Summary1. Differences in energy use between genders is a probable mechanism underlying sexual size dimorphism (SSD), but testing this hypothesis in the field has proven difficult. We evaluated this mechanism as an explanation for SSD in two North American percid species -walleye Sander vitreus and yellow perch Perca flavescens . 2. Data from 47 walleye and 67 yellow perch populations indicated that SSD is associated with the onset of maturation: typically, males of both species matured smaller and earlier and attained a smaller asymptotic size than females. Males also demonstrated equal (perch) or longer (walleye) reproductive life spans compared with females. 3. To examine whether reduced post-maturation growth in males was due to lower energy acquisition or higher reproductive costs we applied a contaminant mass-balance model combined with a bioenergetics model to estimate metabolic costs and food consumption of each sex. Mature males exhibited lower food consumption, metabolic costs and food conversion efficiencies compared with females. 4. We propose that slower growth in males at the onset of maturity is a result of decreased feeding activity to reduce predation risk. Our finding that SSD in percids is associated with the onset of maturity is supported by laboratory-based observations reported elsewhere, showing that changes in growth rate, consumption and food conversion efficiency were elicited by oestrogen (positive effects) or androgen (negative effects) exposure in P. flavescens and P. fluviatilis . 5. Researchers applying bioenergetic models for comparative studies across populations should use caution in applying bioenergetic models in the absence of information on population sex ratio and potential differences between the sexes in energetic parameters.

Human activities have the potential to accelerate population-level decline by contributing to climate warming and decreasing the capacity of species to survive warming temperatures. Here we build a predictive model to test interactions between river warming and catch-and-release mortality in recreational fisheries for Atlantic salmon (Salmo salar L.) by compiling and analyzing published data. We then test if warming has occurred in rivers where angling occurs, and if angling opportunities have been restricted through increased river closures due to high water temperatures. We find that catch and release mortalities are low (< 0.05) at cool river temperatures (< 12°C). At river temperatures often leading to fishery closures (between 18 and 20°C), mortalities range from 0.07 to 0.33 (mean = 0.16). River temperatures on the east and southeast coasts of Newfoundland have warmed leading to an increase in fishery closures in recent years. By contrast, river temperatures in southern Labrador have warmed slightly, with only one documented river closure. Accordingly, increasing temperatures will increase the frequency of river closures and likely result in higher mortality in caught and released Atlantic salmon in rivers that remain open to catch and release angling at warm water temperatures.

Phenotypic plasticity occurs when a genotype produces variable phenotypes under different environments; the shapes of such responses are known as norms of reaction. The genetic scale at which reaction norms can be determined is restricted by the experimental unit that can be exposed to variable environments. This has limited their description beyond the family level in higher organisms, thus hindering our understanding of differences in plasticity at the scale of the individual. Using a three-year common-garden experiment, we quantify reaction norms in sperm performance of individual genotypes within different families of Atlantic cod ( Gadus morhua ). Cod sperm showed phenotypic plasticity in swimming performance across temperatures (3, 6, 11, and 21 °C), but the pattern of the response depended upon how long sperm had been swimming (30, 60, 120, or 180 s), i.e., plasticity in plasticity. Sperm generally swam fastest at intermediate temperatures when first assessed at 30 s after activation. However, a significant genotype × environment interaction was present, indicating inter-individual differences in phenotypic plasticity. To our knowledge, this is the first study to describe variable sperm performance across environmental conditions as a reaction norm. The results have potential theoretical, conservation, and aquaculture implications.

Life history variation among 60 Ontario populations of lake trout (Salvelinus namaycush), walleye (Sander vitreus), cisco (Coregonus artedii), and yellow perch (Perca flavescens) is presented and interpreted using a biphasic model of individual growth that specifically accounts for the significant shift in energy allocation that accompanies sexual maturity. We show that the constraints imposed on life history variation by the character of the biphasic growth model are such that optimal life histories will exhibit associations among growth parameters, reproductive investment, and mortality that are largely consistent with associations evident in both our data set and earlier empirical studies; the von Bertalanffy growth parameter k varies with reproductive investment, and both k and investment vary with adult mortality. Our analysis suggests that within a food web, life history parameters will shift in a predictable fashion with the decreases in mortality expected as one moves from primary consumers up toward top predators. This expectation is supported by the differences in life history parameters that we observe between the two top predators in our data set (lake trout and walleye) and the two mid-trophic level consumers (cisco and yellow perch).

Under common environments, populations of laboratory reared (Grand Banks, GB and Gulf of Maine, GOM) and wild caught (Fortune Bay, FB and Bonavista Bay, BB; Newfoundland) juvenile cod Gadus morhua responded similarly to temperature change in specific growth rates, food conversion efficiencies, condition factors, liver water content, and muscle water content. However, GOM cod had higher condition factors, and showed differences from GB cod in phenotypic plasticity of hepatosomatic index to temperature. These differences were not present in a different population comparison between FB and BB cod. All populations had higher growth rates and food conversion efficiencies at warmer temperatures, and exhibited compensatory growth when temperature was increased. The results suggest relatively larger genetic differences between GB and GOM cod than between FB and BB cod, and indicate that the faster growth of southern populations in the wild is not due to a higher genetic capacity for growth rate. 2001 The Fisheries Society of the British Isles

Geographically separated Atlantic cod (Gadus morhua) stocks in the northwest Atlantic exhibit life history variation and have been shown to differ genetically. The genetic and environmental contributions to phenotypic differences, however, have not yet been measured. We used common environment experiments to evaluate the importance of temperature on the observed growth variation between Grand Banks (GB) and Gulf of Maine (GOM) cod stocks. Larvae from the GB grew faster than GOM larvae at both 7 and 12°C. Growth rates of juveniles were not different, but GB juveniles had higher food conversion efficiencies than those from the GOM (at both ambient and warm temperatures). The results indicate that faster growth of GOM cod in the wild is not due to a higher genetic capacity for growth rate in GOM than in GB fish. The findings give evidence of genetically based phenotypic variation, which is in agreement with molecular studies on population differentiation in cod, and support the theory of countergradient variation in growth rates of larval fish.Résumé : Des stocks de la Morue franche (Gadus morhua) du nord-ouest de l'Atlantique, provenant de régions géo-graphiques distinctes, ont des démographies particulières et possèdent des différences génétiques. Les contributions relatives des gènes et de l'environnement aux différences phénotypiques n'ont cependant pas encore été déterminées. Des montages expérimentaux classiques ont permis d'évaluer l'effet de la température sur les différences de croissance que l'on observe entre les stocks des Grands Bancs (GB) et ceux du golfe du Maine (GOM). Les larves GB croissent plus vite que les larves GOM tant à 7°C qu'à 12°C. Les taux de croissance des juvéniles ne diffèrent pas, mais les juvéniles GB ont une meilleure efficacité de conversion de la nourriture que les juvéniles GOM aussi bien aux tempéra-tures ambiantes que chaudes. Ces résultats indiquent que la croissance plus rapide des morues GOM en nature n'est pas due à une capacité génétique de croissance supérieure à celle des poissons GB. Il s'agit donc d'une variation phé-notypique qui est basée sur des facteurs génétiques; cela s'accorde bien avec les études moléculaires de différentiation des populations chez la Morue franche et confirme la théorie de l'existence d'une variation à contre-gradient des taux de croissance chez les larves de poisson.[Traduit par la Rédaction] 2229Purchase and Brown

Summary Computer assisted sperm analysis (CASA) has greatly enhanced the quality and quantity of data that can be collected on spermatozoa movements. Open source imagej software allows any researcher to have access to such analyses, but CASA systems in general are not designed to ably process large numbers of video sequences. Here we introduce a modified imagej CASA_automated plugin that fundamentally enhances efficiency and will allow higher quality research to be conducted with no extra post‐microscope effort.

Sex-specific life history variation was examined among 72 populations of yellow perch Perca flavescens from Ontario, Canada. We sought to determine whether relationships could be applied to other populations to predict parameter values when life history data are not available. Each of the measured traits (early growth rate, maturation size and age, reproductive investment, and maximum size) varied two-to threefold among populations. Relationships were developed to predict standard calculations of life history traits from population-specific data for use in poorly sampled lakes. Associations between life history traits and environmental variables can be used in unsampled lakes. Early growth rate was positively related to lake surface area, while relative density was positively related to total dissolved solids. For both sexes, maximum body size was positively related to lake surface area and negatively related to growing degree-days. Additional variation in female maximum size was explained by a positive relationship with water hardness. Much variation in yellow perch growth could not be accounted for, despite incorporation of the major hypotheses that appear in the literature relating environmental variation to life history. Although explained variation was too low to generate important management policies, the results indicate types of lakes capable of producing large fish and therefore of interest for future study.