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Mark Recovery Laboratory
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The Mark Recovery Lab provides commercial fishery, test fishery, and escapement survey otolith mark recovery data to fishery biologists for salmon stock management. The Mark Recovery Lab also provides otolith mark characteristic reports to evaluate mark application success for hatchery-reared salmon.

Otoliths

An otolith, or ear stone, is a dense structure composed primarily of calcium carbonate and protein that is located within the inner ear of teleost (e.g. bony) fishes that is essential for maintenance of balance and hearing, similar to the bones in the inner ear of mammals. Otoliths differ size and shape among fish species so they are often used for species identification.

In bony fish, there are three pairs of otoliths: the lapilli, asterisci, and sagittae (Figure 1). In Pacific salmon, the asteriscus and lapillus are usually small, only a millimeter in size, but the sagittae are much larger (approximately 5 mm). Consequently, it is the most frequently studied of the three otoliths. Sagittae are often referred to as "otoliths," although this term applies to all three structures.

Otoliths are formed by the incremental and constant accretion of protein and calcium carbonate. This accretion process is sensitive to environmental and physiological change leaving growth “rings” that can be observed within the otolith structure. These growth rings have been used to estimate fish age and to study fish life history. Hatcheries vary water temperature while salmon eggs and fry are incubating to create a unique pattern of growth rings, or “thermal mark”, within the otolith structure. Unique thermal marks are applied to groups of fish, fish from the same broodyear and facility or stock of origin, so that later brood year and stock of origin can be determined for individual fish. Otolith mark recovery data are used by fisheries biologists for salmon stock management and at hatcheries to assess the efficacy of enhancement programs.

Otolith image
Figure 1. Sockeye salmon otoliths: sagitta, lapillus, and asteriscus.