The spatial scale of density-dependent growth and implications for dispersal from nests in juvenile Atlantic salmon

Multisensory navigational strategies of hatchling fish for dispersal

Animals influence how they disperse in the environment by sensing local cues and adapting how they move. However, controlling dispersal can present a particular challenge early in life when animals tend to be more limited in their capacities to sense and move. To what extent and by what mechanisms can newly hatched fish control how they disperse? Here, we reveal hatchling sensorimotor mechanisms for controlling dispersal by combining swim tracking and precise sensory manipulations of a model species, zebrafish. In controlled laboratory experiments, if we physically constrained hatchlings or blocked sensations of motion through vision and the lateral line, hatchlings responded by elevating their buoyancy and passively moving with faster surface currents. Complementarily, in stagnant water, hatchlings covered more ground using hyperstable swimming, strongly orienting based on graviception. Using experimentally calibrated hydrodynamic simulations, we show that these hatchling behaviors nearly tripled diffusivity and made dispersal robust to local conditions, suggesting this multisensory strategy may provide important advantages for early life in a variable environment.

Habitat restoration weakens negative environmental effects on telomere dynamics

Habitat quality can have far-reaching effects on organismal fitness, an issue of concern given the current scale of habitat degradation. Many temperate upland streams have reduced nutrient levels due to human activity. Nutrient restoration confers benefits in terms of invertebrate food availability and subsequent fish growth rates. Here we test whether these mitigation measures also affect the rate of cellular ageing of the fish, measured in terms of the telomeres that cap the ends of eukaryotic chromosomes. We equally distributed Atlantic salmon eggs from the same 30 focal families into 10 human-impacted oligotrophic streams in northern Scotland. Nutrient levels in five of the streams were restored by simulating the deposition of a small number of adult Atlantic salmon Salmo salar carcasses at the end of the spawning period, while five reference streams were left as controls. Telomere lengths and expression of the telomerase reverse transcriptase (TERT) gene that may act to lengthen telomeres were then measured in the young fish when 15 months old. While TERT expression was unrelated to any of the measured variables, telomere lengths were shorter in salmon living at higher densities and in areas with a lower availability of the preferred substrate (cobbles and boulders). However, the adverse effects of these habitat features were much reduced in the streams receiving nutrients. These results suggest that adverse environmental pressures are weakened when nutrients are restored, presumably because the resulting increase in food supply reduces levels of both competition and stress.

Temperature, emergence phenology and consumption drive seasonal shifts in fish growth and production across riverscapes

Changes in biophysical conditions through time generate spatial and temporal variability in habitat quality across landscapes. For river ecosystems, researchers are increasingly able to characterize spatial and temporal patterns in habitat conditions, referred to as shifting habitat mosaics, yet rarely demonstrate how this translates into corresponding biological processes such as organism growth and production. We assessed spatial patterns and processes determining seasonal changes in juvenile Chinook Salmon Oncorhynchus tshawytscha size, growth and production over 30-40 km in two NE Oregon subbasins. We quantified seasonal patterns of growth by combining estimated emergence dates and body size distributions in July and September. We then used analysis of bioenergetics, empirical fish diets and spatial models incorporating temperature, habitat and population density to evaluate mechanisms driving spatiotemporal patterns of growth. Lastly, we quantified seasonal contributions to individual fish growth and to total production as a function of position within the stream network. Spatial heterogeneity in incubation temperatures corresponded to later estimated emergence timing with distance upstream in both subbasins. During spring, estimated growth rates decreased with distance upstream, and coupled with emergence patterns, resulted in pronounced longitudinal gradients in body size by July. During summer, spatial patterns of growth reversed, with greater diet ration sizes and growth efficiencies upstream than downstream. These opposing spatiotemporal patterns of emergence timing and seasonal growth rates produced longitudinal gradients in the proportion of fish growth achieved in spring versus summer, with up to 80% of an individual's growth occurring in spring at downstream sites but as low as 10% at upstream sites. Coupling longitudinal patterns of fish density and growth revealed that in one subbasin the majority (65%) of total production occurred in spring, while in the other, in which fish were concentrated in headwaters, the majority (60%) of production occurred in summer. While recent work has emphasized inter-annual shifts in fish production across large spatial scales, this study demonstrates that longitudinal gradients of fish growth and production can reverse across seasons, and reveals important contributions of warmer, downstream habitats to overall production that occurred during cooler times of the year.

Adaptive Maternal Investment in the Wild? Links between Maternal Growth Trajectory and Offspring Size, Growth, and Survival in Contrasting Environments

Life-history theory predicts that investment per offspring should correlate negatively with the quality of the environment that offspring are anticipated to encounter; parents may use their own experience as juveniles to predict this environment and may modulate offspring traits, such as growth capacity and initial size. We manipulated nutrient levels in the juvenile habitat of wild Atlantic salmon (Salmo salar) to investigate the hypothesis that the egg size that maximizes juvenile growth and survival depends on environmental quality. We also tested whether offspring traits were related to parental growth trajectory. Mothers that grew fast when young produced more offspring and smaller offspring than mothers that grew slowly to reach the same size. Despite their size disadvantage, offspring of faster-growing mothers grew faster than those of slower-growing mothers in all environments, counter to the expectation that they would be competitively disadvantaged. However, they had lower relative survival in environments where the density of older predatory/competitor fish was relatively high. These links between maternal (but not paternal) growth trajectory and offspring survival rate were independent of egg size, underscoring that mothers may be adjusting egg traits other than size to suit the environment their offspring are anticipated to face.

Farmed Atlantic salmon Salmo salar L. parr may reduce early survival of wild fish

The study examined the density-mediated effects on growth, survival and dispersal of wild and farmed Atlantic salmon Salmo salar offspring in the period immediately following emergence, using a substitutive design. In small confined stream channels, wild parr coexisting with farmed parr had a significantly poorer survival, than wild parr alone. Density did not affect this relationship. In larger unconfined stream channels, wild parr coexisting with farmed parr entered a downstream trap in higher numbers than wild parr in allopatry. The results suggests that during the earliest life stages, farmed S. salar can outcompete wild S. salar, resulting in a reduced survival of wild S. salar.

Spatial variation in the relationship between performance and metabolic rate in wild juvenile Atlantic salmon

Maintenance of metabolic rate (MR, the energy cost of self-maintenance) is linked to behavioural traits and fitness and varies substantially within populations. Despite having received much attention, the causes and consequences of this variation remain obscure. Theoretically, such within-population variation in fitness-related traits can be maintained by environmental heterogeneity in selection patterns, but for MR, this has rarely been tested in nature. Here, we experimentally test whether the relationship between MR and performance can vary spatially by assessing survival, growth rate and movement of Atlantic salmon (Salmo salar L.) juveniles from 10 family groups differing in MR (measured as egg metabolism) that were stocked in parallel across 10 tributaries of a single watershed. The relationship between MR and relative survival and growth rate varied significantly among tributaries. Specifically, the effect of MR ranged from negative to positive for relative survival, whereas it was negative for growth rate. The association between MR and movement was positive and did not vary significantly among tributaries. These results are consistent with a fitness cost of traits associated with behavioural dominance that varies across relatively small spatial scales (within a single watershed). More generally, our results support the hypothesis that spatial heterogeneity in environmental conditions contributes to maintain within-population variation in fitness-related traits, such as MR.

Early maternal experience shapes offspring performance in the wild

Both the environments experienced by a mother as a juvenile and an adult can affect her investment in offspring. However, the implications of these maternal legacies, both juvenile and adult, for offspring fitness in natural populations are unclear. We investigated whether the juvenile growth rate and adult reproductive traits (length, body condition, and reproductive investment at spawning) of female wild Atlantic salmon (Salmo salar) were related to the growth and survival of their offspring. Adult salmon captured on their upstream migration were used to create experimental full-sib clutches of eggs, which were mixed and then placed in artificial nests in a natural stream that lacked salmon due to a migration barrier. Four months later we resampled the stream to obtain family-level estimates of offspring size and survival. Mothers that had grown slowly as juveniles (as determined by scalimetry) but had invested heavily in reproduction (egg production for a given body length) and were in relatively poor body condition (somatic mass for a given body length) at spawning produced the largest eggs. Larger eggs resulted in larger juveniles and higher juvenile survival. However, after controlling for egg size, offspring growth was positively related to maternal juvenile growth rate and reproductive investment. The predictors of offspring survival (i.e., reproductive success) varied with the juvenile growth rate of the mother: If females grew slowly as juveniles, their reproductive success was negatively related to their own body condition. In contrast, the reproductive success of females that grew quickly as juveniles was instead related positively to their own body condition. Our results show that maternal influences on offspring in the wild can be complex, with reproductive success related to the early life performance of the mother, as well as her state at the time of breeding.

Foraging behaviour of brown trout in wild populations: can population density cause behaviourally-mediated foraging specializations?

Brown trout is considered as a territorial fish, in which negative density effects on growth and survival rates can be mediated through competition mechanisms. Here, in order to examine whether competition mechanisms can affect the foraging behaviour of wildSalmo truttawith respect to active-bottom, benthic-drift or surface-drift foraging, three neighbouring populations under different levels of fish density (high, intermediate and low) were studied. We analysed the foraging behaviour of each population according to niche breadth, prey preferences, the modified Costello graphical method and prey trait analysis. The results revealed a remarkable similarity in the feeding behaviour among these feral fish populations, suggesting a foraging behaviour convergence in response to site-specific prey accessibility. A generalist foraging behaviour was the prevailing feeding strategy, independent of fish density. Hence, this study offered evidence for the occurrence of density-independent individual foraging behaviour when food is abundant and available; however, density-dependent foraging behaviour might occur when resource limitation exists. Studies under natural conditions like the present study are needed to increase ecological realism, and indeed this study opens promising research directions for future feeding studies in territorial fish species.

The spatial scale of competition from recruits on an older cohort in Atlantic salmon

Competitive effects of younger cohorts on older ones are frequently assumed to be negligible in species where older, larger individuals dominate in pairwise behavioural interactions. Here, we provide field estimates of such competition by recruits on an older age class in Atlantic salmon (Salmo salar), a species where observational studies have documented strong body size advantages which should favour older individuals in direct interactions. By creating realistic levels of spatial variation in the density of underyearling (YOY) recruits over a 1-km stretch of a stream, and obtaining accurate measurements of individual growth rates of overyearlings (parr) from capture–mark–recapture data on a fine spatial scale, we demonstrate that high YOY density can substantially decrease parr growth. Models integrating multiple spatial scales indicated that parr were influenced by YOY density within 16 m. The preferred model suggested parr daily mass increase to be reduced by 39% when increasing YOY density from 0.0 to 1.0 m⁻², which is well within the range of naturally occurring densities. Reduced juvenile growth rates will in general be expected to reduce juvenile survival (via increased length of exposure to freshwater mortality) and increase generation times (via increased age at seaward migrations). Thus, increased recruitment can significantly affect the performance of older cohorts, with important implications for population dynamics. Our results highlight that, even for the wide range of organisms that rely on defendable resources, the direction of competition among age classes cannot be assumed a priori or be inferred from behavioural observations alone.