Population Abundance Research Articles

Comparing reintroduction strategies for the endangered San Francisco gartersnake (Thamnophis sirtalis tetrataenia) using demographic models.

For endangered species persisting in a few populations, reintroductions to unoccupied habitat are a popular conservation action to increase viability in the long term. Identifying the reintroduction strategy that is most likely to result in viable founder and donor populations is essential to optimally use resources available for conservation. The San Francisco gartersnake (Thamnophis sirtalis tetrataenia) is an endangered sub-species that persists in a small number of populations in a highly urbanized region of California. Most of the extant populations of San Francisco gartersnakes have low adult abundance and effective population size, heightening the need for establishment of more populations for insurance against the risk of extinction. We used simulations from demographic models to project the probability of quasi-extinction for reintroduced populations of San Francisco gartersnakes based on the release of neonate, juvenile, adult, or mixed-age propagules. Our simulation results indicated that the release of head-started juveniles resulted in the greatest viability of reintroduced populations, and that releases would need to continue for at least 15 years to ensure a low probability of quasi-extinction. Releasing captive-bred juvenile snakes would also have less effect on the viability of the donor population, compared to strategies that require more adult snakes to be removed from the donor population for translocation. Our models focus on snake demography, but the genetic makeup of donor, captive, and reintroduced populations will also be a major concern for any proposed reintroduction plan. This study demonstrates how modeling can be used to inform reintroduction strategies for highly imperiled species.

Mechanistic modeling of climate effects on redistribution and population growth in a community of fish species.

Understanding community responses to climate is critical for anticipating the future impacts of global change. However, despite increased research efforts in this field, models that explicitly include important biological mechanisms are lacking. Quantifying the potential impacts of climate change on species is complicated by the fact that the effects of climate variation may manifest at several points in the biological process. To this end, we extend a dynamic mechanistic model that combines population dynamics, such as species interactions, with species redistribution by allowing climate to affect both processes. We examine their relative contributions in an application to the changing biomass of a community of eight species in the Gulf of Maine using over 30 years of fisheries data from the Northeast Fishery Science Center. Our model suggests that the mechanisms driving biomass trends vary across space, time, and species. Phase space plots demonstrate that failing to account for the dynamic nature of the environmental and biologic system can yield theoretical estimates of population abundances that are not observed in empirical data. The stock assessments used by fisheries managers to set fishing targets and allocate quotas often ignore environmental effects. At the same time, research examining the effects of climate change on fish has largely focused on redistribution. Frameworks that combine multiple biological reactions to climate change are particularly necessary for marine researchers. This work is just one approach to modeling the complexity of natural systems and highlights the need to incorporate multiple and possibly interacting biological processes in future models.

Census of breeding birds in the Pyrenees in the early 1980s: A publicly available dataset for ecological research.

The Pyrenees is a mountain range in south-western Europe that supports a rich diversity of bird species. In 1981, point count surveys of breeding birds (passerines and picidae) were carried out in the Vanera valley, a valley in the eastern Pyrenees, resulting in a data set of 228 counts. These data provide valuable information on the distribution and abundance of bird populations on 5350 ha of heterogeneous land (including cropland, grasslands, heaths and river banks, and pine forest) at altitudes ranging from 1100 to 2600 meters. Additional point count surveys were carried out in the pine forest from 1982 to 1985, resulting in a data set of 144 counts. Habitat descriptors (percentage of herbaceous and ligneous plant cover at different heights - 0-1m, 1-4m and more than 8m; vegetation type) and altitude were assessed around each point count. This dataset provides a complete picture of the breeding bird community in a typical valley of eastern Pyrenees in the early 1980s, which could be compared with future censuses to contribute to a variety of research questions, such as quantifying changes in birds between the early 1980s and now in mountain areas, understanding the effects of climate change on bird populations, examining the effects of habitat fragmentation and land-use change, and identifying priority areas for conservation and management. These data could inspire new research and contribute to our collective understanding of bird ecology in the Pyrenees.

ON THE PATTERNS AND CAUSES OF CHRONOGRAPHIC VARIABILITY OF MORPHOLOGICAL CHARACTERISTICS IN SOME MAMMALS: THE ORDERS RODENTIA AND CARNIVORA

Chronographic variations in some craniological features in the Spotted ground squirrel, the Red fox and the Wolf in the middle of the 20th to early 21st centuries have been established. They represent their temporary deviations from the average size towards an increase which, after several generations, is replaced by a return to the original values. The influence of trophic and climatic factors, parasites, diseases and population densities during the development of young individuals is considered. Chronographic changes in the condylobasal length and zygomatic width of the skull are established as temporary and reversible, increases in their values being replaced by their decreases. Increases in the size of the skull of the Spotted ground squirrel and the Red fox occur “suddenly” in individuals of one generation, and their return to the initial values occurs gradually in individuals of several subsequent generations. Chronographic variations in the size of the skull of the Spotted ground squirrel and the Red fox correlate with changes in reproduction rates and some other varying factors. They correspond to certain phases of population abundance. Increased sizes of the Red fox and Wolf skulls are observed in the phases of quitting a depression to the beginning of a population growth. The probable causes of chronographic changes lie in somatic heterosis, caused in natural habitats by the alternation of the prevailing mating types (inbreeding or outbreeding), these being due to massive rearrangements of individuals as a result of deep population reductions from natural or anthropogenic impacts.

Shifts in risk sensitivity and resource availability alter fat stores for a large mammal following extreme winter conditions

Abstract For species that inhabit environments where resource availability may be unpredictable, balance of resource allocation to life‐history traits can have heightened consequences for survival, reproduction, and ultimately, fitness. Acquisition and allocation of energy to maintenance, capital gain and reproduction should be in tune with the landscape an animal inhabits—environmental severity, food availability and population size all influence the resources animals have and dictate the ways they should be allocated. In seasonal environments, animals that experience periods of extreme resource limitation (e.g. harsh winters) may favour allocation of resources to body reserves to secure their survival at the cost of reproduction (i.e. risk averse). In contrast, the same accumulation of body reserves may not be necessary to survive in relatively benign landscapes where instead, allocation to reproduction is favoured (i.e. risk prone). According to the theory of risk‐sensitive allocation of resources, when animals are exposed to unprecedented or life‐threatening conditions, they may shift resource allocation to favour building capital over allocation in reproduction to preempt against encountering another life‐threatening event in the future. Using data from a long‐term project on a highly site‐faithful and long‐lived species, mule deer (Odocoileus hemionus), we evaluated how a life‐threatening winter and the associated changes in resource availability resulting from a population reduction influenced how animals acquired and allocated energy to survival (i.e. fat accumulation). Per capita precipitation, and the associated reduction in population abundance after the severe winter, had a positive influence of accrual of fat over summer. After the extreme physiological stress of a hard winter, deer starting spring with low body reserves accumulated 2.8 percentage points more fat over summer compared with before the experience of a bad winter and had an increased probability of recruiting fewer offspring. Fat stores can interact with environment, life history and behaviour to influence survival during periods of resource scarcity. For a long‐lived herbivore, we documented shifts in risk tolerance associated with fat accrual in preparation for winter, supporting the notion that risk‐sensitive allocation of resources may be plastic—an essential adaptation for animals to cope with rapidly changing landscapes. Read the free Plain Language Summary for this article on the Journal blog.

Біологічні особливості та перспективи штучного відтворення вирезуба (Rutilus frisii frisii Nordman, 1840) в Україні (огляд)

Purpose. The main goal of the thematic review was the analysis and generalization of literary sources on the peculiarities of its biology, technological aspects of artificial reproduction and growing of vyrezub (Rutilus frisii frisii Nordman, 1840) for the restoration of its natural population in water bodies of Ukraine. To evaluate the perspectives of artificial reproduction and growing of vyrezub in aquaculture of Ukraine. Findings. As a result of human impact on river networks of Ukraine, the abundance of vyrezub significantly decreased, it has become a rare and at the same time an endangered species. According to retrospective data, vyrezub previously belonged to the widespread commercial species of the Dnipro and Southern Bug rivers. The biological features of this species have been analyzed, the observance of which can allow obtaining viable fish seeds in aquaculture conditions. The adaptive possibilities of vyrezub to artificial conditions of reproduction have been reviewed. Historical information on the reproduction of this species over the last century has been analyzed. On the territory of Ukraine, first attempts to artificially reproduce vyrezub were carried out by specialists of Ukrcherrybvod. According to the results of studies of Ukrainian scientists, several stages of artificial reproduction of vyrezub have been identified, namely: taking spawners from natural water bodies, obtaining eggs and sperm, incubation of eggs in incubation devices, obtaining 6 g fish seeds, release of viable juveniles into natural water bodies. However, the works on artificial reproduction of vyrezub were not successful, which was the reason for their complete cessation. In particular, this species is promising for growing in polyculture, as it does not create intense trophic competition with most objects of freshwater aquaculture, and as a result of the consumption of mollusks will reduce the likelihood of the spread of some invasive fish diseases. Practical value. The review of information sources on various aspects of the use of vyrezub in aquaculture will contribute to the development of a system of separate links of the technology of its cultivation for the needs of rearing in the conditions of aquaculture enterprises and stocking of internal water bodies with viable juveniles. Key words: vyrezub (Rutilus frisii frisii Nord­man, 1840), aquaculture, biocontrol effect, artificial reproduction, viable juveniles, stocking, restoration of population abundance.

Agent-based diffusion in predation systems with Beddington–DeAngelis response

Understanding dynamical behavior of a spatially distributed population is crucial to conservation and management of endangered species. This paper considers predator–prey systems with Beddington–DeAngelis functional response, where the predator moves between source–sink patches asymmetrically and acts as an agent. Our aim is to show how agent-based diffusion affects dynamics of the system and total population abundance of the species. Using dynamical systems theory, we demonstrate stability of positive equilibria in the system, which implies coexistence of the species and change of abundance by diffusion. Moreover, we show Hopf and Bautin bifurcations with multiple limit cycles, which implies multiple oscillations of populations and even extinction of species. Furthermore, this work demonstrates that diffusion in the system may lead to results reversing those without diffusion. The diffusion could change dynamics of the system between coexistence at a steady state and persistence in periodic oscillation, while evolution in asymmetry of diffusion could make the predator reach a total abundance larger than that without diffusion, even reach the maximal abundance. Our results are consistent with experimental observations and are important in studying conservation of biodiversity.

A review of aerial survey density estimates of bearded seals (Erignathus barbatus) in the Canadian Arctic highlights important knowledge gaps and research needs

AbstractThere has been significant sea ice loss in the Arctic as a result of climate change, with measurable impacts on available habitat for ice-obligate marine mammals. Bearded seals (Erignathus barbatus) have a circumpolar distribution and primarily inhabit coastal areas of shallower depths with seasonal pack ice cover. However, there is limited information and no formal estimates on population abundance and densities of bearded seals within Canada. Here, we review historic field reports and publications from aerial surveys conducted from 1974 to 2022 to compile the density and abundance estimates of bearded seals across Canadian waters. Aerial surveys for marine mammals have been flown in several areas across the Canadian Arctic, including the Beaufort Sea (1974–2006), Canadian High Arctic (1979–2022), Baffin Bay-Davis Strait (1979–2014), and the Hudson Complex (1994–2017). After reviewing all published data sources, we report that the density of bearded seals in Canada were highest (and highly variable) in the Beaufort Sea (0.01–8.68 seals*km−2), then in Baffin Bay-Davis Strait (0.004–8.3 seals*km−2), and lowest and less variable inter-annually in the Hudson Complex (0.02 to 0.12 seals*km−2) at the time of these surveys. We also determined that bearded seals are more often found in areas of patchy ice cover (50–75%) with shallow water depths ≤ 500 m. Further, this review identifies regions within the Canadian Arctic and sub-Arctic that require updated aerial survey information. Quantifying the abundance and density estimates of bearded seals in Canada is essential for monitoring population status over time to better understand how this species is responding to environmental variation from anthropogenic activity and climate change.

Diverse pathways for climate resilience in marine fishery systems

AbstractBoth the ecological and social dimensions of fisheries are being affected by climate change. As a result, policymakers, managers, scientists and fishing communities are seeking guidance on how to holistically build resilience to climate change. Numerous studies have highlighted key attributes of resilience in fisheries, yet concrete examples that explicitly link these attributes to social‐ecological outcomes are lacking. To better understand climate resilience, we assembled 18 case studies spanning ecological, socio‐economic, governance and geographic contexts. Using a novel framework for evaluating 38 resilience attributes, the case studies were systematically assessed to understand how attributes enable or inhibit resilience to a given climate stressor. We found population abundance, learning capacity, and responsive governance were the most important attributes for conferring resilience, with ecosystem connectivity, place attachment, and accountable governance scoring the strongest across the climate‐resilient fisheries. We used these responses to develop an attribute typology that describes robust sources of resilience, actionable priority attributes and attributes that are case specific or require research. We identified five fishery archetypes to guide stakeholders as they set long‐term goals and prioritize actions to improve resilience. Lastly, we found evidence for two pathways to resilience: (1) building ecological assets and strengthening communities, which we observed in rural and small‐scale fisheries, and (2) building economic assets and improving effective governance, which was demonstrated in urban and wealthy fisheries. Our synthesis presents a novel framework that can be directly applied to identify approaches, pathways and actionable levers for improving climate resilience in fishery systems.

The impact of stand composition and tree density on topsoil characteristics and soil microbial activities

The assessment of long-term effects of forest management practices, particularly species mixing and stand density, provides valuable information for the forestry sectors. This study evaluated and compared the effect of stand composition and density on organic horizon (i.e., OL, OF, OH) and organo-mineral horizon characteristics of nine stands in the Orleans State Forest (France), seven years after the first thinning treatments. To this end, three triplets of stands of pure Quercus petraea Matt., pure Pinus sylvestris L. and a mixture of both species were selected. Each stand consisted of two plots with different tree densities: low and normal. Physicochemical variables were measured on the organic humus horizon (OH), while microbial biomass carbon (MBC) and nitrogen (MBN), and soil microbial metabolic profile were evaluated on the organo-mineral horizon; the abundance of soil microbial populations (i.e., bacteria, fungi and archaea) in each plot was also assessed by qPCR. The OH thickness consistently increased under pure pine stands (25–35 mm), while other OH characteristics showed no variation based on stand composition and tree density. Low-density plots exhibited changes in microbial biomass, with a significant decrease in both MBC and MBN. Moreover, the highest MBC was recorded under pure pine stands (1241 mg C.kg−1 DW soil), and the highest MBN under pure oak stands (24–39 mg N.kg−1 DW soil). The highest C assimilation rates were recorded in the mixed stands, especially under low tree density. Bacteria and archea were similarly abundant across stand compositions and tree densities, while fungi tended to be more abundant in the mixed coniferous-broadleaf stands. Our findings should be considered by the forestry sectors of European countries where these two species are distributed, and suggest that EU forestry strategies should promote biodiversity in the context of tree plantations.

Isolation and Identification of Biocontrol Bacteria against Atractylodes Chinensis Root Rot and Their Effects.

Fusarium root rot (FRR) seriously affects the growth and productivity of A. chinensis. Therefore, protecting A. chinensis from FRR has become an important task, especially for increasing A. chinensis production. The purpose of this study was to screen FRR control strains from the A. chinensis rhizosphere soil. Eighty-four bacterial strains and seven fungal strains were isolated, and five strains were identified with high inhibitory effects against Fusarium oxysporum (FO): Trichoderma harzianum (MH), Bacillus amyloliquefaciens (CJ5, CJ7, and CJ8), and Bacillus subtilis (CJ9). All five strains had high antagonistic effects in vitro. Results showed that MH and CJ5, as biological control agents, had high control potential, with antagonistic rates of 86.01% and 82.78%, respectively. In the pot experiment, the growth levels of roots and stems of A. chinensis seedlings treated with MH+CJ were significantly higher than those of control plants. The total nitrogen, total phosphorus, total potassium, indoleacetic acid, and chlorophyll contents in A. chinensis leaves were also significantly increased. In the biocontrol test, the combined MH + CJ application significantly decreased the malondialdehyde content in A. chinensis roots and significantly increased the polyphenol oxidase, phenylalanine ammonolyase, and peroxidase ability, indicating a high biocontrol effect. In addition, the application of Bacillus spp. and T. harzianum increased the abundance and diversity of the soil fungal population, improved the soil microbial community structure, and significantly increased the abundance of beneficial strains, such as Holtermanniella and Metarhizium. The abundance of Fusarium, Volutella, and other pathogenic strains was significantly reduced, and the biocontrol potential of A. chinensis root rot was increased. Thus, Bacillus spp. and T. harzianum complex bacteria can be considered potential future biocontrol agents for FRR.

Denitrifying phosphorus removal (DPR) performance and metabolic mechanisms of Tetrasphaera at long-term nitrite exposure

Tetrasphaera has been identified as the main polyphosphate-accumulating organism (PAOs) in enhanced biological phosphate removal (EBPR), but the ability of Tetrasphaera to use nitrite as electron acceptor for denitrifying phosphorus removal (DPR) is still unclear. This study investigated the DPR performance and mechanism, microbial structures, and FNA inhibition of Tetrasphaera-dominated culture in the long-term acclimatization at different nitrite concentrations (10, 20, 40 and 50 mgN/L). The results showed that the most optimal influent nitrite concentration was 20 mgN/L (C/N = 6), and the removal efficiency of nitrite and phosphorus reached above 90 % and 95 %, respectively. Fluorescence In Situ Hybridization-Flow Cytometry (FISH-FCM) showed that the population abundance of Tetrasphaera was 80.95 %, in which Tet2-892 clade was the most abundant and mainly responsible for DPR with nitrite as electron acceptor. When the nitrite concentration reached 50 mgN/L (FNA = 12.80 µgHNO2-N/L, pH = 7.0 ± 0.1), electron supply deficiency and FNA inhibition resulted in only 43.0 % and 68.4 % of nitrite and phosphorus removal efficiency, respectively. The intracellular amino acid and glycogen metabolism were inhibited by FNA under this condition. A significant accumulation of N2O was observed, resulting from a combination of the insufficient electron supply, absence of nos-Z gene, and long-term FNA inhibition. DPR dominated by Tetrasphaera with appropriate nitrite as electron acceptor provides a new way to achieve nitrogen and phosphorus simultaneous removal. In addition, this study found that Tet2-892 clade of Tetrasphaera was more tolerant to FNA compared to Accumulibacter and GAOs, which provides a strategy for inhibiting GAOs activity to maintain the stability of the EBPR system.

Models for linking hunter retention and recruitment to regulations and game populations

IntroductionDeclining hunter populations across North America present wildlife management agencies with the prospect of declining revenues for wildlife conservation and management and the need for new tools to evaluate management strategies and predict future status of game species and hunters.MethodsHere we present a modeling framework and potential decision support tool for managers to link future hunter population dynamics to regulatory restrictiveness, prey abundance, and harvest success. Our hunter model is parameterized based on the authors’ judgment and can be used for demonstration purposes. We simulated three scenarios of restricted harvest, moderate harvest and liberal harvest.ResultsOur simulations show that even though liberal harvest predicts higher cumulative license sales revenue, it corresponds with a slight decline in buck abundance over 10 years. In contrast, highly restrictive harvest corresponds with deer population growth, but a near collapse of hunter populations. Our model demonstrates that managers might face tradeoffs between managing for deer population abundance and hunting revenue and clarifies how these factors might affect decision making.DiscussionThe utility of our tool would be dependent on accessing data on hunter retention and recruitment, however, the strength of our paper is in highlighting a new way of thinking about and potentially addressing these potential tradeoffs. Further, these simulations demonstrate that these tools could be used to evaluate management strategies but also highlight uncertainties, establish research priorities, and potentially design an adaptive management framework.

Primary parasitoids of Aulacaspis tubercularis Newstead (Hemiptera: Diaspididae) occurring in Spanish mango orchards

The white mango scale (WMS), Aulacaspis tubercularis, is the main pest of mango crops in Southern Spain. Field surveys were carried out from July 2019 to May 2022 to investigate the identity, seasonal abundance and active parasitism rates of A. tubercularis primary parasitoid species, using yellow sticky traps and periodic leaf sampling. The combination of morphological characters and DNA sequences of the mitochondrial cytochrome c oxidase subunit 1 (mtCOI) and 28S ribosomal (28S rDNA) genes allowed the identification of only two parasitoid species belonging to the genus Encarsia Förster (Hymenoptera: Aphelinidae). The most prevalent, E. citrina (Crawford), is a widespread species and proved to be the dominant species in the sampled mango orchards. Next, Encarsia sp., is an unidentified species genetically close to E. lounsburyi Berlese but significantly differs in the sequences of the mtCOI and 28S rDNA regions. Population abundance and active parasitism rates of Encarsia parasitoids depended on the presence of susceptible WMS stages for parasitism and on the abiotic conditions. The seasonal abundance of these parasitoid species fluctuated around the year, being lower during the winter but it increased in late spring, reaching the highest values at the end of summer. The average percentage of active parasitism was 12.48% for female scales and 13.97% for male scales but reached maximum values of 38.56 and 43.52%, respectively at specific seasons in the year. The highest rates of parasitism for female scales were observed in summer, while for male scales were observed in winter. The natural occurrence of parasitoids was unable to control the WMS populations and, consequently, complementary measures should be implemented to reduce its density levels.

Comparative abundance and diversity of populations of the Pseudomonas syringae and Soft Rot Pectobacteriaceae species complexes throughout the Durance River catchment from its French Alps sources to its delta

Comparative abundance and diversity of populations of the Pseudomonas syringae and Soft Rot Pectobacteriaceae species complexes throughout the Durance River catchment from its French Alps sources to its delta

Recent Advances in Bacterial Persistence Mechanisms.

The recurrence of bacterial infectious diseases is closely associated with bacterial persisters. This subpopulation of bacteria can escape antibiotic treatment by entering a metabolic status of low activity through various mechanisms, for example, biofilm, toxin-antitoxin modules, the stringent response, and the SOS response. Correspondingly, multiple new treatments are being developed. However, due to their spontaneous low abundance in populations and the lack of research on in vivo interactions between persisters and the host's immune system, microfluidics, high-throughput sequencing, and microscopy techniques are combined innovatively to explore the mechanisms of persister formation and maintenance at the single-cell level. Here, we outline the main mechanisms of persister formation, and describe the cutting-edge technology for further research. Despite the significant progress regarding study techniques, some challenges remain to be tackled.

Frequency of change determines effectiveness of microbial response strategies

Nature challenges microbes with change at different frequencies and demands an effective response for survival. Here, we used controlled laboratory experiments to investigate the effectiveness of different response strategies, such as post-translational modification, transcriptional regulation, and specialized versus adaptable metabolisms. For this, we inoculated replicated chemostats with an enrichment culture obtained from sulfidic stream microbiomes 16 weeks prior. The chemostats were submitted to alternatingly oxic and anoxic conditions at three frequencies, with periods of 1, 4 and 16 days. The microbial response was recorded with 16S rRNA gene amplicon sequencing, shotgun metagenomics, transcriptomics and proteomics. Metagenomics resolved provisional genomes of all abundant bacterial populations, mainly affiliated with Proteobacteria and Bacteroidetes. Almost all these populations maintained a steady growth rate under both redox conditions at all three frequencies of change. Our results supported three conclusions: (1) Oscillating oxic/anoxic conditions selected for generalistic species, rather than species specializing in only a single condition. (2) A high frequency of change selected for strong codon usage bias. (3) Alignment of transcriptomes and proteomes required multiple generations and was dependent on a low frequency of change.

Low-coverage whole genome sequencing for highly accurate population assignment: Mapping migratory connectivity in the American Redstart (Setophaga ruticilla).

Understanding the geographic linkages among populations across the annual cycle is an essential component for understanding the ecology and evolution of migratory species and for facilitating their effective conservation. While genetic markers have been widely applied to describe migratory connections, the rapid development of new sequencing methods, such as low-coverage whole genome sequencing (lcWGS), provides new opportunities for improved estimates of migratory connectivity. Here, we use lcWGS to identify fine-scale population structure in a widespread songbird, the American Redstart (Setophaga ruticilla), and accurately assign individuals to genetically distinct breeding populations. Assignment of individuals from the nonbreeding range reveals population-specific patterns of varying migratory connectivity. By combining migratory connectivity results with demographic analysis of population abundance and trends, we consider full annual cycle conservation strategies for preserving numbers of individuals and genetic diversity. Notably, we highlight the importance of the Northern Temperate-Greater Antilles migratory population as containing the largest proportion of individuals in the species. Finally, we highlight valuable considerations for other population assignment studies aimed at using lcWGS. Our results have broad implications for improving our understanding of the ecology and evolution of migratory species through conservation genomics approaches.

Spawner weight and ocean temperature drive Allee effect dynamics in Atlantic cod, Gadus morhua: inherent and emergent density regulation

Abstract. Stocks of Atlantic cod, Gadus morhua, show diverse recovery responses when fishing pressure is relieved. The expected outcome of reduced fishing pressure is that the population regains its size. However, there are also cod stocks that seem to be locked in a state of low abundance from which population growth does not occur (or only slowly occurs). A plausible explanation for this phenomenon can be provided by the Allee effect, which takes place when recruitment per capita is positively related to population density or abundance. However, because of methodological limitations and data constraints, such a phenomenon is often perceived as being rare or non-existent in marine fish. In this study, we used time series of 17 Atlantic cod stocks to fit a family of population equations that consider the abundance of spawners, their body weight and sea water temperature as independent components of recruitment. The developed stock-recruitment function disentangles the effects of spawner abundance, spawner weight and temperature on recruitment dynamics and captures the diversity of density dependencies (compensation, Allee effect) of the recruitment production in Atlantic cod. The results show for 13 cod stocks an inherent spawner-abundance-related Allee effect. Allee effect strength, i.e., the relative change between maximum and minimum recruitment per capita at low abundance, was increased when recruitment production was suppressed by unfavorable changes in water temperature and/or in spawner weight. The latter can be a concomitant of heavy fishing or a result of temperature-related altered body growth. Allee effect strength was decreased when spawner weight and/or temperature elevated recruitment production. We show how anthropogenic stress can increase the risk of Allee effects in stocks where ocean temperature and/or spawner weight had been beneficial in the past but are likely to unmask and strengthen an inherent Allee effect under future conditions.

Effects of exposure to nanoplastics on the gill of mussels Mytilus galloprovincialis: An integrated perspective from multiple biomarkers

The pervasive presence of nanoplastics (NPs) in marine environments poses a threat to marine organisms. Gills, as the organ in direct contact with the environment in marine invertebrates, maybe the first to accumulate NPs. To date, the toxic effects of NPs on the gills of marine invertebrates are still largely unknown. In this study, the response of multiple biomarkers (i.e., total antioxidant capacity, the activity of acetylcholine, ion content and transport enzyme, metabolic enzymes, and lipids content) in mussels Mytilus galloprovincialis exposed to polystyrene nanoplastics (PS-NPs) for 7 days were evaluated. Significant inductions of total antioxidant capacity (T-AOC) and inhibition of acetylcholine (AChE) activity were detected after 7 days of PS-NPs exposure. PS-NPs also triggered significant alteration in ion content (Na+ and K+) and suppressed the activities of the ion transport enzyme (Na+/K+-ATPase). Moreover, we found the activity of metabolic enzymes (succinate dehydrogenase and pyruvate kinase) and lipids content (triacylglycerol and cholesterol) were significantly altered, suggesting the interference of PS-NPs on energy metabolism and lipid metabolism. This investigation provides substantial information to understand the physical responses of invertebrate gills to PS-NPs exposure. Given the crucial ecological roles of invertebrates, the presence of PS-NPs in the marine environment may have far-reaching impacts on population abundance, biodiversity, and stability of the marine ecosystem.