Latitudinal populations of the Atlantic silverside, Menidia menidia, show substantial genetic variation in rates of energy acquistion and allocation. Reared in common environments, silversides from northern latitudes consume more food, grow faster and more efficiently, store more energy, and produce greater quantities of eggs than their southern conspecifics. The persistence of seemingly inferior southern genotypes in the face of ostensibly superior northern genotypes suggest that there are hidden evolutionary trade-offs associated with these elevated acquisition and allocation rates. We tested the hypothesis that rapid growth and high levels of food consumption trade-off against locomotory performance in M. menidia. We compared both aerobic (prolonged and endurance) and anaerobic (burst) swimming capacities between intrinsically fast-growing fish from the north (Nova Scotia, NS) and intrinsically slow-growing fish from the south (South Carolina, SC) and between growth-manipulated phenotypes within each population. We also compared swimming speeds and endurance between fasted and recently fed fish within populations. Maximum prolonged and burst swimming speeds of NS fish were significantly lower than those of SC fish, and swimming speeds of fast-growing phenotypes were lower than those of slow-growing phenotypes within populations. Fed fish had lower burst speeds and less endurance than fasted fish from the same population. Thus, high rates of growth and the consumption of large meals clearly diminish swimming performance, which likely increases vulnerability to predation and decreases survival and relative fitness. The submaximal growth rate of southern M. menidia appears to be adaptive, resulting from balancing selection on rates of somatic growth.

Abstract. The Atlantic silverside (Menidia menidia) exhibits countergradient latitudinal variation in somatic growth rate along the East Coast of North America. Larvae and juveniles from high-latitude populations display higher intrinsic rates of energy consumption and growth than genotypes from low-latitude populations. The existence of submaximal growth in some environments suggests that trade-offs must counter the oft-cited theoretical benefits of energy and growth maximization (e.g., ''bigger is better,'' ''faster is better'') in the immature life stages. We hypothesized that energy and growth maximization trades off against investment in defense from predators. We conducted laboratory selection experiments to compare vulnerability to predation of silversides from: (1) fast-growing northern (Nova Scotia, NS) versus slow-growing southern (South Carolina, SC) source populations; (2) phenotypically manipulated fast-growing versus moderately-growing NS fish; and (3) recently fed versus unfed NS and SC fish. Tests involved fish drawn from common-garden environments and were conducted by subjecting mixed-treatment schools of sizematched silversides to natural, common piscine predators. NS silversides suffered significantly higher predation mortality than SC silversides. Parallel results were found in phenotypic manipulation of growth: NS silversides reared on a fast-growth trajectory (ϳ1.0 mm/day) were significantly more vulnerable to predation than those growing at a moderate rate (ϳ0.5 mm/day). Food consumption also affected vulnerability to predators: Silversides with large meals in their stomachs suffered significantly higher predation mortality than unfed silversides. Differences in predation vulnerability were likely due to swimming performance, not attractiveness to predators. Our findings demonstrate that maximization of energy intake and growth rate engenders fitness costs in the form of increased vulnerability to predation.Key words. Countergradient variation, evolutionary physiology, genotype-environment covariance, growth rate, latitude, life-history evolution, locomotory performance, optimal foraging theory. At what rate should immature organisms grow to maximize their fitness? Ecological theory suggests that natural selection favors genotypes that maximize net energy acquisition and growth (Lotka 1922;Ware 1982;Stearns 1992). Because rates of survival and reproductive output generally increase with size in many organisms, individuals that grow faster would reap the benefits of large size earlier in life and experience higher survival and fitness (Sauer and Slade 1987;Sogard 1997). Despite evidence and theoretical arguments to the contrary (Priede 1977;Calow 1982;Sibly et al. 1985;Abrams et al. 1996;Arendt 1997;Conover and Schultz 1997;Gotthard 2000), this faster-is-better paradigm is pervasive: Many life-history studies employ growth rate as a direct scalar of relative fitness among juveniles (e.g., Hatfield and Schluter 1999), and habitat quality is routinely evaluated based on positive associations...

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Age‐0 Atlantic croakers Micropogonias undulatus overwinter in Mid‐Atlantic Bight (MAB) estuaries, where interannual variability in year‐class strength is pronounced. Previous researchers have hypothesized that this variability is the result of cold‐induced winter mortality of age‐0 Atlantic croakers within estuarine nurseries. We tested this hypothesis by means of laboratory experiments designed to (1) estimate the effects of winter severity and duration on the probability of survival of age‐0 fish, (2) determine the relationship between body size and cold tolerance, and (3) examine the effects of temperature decline rate and salinity on susceptibility to low‐temperature mortality. Age‐0 Atlantic croakers (15–65 mm standard length, SL) were collected from Delaware Bay during October, acclimated to 8°C in the laboratory, and then cooled (1.0°C/d) to treatment levels of 1, 3, 5, or 7°C that were designed to simulate varying levels of winter severity in MAB estuaries. Survival ranged from 0% at 1°C to 99.3% at 7°C, increasing dramatically between 3°C (1.3%) and 5°C (86.8%). Cold tolerance was size dependent, with smaller individuals surviving significantly longer than larger ones. The rate of temperature decline (1.0°C/d versus 0.2°C/d) did not affect survival at lower winter temperatures. Atlantic croakers remained intolerant of temperatures of 3°C or less even when these temperatures were approached gradually. Cold tolerance increased with increasing salinity, implying that the risk of overwinter mortality varies spatially along an estuarine salinity gradient. The mean survival duration at 3°C increased from 5.9 d at 2‰ to 14.7 d at 10‰ and to 17.1 d at 26‰. Our findings provide empirical support for the hypothesis that cold‐induced overwinter mortality regulates the year‐class strength and recruitment success of Atlantic croakers in the Mid‐Atlantic Bight.