum sustainable yield (MSY) is a classic concept from fish population dynamics.  It operates under the assumption that populations have a carrying capacity, or equilibrium size, where the the resources available match the population requirements and if individuals are removed from the population, they will be replaced (i.e., are renewable).  When harvest removes a portion of the population, it  lowers pressures on the population’s associated resources, such as food and habitat, allowing the remainder of the population to benefit from less competition and balance the population back to the carrying capacity.  By this theory, the surplus biomass can be harvested sustainably, without long-term impacts to the population.

MSY is a commonly applied fisheries management tool, and even a guiding principle of the Magnuson-Stevens Fishery Conservation and Management Act, the primary legislation governing marine fisheries management in United States federal waters.  However, it is important to note that there are many situations in which the assumptions of MSY are not met which result in overfishing and unsustainable practices.

Gill rakers

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Gill rakers (above gill arch) from an American Shad Alosa sapidissima are not to be confused with gill filaments (below); they are particularly important for filter-feeding fish (photo credit: NOAA).

While gill rakers have no role in gas exchange, the predominant function of gills,  they do perform an equally important function for filter-feeding fish – food acquisition.  These bony projections serve as a sieve to trap food particles.  They vary greatly in length and number and these characteristic differences have evolved with different feeding strategies for fish.  A plankton feeder, for example, such as American Shad Alosa sapidissima, has very tightly-packed, comb-like, gill rakers to efficiently filter their food from the water column. An omnivore or piscivore, on the other hand, has shorter, more widely-spaced gill rakers, better for larger prey items.  In fact, the morphology of gill rakers is so diverse that they are often used as a taxonomic tool to identify and classify fish species (e.g., gill raker counts can differentiate species on a dichotomous key).

Escapement

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This spawned out Chinook Salmon contributes to the escapement estimate for its particular run.

Escapement is a term used in salmon management to mean, quite simply, how many salmon are able to “escape” premature death and complete their full life cycle.  These escapees have run the gauntlet of natural mortality, recreational fishing, commercial fishing, and other life-threatening episodes for the opportunity to spawn in freshwater.  Escapement is estimated by a number of different methods including fish counts (e.g., at a weir or dam viewing window) and carcass surveys (i.e., counting the dead salmon on spawning grounds).  For salmon managers, the amount of escapement for each salmon run informs population estimates and appropriate fishing quotas and other management strategies for future fishing seasons.

Gills

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The underside of a Walleye’s gills (photo credit: USFWS).

Someone may tell you that you look “green around the gills” when you look ill but the expression doesn’t translate well to fish gills…

No matter if a fish is sick or not, fish gills are often red – blood red, to be exact.  That is because gills have blood vessels very close to their external surface.  As the primary mode of gas exchange for most fish (but not all!), gills absorb oxygen from water that passes over them and release carbon dioxide usually facilitated through a counter-current exchange of blood and water flowing in opposite directions.  Gill lamellae provide increased surface area for gas exchange, which is particularly important because there is less dissolved oxygen in water than there is in air.  For fish that breathe air, lungs and skin provide alternative methods for oxygen absorption.

Trophic levels

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Marine trophic levels (diagram credit: University of Waikato).

Trophic levels are a defined by the position of organisms in a food chain or food web.  The different levels help define the flow of energy within an ecosystem.  The lowest trophic level is primary producers, such as algae and phytoplankton, which generate their own energy from the sun via photosynthesis.  Primary consumers, such as herbivorous zooplankton, must eat primary producers as their source of energy.  As a general rule, each trophic level up equates to capturing 10% of the energy from the trophic level below it (e.g., a juvenile fish uses 10% of the energy it gets from eating zooplankton and only 1% of the energy the zooplankton gets from eating phytoplankton).

Many food webs contain organisms, such as fish that eat other fish and zooplankton, that do not fall into simple trophic level constructs.  Stable isotope analyses can be used to infer diet and trophic level for these complex diets by comparing isotopic signatures in tissue from the organisms of interest to those from samples of known origin.  Commonly used isotopes for aquatic ecosystem food web analyses include 13C, 15N and 34S.