•A framework is developed to assess fish and invertebrate stocks that includes age-, size- and age-size-structured population dynamics models.•A variety of data types can be used for parameter ...estimation.•Uncertainty due to model type dominates when changing specifications within a model type.
An assessment framework is developed that allows analysts to conduct stock assessments for fish and invertebrate stocks based on age-, size- and age-size-structured population dynamics models. The size-structured model is nested within the age-size-structured model. The framework can use catch, discard, index of abundance, size- and age-composition, conditional age-at-length, mean length-at-age, and tagging data to estimate model parameters. It is used to explore the sensitivity of key model outputs for Pacific cod in the Eastern Bering Sea by applying model configurations that use the same data, same likelihood functions, and same data weighting schemes. Base model configurations using the three model types all fit the available data adequately, but the age-structured model fits the data better than the size-structured model. Variation in estimates of spawning biomass and the overfishing level was higher among model-types than within model-types. This result highlights the need for assessment analysts to focus more on applying and presenting results for multiple models.
Stage structured models, by grouping individuals with similar demographic characteristics together, have proven useful in describing population dynamics. This manuscript starts from reviewing two ...widely used modeling frameworks that are in the form of integral equations and age-structured partial differential equations. Both modeling frameworks can be reduced to the same differential equation structures with/without time delays by applying Dirac and gamma distributions for the stage durations. Each framework has its advantages and inherent limitations. The net reproduction number and initial growth rate can be easily defined from the integral equation. However, it becomes challenging to integrate the density-dependent regulations on the stage distribution and survival probabilities in an integral equation, which may be suitably incorporated into partial differential equations. Further recent modeling studies, in particular those by Stephen A. Gourley and collaborators, are reviewed under the conditions of the stage duration distribution and survival probability being regulated by population density.
The population dynamics of many colonially breeding seabirds are characterized by large interannual fluctuations that cannot be explained by environmental conditions alone. This variation may be ...particularly confounded by the use of skipped breeding by seabirds as a life‐history strategy, which directly impacts the number of breeding pairs and may affect the accuracy of breeding abundance as a metric of population health. Additionally, large fluctuations in time series may suggest that the underlying population dynamics are heavy tailed, allowing for a higher likelihood of extreme events than expected under Gaussian dynamics.
Here, we investigated the effect of demography on time series for abundance of the Adélie penguin Pygoscelis adeliae and explored the occurrence of heavy‐tailed dynamics in observed Adélie time series. We focus this study on the Adélie penguin as it is an important bellwether species long used to track the impacts of climate change and fishing on the Southern Ocean ecosystem and shares life‐history traits with many colonial seabirds.
We quantified the impacts of demographic rates, including skipped breeding, on time series of Adélie abundance simulated using an age‐structured model. We also used observed time series of Adélie breeding abundance at all known Antarctic colonies to classify distributions for abundance as Gaussian or non‐Gaussian heavy tailed. We then identified the cause of such heavy‐tailed dynamics in simulated time series and linked these to spatial patterns in Adélie food resource variability.
We found that breeding propensity drives observed breeding fluctuations more than any other vital rate, with high variability in skipped breeding decoupling true abundance from observed breeding abundance. We also found several Antarctic regions characterized by heavy‐tailed dynamics in abundance. These regions were often also characterized by high variability in zooplankton availability. In simulated time series, heavy‐tailed dynamics were strongly linked to high variability in adult survival.
Our results illustrate that stochastic variability in abundance dynamics, particularly the presence of variable rates of skipped breeding, can challenge our interpretation of fluctuations in abundance through time and obscure the relationship between key environmental drivers and population abundance.
The authors illustrate that the presence of variable rates of skipped breeding of Adélie penguins can challenge our interpretation of abundance fluctuations and obscure relationships with key environmental drivers. While valuable for assessing long‐term trends over time, time series of abundance may have limited value for identifying causal drivers.
Objectives
This study aimed to predict mortality in children with pneumonia who were admitted to the intensive care unit (ICU) to aid decision‐making.
Study Design
Retrospective cohort study ...conducted at a single tertiary hospital.
Patients
This study included children who were admitted to the pediatric ICU at the National Taiwan University Hospital between 2010 and 2019 due to pneumonia.
Methodology
Two prediction models were developed using tree‐structured machine learning algorithms. The primary outcomes were ICU mortality and 24‐h ICU mortality. A total of 33 features, including demographics, underlying diseases, vital signs, and laboratory data, were collected from the electronic health records. The machine learning models were constructed using the development data set, and performance matrices were computed using the holdout test data set.
Results
A total of 1231 ICU admissions of children with pneumonia were included in the final cohort. The area under the receiver operating characteristic curves (AUROCs) of the ICU mortality model and 24‐h ICU mortality models was 0.80 (95% confidence interval CI, 0.69–0.91) and 0.92 (95% CI, 0.86–0.92), respectively. Based on feature importance, the model developed in this study tended to predict increased mortality for the subsequent 24 h if a reduction in the blood pressure, peripheral capillary oxygen saturation (SpO2), or higher partial pressure of carbon dioxide (PCO2) were observed.
Conclusions
This study demonstrated that the machine learning models for predicting ICU mortality and 24‐h ICU mortality in children with pneumonia have the potential to support decision‐making, especially in resource‐limited settings.
•The first formal stock assessments for cisco in the Great Lakes were developed, in Thunder Bay, Ontario.•An age-structured assessment model was preferred to a size-structured model largely due to ...the ability to estimate Μ.•Estimation of Μ in the size-structured model was likely limited due to the fishery targeting individuals near asymptotic size.
Stock assessments are critical to modern fisheries management, supporting the calculation of key reference variables used to make informed management decisions. However, there is still considerable uncertainty as to which class of assessment models is appropriate to use under different circumstances. A common class of models used when age data are available are statistical catch-at-age assessment (SCAA) models, which track annual cohorts through time. When age data are unavailable, as is often the case in invertebrate fisheries where the lack of a bony structure such as otoliths makes aging difficult, statistical catch-at-size assessment (SCSA) models are more often employed, tracking fish or invertebrates through time by size-classes rather than ages. Do SCAA models actually perform better than SCSA models when age data are available, or is this just an assumption we make in fisheries research and management? We examined this question by evaluating the effectiveness of both SCAA and SCSA models in characterizing cisco, Coregonus artedi, population dynamics in Thunder Bay, Ontario. Both models were fit using an integrated framework with multiple sources of data including hydroacoustic estimates of spawning stock, fishery-dependent and -independent age/length compositions, and harvest data. Our results suggest that for cisco in Thunder Bay, data-limitations related to lack of size-composition data over the size range for which cisco growth is rapid resulted in difficulty estimating relative year-class strength within a SCSA. This led to parameter confounding and ultimately the inability to estimate natural mortality within a SCSA. This hampered the utility of a SCSA model in comparison with a SCAA model when age-composition data were available.
Adult mortality is often the most sensitive vital rate affecting at‐risk wildlife populations. Therefore, road ecology studies often focus on adult mortality despite the possibility for roads to be ...hazardous to juvenile individuals during natal dispersal. Failure to quantify concurrent variation in mortality risk and population sensitivity across demographic states can mislead the efforts to understand and mitigate the effects of population threats. To compare relative population impacts from road mortality among demographic classes, we weighted mortality observations by applying reproductive value analysis to quantify expected stage‐specific contributions to population growth. We demonstrate this approach for snapping turtles (Chelydra serpentina) observed on roads at two focal sites in Ontario, Canada, where we collected data for both live and dead individuals observed on roads. We estimated reproductive values using stage‐classified matrix models to compare relative population‐level impacts of adult and juvenile mortality. Reproductive value analysis is a tractable approach to assessing demographically variable effects for applications covering large spatial scales, nondiscrete populations, or where abundance data are lacking. For one site with long‐term life‐history data, we compared demographic frequency on roads to expected general population frequencies predicted by the matrix model. Our application of reproductive value is sex specific but, as juvenile snapping turtles lack external secondary sex characters, we estimated the sex ratio of road‐crossing juveniles after dissecting and sexing carcasses collected on roads at five sites across central Ontario, Canada. Juveniles were more abundant on roads than expected, suggesting a substantial dispersal contribution, and the road‐killed juvenile sex ratio approached 1:1. A higher proportion of juveniles were also found dead compared with adults, and cumulative juvenile mortality had similar population‐level importance as adult mortality. This suggests that the impact of roads needs to be considered across all life stages, even in wildlife species with slow life histories, such as snapping turtles, that are particularly sensitive to adult mortality.
•An operating model based on an age- and size-structured population dynamics model that included growth platoons is developed.•Assessment methods based on age-, size-, and age- and size-structured ...population dynamics models are evaluated.•Simpler models of those considered lead to least-worst performance.
Stock assessments estimate biomass and fishing mortality in absolute terms and relative to reference points. Most of the world’s stock assessments are based on age- or size-structured population dynamics models, with few stock assessments directly accounting for both age and size dynamics. However, the life history parameters of fish and invertebrate populations are often functions of both age and size. An operating model that is based on an age- and size-structured population dynamics model is used to evaluate the performance of assessment methods based on age-, size- and age- and size-structured population dynamics models. Variants of the operating model and the assessment methods, which include ‘platoons’ to better represent individual variation in growth, are considered to explore the impact of this source of uncertainty on the performance of stock assessment methods. Simulation experiments based conceptually on the assessment for Pacific cod in the Eastern Bering Sea are used to explore the consequences of applying assessment methods that are mis-specified in terms of the population dynamics model and the way variation in growth is modelled. The age-structured assessment methods perform poorest, most likely because they mis-represent how mortality, due to fishing, is removed from the population in the operating model, but also because of mis-specification of the growth curve. The assessment methods with platoons can outperform those that ignore platoon structure when this is present, but their performance when there are no platoons is such that, overall, simpler assessment methods based on size-structured, or age- and size-structured population dynamics models, appear best. Conducting assessments using multiple model types and selecting the best model based on the lowest AIC was, however, the best approach overall.
This paper concerns an age-structured juvenile-adult model incorporating harvesting pulse and moving boundaries in a heterogeneous environment, in which harvesting reflects human periodic pulse ...intervention on adults and the moving boundaries describe the natural expanding front of species. The principal eigenvalue is firstly defined and its properties involving the intensity of harvesting and length of habitat sizes are analyzed. Then the criteria to determine whether the species spread or vanish is discussed, and some relevant sufficient conditions characterized by pulse are established. Our results reveal that the co-extinction or coexistence of species is influenced by internal expanding capacity from species itself and external harvesting pulse from human intervention, in which the intensity and timing of harvesting play key roles. Our numerical simulations validate that the larger the harvesting rate and the shorter the harvesting period, the worse the survival of the species due to the cooperation among juveniles and adults, and such harvesting pulse can even alter the situation of species from persistence to extinction. In addition, expanding capacities also affect or alter the outcomes of spreading and vanishing.
