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OTOLITH MICROSTRUCTURE OF COLORADO SQUAWFISH 115 Results of experiments 2 and 3 apparently con- tradict one another because otolith growth rates did not change during starvation in the former but did in the latter. The smaller (mean TL, 8.2 mm) and younger (6 d posthatch) fish in experiment 3 were likely to show effects of longer starvation periods sooner than the larger (11.1 mm) and older (17 d) fish used in experiment 2 because the youn- ger fish had smaller energy reserves. Otoliths of larger and older Colorado squawfish larvae would likely exhibit reduced growth if fish were starved longer than 6 d. Thorrold and Williams (1989) also suggested that changes in food abundance may affect otolith growth more quickly in smaller, younger larvae than in larger and older juveniles, presumably be- cause larvae deplete energy reserves more quickly. In support of this, Maillet and Checkley (1990) and ~Eckmann and Rey (1987) found that otolith growth rates of small larvae less than 10 d old were reduced almost immediately after starvation. Reduced increment widths were not statistically detectable in larger and older glass fish Ambassis vachelli or chinook salmon for at least 2 weeks after starvation (Molony and Choat 1990; Brad- ford and Geen 1992). Thus, small-bodied life stages or species that are more vulnerable to star- vation (Miller et al. 1988) may display reduced otolith growth more immediately than larger-bod- ied life stages or species due to differences in en- ergy or calcium reserves. Laboratory conditions in experiment 4 simulat- ed environmental changes experienced by wild Colorado squawfish when larvae are transported downstream of spawning areas in the relatively cool, turbid, and food-poor main channel and then move into relatively warm, low-velocity channel margin habitats (backwaters) with higher food abundance (Haines and Tyus 1990; Tyus and Haines 1991). Increased widths of otolith daily increments (result of increased otolith growth) in wild fish may signal the arrival and feeding by Colorado squawfish larvae in backwaters, in which case, they may reveal the effects of different flows on transport rates of larvae from spawning to nurs- ery areas. Similarly, settlement marks in otolitlts of some marine fishes record shifts from a pelagic life style to a benthic one. (Victor 1991). Age of Colorado squawfish larvae was deter- mined by simply counting otolith daily increments. Numbers and patterns of daily increments will be useful for determining the relative importance of cohorts in age-classes and the distributions of hatching dates for populations. However, age de- terminations made for Colorado squaw fish from counting daily otolith increments should incor- porate estimates of error, determined by prediction intervals for size-classes, into analyses. Similarly, growth rate estimates based on fish length at cap- ture and age (e.g., growth/day) should incorporate the uncertainty of age estimates into calculations (Rice 1987). Otolith growth rates in Colorado squawfish de- pended on water temperature, somatic growth rate, food availability, and perhaps many other envi- ronmental factors. Ololith growth rates were af- fected by age- and size-specific responses to star- vation, and by the duration and delayed effects of starvation. Complex patterns of otolith growth, and the non synchrony of otolith and somatic growth during periods of variable food abundance, suggest that accurate back-calculation of daily growth for individual fish larvae and the correla- tion of growth rate changes with time-specific en- vironmental events may be difficult. This is so because most techniques for back-calculating length require a proportional relationship of otolith to somatic growth such that a severe reduction (or increase) in growth would be recorded as corre- spondingly reduced (or increased) otolith growth. Although length back-calculation may yield useful growth rate information for populations rather than for individuals, the bias of these techniques should be evaluated before they are used with Colorado squawfish (Francis 1990; 1995). Delayed reduc- tions of otolith growth after starvation periods (e.g., experiment 3) reflected reduced somatic growth in the history of a fish, so it may be possible to calculate reductions in growth for periods of time longer than a day from otolith growth pat- terns. Changes in otolith microstructure of wild fish that are induced by habitat shifts or changes in food abundance or temperature may also yield insights into processes that affect growth and sur- vival of early life stages of Colorado squawfish (Bestgen 1997). Acknowledgments Funding for this project was provided by the U.S. Bureau of Reclamation, Salt Lake City, Utah, under cooperative agreement 8-FC-40-06460 with the Larval Fish Laboratory (LFL) of Colorado State University, and by the Recovery Implemen- tation Program of the Upper Colorado River Basin. Project administration was facilitated by R. Wil- liams, L. Crist, and R. Muth. Fish embryos and larvae were provided by Dexter National Fish Hatchery and Technology Center, U.S. Fish and