Osteichthyes are a taxonomic grouping of bony fishes. This group includes ray-finned fishes (class: Actinopterygii) and lobe-finned fishes (class: Sarcopterygii). This highly diverse group of fishes, which contains almost all fish species, is the most diverse group of vertebrates today. Osteichthyes differ from chondrichthyes by (in most cases) possessing a bony skeleton, a swim bladder, scales (ctenoid, cycloid, or ganoid scales), and external fertilization.
An oceanodromous fish, like an anadromous or catadromous fish, is a migratory fish. Unlike anadromous or catadromous fish, an oceanodromous fish spends its whole life in salt water. Many oceanodromous fishes are termed highly migratory species (HMS) because of their ocean basin-sized migration routes. Many HMS are high trophic level fish and they migrate in search of food in addition to spawning. Endangered Atlantic Bluefin Tuna (Thunnus thynnus) are one example of a HMS, high trophic level, oceanodromous fish. They spawn in the Gulf of Mexico and the Mediterranean Sea. Tagging studies show that the feed all around the Atlantic basin. Highly prized on the sashimi markets, bluefin spawning aggregations and schooling behavior make them particularly vulnerable to modern fishing techniques.
For more information, check out the “CAN YOU SAY ANADROMOUS, CATADROMOUS, AMPHIDROMOUS, OCEANODROMOUS, OR POTAMODROMOUS?” post on The Fisheries Blog!
The terms overfishing and overfished are confusing because they address a similar subject. The difference between the two terms is subtle but significant.
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.
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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.
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).