Native and Introduced Populations of Smallmouth Bass Differ in the Concordance Between Climate and Somatic Growth

We characterized the association between climate and somatic growth in 125 North American populations of smallmouth bass, Micropterus dolomieu. Using multivariate techniques (i.e., principal components and Procrustes analyses), we found an overall significant concordance between 8 climate variables (cloud cover, frost frequency, precipitation, mean air temperature, minimum air temperature, maximum air temperature, mean summer air temperature, and growing degree days above 10 degrees Celsius) and 4 growth variables (body length increments for ages 1 to 4). Bivariate linear regressions revealed that there was a significant positive relationship between air temperature variables and early growth while growth at later ages was generally less influenced by climate. Given that the geographical range of smallmouth bass has been rapidly expanding over the past century, we also examined how the climate-growth relationships differed in populations that have been introduced outside the native distribution. Analysis of residuals from the Procrustes test indicated that the concordance between climate and growth was likely higher for populations within the native range and lower for introduced populations. Mechanisms that might generate this pattern include the possibility that the introduced populations have not had time to adapt to their new environments and the possibility that growth might respond atypically to the more extreme climates experienced outside the native range of the species

Isolating the influence of growth rate on maturation patterns in the smallmouth bass (Micropterus dolomieu)

In this study, we examine the divergence in growth and maturation between two populations of smallmouth bass (Micropterus dolomieu) introduced from a common source a century ago. To determine if the divergence in maturation is simply a plastic response to differences in growth rate, we use a new approach to estimate and then compare the probabilistic maturation reaction norms (PMRNs) for each population. The PMRNs for 5-year-old males are similar in the two populations, suggesting that the observed divergence in maturation is largely a plastic response to growth rate differences. For one population, we document the time course of maturation changes for the 60-year period from 1937 through 1990; while the mean length at maturation for 5-year-old males exhibits a steady downward trend (beginning at 31 cm and ending at 26 cm), their PMRNs vary over a much narrower range (25-27 cm) and do not exhibit a consistent temporal trend. These observations are consistent with the hypothesis that most of the observed change in maturation since introduction is a product of phenotypic plasticity, driven by environmentally based differences in growth rate. Our study provides an instructive example of how the PMRN approach can be used to isolate the role of growth rate variation in generating life history differences

Biphasic growth in fish I: Theoretical foundations

We develop the theory of biphasic somatic growth in fish using models based on the distinction between pre- and post-maturation growth and an explicit description of energy allocation within a growing season. We define a 'generic biphasic' (GB) model that assumes post-maturation growth has a von Bertalanffy (vB) form. For this model we derive an explicit expression for the gonad weight/somatic weight ratio (g) which may either remain fixed or vary with size. Optimal biphasic models are then developed with reproductive strategies that maximise lifetime reproductive output. We consider two optimal growth models. In the first (fixed g optimal), gonad weight is constrained to be proportional to somatic weight. In the second (variable g optimal) model, allocation to reproduction is unconstrained and g increases with size. For the first of these two models, adult growth in a scaled measure of length has the exact vB form. When there are no constraints on allocation, growth is vB to a very good approximation. In both models, pre-maturation growth is linear. In a companion paper we use growth data from lake trout (Salvelinus namaycush) to test the bioenergetics assumptions used to develop these models, and demonstrate that they have advantages over the vB model, both in quality of fit, and in the information contained in the fitted parameter

Diet and divergence of introduced smallmouth bass (Micropterus dolomieu) populations

We examine the degree and causes of divergence in growth and reproduction in two populations of smallmouth bass (Micropterus dolomieu) introduced a century ago. Despite a common source, the Provoking Lake population now has a higher population density and slower growing individuals than the Opeongo Lake population. Using this system, we test the predictions of life history theory that delayed maturation and reduced reproductive investment are expected in high density populations with slow individual growth rates. Observations on both populations run directly counter to the aforementioned expectations. Instead, Provoking males have smaller sizes and younger ages at nesting and higher gonad masses than Opeongo males; Provoking females have smaller sizes at maturity, larger egg sizes, and higher ovarian dry masses than Opeongo females. Temperature, food availability, diet ontogeny, young-of-the-year mortality, and adult mortality were examined as plausible contributors to the divergence. Results suggest that low food availability, likely caused or mediated by intraspecific competition for prey, and lack of large prey in the diet are contributing to the slow growth, increased reproductive investment, and higher mortality following reproduction in Provoking. This study provides insight into the processes that produce rapid divergence of life history in a species exhibiting parental care

Invariant scaling of phytoplankton abundance and cell size in contrasting marine environments

Scaling relationships such as the variation of population abundance with body size provide links between individual organisms and ecosystem functioning. Previous work, in marine pelagic ecosystems, has focused on the relationship between total phytoplankton abundance and the assemblage mean cell size. However, the relationship between specific population abundance and cell size in marine phytoplankton has received little attention. Here, we show that cell size accounts for a significant amount of variability in the population abundance of phytoplankton species across a cell volume range spanning seven orders of magnitude. The interspecific scaling of population abundance and cell size takes a power exponent near -3/4. Unexpectedly, despite the constraints imposed on large phytoplankton by limited resource acquisition, the size scaling exponent does not differ between contrasting marine environments such as coastal and subtropical regions. These findings highlight the adaptive abilities of individual species to cope with different environmental conditions and suggest that a general rule such as the `energetic equivalence' constrains the abundance of phytoplankton populations in marine pelagic ecosystems