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295 factors. Serns (1982a, b) showed a similar relation between growth (as well as year-class strength) of age-0 walleye, Stizostedion vitreum, and small- mouth bass, M. dolomieui, and aspects of the an- nual water temperature regime. Water temper- ature is a cue for spawning of temperate-zone fish- es. Colorado squawfish begin spawning when tem- peratures reach 20-22° C (Hamman 1981, Tyus & McAda 1984, Haynes et a1.1984) -normally during July or August in the upper basin (Fig. 3) -and embryos hatch 4-5 days later (Hamman 1981). As the relative suitability of the temperature regime increases, the date when spawning temperatures are achieved advances and the length of the grow- ing season increases (Fig. 3). In the historic lower Colorado River, spawning temperatures were reached in early May (Fig. 3) and young fish had most of the longer growing season available for first-year growth. Although its precise causal fac- tors are unknown, the relation shown in Figure 4 provides a perspective for the much larger differ- ences in age-0 growth that probably occurred his- torically between the upper and lower basins. In the upstream regions of historic range that constitute the remaining habitat of Colorado squawfish, the small size of the age-0 fish going into winter might be an important factor affecting recruitment to the adult stock. Studies onage-0 fish have shown that overwinter survival is directly re- lated to fish size (Toneys & Coble 1979, Oliver et al. 1979, Shuter et al. 1980), and that first-year growth can directly affect adult year-class strength in smallmouth bass and largemouth bass (Shuter et al. 1980, Gutreuter & Anderson 1985). The largest age-0 largemouth bass most often recruit to the adult stock (Gutreuter & Anderson 1985 and refer- ences therein). If a similar relation holds true for Colorado squawfish in the upper basin, the more frequent occurrence in the Green River than in the Colorado of comparatively large age-0 young in fall (Fig. 4) might explain the relatively large adult squawfish stock of the Green (Holden &Stalnaker 1975, USFWS, unpublished data). But examples probably exist of species that live under conditions well below their optimum for growth -where age-0 growth is slow and subse- quentrecruitment to the adult stock might be quite restricted -yet their populations are large. Our simulations provide insight into why this may no longer be the case for Colorado squawfish, a spe- cies presumed to have formerly had large pop- ulations in the upper basin. They show how slow growth in the upper basin can make Colorado squawfish there especially vulnerable to the effects of increased early-life mortality. Introduced fish species and other habitat manipulations of techn- ologicman have doubtless contributed to increased early-life mortality of Colorado squawfish, though the precise nature of these negative interactions and their relative importance is unknown. River reaches inhabited by Colorado squawfish have been successfully colonized by numerous intro- ducedspecies, including piscivores such as channel catfish, Ictalurus punctatus, green sunfish, Lep- omis cyanellus, and largemouth bass (Holden & Stalnaker 1975, Tyus et al. 1982). Green sunfish, for example, can greatly suppress native cyprinid populations in rivers (Lernly 1985). Our simulations also show how an increase in early-life mortality can reduce both the number of age groups (eliminating the oldest, largest fish) and the relative abundance of those that remain (Table 1) -changes that have indeed occurred in the up- per-basin Colorado squawfish population. But why are the natural compensatory mechanisms that oc- cur in populations of numerous other species not operating to offset the effects of increased early-life mortality on the Colorado squawfish population (e.g. McFadden 1977)? Most common among these are the related responses of increased growth and fecundity in the fish that escape early-life mortal- ity. Each of these would have the effect of in- creasing the potential for population growth. Such compensatory responses would be anticipated if competition for resources had a limiting effect on growth and fecundity. However, if growth of Col- orado squawfish in the upper basin is ultimately limited by the relative scarcity of optimal temper- atures - a resource whose availability to individual fish is independent of population density -such compensatory mechanisms would not be oper- ative. The capacity for compensatory responses in a population might be severely limited when condi- tions for growth of individual fish are appreciably