Capture probability of fishes in Central European (Hungary) wadeable lowland streams

Neglect of imperfect capture efficiency leads to biased inferences on population abundance, and correspondingly, seriously affects ecological research, bioassessment, conservation, and fisheries management. To date, many research studies have studied capture efficiency of salmonid fishes, but the catchability of fishes living in non-salmonid streams has received much less attention. This paper estimates capture probability for seven fish species in densely vegetated lowland streams by using double-pass electrofishing data and an N-mixture removal model. Results show that capture probability can vary among species, and between-stream differences have a stronger influence on the abundance and the catchability than within-stream variability. Estimation uncertainty decreases with observed abundance, and the mean catchability tends to be the highest for the medium abundant species. These findings suggest that relative abundances from single-pass data are biased to a species- and habitat-specific degree. Therefore, plausible estimation of capture probability from double-pass electrofishing requires data collected from numerous sites that cover a wide range of the environmental gradient in lowland streams.     

Divergent trends in anadromous salmonid populations in Norwegian and Scottish rivers

The Atlantic salmon (Salmo salar) is a charismatic anadromous fish of high conservation and economic value. Concerns have been expressed regarding the long-term viability of fisheries throughout the species''s distributional range because of abundance variations that cannot currently be explained or predicted. Here, we analyse long-term catch data obtained over a wide geographical range and across a range of spatial subscales to understand more fully the factors that drive population abundance. We use rod catch data from 84 Norwegian rivers over 125 years (1876–2000) and 48 Scottish rivers over 51 years (1952–2002). The temporal correlation in catches is very long-term, with trends persisting over several decades. The spatial correlation is relatively short-range, indicating strong local-scale effects on catch. Furthermore, Scottish salmon populations exhibit recent negative trends in contrast to some more positive trends in Norway—especially in the north.     

Estimating abundance and population trends when detection is low and highly variable: A comparison of three methods for the Hermann's tortoise

Assessing population trends is a basic prerequisite to carrying out adequate conservation strategies. Selecting an appropriate method to monitor animal populations can be challenging, particularly for low-detection species such as reptiles. This study compares 3 detection-corrected abundance methods (capture–recapture, distance sampling, and N-mixture) used to assess population size of the threatened Hermann's tortoise. We used a single dataset of 432 adult tortoise observations collected at 118 sampling sites in the Plaine des Maures, southeastern France. We also used a dataset of 520 tortoise observations based on radiotelemetry data collected from 10 adult females to estimate and model the availability (g0) needed for distance sampling. We evaluated bias for N-mixture and capture–recapture, by using simulations based on different values of detection probabilities. Finally, we conducted a power analysis to estimate the ability of the 3 methods to detect changes in Hermann's tortoise abundances. The abundance estimations we obtained using distance sampling and N-mixture models were respectively 1.75 and 2.19 times less than those obtained using the capture–recapture method. Our results indicated that g0 was influenced by temperature variations and can differ for the same temperature on different days. Simulations showed that the N-mixture models provide unstable estimations for species with detection probabilities <0.5, whereas capture–recapture estimations were unbiased. Power analysis showed that none of the 3 methods were precise enough to detect slow population changes. We recommend that great care should be taken when implementing monitoring designs for species with large variation in activity rates and low detection probabilities. Although N-mixture models are easy to implement, we would not recommend using them in situations where the detection probability is very low at the risk of providing biased estimates. Among the 3 methods allowing estimation of tortoise abundances, capture–recapture should be preferred to assess population trends. © 2013 The Wildlife Society.     

Methodology matters when estimating deer abundance: a global systematic review and recommendations for improvements

Deer (Cervidae) are key components of many ecosystems and estimating deer abundance or density is important to understanding these roles. Many field methods have been used to estimate deer abundance and density, but the factors determining where, when, and why a method was used, and its usefulness, have not been investigated. We systematically reviewed journal articles published during 2004–2018 to evaluate spatio-temporal trends in study objectives, methodologies, and deer abundance and density estimates, and determine how they varied with biophysical and anthropogenic attributes. We also reviewed the precision and bias of deer abundance estimation methods. We found 3,870 deer abundance and density estimates. Most estimates (58%) were for white-tailed deer (Odocoileus virginianus), red deer (Cervus elaphus), and roe deer (Capreolus capreolus). The 6 key methods used to estimate abundance and density were pedestrian sign (track or fecal) counts, pedestrian direct counts, vehicular direct counts, aerial direct counts, motion-sensitive cameras, and harvest data. There were regional differences in the use of these methods, but a general pattern was a temporal shift from using harvest data, pedestrian direct counts, and aerial direct counts to using pedestrian sign counts and motion-sensitive cameras. Only 32% of estimates were accompanied by a measure of precision. The most precise estimates were from vehicular spotlight counts and from capture–recapture analysis of images from motion-sensitive cameras. For aerial direct counts, capture–recapture methods provided the most precise estimates. Bias was robustly assessed in only 16 studies. Most abundance estimates were negatively biased, but capture–recapture methods were the least biased. The usefulness of deer abundance and density estimates would be substantially improved by 1) reporting key methodological details, 2) robustly assessing bias, 3) reporting the precision of estimates, 4) using methods that increase and estimate detection probability, and 5) staying up to date on new methods. The automation of image analysis using machine learning should increase the accuracy and precision of abundance estimates from direct aerial counts (visible and thermal infrared, including from unmanned aerial vehicles [drones]) and motion-sensitive cameras, and substantially reduce the time and cost burdens of manual image analysis.     

Long-term population trends in widespread British moths

The Rothamsted Insect Survey has operated a Great Britain-wide network of light-traps since 1968. From these data we estimated the first ever national abundance indices and 35-year population trends for 338 species of common macro-moths. Although the number of trap sites which run each year is not constant, there is a representative, well-distributed core of traps that have run for 15 years. The proportion of operating sites catching a species and the annual geometric mean catch of successful traps were used to provide estimates of species range and absolute abundance. T, an index of long-term population trends, was used to compare trends among species. T was not biased by trap site turnover. The percentage of species displaying significant decreases (54%) was more than double that displaying increases (22%). Species found throughout Great Britain are decreasing most rapidly in the south and especially the southeast but species with a southerly distribution are increasing. Results of a preliminary overview suggest habitat and climate change may both play a role in changing species dynamics. The existence of estimates of abundances and trends for such a large species pool opens the way for much further research, linking trends with land-use changes, climate change and inter-specific dynamics.     

When are genetic methods useful for estimating contemporary abundance and detecting population trends?

The utility of microsatellite markers for inferring population size and trend has not been rigorously examined, even though these markers are commonly used to monitor the demography of natural populations. We assessed the ability of a linkage disequilibrium estimator of effective population size (N_e) and a simple capture-recapture estimator of abundance (N) to quantify the size and trend of stable or declining populations (true N = 100–10,000), using simulated Wright–Fisher populations. Neither method accurately or precisely estimated abundance at sample sizes of S = 30 individuals, regardless of true N. However, if larger samples of S = 60 or 120 individuals were collected, these methods provided useful insights into abundance and trends for populations of N = 100–500. At small population sizes (N = 100 or 250), precision of the N_e estimates was improved slightly more by a doubling of loci sampled than by a doubling of individuals sampled. In general, monitoring N_e proved a more robust means of identifying stable and declining populations than monitoring N over most of the parameter space we explored, and performance of the N_e estimator is further enhanced if the N_e/N ratio is low. However, at the largest population size (N = 10,000), N estimation outperformed N_e. Both methods generally required ≥ 5 generations to pass between sampling events to correctly identify population trend.     

The sampling efficiency of electrofishing for Neogobius species in a riprap habitat: a field experiment

Even though electrofishing is commonly used to sample Neogobius spp. and other swimbladder‐lacking benthic fishes, its efficiency is considered poor especially in habitats with abundant interstitial spaces. To determine the efficiency of electrofishing Neogobius spp. and quantitatively estimate sampling bias in a riprap (shot rock used to armor shorelines against water erosion) mesohabitat, riprap fragments were set up in a natural riverine environment. The experimental setting enabled us to collect all fish remaining in the riprap fragments after these areas had been electrofished. The sampling efficiency of electrofishing Neogobius spp. (dominated by Neogobius melanostomus) varied between 17.6 and 47.4% (mean 29.7%), while percids (possessing a well‐developed swim bladder) were collected with 74.6% efficiency. Fish size had no effect on the probability of capturing Neogobius spp. by electrofishing. Within Neogobius spp., N. melanostomus was less susceptible to electrofishing than Neogobius gymnotrachelus (23.7% and 50.1%, respectively). Decreased electrofishing efficiency in areas of rocky substrate should be considered in estimates of total abundance of Neogobius spp., especially if they are to be compared with catches of other species possessing swim bladders.     

Heterogeneous capture rates in low density populations and consequences for capture-recapture analysis of camera-trap data

Closed population capture-recapture analysis of camera-trap data has become the conventional method for estimating the abundance of individually recognisable cryptic species living at low densities, such as large felids. Often these estimates are the only information available to guide wildlife managers and conservation policy. Capture probability of the target species using camera traps is commonly heterogeneous and low. Published studies often report overall capture probabilities as low as 0.03 and fail to report on the level of heterogeneity in capture probability. We used simulations to study the effects of low and heterogeneous capture probability on the reliability of abundance estimates using the M_h jack-knife estimator within a closed-population capture-recapture framework. High heterogeneity in capture probability was associated with under- and over-estimates of true abundance. The use of biased abundance estimates could have serious conservation management consequences. We recommend that studies present capture frequencies of all sampled individuals so that policy makers can assess the reliability of the abundance estimates.     

Abundance of rare and elusive species: Empirical investigation of closed versus spatially explicit capture–recapture models with lynx as a case study

Effective conservation and management require reliable monitoring methods and estimates of abundance to prioritize human and financial investments. Camera trapping is a non-invasive sampling method allowing the use of capture–recapture (CR) models to estimate abundance while accounting for the difficulty of detecting individuals in the wild. We investigated the relative performance of standard closed CR models and spatially explicit CR models (SECR) that incorporate spatial information in the data. Using simulations, we considered 4 scenarios comparing low versus high detection probability and small versus large populations and contrasted abundance estimates obtained from both approaches. Standard CR and SECR models both provided minimally biased abundance estimates, but precision was improved when using SECR models. The associated confidence intervals also provided better coverage than their non-spatial counterpart. We concluded SECR models exhibit better statistical performance than standard closed CR models and allow for sound management strategies based on density maps of activity centers. To illustrate the comparison, we considered the Eurasian lynx (Lynx lynx) as a case study that provided the first abundance estimates of a local population in France. © 2012 The Wildlife Society.     

Using the double-observer method to estimate detection probability of two cavity-nesting seabirds in Antarctica: the snow petrel (Pagodroma nivea) and the Wilson��s storm petrel (Oceanites oceanicus)

When estimating the size of seabird populations, count data may be biased due to various factors such as detection probability. Failing to account for detection probability in surveys may lead to an underestimate of population size and may compromise the ability to monitor trends if detection probability varies among surveys. Here, we use the double-observer method to estimate detection probability of cavity-nesting snow petrels (Pagodroma nivea) and Wilson’s storm petrels (Oceanites oceanicus) in East Antarctica. Estimates of single-visit detection probability of nesting/roosting adult snow petrels during the incubation stage of the breeding cycle ranged from 0.86 (SE = 0.04) to 0.87 (SE = 0.04) depending upon observers. Both observers found snow petrel chicks were easier to detect than adults, with estimated detection probability for chicks ranging from 0.92 (SE = 0.03) to 1.00 (SE = 0.34 × 10⁻⁵). Detection probability of adult and chick snow petrels increased as cavity volume increased. Compared to snow petrels, estimated detection probability was considerably lower for nesting/roosting Wilson’s storm petrels, ranging from 0.27 (SE = 0.09) to 0.50 (SE = 0.13) for each observer. These estimates of detection probability apply only to those individuals in the population that were potentially viewable or audible. Nevertheless, our results indicate that double-observer counts for ground surveys of cavity-nesting seabirds should improve estimates of population abundance in comparison with single-visit counts. Accounting for observer effects, habitat characteristics and stage of the breeding season on detection probability should also improve estimation of population trends.     

Potential for parasite-induced biases in aquatic invertebrate population studies

Recent studies highlight the need to include estimates of detection/capture probability in population studies. This need is particularly important in studies where detection and/or capture probability is influenced by parasite-induced behavioral alterations. We assessed potential biases associated with sampling a population of the amphipod Gammarus lacustris in the presence of Polymorphus spp. acanthocephalan parasites shown to increase positive phototaxis in their amphipod hosts. We trapped G. lacustris at two water depths (benthic and surface) and compared number of captures and number of parasitized individuals at each depth. While we captured the greatest number of G. lacustris individuals in benthic traps, parasitized individuals were captured most often in surface traps. These results reflect the phototaxic movement of infected individuals from benthic locations to sunlit surface waters. We then explored the influence of varying infection rates on a simulated population held at a constant level of abundance. Simulations resulted in increasingly biased abundance estimates as infection rates increased. Our results highlight the need to consider parasite-induced biases when quantifying detection and/or capture probability in studies of aquatic invertebrate populations.     

Influence of sea temperature and initial marine feeding on survival of Atlantic salmon Salmo salar post-smolts from the Rivers Orkla and Hals, Norway

The abundance of returning adult Atlantic salmon Salmo salar, in the River Orkla in mid‐norway (1 sea‐winter, SW, fish) and River Hals in north Norway (1–3 SW fish), was tested against the early marine feeding and the seawater temperature experienced by their corresponding year classes of post‐smolts immediately after entry into the Trondheimsfjord (Orkla smolts, 22 years of data) and Altafjord (Hals smolts, 17 years of data). In both river–fjord systems, there was a significant positive correlation between the abundance of returning S. salar and the mean seawater temperature at the time of smolts descending to the sea. The number of 1SW fish reported caught in River Orkla was positively correlated to the proportion of fish larvae in the post‐smolt stomachs in Trondheimsfjord. The abundance of returning S.salar was, however, neither correlated to forage ratio (R_F) nor other prey groups in post‐smolt stomachs in the two fjord systems. In the Altafjord, the post‐smolts fed mainly on pelagic fish larva (70–98%) and had a stable R_F (0·009–0·023) over the 6 years analysed. In the Trondheimsfjord, however, there was a higher variation in R_F (0·003–0·036), and pelagic fish larvae were dominant prey in only two (50 and 91%) of the 8 years analysed. These 2 years also showed the highest return rates of S. salar in River Orkla. These results demonstrate that the thermal conditions experienced by post‐smolts during their early sea migration may be crucial for the subsequent return rate of adults after 1–3 years at sea. Pelagic marine fish larvae seem to be the preferred initial prey for S. salar post‐smolts. As the annual variation in abundance of fish larvae is related to seawater temperature, it is proposed that seawater temperature at sea entry and the subsequent abundance of returning adult S. salar may be indirectly linked through variation in annual availability of pelagic fish larvae or other suitable food items in the early post‐smolt phase.     

Electrofishing as a sampling technique for coastal stream fish populations and communities in the Southeast of Brazil

Electrofishing adequacy was tested as a technique to obtain quantitative data of coastal stream fish populations and communities in the Southeast of Brazil. Seven field trips, between July/94 and July/95, were done in 5 localities of the Ubatiba fluvial system (Maricá, RJ). Seventeen species, among the 22 collected, had their numbers estimated through the Zipping method, the model used to test the sampling methodology. At each field trip, three removals with electrofishing were done in each locality and, according to the number of obtained species at each locality/field trip, we analysed 315 cases. Nineteen cases, among 315, showed failure condition. Estimates were significant (p < 0.01) in 96% of the studied cases. Non-significant cases were obtained for rare species due to over and randomly efficient electrofishing in 63.3% and 36.4% of the cases, respectively. No correlation was found between catchability and the estimated number of individuals and/or environmental characteristics. High values for sampling efficiency (> 85%) were found for all estimates. An experimental analyses were done for one locality and, the comparison between the estimates for 3 and 6 successive removals showed a mean error and a standard deviation of 5.5% and 2.1% respectively. Therefore, it can be concluded that electrofishing was an efficient method for quantitative data analysis of fish populations and communities in the Ubatiba fluvial system.     

Electrofishing and salmonid movement: reciprocal effects in two small montane streams

The effects of electrofishing on salmonid movement and of salmonid movement on electrofishing‐derived abundance estimates were studied in two streams in western Montana, U.S.A. Electrofishing increased emigration of salmonids from study reaches for 1 day, but not for succeeding days, whereas immigration to study reaches was unaffected. Movement of most emigrating fishes was downstream. On these small streams, electrofishing did not appear to cause fishes to flee during sampling. Numbers of salmonids migrating between mark and recapture runs were small relative to the fish abundance estimates in study reaches, usually much less than the 95% CL for those estimates, thus disregarding movements of marked fishes from the study reaches would have produced small positive biases in abundance estimates. Overall, for this suite of salmonid species in mid‐summer in these streams, the effects of electrofishing on fish movement and of fish movement on abundance estimates were minor.     

Estimating Abundance of an Unmarked,Low-Density Species using Cameras

Estimating abundance of wildlife populations can be challenging and costly, especially for species that are difficult to detect and that live at low densities, such as cougars (Puma concolor). Remote, motion-sensitive cameras are a relatively efficient monitoring tool, but most abundance estimation techniques using remote cameras rely on some or all of the population being uniquely identifiable. Recently developed methods estimate abundance from encounter rates with remote cameras and do not require identifiable individuals. We used 2 methods, the time-to-event and space-to-event models, to estimate the density of 2 cougar populations in Idaho, USA, over 3 winters from 2016–2019. We concurrently estimated cougar density using the random encounter model (REM), an existing camera-based method for unmarked populations, and genetic spatial capture recapture (SCR), an established method for monitoring cougar populations. In surveys for which we successfully estimated density using the SCR model, the time-to-event estimates were more precise and showed comparable variation between survey years. The space-to-event estimates were less precise than the SCR estimates and were more variable between survey years. Compared to REM, time-to-event was more precise and consistent, and space-to-event was less precise and consistent. Low sample sizes made the space-to-event and SCR models inconsistent from survey to survey, and non-random camera placement may have biased both of the camera-based estimators high. We show that camera-based estimators can perform comparably to existing methods for estimating abundance in unmarked species that live at low densities. With the time- and space-to-event models, managers could use remote cameras to monitor populations of multiple species at broader spatial and temporal scales than existing methods allow. © 2020 The Wildlife Society.     

Fort Peck paddlefish population survival and abundance in the Missouri River

Excessive fishing pressure can induce population declines or complete collapse of fisheries. Unless commercial and recreational fisheries for K-selected fishes, or those with slow growth and late maturation, are carefully managed, declines in abundance or fishery collapse is probable. Paddlefish Polyodon spathula,are a K-selected species that experienced historical declines in abundance as a result of habitat degradation and overfishing. Mark-recapture studies are well-suited for long-lived fishes by providing information on population density and vital rates. For sustainable commercial or recreational fisheries targeting species such as the paddlefish, managers require accurate estimates of population vital rates including survival, abundance, and exploitation. We used a Montana Fish, Wildlife & Parks (MFWP) mark-recapture dataset and modified Jolly-Seber (POPAN) models to estimate survival, recapture, probability of entry, and abundance of 8,518 tagged paddlefish over a 25-year period. With many supporting estimates including stable survival (0.92 for females, mean of 0.82 for males), low exploitation rates (means of 2.6% for females and 2.9% for males), and stable abundance estimates (25-year mean of 12,309 individuals for both sexes), the Fort Peck paddlefish population appears to be stable and well-managed over the past 25 years. Presently, this is the only study focused on paddlefish in North America that has estimated survival and abundance for both male and female paddlefish using contemporary analyses. This research provided a unique opportunity to highlight that the effort exerted by management agencies to collect long-term field data is extremely useful to our understanding of fish populations and management.     

Evaluating the potential of night spotlighting as a method for assessing species composition and brown trout abundance: a comparison with electrofishing in small streams

Direct counts of fish obtained by night spotlighting were compared with species composition and population estimates obtained from three-pass electrofishing obtained across 29 sites along small clear streams in the Otago region of New Zealand. The influence of habitat variables on the relative efficiency of each method was also examined. The same seven species of fishes were identified by both methods. Juvenile brown trout Salmo trutta were the only species present in sufficient numbers to allow comparison of abundance estimates using the two methods. A total of 777 brown trout were counted by spotlight and 803 brown trout were caught using electrofishing. Estimates of abundance obtained by spotlighting reflected population estimates obtained by three-pass electrofishing across most habitats. Electrofishing produced higher population estimates relative to spotlighting in fast-flowing turbulent riffle habitats, whereas counts obtained by spotlighting tended to be higher relative to electrofishing in slow-flowing pool habitats. The results suggest that spotlighting is an effective method for assessing fish composition and brown trout abundance in small clear water streams, although the extremes of water velocity may influence efficiency of both spotlighting and electrofishing.     

Mortality induced by electrofishing and handling in five young‐of‐the‐year cyprinids: effect of the fish size,species and anode size

Immediate mortality induced by electrofishing and handling of young‐of the‐year (YOY) fish of five cyprinid species (bitterling Rhodeus amarus, chub Leuciscus cephalus, bleak Alburnus alburnus, barbel Barbus barbus, roach Rutilus rutilus) was quantified during routine YOY fish monitoring (point abundance sampling with backpack electrofisher, in situ identification and measurement). Sampled, identified and measured fish were held in screened containers immersed in the river for 5 h after electrofishing. Bleak exhibited significantly higher mortality (17%) than all other species (1–3%). Mortality decreased significantly with fish length; fish <35 mm exhibited the highest mortality. Anode size (10, 20, and 30 cm diameter) had no direct effect on mortality. Point abundance electrofishing with in situ identification and measurement is a safe method for sampling riverine YOY cyprinids.     

Monitoring of the Flat-Tailed Horned Lizard With Methods Incorporating Detection Probability

Abstract: Difficulty in monitoring the flat-tailed horned lizard (Phrynosoma mcallii) has led to controversy over its conservation status. The difficulty in detecting this species has discouraged large-scale estimates of abundance and led to uncertainty over whether the species exists in population sizes of sufficient size for long-term persistence. We incorporated detection probability into monitoring of this species using closed mark—recapture and distance-sampling methods. Density estimation from mark—recapture abundance estimates was improved using an estimate of the proportion of time lizards were on the plot. We estimated the probability of detection on the line for distance sampling and adjusted density estimates accordingly. We estimated the populations of the Yuha Basin Management Area in 2002 and the East Mesa Management Area, Imperial County, California, USA, in 2003 to be 25,514 (95% CI 14,444-38,970) and 42,619 (95% CI 23,161-67,639), respectively. Two estimates of detection probability on the line in distance sampling by different methods were 0.45 and 0.65. Density estimates derived from distance analyses for 3 East Mesa Management Area plots and the Yuha Basin Management Area were 1.55 per ha (95% CI 0.64-3.76) and 0.41 per ha (95% CI 0.22-0.7), respectively. These are the first large-scale estimates of abundance and density for P. mcallii.     

Retrospective growth analysis of Atlantic salmon Salmo salar and implications for abundance trends

Scale archives of Atlantic salmon Salmo salar from Maine, U.S.A., were examined to determine whether ocean conditions affected the long‐term trends in S. salar populations in the southern tier of the species' range in North America. To date, scale analyses of southern tier populations have been limited to hatchery fish; previous studies suggest that post‐smolt growth does not influence recruitment, with the exception that winter growth may play a role in stock maturation rate. A time series of scales from the Machias and Narraguagus Rivers spanning the years 1946 to 1999 was analysed. Image analysis was used to measure intercirculi spacing, which provided proxy variables of growth rate. Post‐smolt growth increment has increased since the early 1990s, as returns have decreased, suggesting that survival factors act on post‐smolts independent of growth. The data support the hypothesis of a decoupling between freshwater size and early marine growth. Growth during the second sea winter was independent of post‐smolt growth, suggesting that individuals are capable of significant compensatory growth. Southern tier North American stocks exhibit a similar pattern of independence between growth and survival as observed for northern tier North American stocks. These data support the inference that the recruitment of the North American and European subspecies is governed by fundamentally different mechanisms.