Bony fish otoliths are complex polycrystalline bodies which act as organs of balance in the inner ear (Carlstrom, 1963; Gauldie, 1988). The otoliths are primarily composed of crystallized calcium carbonate in the form of aragonite and of a fibrous, collagen-like protein: otoline (Degens et al., 1969; Morales-Nin, 1986 a; 1986 b). Partly or wholly abnormal otoliths made up of calcite are relatively common (Morales-Nin, 1985 a). These crystalline otoliths are transparent and lack clearly defined growth marks.
The otolith grows by the surface deposition of materials, a cyclical process dependent on internal calcium metabolism rates (Simkiss, 1974) and on amino-acid synthesis. The result is the formation of daily growth increments in the otolith, made up of a continuous or incremental unit, and a check unit (Pannella, 1971; 1974; Dunkelberguer et al., 1980). The incremental zone is made up of needle-like aragonite microcrystals surrounded by the organic matrix and laid down across the surface of the otolith. The check zone or unit is mainly made up of concentric shells of organic matter (Mugiya et al., 1981; Watabe et al., 1982; Morales-Nin, 1986 b).
The thickness of the increments and density of the microcrystals depends on the stage of growth (Irie, 1960). In active periods of growth, for example, the increments are thick with well-developed check units and in slow periods the increments are finer and the microcrystals more compact and continuous. Often, there are two or more sub-increments, probably caused by migrations, feeding rates (Pannella, 1974; 1980) and temperature changes (Brothers, 1978; Pannella, 1980; Campana, 1983; Campana and Neilson, 1982; Geffen, 1982, 1983), etc.
As bodily growth and otolith growth are closely linked, the increment thickness will reflect the rate of growth, recording periods of environmental and physiological stress and growth fluctuations caused by age-linked metabolic slowdown (Gutiérrez and Morales-Nin, 1986). Bodily growth and otolith growth do in some cases, however, appear to occur independently (Wright et al., 1990).
The daily deposition of increments depends on circadian endocrine rhythms which are synchronized at an early age with photo-periodicity or other external daily factors (Tanaka et al., 1981; Radtke and Dean, 1982; Campana and Nielson, 1985). The synchronizing stimulus must either not vary in periodicity by more than 2–4 hours from the 24-hour cycle, or else must consist of harmonious multiple 24-hour cycles. Only an environmental factor can act as synchronizer, although other factors may mask or reinforce the endogenous rhythm.
The daily deposition of increments should, at least in theory, allow an extremely precise determination of age. Many authors have used these increases to determine the age of larvae and juveniles (see references in Ré, 1983; Campana and Nielson, 1985; Palomera et al., 1988; Bergstad, 1984; inter al.) and in some studies on adult fish (Darayatne and Gjosaeter, 1986; Ralston and Miyamoto, 1981; Randall, 1961; Morales-Nin and Ralston, 1990; Strushaker and Uchiyama, 1976; Taubert and Colbe, 1977; Uchiyama et al., 1986; Uchiyama and Struchaker, 1981). However, after the first year of life the thinness of the increments and otolith morphology can make interpretation difficult (Morales-Nin, 1988).
The daily periodicity of increments has not been determined in some cases, and the authors have simply assumed that it does occur, daily rhythms being common in marine organisms. This can induce error, and although determining the increment deposition rate at all ages is more work, it is essential if the results are to be considered valid.
In addition to age determination, increments have been used to validate annulae periodicity (Pannella, 1980; Victor and Brothers, 1982), to determine changes in growth (Gutiérrez and Morales-Nin, 1986), to detect life transitions (Radtke, 1984), to estimate recruitment and mortality (Methot, 1981, 1983; Robertson et al., 1988; Thomas, 1983) and in taxonomic studies.
