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.
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.
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 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.
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.
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.
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.
Cycloid scales are smooth-edged scales predominately found in lower order teleost fishes, such as salmon, carp and other soft fin rayed fish. Similar to ctenoid scales, they are overlapping which allow 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.
Otoliths are ear bones in fish. Bony fish (not sharks or rays) have three pairs of otoliths:
- Sagitta: detects sound and converts sound waves into electrical signals (i.e., hearing);
- Asteriscus: detects sound and is involved in hearing; and,
- Lapillus: detects gravitational force and sound.
Sagittal otoliths are sometimes used for aging fish because they add “growth rings,” similar to tree rings for periods of faster and slower growth. Before using these growth rings, or annuli, as an age estimate, fisheries scientists must validate that each annulus is equivalent to an annual ring. There are a number of ways to do this, including raising a fish in an experimental setting, “tagging” an otolith with a fluorescent dye at a known age, and marginal increment analysis (measuring the distance from the last annulus to the edge of the otolith at different months during the year; if the distance peaks only once a year, the annulus is a yearly measure).
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.