Escapement Estimates Research Articles

Effects of ion composition on escape and morphology on Mars

Abstract. We refine a recently presented method to estimate ion escape from non-magnetized planets and apply it to Mars. The method combines in situ observations and a hybrid plasma model (ions as particles, electrons as a fluid). We use measurements from the Mars Atmosphere and Volatile Evolution (MAVEN) mission and Mars Express (MEX) for one orbit on 1 March 2015. Observed upstream solar-wind conditions are used as input to the model. We then vary the total ionospheric ion upflux until the solution fits the observed bow shock location. This solution is a self-consistent approximation of the global Mars–solar-wind interaction at the time of the bow shock crossing for the given upstream conditions. We can then study global properties, such as the heavy-ion escape rate. Here, we investigate in a case study the effects on escape estimates of assumed ionospheric ion composition, solar-wind alpha-particle concentration and temperature, solar-wind velocity aberration, and solar-wind electron temperature. We also study the amount of escape in the ion plume and in the tail of the planet. Here, we find that estimates of total heavy-ion escape are not very sensitive to the composition of the heavy ions or to the number and temperature of the solar-wind alpha particles. We also find that velocity aberration has a minor influence on escape but that it is sensitive to the solar-wind electron temperature. The plume escape is found to contribute 29 % of the total heavy-ion escape, in agreement with observations. Heavier ions have a larger fraction of escape in the plume compared to the tail. We also find that the escape estimates scale inversely with the square root of the atomic mass of the escaping ion species.

Modifying a pelagic trawl to better retain small Arctic fishes

Pelagic trawls are one of the primary methods of sampling midwater fishes. However, these trawls are species- and size-selective, and small fish can escape through trawl meshes. This can introduce uncertainty and bias into survey abundance estimates if not accounted for. The small, abundant pelagic fishes of the Alaska Arctic are challenging to sample with trawls as they are sufficiently motile to avoid small fine-mesh trawls but are also small enough to escape through the meshes of trawls designed to capture larger fishes. A pelagic herring trawl equipped with a fine-mesh codend liner was used to quantify the size and species composition of pelagic fishes during a baseline acoustic-trawl survey of the Chukchi Shelf. Subsequent experiments with recapture nets attached to the outside of the trawl netting suggested that escapement of small fishes was substantial, particularly in the aft net section. Thus, the trawl was further modified by reducing the taper in the aft net section and adding a small-mesh section in front of the codend to potentially reduce escapement. Further use of recapture nets during two subsequent acoustic-trawl surveys confirmed that this trawl modification substantially increased retention of small fishes and resulted in less size selectivity. These improvements will reduce biases in estimates of abundance, size, and species composition of pelagic Arctic fishes. This work highlights the importance of quantifying escapement from survey trawls and demonstrates that escapement estimates can guide successful trawl modifications.

Reservoir Fish Escapement in North America: A Historical Review and Future Directions

AbstractDownstream escapement of fishes from reservoirs via release structures can represent a loss to populations that is comparable to natural and harvest mortality. Consequently, quantifying and managing fish escapement constitute a critical component of sustainable reservoir fish management. We reviewed existing literature to assess the state of knowledge of reservoir fish escapement and found 57 unique papers published from 1942 through 2021. Early studies sought to evaluate escapement by directly capturing fish below release structures. More recently, advances in technology have enabled more detailed studies on factors influencing escapement and the influence it has on regulating reservoir fisheries. Evaluations have occurred throughout North America, assessing escapement of 49 species through a number of different outlet structures. Annual escapement estimates ranged from 0% to 100%, and escapement tended to be higher from spillways and surface release outlets compared to other outlets. Further, smaller‐bodied individuals tended to escape at higher rates than larger fish and escapement was generally positively related to reservoir discharge metrics. Sixteen papers assessed benefits of physical and nonphysical barriers for reducing reservoir fish escapement and determined that the barriers were effective for retaining fish in reservoirs. We conclude by describing management options to address escapement as well as three pressing research needs that will broaden the existing knowledge base regarding fish escapement. In light of predicted changes in precipitation events and subsequent adaptations to reservoir management, quantifying and mitigating fish escapement will be a critical component of sustainable reservoir fish management in the future.

Zodiacal exoplanets in time – XIII. Planet orbits and atmospheres in the V1298 Tau system, a keystone in studies of early planetary evolution

ABSTRACT Studies of planetary systems of stars in star-forming regions and young clusters open a window on the formative stages of planetary evolution. We obtained high-cadence high-resolution infrared spectroscopy of the solar-mass Taurus association-member V1298 Tau during a transit of its 10R⊕-size ‘b’ planet. We measured the systemic radial velocity (RV) and find that the kinematics of V1298 Tau suggest an affiliation with a ≳6 Myr-old subgroup. A comparison of V1298 Tau and the nearby, co-moving star 2M0405 with stellar evolution models suggests an age of ∼10–25 Myr. We measured the projected spin-orbit angle of ‘b’ as $\lambda =15_{-16}^{+15}$ and $\lambda = 2_{-4}^{+12}$ degrees using the apparent RV shift and change in line profile, respectively, induced by the transient occultation of the rotating star by the planet. These values indicate a prograde orbit like that of the interior ‘c’ planet of V1298 Tau and point to a co-planar multiplanet system that formed within a disc. We also measured variation in the strength of the 1083 nm triplet of neutral orthohelium as a probe of any extended/escaping atmosphere around ‘b’. We detect a steady decrease in absorption over the transit that appears to arise from the star or its planetary system. While this variation could be ascribed to ‘b’ or possibly to the immediately preceding transit of ‘d’, we cannot rule out that this is due to rapid variation in the stellar disc-integrated flux in the triplet. The amplitude of variation (∼0.04 nm) is consistent with moderate estimates of atmospheric escape driven by XUV radiation from the central star.

Maximum Likelihood Estimation of Kokanee and Sockeye Salmon Spawners in a Stream Using Visual Survey Data, 2003-2017

A Maximum Likelihood estimation procedure was developed for the joint assessment of resident kokanee and anadromous sockeye salmon in the Okanagan River, British Columbia. The model uses visual survey counts and is an extension of other areaunder- the type models used for estimation of salmon escapement. Alternative hypotheses were tried concerning observation error structure, arrival times, and survival patterns for a common observer efficiency estimate. The final model is described here, with information on setting bounds and constraints for parameter estimation, comparisons made and the results obtained. Estimates of abundance and approximate confidence intervals are comparable to those obtained from other investigations. Since 2001, Sockeye accounted from 6%-38% of the spawners, with no evidence from surveys and estimates to indicate kokanee are negatively impacted Sockeye, perhaps because of the estimated spatio-temporal segregation pattern. Recommendations are made concerning future adjustments and surveys for systems with two Nerkids that cannot always be readily distinguished in visual surveys. The results obtained are important to justify continued efforts to re-introduce Sockeye into Canada, maintain hatchery production to supplement Columbia River Sockeye fisheries, and protect existing biodiversity in this multi-species ecosystem. Management implications for the Columbia River fisheries in the US and Canada are discussed.

Conservation restocking of the imperilled European eel does not necessarily equal conservation

Abstract To stop the decline of the European eel population, one of the measures taken is translocating eels for restocking, despite its conservational value being largely unknown. We aimed to contribute to this knowledge gap by (i) investigating the origin of eels caught in coastal waters of Estonia and Finland using otolith microchemistry and (ii) directly estimating restocked eel escapement from Narva River Basin District (NRBD), which is part of the primary Eel Management Unit in Estonia. In Estonia, 74% of the sampled eels (n = 140) were natural recruits and 26% were restocked. In Finland, 27% of the sampled eels (n = 235) were natural recruits and 73% were restocked. Only 1% of all the coastally collected eels were originally restocked to NRBD. These new data together with the reported commercial landings from the escapement route conflict with the current silver eel escapement estimation for NRBD and question the accuracy and value of such indirect calculations compiled for most Eel Management Units throughout the European Union. It is concluded that restocking eels to freshwaters may be futile as a conservation measure in some situations, and better escapement is likely achieved in restocking eels to coastal waters or undammed freshwater systems with a direct connection to the sea.

Higher Martian Atmospheric Temperatures at All Altitudes Increase the D/H Fractionation Factor and Water Loss

AbstractMuch of the water that once flowed on the surface of Mars was lost to space long ago, and the total amount lost remains unknown. Clues to the amount lost can be found by studying hydrogen (H) and its isotope deuterium (D), which are produced when atmospheric water molecules H2O and HDO dissociate. The difference in escape efficiencies of H and D (which leads to an enhanced D/H ratio) is referred to as the fractionation factor f. Both the D/H ratio and f are necessary to estimate water loss; thus, if we can constrain the range of f and understand what controls it, we will be able to estimate water loss more accurately. In this study, we use a 1D photochemical model of the neutral Martian atmosphere to determine how f depends on assumed temperature and water vapor profiles. We find that the exobase temperature most strongly controls the value of f for thermal escape processes. When we include estimates of nonthermal escape from other studies, we find that the tropopause temperature is also important. Overall, for the standard Martian atmosphere, f = 0.002 for thermal escape, and f = 0.06 for thermal + nonthermal escape. We estimate that Mars has lost at minimum 66–122 m global equivalent layer of water. Importantly, our results demonstrate that the value of f depends critically on nonthermal escape of D, and that modeling studies that include D/H fractionation must model both neutral and ion processes throughout the atmosphere.

Parentage-based tagging improves escapement estimates for ESA-listed adult Chinook salmon and steelhead in the Snake River basin

Parentage-based tagging (PBT) is a nonlethal, genetic tagging method that has been successfully applied in hatchery-supplemented populations to manage hatchery brood stock and monitor hatchery harvest and straying rates. We show that PBT can also improve the accuracy of escapement estimates by significantly reducing the number of hatchery-origin fish falsely classified as natural-origin. Unlike conventional abundance estimates, which use physical marks and tags to distinguish hatchery individuals from their wild counterparts, PBT identifies origin independent of physical form. We applied PBT to populations of Chinook salmon (Oncorhynchus tshawytscha) and steelhead (Oncorhynchus mykiss), which are classified as Threatened under the Endangered Species Act and subject to extensive hatchery supplementation efforts. For spawn years 2014–2018, 16 511 adipose-intact Chinook salmon and 21 953 adipose-intact steelhead were sampled, and PBT identified 19.6% of returning Chinook salmon and 8.3% of steelhead were of hatchery-origin, despite having no physical or mechanical marks. The 90% confidence intervals for escapement estimates of natural-origin Chinook salmon and steelhead made with and without corrections using PBT were nonoverlapping for nine of ten comparisons, indicating that failing to account for unmarked, untagged hatchery-origin fish would result in a significant overestimation of natural abundance.

Conservation implications of using an imitation carnivore to assess rarely used refuges as critical habitat features in an alpine ungulate.

Understanding relationships between animals and their habitat is a central goal in ecology with important implications for conservation. Misidentified habitat requirements can have serious repercussions because land protection or reintroductions might occur in less than optimal habitat. Studies of resource selection have greatly facilitated an understanding of ecological relationships but can be improved when vital yet infrequently utilized habitat features are more fully described. A critical element for many prey species is escape terrain or some other form of refuge to avoid predation. Mountain goats (Oreamnos americanus) are well known for their use of cliffs to avoid predation, but a survey of the literature revealed at least twelve different approximations of goat escape terrain, ranging from > 25° to > 50° slopes. Here, we seek to (1) enhance estimates of mountain goat escape terrain and antipredator behavior, and (2) highlight the limitations of the assumption that the time an animal spends in an area is proportional to importance. To improve estimates of goat escape terrain, we conducted field work across two years (2014–15) in Glacier National Park, Montana USA and manipulated apparent predation risk by exposing mountain goats to a threatening simulated grizzly bear (Ursus arctos) treatment and a non-threatening ungulate (control) treatment. Mountain goats moved in response to the simulated bear but not in response to the simulated ungulate, with shorter latencies to move for subjects in larger groups and at shorter distances to the simulated threat. Through a used-unused resource selection function we tested 22 landscape variables to describe the use of escape terrain. Proximity to slopes greater than 60° best explained the locations to which mountain goats fled after exposure to the simulated bear, and the average slope of these escape locations was 56.5° (±14.1 S.D.). Our results suggest that mountain goat escape terrain be considered at slopes of 60° as a minimum because our simulated threat did not include pursuit of goats and, thus, slopes of 60° are likely underestimates of actual escape terrain. Additionally, because direct interactions between carnivores and goats seldom occur, serious escape terrain is infrequently used. Past estimates of escape may have miscalculated the slopes which goats select for in response to predation risk. Based on experimental approaches in the wild, we suggest that anti-predator behavior should be included in studies of resource selection when the goal is to consider habitat as a predictor for conservation success. Finally, we discuss evidence suggesting a past mountain goat introduction failed due to lack of adequate escape terrain and subsequent recolonization of a predator.

A Coupled Analysis of Atmospheric Mass Loss and Tidal Evolution in XUV Irradiated Exoplanets: The TRAPPIST-1 Case Study

Abstract Exoplanets residing close to their stars can experience evolution of both their physical structures and their orbits due to the influence of their host stars. In this work, we present a coupled analysis of dynamical tidal dissipation and atmospheric mass loss for exoplanets in X-ray and ultraviolet (XUV) irradiated environments. As our primary application, we use this model to study the TRAPPIST-1 system and place constraints on the interior structure and orbital evolution of the planets. We start by reporting on an ultraviolet continuum flux measurement (centered around ∼1900 Å) for the star TRAPPIST-1, based on 300 ks of Neil Gehrels Swift Observatory data, and which enables an estimate of the XUV-driven thermal escape arising from XUV photodissociation for each planet. We find that the X-ray flaring luminosity, measured from our X-ray detections, of TRAPPIST-1 is 5.6 × 10−4 L *, while the full flux including non-flaring periods is 6.1 × 10−5 L *, when L * is TRAPPIST-1's bolometric luminosity. We then construct a model that includes both atmospheric mass loss and tidal evolution and requires the planets to attain their present-day orbital elements during this coupled evolution. We use this model to constrain the ratio for each planet. Finally, we use additional numerical models implemented with the Virtual Planet Simulator VPLanet to study ocean retention for these planets using our derived system parameters.

Challenges to reconcile conservation and exploitation of the threatened Anguilla anguilla (Linnaeus, 1758) in Santo André lagoon (Portugal)

The adoption of the EC Regulation 1100/2007 for the recovery of the European eel required EU member states to implement and periodically report on their Eel Management Plans (EMPs) with the goal of achieving at least 40% of the silver eel escapement relative to the best estimate of escapement that would have existed if no anthropogenic influences had impacted the stock. The specific measures to achieve such a goal have spurred a harsh debate among the different stakeholders with the need to mediate among multiple conflicting interests. In this investigation the study of the dynamics of the eel fishery in the Santo André Lagoon, a small-scale fishery in the southwest of Portugal, is presented. Through the description of the management structure, the analysis of 40 years of catch and effort data, and the analysis of socio-economic data, the existing enforcement and the way forward is discussed. Day-to-day logbooks of catch and effort data from five professional fishermen, for two fishing seasons (that is, 2011/2012 and 2016/2017), and surveys to the community, were used to independently estimate total harvest and income of the eel fishery in Santo André Lagoon, and the perception of the fishermen toward regulation and management. The results showed that both catch per unit effort and fishing yield increased during the study period and now ranges between 68 and 169 kg/ha, with eel fishing making a significant fraction of the fishermen's revenue. Although the existing regulation does not satisfy the fishing community, their disagreement decreased during the studied period. The analysis of the fisheries management in the lagoon and its impact on eel harvesting and, on the stock, revealed opportunities to adjust the set of regulations enforced at the national level under the Portuguese EMP to the specific context of the Santo André Lagoon. Specifically, a co-management approach would promote legitimacy and contribute to a better acceptance of the regulations by the fishing community.

Use of an acoustic camera to monitor seaward migrating silver-phase eels (Anguilla anguilla) in a regulated river

Downstream migration dynamics of silver-phase eels have traditionally been documented using fisheries catch data. However, improved monitoring protocols and more comprehensive analyses are needed because of widespread declines in Anguillid eel stocks. Therefore, daily catches of silver-phase European eel (Anguilla anguilla) were recorded at an eel fishing weir on the lower River Erne in northwest Ireland. In parallel, eels detected downstream with an acoustic (DIDSON) camera were surveyed and the results were compared with catch data obtained at the eel weir. The principal objective was to evaluate the use of the acoustic camera to monitor migration patterns and eel swimming behaviour in a regulated river. A highly significant relationship between the acoustic camera eel counts and fishing weir catches was observed (p < 0.001). This indicates that fisheries-independent estimates of eel spawner biomass escapement will be possible in the future using acoustic cameras. Acoustic camera observations on eel swimming behaviour indicated significantly more eels swimming downstream when discharge was continuous through the night (p < 0.05). On nights with no discharge more eels were observed swimming upstream (p < 0.05). The swimming speeds of eels differed significantly (p < 0.05) between continuous and low discharge conditions. European rivers are increasingly affected by anthropogenic control of discharge to facilitate hydropower generation, flood control and navigation, and this affects natural patterns of eel migration. The results of this study demonstrate the potential use of acoustic cameras for monitoring eel migration dynamics and for behavioural analyses of eel migration in such regulated rivers.

Hierarchical Bayesian estimation of unobserved salmon passage through weirs

We developed a hierarchical Bayesian model (HBM) to estimate missing counts of Chinook salmon (Oncorhynchus tshawytscha (Walbaum in Artedi, 1792)) at a weir on the Kogrukluk River, Alaska, between 1976 and 2015. The model assumed that fish passage during a breach of the weir was typical of passage during normal operations. Counts of fish passing the weir were missing for some days during the runs, or only partial counts for a given 24-hour period were available. The HBM approach provided more defensible estimates of missing data and total escapement than ad hoc or year-by-year model estimates, because estimates of passage for a given year were informed not only by counts for the current year, but also by counts for all previous years. The results of the HBM yielded less variable estimates of escapement than did ad hoc or year-by-year model estimates. The HBM represents a standardized approach to estimate missing counts and total escapement and eliminates the need for ad hoc estimates of missing counts. The model also provides managers with a measure of uncertainty around estimates of escapement and around estimates of hyper-parameters to define run curves in subsequent years with incomplete fish counts.

Declining spring usage of core habitat by endangered fish-eating killer whales reflects decreased availability of their primary prey

The salmon-eating Southern Resident killer whales (Orcinus orca) of the north-eastern Pacific Ocean are listed as endangered both in the United States and Canada. Their critical habitat has been defined as the region of the inland waters of Washington State and British Columbia known as the Salish Sea, where they have traditionally spent much of their time from spring through fall. Using reports from experienced observers to sightings networks, we tracked the daily presence of the Southern Residents in these waters from 1 April to 30 June from 1994 through 2016. We found that the escapement estimates of spring Chinook salmon (Oncorhynchus tshawytscha) on the Fraser River in British Columbia were a significant predictor of the cumulative presence/absence of the whales throughout the spring season. There was also a difference in both whale presence and salmon abundance before and after 2005, suggesting that the crash in Chinook salmon numbers has fallen below threshold where it is worthwhile for the whales to spend as much time in the Salish Sea. The use of the Salish Sea by the Southern Residents has declined in the spring months as they are either foraging for Chinook salmon elsewhere or are shifting to another prey species. In order to continue providing necessary protections to this endangered species, critical habitat designations must be re-evaluated as this population of killer whales shifts its range in response to prey availability.

Catalog of Chinook Salmon Spawning Areas in Yukon River Basin in Canada and United States

Abstract Chinook Salmon Oncorhynchus tshawytscha return to the Yukon River in northwestern North America each summer, migrating to spawning destinations from the lower river to more than 3,000 km upstream. These returns support numerous fisheries throughout the basin. Despite a long history of fisheries research and management, there is no comprehensive account of Chinook Salmon spawning areas in the basin. To address this issue, we cataloged, summarized, and mapped the known spawning areas of Yukon River Chinook Salmon by using a variety of sources including published articles, gray literature, and information archived in agency databases. Most of our sources were published within the past 30 y, but some refer to observations that were recorded as long ago as the late 1800s. We classified spawning areas as major or minor producers with three indicators of abundance: 1) quantitative estimates of escapement (major producer if ≥500 fish, minor producer if &amp;lt;500 fish), 2) radiotelemetry-based proportions of annual production (major producer if ≥1% of the run, minor producer if &amp;lt;1% of the run), and 3) aerial survey index counts (major producer if ≥165 fish observed, minor producer if &amp;lt;165 fish observed). We documented 183 spawning areas in the Yukon River basin, 79 in the United States, and 104 in Canada. Most spawning areas were in tributary streams, but some were in main-stem reaches as well. We classified 32 spawning areas as major producers and 151 as minor producers. The Chinook Salmon spawning areas cataloged here provide a baseline that makes it possible to strategically direct abundance, biological sampling, and genetics projects for maximum effect and to assess both spatial and temporal changes within the basin.

The role of density-dependent and -independent processes in spawning habitat selection by salmon in an Arctic riverscape.

Density-dependent (DD) and density-independent (DI) habitat selection is strongly linked to a species’ evolutionary history. Determining the relative importance of each is necessary because declining populations are not always the result of altered DI mechanisms but can often be the result of DD via a reduced carrying capacity. We developed spatially and temporally explicit models throughout the Chena River, Alaska to predict important DI mechanisms that influence Chinook salmon spawning success. We used resource-selection functions to predict suitable spawning habitat based on geomorphic characteristics, a semi-distributed water-and-energy balance hydrologic model to generate stream flow metrics, and modeled stream temperature as a function of climatic variables. Spawner counts were predicted throughout the core and periphery spawning sections of the Chena River from escapement estimates (DD) and DI variables. Additionally, we used isodar analysis to identify whether spawners actively defend spawning habitat or follow an ideal free distribution along the riverscape. Aerial counts were best explained by escapement and reference to the core or periphery, while no models with DI variables were supported in the candidate set. Furthermore, isodar plots indicated habitat selection was best explained by ideal free distributions, although there was strong evidence for active defense of core spawning habitat. Our results are surprising, given salmon commonly defend spawning resources, and are likely due to competition occurring at finer spatial scales than addressed in this study.

A time-lapse photography method for monitoring salmon (Oncorhynchus spp.) passage and abundance in streams.

Accurately estimating population sizes is often a critical component of fisheries research and management. Although there is a growing appreciation of the importance of small-scale salmon population dynamics to the stability of salmon stock-complexes, our understanding of these populations is constrained by a lack of efficient and cost-effective monitoring tools for streams. Weirs are expensive, labor intensive, and can disrupt natural fish movements. While conventional video systems avoid some of these shortcomings, they are expensive and require excessive amounts of labor to review footage for data collection. Here, we present a novel method for quantifying salmon in small streams (<15 m wide, <1 m deep) that uses both time-lapse photography and video in a model-based double sampling scheme. This method produces an escapement estimate nearly as accurate as a video-only approach, but with substantially less labor, money, and effort. It requires servicing only every 14 days, detects salmon 24 h/day, is inexpensive, and produces escapement estimates with confidence intervals. In addition to escapement estimation, we present a method for estimating in-stream salmon abundance across time, data needed by researchers interested in predator--prey interactions or nutrient subsidies. We combined daily salmon passage estimates with stream specific estimates of daily mortality developed using previously published data. To demonstrate proof of concept for these methods, we present results from two streams in southwest Kodiak Island, Alaska in which high densities of sockeye salmon spawn.

Evaluation of a random displacement model for predicting particle escape from canopies using a simple eddy diffusivity model

There is a need for more practical tools for estimating spore escape from crop canopies, which is essential in forecasting the propagation of disease to other fields. In this paper, we evaluated whether a random displacement model (RDM) parameterized with an eddy diffusivity Kz(z) could be used to predict spore escape probability. The proposed RDM does not require detailed turbulence measurements for parameterization. Instead, it constructs profiles of velocity and eddy diffusivity from a simple set of parameters [canopy height, canopy density, vegetation length scale, and wind speed]. The RDM was validated using field measurements of spore concentration. On average, the model predictions matched the field measurements within 28% inside the canopy and 42% above it, comparable to LES results over the same canopy. Once validated, the RDM was used to explore particle escape across a range of canopy densities and particle settling velocities, in order to inform estimates of particle escape from crops of varying maturity or area density. Escape fraction as calculated by the RDM increased as canopy density decreased, as the ratio of particle settling velocity to turbulent shear velocity ratio decreased, and as the source height within the canopy increased.

Overdispersion in the Estimation of Salmon Escapements by Mark–Recapture Surveys and Its Related Factors

AbstractThe Petersen method, a well-known mark–recapture method, is frequently used in the estimation of escapement of Pacific salmon Oncorhynchus spp. Robust estimates and associated variances can be obtained when the underlying assumptions of this method are met. However, in field surveys, it is often difficult to meet these assumptions because wild animals are often not randomly distributed. When the sampling variance of the estimator exceeds its binomial variance (resulting in overdispersion), the Petersen model underestimates variance. We assessed the occurrence of overdispersion in the mark–recapture data collected to estimate the escapements of Masu Salmon Oncorhynchus masou in a river in northern Japan in 1998 and 1999. The dispersion parameters estimated from 2-year data were 2.89 and 2.51 for males and 1.45 and 1.52 for females, indicating that overdispersion occurred in all cases. The magnitude of overdispersion for males exceeded that for females, probably related to differences in the postspa...

Robust and Defensible Mark–Recapture Methodologies for Salmonid Escapement: Modernizing the Use of Data and Resources

AbstractEstimates of population size, required for most ecological investigations, are often achieved by mark–recapture experiments, frequently by applying pooled or stratified Petersen estimators. Unfortunately, the closure assumption required by Petersen estimators is frequently violated in the estimation of salmonid escapement, even though the consequences of this violation have been known for decades. We illustrate how biologists and analysts can and should make better use of statistical, mathematical, and computational advances in their analysis of mark–recapture data. Modern, easily applied approaches address and minimize the effects of violations to the model assumptions on which abundance estimators are based. Using examples from research estimating the numbers of Chinook Salmon Oncorhynchus tshawytscha escaping fisheries to spawn, this study demonstrates and provides evidence in support of the use of a robust and defensible approach to salmonid escapement estimation based on the analysis of individual encounter histories. The main attributes of the approach include (1) testing for demographic closure, (2) allowing different hypotheses about the demographic attributes and capture history of the studied population to be expressed within a model selection framework, encompassing suites of open‐ or closed‐population approaches, and (3) optimizing the use of information by embracing the opportunities that mark–recapture experiments generate to increase our knowledge of salmonid ecology and hence improve both future study designs and management decisions. This study also demonstrates that discrepancies (positive) in abundance estimates produced with the Petersen estimator relative to those produced by the “best models” from robust estimators are inversely proportional to sampling rates.Received May 20, 2015; accepted October 22, 2015