Abstract
Analysis of the alternating opaque and translucent zones in fish otoliths is a widely used method to determine age in fish. The mechanisms underlying the annual periodicities in the formation of these zones remain unknown, although various interpretations and explanations have been presented to explain the appearance of the opaque zones. Here I review the biological and structural characteristics of the opaque zones in otoliths from a number of fish species. The results of this review suggest that the opaque zones can be classified into four different types. Type A is a dark opaque zone, displaying minute, dense crystals, and is typically formed at a young life-history stage. Type B comprises an aggregation of grooves and discontinuous crystals that are formed during growth-stagnant periods. Type C can be described as a washy black zone that has the appearance of an object that is smeared with ink; it is formed during seasons of active growth. Type D has deep grooves, appears luminous in transmitted light in etched otolith sections, and is formed during the spawning season. Types A and C otolith opaque zones are typical of younger fish and are formed during growing periods. They are complementary to Types B and D otolith opaque zones which are formed during periods of stagnant growth and/or during the spawning seasons and which are typical of older fish, which generally lack Type A zones. To ensure precise age determination in fish, it is therefore necessary to understand the structural and biological characteristics that produce these four distinct types of opaque zones.
Highlights
Age determination is an essential component of fishery assessment and fish population analysis, as well as important to a better understanding of animal life history and biology
Type A is a dark opaque zone, displaying minute, dense crystals, and is typically formed at a young life-history stage; Type B comprises an aggregation of grooves and discontinuous crystals that are formed during growth-stagnant periods; Type C has the appearance of an object that is smeared with ink; it is formed during seasons of active growth; Type D has deep grooves, appears luminous in transmitted light in etched otolith sections, and is formed during the spawning season
These four types of otolith opaque zone are described in detail
Summary
Age determination is an essential component of fishery assessment and fish population analysis, as well as important to a better understanding of animal life history and biology. Detailed information on age is required for life-history tables which document an organism’s mortality (or survival) and reproduction rate (maturation rate) as a function of age. Molting times are utilized in life-history tables for some crustaceans or insects, fisheries generally use the age-dependent year. Age information is required the aim is to examine the growth patterns of fish. Fisheries researchers generally express the relationship of body length to age using growth curves and/or growth formulas. The Bertalanffy growth formula, Gompertz formula, logistic formula, and other sigmoid formulas relate regressed body
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