Rick J. Stoffels

The effects of turbidity, prey density and environmental complexity on the feeding of juvenile Murray cod Maccullochella peelii

Juvenile Murray cod Maccullochella peelii exhibited a type II functional response while preying on blackworms Lumbriculus variegatus, and the parameters of the type II model did not differ significantly between clear (0 NTU) and turbid (150 NTU) treatments. Further experiments showed that vision may not be necessary for prey detection and capture by juvenile M. peelii; consumption of inanimate prey was not significantly different between light and dark (<1 × 10(-4) µE m(-2) s(-1)) trials. These results imply that the sensory physiology of M. peelii is well adapted to a turbid visual environment. In addition, habitat complexity increased the food consumption rate of juvenile M. peelii, perhaps by relaxing innate predator avoidance behaviours that depress foraging in more open environments.

Physiological Trade-Offs Along a Fast-Slow Lifestyle Continuum in Fishes: What Do They Tell Us about Resistance and Resilience to Hypoxia?

It has recently been suggested that general rules of change in ecological communities might be found through the development of functional relationships between species traits and performance. The physiological, behavioural and life-history traits of fishes are often organised along a fast-slow lifestyle continuum (FSLC). With respect to resistance (capacity for population to resist change) and resilience (capacity for population to recover from change) to environmental hypoxia, the literature suggests that traits enhancing resilience may come at the expense of traits promoting resistance to hypoxia; a trade-off may exist. Here I test whether three fishes occupying different positions along the FSLC trade-off resistance and resilience to environmental hypoxia. Static respirometry experiments were used to determine resistance, as measured by critical oxygen tension (Pcrit), and capacity for (RC) and magnitude of metabolic reduction (RM). Swimming respirometry experiments were used to determine aspects of resilience: critical (U crit) and optimal swimming speed (U opt), and optimal cost of transport (COTopt). Results pertaining to metabolic reduction suggest a resistance gradient across species described by the inequality Melanotaenia fluviatilis (fast lifestyle) < Hypseleotris sp. (intermediate lifestyle) < Mogurnda adspersa (slow lifestyle). The Ucrit and COTopt data suggest a resilience gradient described by the reverse inequality, and so the experiments generally indicate that three fishes occupying different positions on the FSLC trade-off resistance and resilience to hypoxia. However, the scope of inferences that can be drawn from an individual study is narrow, and so steps towards general, trait-based rules of fish community change along environmental gradients are discussed.

Contrasting fundamental and realized niches: two fishes with similar thermal performance curves occupy different thermal habitats

Human alteration of thermal regimes of freshwater ecosystems is creating an urgent need to understand how freshwater ectotherms will fare under different thermal futures. Two key questions are: 1) how well do the fundamental thermal niches of ectotherms map to their realized thermal niches, and 2) which axes of the fundamental thermal niche must be modeled to predict temperature-dependent fitness in real ecosystems? The first question is particularly challenging in riverine systems, where gradients in temperature are strongly confounded by gradients in other biotic and abiotic drivers. To address these questions, we compared the realized and fundamental thermal niches of 2 congeneric riverine fish: Gadopsis marmoratus and Gadopsis bispinosus. We characterized their realized thermal niches by examining their distributions in relation to environmental temperature at multiple scales. We characterized their fundamental thermal niches by doing laboratory experiments on the thermal sensitivity of swimming performance and metabolic rates, particularly aerobic scope. The distributions of the 2 species supported the idea that they have different realized thermal niches, with G. bispinosus occupying cooler habitats than G. marmoratus. Despite this, we detected no significant differences in the shapes of thermal performance curves defining 2 axes of their fundamental niches: swimming performance and aerobic scope. Our results suggest that either the distributions of these 2 species are driven by factors other than temperature or that swimming performance and aerobic scope were not suitable proxies of their fundamental thermal niches. Our study shows that modeling the thermal niches of ectotherms along the river continuum is not straightforward. If we are to forecast effects of thermal futures effectively and efficiently, we must do more to decipher the relative influence of temperature and other abiotic drivers on the fitness of riverine ectotherms.

A predicted change in the amino acid landscapes available to freshwater carnivores

Anthropogenic effects lead to nonrandom alterations to macroinvertebrate communities, which may lead to alteration of the ‘micronutrient landscapes’ experienced by higher consumers. Understanding how amino acid composition varies among taxa and guilds is an essential step toward predicting how micronutrient landscape alteration will affect carnivores. We assessed whether: 1) wild macroinvertebrate prey varied in their amino acid compositions, 2) variation in amino acid composition was correlated with economical selection pressures or functional traits that may separate the compositions of functional feeding groups (FFGs) or is simply a result of phylogeny, and 3) simulated anthropogenic change in the composition of macroinvertebrate communities affects the amino acid composition of the nutrient landscape. Amino acid composition varied significantly among taxa and was strongly correlated with phylogeny but not FFGs. The amino acid compositions of holometabolous insects (those that undergo complete metamorphosis; Trichoptera and Diptera) differed from those of hemimetabolous insects (those that undergo incomplete metamorphosis; Ephemeroptera and Hemiptera). This separation can be explained by morphological and physiological traits involved in pupation or nymphal development. Simulated deterioration of macroinvertebrate communities changed the amino acid landscape, resulting in lower availability of threonine, phenylalanine, proline, and tyrosine to carnivores. Amino acid availability to carnivores is likely to respond more to the loss of taxonomic lineages than to loss of prey FFGs. Our study provides a critical first step toward understanding how changes to macroinvertebrate communities might affect the availability of amino acids to higher consumers.