Underestimation Of Abundance Research Articles

Aspectos comportamentais dos machos de abelhas euglossíneas da região cacaueira da Bahia

This work investigated the behavior of euglossine males from the cacao region of Bahia, related the studied group to the environment to evaluate a sampling method and identify possible sources of subjectivity in the interpretation of results related to the ecology of these bees. The study area lies between the coordinates 14o36'0" S; 39o17'0" W e 15o18'0" S, 39o5'00" W. covering Ilhéus, Itabuna, Una and Uruçuca. Bees were collected from February to April 2000, attracted with essences (cineole 1.8, methyl salicylate, benzyl acetate, eugenol and vanillin) in traps 1.3 m from the ground and placed 5 to 10 m apart. These areas were sampled for two hours on sunny mornings. The results show the period of greatest activity - 9:00 to 11:00; Existence of competition between species, possibly for territory and odorous resource, which may result in underestimation of abundance and richness; The number of individuals collected with pollinaria attached to the body, in March, indicates the orchid flowering period. In December and March, there was greater abundance and richness of euglosine males, coinciding with the peak of orchid abundance. Interiors of forests and extensive fragments, with preserved canopy and complex matrix were the most preferred environments by euglossine males.

Bias in estimates of electrofishing capture probability of juvenile Atlantic salmon

We evaluated the effect of the total number of passes used, and the application of block nets, on multi-pass electrofishing removal sampling for estimating juvenile Atlantic salmon (Salmo salar L.) abundance and body size distribution. Sites within selected salmon-bearing Norwegian rivers were enclosed by block nets and electrofished for multiple passes (range: 7–13), and capture probabilities and abundances were estimated using the Carle and Strub removal method. We examined for different body size classes: (1) bias in the estimated capture probability and abundance associated with the number of passes used; (2) the potential for bias to be minimized by the use of block nets; and (3) electrofishing-induced mortality. We found that the capture probability estimate was strongly dependent upon the number of passes used, tending to decline with successive pass, with the effect depending on size class. Thus, estimates made using the traditional three-pass approach would result in underestimates of abundance, and biased estimates of size distribution. Smaller juveniles were both more likely to impinge on the block nets and more likely to experience mortality than larger juveniles. Mortality was greatest when water temperature was high (>18 °C). Our findings indicate that quantitative electrofishing for small juveniles may be unreliable, and that electrofishing at high temperatures should be avoided due to potential high mortality.

The ZooCAM, a new in-flow imaging system for fast onboard counting, sizing and classification of fish eggs and metazooplankton

In this paper we present the ZooCAM, a system designed to digitize and analyse on board large volume samples of preserved and living metazooplankton (i.e. multicellular zooplankton) and fish eggs >300 µm ESD. The ZooCAM has been specifically designed to overcome the difficulties to analyse zooplankton and fish eggs in the framework of the PELGAS survey, and provide high frequency data. The ZooCAM fish eggs counts were comparable to those done with a dissecting microscope. The ZooCAM enabled the accurate prediction and fast on board validation of staged anchovy and sardine eggs in almost real time after collection. A comparison with the ZooScan, on a more complex zooplanktonic community, provided encouraging results on the agreement between the 2 instruments. ZooCAM and ZooScan enabled the identification of similar communities and produced similar total zooplankton abundances, size distributions, and size spectra slopes, when tested on the same samples. However these results need to be further refined due to the small number of samples used to compare the two instruments. The main ZooCAM drawback resides in a slight but sensible underestimation of abundances and sizes, and therefore individual and community biovolumes. The ZooCAM have been successfully deployed over 4 years, on numerous surveys without suffering any major failure. When used in line with the CUFES it provided high resolution maps of staged fish eggs and zooplanktonic functional groups. Hence the ZooCAM is an appropriate tool for the development of on board, high frequency, high spatial coverage zooplanktonic and ecosystemic studies.

Computational aspects of N-mixture models.

The N-mixture model is widely used to estimate the abundance of a population in the presence of unknown detection probability from only a set of counts subject to spatial and temporal replication (Royle, 2004, Biometrics 60, 105–115). We explain and exploit the equivalence of N-mixture and multivariate Poisson and negative-binomial models, which provides powerful new approaches for fitting these models. We show that particularly when detection probability and the number of sampling occasions are small, infinite estimates of abundance can arise. We propose a sample covariance as a diagnostic for this event, and demonstrate its good performance in the Poisson case. Infinite estimates may be missed in practice, due to numerical optimization procedures terminating at arbitrarily large values. It is shown that the use of a bound, K, for an infinite summation in the N-mixture likelihood can result in underestimation of abundance, so that default values of K in computer packages should be avoided. Instead we propose a simple automatic way to choose K. The methods are illustrated by analysis of data on Hermann's tortoise Testudo hermanni.

Capture–recapture population growth rate as a robust tool against detection heterogeneity for population management

Managing large carnivores is one of the most controversial issues in wildlife conservation, as the sociopolitical challenges it raises are as important as the biological ones. Such controversial issues in wildlife conservation require objective biological components to be implemented within the management decision process, in particular, a reliable way of estimating trends in abundance. However, these species usually exhibit territoriality, low densities, and social constraints that can generate individual detection heterogeneity (IDH) of methodological (sampling) or biological (social status, marking behavior) origin. If not accounted for, IDH can lead, in turn, to strong bias in the estimation of population abundance. As a complement to population size, we propose to use the population growth rate (λ) estimated with capture–recapture (CR) data, a robust method to detect and account for IDH, to monitor and manage elusive species. As a case study, we consider the natural recovery of the wolf (Canis lupus) population in France, for which a previous study has shown strong IDH leading to a 27% underestimation of abundance. Analyzing a nine-year data set based on a noninvasive sampling protocol relying on wolf scat genotyping, we adopt a new CR approach to estimate λ while explicitly accounting for IDH. The annual population growth rate was estimated at 1.271 ± 0.087 (mean ± SE) vs. 1.270 ± 0.095 when not accounting for IDH, indicating that λ is much less sensitive to IDH than is abundance. We evaluated the robustness of our approach in a simulation study using increasing levels of IDH. The bias in λ increased with detection heterogeneity but was low whether we used a model with IDH or not. Finally, we discuss the management implications of our findings in terms of sampling protocols and management strategies of elusive species.

Effect of surfacing patterns on abundance estimates of long-diving animals

Population sizes of long-diving marine mammals can be estimated with minimal bias using line transect sampling techniques in conjunction with data on surfacing/diving, even though the detection probability on the trackline is considerably less than 1. Using a simulation, it was investigated whether population sizes can be accurately estimated when only the mean cue production rate, without information on surfacing/diving patterns, is available. In addition, a new model that incorporates information on surfacing/diving patterns into the estimation procedure was developed. The simulation showed that the new model can provide less biased abundance estimates, whereas neglecting surfacing/diving patterns can result in serious underestimates of abundance, in particular for longdiving animals.

Measurement of larval striped bass (Morone saxatilis) net avoidance using evasion radius estimation to improve estimates of abundance and mortality

Net avoidance rate increases as a function of larval striped bass size. This causes under-estimation of abundance and overestimation of mortality rate. We modeled net avoidance by assuming that fish avoid the net by swimming a radial distance at a right angle to the net axis. This distance, the evasion radius, was estimated by comparing the calculated densities of striped bass larvae from a series of paired tows involving a large and a small net. Iteration and solution models were used to estimate the evasion radius for each millimeter size group of fish in order to estimate the actual density in the environment. Avoidance of the nets increased with fish length. The ratio of actual density in the environment to the measured density in the small net was used to adjust abundances measured in our ichthyoplankton surveys. After adjusting for net avoidance, mortality rates of striped bass larvae from the Sacramento-San Joaquin Estuary were reduced by 10% compared to the unadjusted rates.