A synthesis of senescence predictions for indeterminate growth, and support from multiple tests in wild lake trout.

Senescence-the deterioration of functionality with age-varies widely across taxa in pattern and rate. Insights into why and how this variation occurs are hindered by the predominance of laboratory-focused research on short-lived model species with determinate growth. We synthesize evolutionary theories of senescence, highlight key information gaps and clarify predictions for species with low mortality and variable degrees of indeterminate growth. Lake trout are an ideal species to evaluate predictions in the wild. We monitored individual males from two populations (1976-2017) longitudinally for changes in adult mortality (actuarial senescence) and body condition (proxy for energy balance). A cross-sectional approach (2017) compared young (ages 4-10 years) and old (18-37 years) adults for (i) phenotypic performance in body condition, and semen quality-which is related to fertility under sperm competition (reproductive senescence)-and (ii) relative telomere length (potential proxy for cellular senescence). Adult growth in these particular populations is constrained by a simplified foodweb, and our data support predictions of negligible senescence when maximum size is only slightly larger than maturation size. Negative senescence (aka reverse senescence) may occur in other lake trout populations where diet shifts allow maximum sizes to greatly exceed maturation size.

Assessing the potential for post-ejaculatory female choice in a polyandrous beach-spawning fish.

In species with limited opportunities for pre-ejaculatory sexual selection (behavioural components), post-ejaculatory mechanisms may provide opportunities for mate choice after gametes have been released. Recent evidence from a range of taxa has revealed that cryptic female choice (i.e., female-mediated differential fertilization bias), through chemical cues released with or from eggs, can differentially regulate the swimming characteristics of sperm from various males and ultimately determine male fertilization success under sperm competition. We assessed the potential role that such female-modulated chemical cues play in influencing sperm swimming characteristics in beach-spawning capelin (Mallotus villosus), an externally fertilizing fish that mates as couples (one male and one female) or threesomes (two males and one female) with presumably limited opportunities for pre-ejaculatory sexual selection. We assayed sperm swimming characteristics under varying doses and donor origins of egg cues and also examined the possibility of assortative mating based on body size. We found mating groups were not associated by size, larger males did not produce better quality ejaculates, and egg cues (regardless of dosage or donor identity) did not influence sperm swimming characteristics. Our findings suggest that intersexual pre-ejaculatory sexual selection and cryptic female choice mediated by female chemical cues are poorly developed in capelin, possibly due to unique natural selection constraints on reproduction.

Freezing ovarian fluid does not alter how it affects fish sperm swimming performance: creating a cryptic female choice 'spice rack' for use in split-ejaculate experimentation.

Cryptic female choice is often mediated chemically in external fertilizers by ovarian fluid (OF), which can change sperm swimming performance and bias paternity under sperm competition. Assessing cryptic female choice is hindered by the necessity of using fresh gametes and the short time window available to obtain diverse samples from wild animals. Using split-ejaculate experimental designs and samples from lake trout, brown trout and Atlantic salmon, we evaluated whether freezing OF alters the way in which it modifies sperm swimming. Sperm had improved swimming performance in the presence of OF over plain water, and the effect did not depend on whether the OF had previously been frozen. Freezing OF does not seem to alter the way it influences sperm. This allows the researcher to create a 'spice rack' of OF samples that can be used in studies on cryptic female choice, and opens up the possibility to compare animals mating under large spatial and temporal variability.

Thermal physiology of native cool-climate, and non-native warm-climate Pumpkinseed sunfish raised in a common environment.

Contemporary evolution of thermal physiology has the potential to help limit the physiological stress associated with rapidly changing thermal environments; however it is unclear if wild populations can respond quickly enough for such changes to be effective. We used native Canadian Pumpkinseed (Lepomis gibbosus) sunfish, and non-native Pumpkinseed introduced into the milder climate of Spain ~100 years ago, to assess genetic differences in thermal physiology in response to the warmer non-native climate. We compared temperature performance reaction norms of two Canadian and two Spanish Pumpkinseed populations born and raised within a common environment. We found that Canadian Pumpkinseed had higher routine metabolic rates when measured at seasonally high temperatures (15°C in winter, 30°C in summer), and that Spanish Pumpkinseed had higher critical thermal maxima when acclimated to 30°C in the summer. Growth rates were not significantly different among populations, however Canadian Pumpkinseed tended to have faster growth at the warmest temperatures measured (32°C). The observed differences in physiology among Canadian and Spanish populations at the warmest acclimation temperatures are consistent with the introduced populations being better suited to the warmer non-native climate than native populations. The observed differences could be the result of either founder effects, genetic drift, and/or contemporary adaptive evolution in the warmer non-native climate.

Morphological change and phenotypic plasticity in native and non-native pumpkinseed sunfish in response to competition.

Non-indigenous species are oftentimes exposed to ecosystems with unfamiliar species, and organisms that exhibit a high degree of phenotypic plasticity may be better able to contend with the novel competitors that they may encounter during range expansion. In this study, differences in morphological plasticity were investigated using young-of-year pumpkinseed sunfish (Lepomis gibbosus) from native North American and non-native European populations. Two Canadian populations, isolated from bluegill sunfish (L. macrochirus) since the last glaciation, and two Spanish populations, isolated from bluegill since their introduction in Europe, were reared in a common environment using artificial enclosures. Fish were subjected to allopatric (without bluegill) or sympatric (with bluegill) conditions, and differences in plasticity were tested through a MANOVA of discriminant function scores. All pumpkinseed populations exhibited dietary shifts towards more benthivorous prey when held with bluegill. Differences between North American and European populations were observed in body dimensions, gill raker length and pelvic fin position. Sympatric treatments induced an increase in body width and a decrease in caudal peduncle length in native fish; non-native fish exhibited longer caudal peduncle lengths when held in sympatry with bluegill. Overall, phenotypic plasticity influenced morphological divergence less than genetic factors, regardless of population. Contrary to predictions, pumpkinseeds from Europe exhibited lower levels of phenotypic plasticity than Canadian populations, suggesting that European pumpkinseeds are more canalized than their North American counterparts.

The impact of a changing winter climate on the hatch phenology of one of North America's largest Atlantic salmon populations.

In northern temperate regions, some of the most dramatic effects of climate change are expected during the winter. Understanding how changing winter climates influence the seasonal timing of key life events is critical for implementing effective conservation strategies, especially for poikilotherms, whose physiology and development are particularly sensitive to changes in thermal environment. Four mathematical models are available to predict the timing of hatch and emergence in Atlantic salmon (Salmo salar); however, such models are only useful if the effect of temperature is both repeatable within and among maternal families, and predictable across variable temperature regimes. Using a split-brood experiment, we found the timing of hatch to be repeatable and predictable in Atlantic salmon from the Exploits River, one of the largest remaining wild populations in North America. Three of the available mathematical models under-estimated the timing of hatch by an average of 21-26 accumulated thermal units (ATU); however, we identified one model that provided reasonable estimates of hatch timing (average under-estimate 7 ATU) under the three incubation temperature regimes we tested. We applied this model to daily water temperature profiles from 2006-18 at four sites within the Exploits River watershed. Across all years and sites, the predicted dates at 50% hatch ranged between 8 March and 23 May, while predicted dates of 50% emergence ranged from 11 May to 13 June. By identifying the seasonal timing of these particularly vulnerable early life stages, this model can aid the implementation of conservation efforts for this ecologically and economically important population.