Atlantic cod individual spatial behaviour and stable isotope associations in a no-take marine reserve.

Foraging is a behavioural process and, therefore, individual behaviour and diet are theorized to covary. However, few comparisons of individual behaviour type and diet exist in the wild. We tested whether behaviour type and diet covary in a protected population of Atlantic cod, Gadus morhua. Working in a no-take marine reserve, we could collect data on natural behavioural variation and diet choice with minimal anthropogenic disturbance. We inferred behaviour using acoustic telemetry and diet from stable isotope compositions (expressed as δ13 C and δ15 N values). We further investigated whether behaviour and diet could have survival costs. We found cod with shorter diel vertical migration distances fed at higher trophic levels. Cod δ13 C and δ15 N values scaled positively with body size. Neither behaviour nor diet predicted survival, indicating phenotypic diversity is maintained without survival costs for cod in a protected ecosystem. The links between diet and diel vertical migration highlight that future work is needed to understand whether the shifts in this behaviour during environmental change (e.g. fishing or climate), could lead to trophic cascades.

Attuning to a changing ocean.

The ocean is a lifeline for human existence, but current practices risk severely undermining ocean sustainability. Present and future social-ecological challenges necessitate the maintenance and development of knowledge and action by stimulating collaboration among scientists and between science, policy, and practice. Here we explore not only how such collaborations have developed in the Nordic countries and adjacent seas but also how knowledge from these regions contributes to an understanding of how to obtain a sustainable ocean. Our collective experience may be summarized in three points: 1) In the absence of long-term observations, decision-making is subject to high risk arising from natural variability; 2) in the absence of established scientific organizations, advice to stakeholders often relies on a few advisors, making them prone to biased perceptions; and 3) in the absence of trust between policy makers and the science community, attuning to a changing ocean will be subject to arbitrary decision-making with unforeseen and negative ramifications. Underpinning these observations, we show that collaboration across scientific disciplines and stakeholders and between nations is a necessary condition for appropriate actions.

Multiple-batch spawning: a risk-spreading strategy disarmed by highly intensive size-selective fishing rate.

Can the advantage of risk-managing life-history strategies become a disadvantage under human-induced evolution? Organisms have adapted to the variability and uncertainty of environmental conditions with a vast diversity of life-history strategies. One such evolved strategy is multiple-batch spawning, a spawning strategy common to long-lived fishes that 'hedge their bets' by distributing the risk to their offspring on a temporal and spatial scale. The fitness benefits of this spawning strategy increase with female body size, the very trait that size-selective fishing targets. By applying an empirically and theoretically motivated eco-evolutionary mechanistic model that was parameterized for Atlantic cod (Gadus morhua), we explored how fishing intensity may alter the life-history traits and fitness of fishes that are multiple-batch spawners. Our main findings are twofold; first, the risk-spreading strategy of multiple-batch spawning is not effective against fisheries selection, because the fisheries selection favours smaller fish with a lower risk-spreading effect; and second, the ecological recovery in population size does not secure evolutionary recovery in the population size structure. The beneficial risk-spreading mechanism of the batch spawning strategy highlights the importance of recovery in the size structure of overfished stocks, from which a full recovery in the population size can follow.

Species interactions, environmental gradients and body size shape population niche width.

Competition for shared resources is commonly assumed to restrict population-level niche width of coexisting species. However, the identity and abundance of coexisting species, the prevailing environmental conditions, and the individual body size may shape the effects of interspecific interactions on species' niche width. Here we study the effects of interspecific and intraspecific interactions, lake area and altitude, and fish body size on the trophic niche width and resource use of a generalist predator, the littoral-dwelling large, sparsely rakered morph of European whitefish (Coregonus lavaretus; hereafter LSR whitefish). We use stable isotope, diet and survey fishing data from 14 subarctic lakes along an environmental gradient in northern Norway. The isotopic niche width of LSR whitefish showed a humped-shaped relationship with increasing relative abundance of sympatric competitors, suggesting widest population niche at intermediate intensity of interspecific interactions. The isotopic niche width of LSR whitefish tended to decrease with increasing altitude, suggesting reduced niche in colder, less productive lakes. LSR whitefish typically shifted to a higher trophic position and increased reliance on littoral food resources with increasing body size, although between-lake differences in ontogenetic niche shifts were evident. In most lakes, LSR whitefish relied less on littoral food resources than coexisting fishes and the niche overlap between sympatric competitors was most evident among relatively large individuals (>250 mm). Individual niche variation was highest among >200 mm long LSR whitefish, which likely have escaped the predation window of sympatric predators. We demonstrate that intermediate intensity of interspecific interactions may broaden species' niche width, whereas strong competition for limited resources and high predation risk may suppress niche width in less productive environments. Acknowledging potential humped-shaped relationships between population niche width and interspecific interactions can help us understand species' responses to environmental disturbance (e.g. climate change and species invasions) as well as the driving forces of niche specialization.

Allee effects and the Allee-effect zone in northwest Atlantic cod.

According to the theory of compensatory dynamics, depleted populations should recover when the threat responsible for their decline is removed because per capita population growth is assumed to be highest when populations are at their smallest viable sizes. Yet, many seriously depleted fish populations have failed to recover despite threat mitigation. Atlantic cod (Gadus morhua) stocks off Newfoundland, despite 30 years of dramatically reduced fishing mortality and numerous fishery closures, have not recovered, suggesting that drivers other than fishing can regulate the growth of collapsed fish populations, inhibiting or preventing their recovery. Here, using Bayesian inference, we show strong evidence of Allee effects in a south Newfoundland cod population, based on data on recruitment and spawning stock biomass. We infer the Allee-effect threshold, below which recovery is impaired. We demonstrate the necessity of data at low population sizes to make inferences about the nature of low-abundance dynamics. Our work indicates that Allee effects are not negligible in commercially exploited fish populations, as commonly projected, and that they represent an inhibitory force that can effectively prevent recovery from overfishing. Our findings contrast with prevailing fisheries management practices that assume compensatory dynamics at low abundances with potential to seriously overestimate the recovery potential of collapsed populations.

Gill area explains deviations from body size-metabolic rate relationship in teleost fishes.

Whether gill area constrains fish metabolism through oxygen limitation is a debated topic. Here, the authors provide insights into this question by analysing mass-specific metabolic rates across 44 teleost fishes extracted from FishBase. They explore whether species deviations from metabolic rates predicted by body mass can be explained by species gill area. They show that the gill area explains c. 26%-28% of species-level deviations from mass-specific metabolic rates. Their findings suggest that gill area might indeed be one of the factors limiting metabolic rate in fishes.

Consequences of Single-Locus and Tightly Linked Genomic Architectures for Evolutionary Responses to Environmental Change.

Genetic and genomic architectures of traits under selection are key factors influencing evolutionary responses. Yet, knowledge of their impacts has been limited by a widespread assumption that most traits are controlled by unlinked polygenic architectures. Recent advances in genome sequencing and eco-evolutionary modeling are unlocking the potential for integrating genomic information into predictions of population responses to environmental change. Using eco-evolutionary simulations, we demonstrate that hypothetical single-locus control of a life history trait produces highly variable and unpredictable harvesting-induced evolution relative to the classically applied multilocus model. Single-locus control of complex traits is thought to be uncommon, yet blocks of linked genes, such as those associated with some types of structural genomic variation, have emerged as taxonomically widespread phenomena. Inheritance of linked architectures resembles that of single loci, thus enabling single-locus-like modeling of polygenic adaptation. Yet, the number of loci, their effect sizes, and the degree of linkage among them all occur along a continuum. We review how linked architectures are often associated, directly or indirectly, with traits expected to be under selection from anthropogenic stressors and are likely to play a large role in adaptation to environmental disturbance. We suggest using single-locus models to explore evolutionary extremes and uncertainties when the trait architecture is unknown, refining parameters as genomic information becomes available, and explicitly incorporating linkage among loci when possible. By overestimating the complexity (e.g., number of independent loci) of the genomic architecture of traits under selection, we risk underestimating the complexity (e.g., nonlinearity) of their evolutionary dynamics.

Cyclical and stochastic thermal variability affects survival and growth in brook trout.

Directional changes in temperature have well-documented effects on ectotherms, yet few studies have explored how increased thermal variability (a concomitant of climate change) might affect individual fitness. Using a common-garden experimental protocol, we investigated how bidirectional temperature change can affect survival and growth of brook trout (Salvelinus fontinalis) and whether the survival and growth responses differ between two populations, using four thermal-variability treatments (mean: 10 °C; range: 7-13 °C): (i) constancy; (ii) cyclical fluctuations every two days; (iii) low stochasticity (random changes every 2 days); (iv) high stochasticity (random changes daily). Recently hatched individuals were monitored under thermal variability (6 weeks) and a subsequent one-month period of thermal constancy. We found that variability can positively influence survival, relative to thermal constancy, but negatively affect growth. The observations reported here can be interpreted within the context of Jensen's Inequality (performance at average conditions is unequal to average performance across a range of conditions). Projections of future population viability in the context of climate change would be strengthened by increased experimental attention to the fitness consequences of stochastic and non-stochastic thermal variability.

The mechanistic basis of demographic Allee effects: The search for mates.

In Focus: Berec, L., Kremer, A.M., Bernhauverova, V., & Drake, J.M. (2017). Density-dependent selection on mate-finding Allee effects. Journal of Animal Ecology, 87, 24-35. https://doi.org/10.1111/1365-2656.12662 In Focus: Shaw, A.K., Kokko, H., & Neubert, M.G. (2017). Details of mate finding drive dynamics of sex structured invasions. Journal of Animal Ecology, 87, 36-46. https://doi.org/10.1111/1365-2656.12658 Lowered population growth ability at low abundances is called the demographic Allee effect. The difficulty of finding mates in a sparse population is the best documented pathway through which a demographic Allee effect might arise. The articles in focus here aim to establish the mechanistic links between mate search component Allee effects and the emergent demographic Allee effect manifested at the level of population growth rate. The authors discover that limitations in the time invested in mate searching generates demographic Allee effects and that the population level adaptations of mate search time are likely to be dependent on the prevailing population density. Trade-offs between mate search, survival and reproductive outputs are key in understanding optimal mate search strategies and their fitness consequences. The present studies provide guidelines to identify populations at risk of experiencing demographic Allee effects at low abundances.

Detection of Allee effects in marine fishes: analytical biases generated by data availability and model selection.

The demographic Allee effect, or depensation, implies positive association between per capita population growth rate and population size at low abundances, thereby lowering growth ability of sparse populations. This can have far-reaching consequences on population recovery ability and colonization success. In the context of marine fishes, there is a widespread perception that Allee effects are rare or non-existent. However, studies that have failed to detect Allee effects in marine fishes have suffered from several fundamental methodological and data limitations. In the present study, we challenge the prevailing perception about the rarity of Allee effects by analysing nine populations of Atlantic herring (Clupea harengus), using Bayesian statistical methods. We find that populations of the same species can show either strong evidence for Allee effects or compensation. We explicitly demonstrate how the evidence for Allee effects is strongly provisional on observations made at low population abundances. We contrast our statistical approach with previous attempts to detect Allee effects and illustrate methodological issues that can lead to erroneous conclusions about the nature of population dynamics at low abundance. The present study demonstrates that there is no substantive scientific basis to support the perception that Allee effects are rare or non-existent in marine fishes.

Empirical links between natural mortality and recovery in marine fishes.

Probability of species recovery is thought to be correlated with specific aspects of organismal life history, such as age at maturity and longevity, and how these affect rates of natural mortality (M) and maximum per capita population growth (rmax). Despite strong theoretical underpinnings, these correlates have been based on predicted rather than realized population trajectories following threat mitigation. Here, we examine the level of empirical support for postulated links between a suite of life-history traits (related to maturity, age, size and growth) and recovery in marine fishes. Following threat mitigation (medium time since cessation of overfishing = 20 years), 71% of 55 temperate populations had fully recovered, the remainder exhibiting, on average, negligible change (impaired recovery). Singly, life-history traits did not influence recovery status. In combination, however, those that jointly reflect length-based mortality at maturity, Mα , revealed that recovered populations have higher Mα , which we hypothesize to reflect local adaptations associated with greater rmax But, within populations, the smaller sizes at maturity generated by overfishing are predicted to increase Mα , slowing recovery and increasing its uncertainty. We conclude that recovery potential is greater for populations adapted to high M but that temporal increases in M concomitant with smaller size at maturity will have the opposite effect. The recovery metric documented here (Mα ) has a sound theoretical basis, is significantly correlated with direct estimates of M that directly reflect rmax, is not reliant on data-intensive time series, can be readily estimated, and offers an empirically defensible correlate of recovery, given its clear links to the positive and impaired responses to threat mitigation that have been observed in fish populations over the past three decades.

Age at maturation has sex- and temperature-specific effects on telomere length in a fish.

Telomeres are highly conserved nucleoprotein structures which protect genome integrity. The length of telomeres is influenced by both genetic and environmental factors, but relatively little is known about how different hereditary and environmental factors interact in determining telomere length. We manipulated growth rates and timing of maturation by exposing full-sib nine-spined sticklebacks (Pungitius pungitius) to two different temperature treatments and quantified the effects of temperature treatments, sex, timing of maturation, growth rate and family (genetic influences) on telomere length. We did not find the overall effect of temperature treatment on the relative telomere length. However, we found that variation in telomere length was related to timing of maturation in a sex- and temperature-dependent manner. Telomere length was negatively related to age at maturation in elevated temperature and early maturing males and females differed in telomere length. Variation in growth rate did not explain any variation in telomere length. The broad sense heritability (h 2) of telomere length was estimated at h 2 = 0.31 - 0.47, suggesting predominance of environmental over genetic determinants of telomere length variability. This study provides the first evidence that age at maturation together with factors associated with it are influencing telomere length in an ectotherm. Future studies are encouraged to identify the extent to which these results can be replicated in other ectotherms.

Altered trait variability in response to size-selective mortality.

Changes in trait variability owing to size-selective harvesting have received little attention in comparison with changes in mean trait values, perhaps because of the expectation that phenotypic variability should generally be eroded by directional selection typical for fishing and hunting. We show, however, that directional selection, in particular for large body size, leads to increased body-size variation in experimentally harvested zebrafish (Danio rerio) populations exposed to two alternative feeding environments: ad libitum and temporarily restricted food availability. Trait variation may influence population adaptivity, stability and resilience. Therefore, rather than exerting selection pressures that favour small individuals, our results stress the importance of protecting large ones, as they can harbour a great amount of variation within a population, to manage fish stocks sustainably.

The role of life histories and trophic interactions in population recovery.

Factors affecting population recovery from depletion are at the focus of wildlife management. Particularly, it has been debated how life-history characteristics might affect population recovery ability and productivity. Many exploited fish stocks have shown temporal changes towards earlier maturation and reduced adult body size, potentially owing to evolutionary responses to fishing. Whereas such life-history changes have been widely documented, their potential role on stock's ability to recover from exploitation often remains ignored by traditional fisheries management. We used a marine ecosystem model parameterized for Southeastern Australian ecosystem to explore how changes towards "faster" life histories might affect population per capita growth rate r. We show that for most species changes towards earlier maturation during fishing have a negative effect (3-40% decrease) on r during the recovery phase. Faster juvenile growth and earlier maturation were beneficial early in life, but smaller adult body sizes reduced the lifetime reproductive output and increased adult natural mortality. However, both at intra- and inter-specific level natural mortality and trophic position of the species were as important in determining r as species longevity and age of maturation, suggesting that r cannot be predicted from life-history traits alone. Our study highlights that factors affecting population recovery ability and productivity should be explored in a multi-species context, where both age-specific fecundity and survival schedules are addressed simultaneously. It also suggests that contemporary life-history changes in harvested species are unlikely to increase their resilience and recovery ability.

Allee effect and the uncertainty of population recovery.

Recovery of depleted populations is fundamentally important for conservation biology and sustainable resource harvesting. At low abundance, population growth rate, a primary determinant of population recovery, is generally assumed to be relatively fast because competition is low (i.e., negative density dependence). But population growth can be limited in small populations by an Allee effect. This is particularly relevant for collapsed populations or species that have not recovered despite large reductions in, or elimination of, threats. We investigated how an Allee effect can influence the dynamics of recovery. We used Atlantic cod (Gadus morhua) as the study organism and an empirically quantified Allee effect for the species to parameterize our simulations. We simulated recovery through an individual-based mechanistic simulation model and then compared recovery among scenarios incorporating an Allee effect, negative density dependence, and an intermediate scenario. Although an Allee effect significantly slowed recovery, such that population increase could be negligible even after 100 years or more, it also made the time required for biomass rebuilding much less predictable. Our finding that an Allee effect greatly increased the uncertainty in recovery time frames provides an empirically based explanation for why the removal of threat does not always result in the recovery of depleted populations or species.

Environmental forcing alters fisheries selection.

Fishing-induced evolution (FIE) threatens the ecology, resilience, and economic value of fish populations. Traits under selection, and mechanisms of selection, can be influenced by abiotic and biotic perturbations, yet this has been overlooked. Here, we present the fishery selection continuum, where selection ranges from rigid fisheries selection to flexible fisheries selection. We provide examples on how FIE may function along this continuum, and identify selective processes that should be considered less or more flexible. We also introduce fishery reaction norms, which serve to conceptualise how selection from fishing may function in a dynamic context. Ultimately, we suggest an integrative approach to studying FIE that considers the environmental conditions in which it functions.

Food-web complexity, consumer behavior, and diet specialism: impacts on ecosystem stability.

Ecological stability is a fundamental aspect of food web dynamics. In this study, we explore the factors influencing stability in complex ecological networks, characterizing it through biomass oscillations and species persistence. Using an Extended Niche model, we generate diverse food web structures and investigate the effects of intraspecific consumer interference, network size, connectance, and diet specialism on stability. Our findings reveal that intraspecific consumer interference plays a pivotal role in shaping stability. Higher interference results in stable dynamics, reducing oscillations and extinctions. Additionally, differences emerge between food webs comprised of invertebrate consumers and those of ectotherm vertebrates, with the latter showing higher oscillations. Network size and connectance also influence stability, where larger and more connected webs tend to exhibit reduced oscillations. Overall, our study sheds light on the complex interplay of factors affecting ecological stability in food webs. Understanding these dynamics is crucial for biodiversity conservation and ecosystem management. Supplementary Information: The online version contains supplementary material available at 10.1007/s12080-024-00580-w.

Fish with slow life-history cope better with chronic manganese exposure than fish with fast life-history.

Animals with different life-history types vary in their stress-coping styles, which can affect their fitness and survival in changing environments. We studied how chronic exposure to manganese sulfate (MnSO4), a common aquatic pollutant, affects life-history traits, physiology, and behavior of zebrafish (Danio rerio) with two life-history types: fast (previously selected for fast juvenile growth, early maturation, and small adult body size) and slow life histories (selected for slow juvenile growth, late maturation, and large adult body size). We found that MnSO4 had negative effects on growth and condition factors, but the magnitude of these effects depended on the life-history type. Individuals with fast life histories were more susceptible to MnSO4 than fish with slow life histories as they had lower growth rate, condition factor and feeding probability in high MnSO4 concentrations. Our results demonstrate that MnSO4 can impair fish performance, and life-history variation can modulate the stress-coping ability of individuals.

Ecological consequences of body size decline in harvested fish species: positive feedback loops in trophic interactions amplify human impact.

Humans are changing marine ecosystems worldwide, both directly through fishing and indirectly through climate change. One of the little explored outcomes of human-induced change involves the decreasing body sizes of fishes. We use a marine ecosystem model to explore how a slow (less than 0.1% per year) decrease in the length of five harvested species could affect species interactions, biomasses and yields. We find that even small decreases in fish sizes are amplified by positive feedback loops in the ecosystem and can lead to major changes in natural mortality. For some species, a total of 4 per cent decrease in length-at-age over 50 years resulted in 50 per cent increase in predation mortality. However, the magnitude and direction in predation mortality changes differed among species and one shrinking species even experienced reduced predation pressure. Nevertheless, 50 years of gradual decrease in body size resulted in 1-35% decrease in biomasses and catches of all shrinking species. Therefore, fisheries management practices that ignore contemporary life-history changes are likely to overestimate long-term yields and can lead to overfishing.