Acoustic Survey (acoustic + survey)

Distribution by Scientific Domains
Life Sciences50%
Earth and Environmental Science50%

Selected Abstracts

Distribution patterns of fish biomass by acoustic survey in three Tunisian man-made lakes

JOURNAL OF APPLIED ICHTHYOLOGY, Issue 3 2010
I. Djemali
Summary In order to test the hypothesis that fish distribution is related to water depth, three exploited reservoirs were sampled at day and at night using a Simrad EK60 echosounder. Acoustic surveys were carried out between February and June 2008, which focused on the fish size, species density (fish per ha) and biomass (kg ha,1) along vertical and longitudinal gradients. It was evident in the surface layer (0,3 m) during daytime that the larger fish sizes occupy waters near the dam or the middle of Lakhmess and Sejnane reservoirs. In the upper layer at nighttime, a gradient of fish size is shown proportional to the depth at Lakhmess Reservoir and inversely proportional at the man-made Sejnane Lake. At Lakhmess and Laabid reservoirs, fish density was significantly higher in deep water (3926 ± 1409 and 624 ± 258 fish per ha) rather than in the upper layer (988 ± 2 and 8 ± 2 fish per ha in daytime, respectively), while at Sejnane Reservoir the number of fish per area was similar along the vertical gradient. The biomass was significantly higher in waters deeper than 3 m at Sejnane Reservoir whereas at Lakhmess it was higher in the surface layer. Fish biomass increased from upstream (0.16 ± 0.05 kg ha,1) to middle (3.07 ± 2.96 kg ha,1) and downstream (3.82 ± 2.30 kg ha,1 at night) areas in the man-made Laabid Lake while a similar longitudinal gradient occurred in the deepest Sejnane Reservoir. The vertical gradient in fish distribution is discussed. From the dam to the tributary of the entire water column, it was concluded that fish biomass distribution was governed by depth and was most abundant in areas with deep waters. [source]

Heard but not seen: an acoustic survey of the African forest elephant population at Kakum Conservation Area, Ghana

AFRICAN JOURNAL OF ECOLOGY, Issue 1 2010
Mya E. Thompson
Abstract This study, designed to survey forest elephants (Loxodonta africana cyclotis) at Kakum Conservation Area, Ghana, is the first to apply acoustic methods to elephant abundance estimation and to compare results with independent survey estimates. Nine acoustic sensors gathered sound continuously for 38 days. Low-frequency calling rates have been established as useful elephant abundance indices at a Namibian watering hole and a central African forest clearing. In this study, we estimated elephant population size by applying an abundance index model and detection function developed in central Africa to data from simultaneous sampling periods on Kakum sensors. The sensor array recorded an average of 1.81 calls per 20-min sampling period from an effective detection area averaging 10.27 km2. The resulting estimate of 294 elephants (95% CI: 259,329) falls within confidence bounds of recent dung-based surveys. An extended acoustic model, estimating the frequency with which elephants are silent when present, yields an estimate of 350 elephants (95% CI: 315,384). Acoustic survey confidence intervals are at least half as wide as those from dung-based surveys. This study demonstrates that acoustic surveying is a valuable tool for estimating elephant abundance, as well as for detecting other vocal species and anthropogenic noises that may be associated with poaching. Résumé Cette étude, conçue pour étudier les éléphants de forêt (Loxodonta africana cyclotis) de l'Aire de conservation de Kakum, au Ghana, est la première qui applique des méthodes acoustiques pour l'estimation de l'abondance des éléphants et qui compare les résultats avec des estimations indépendantes. Pendant 38 jours, neuf senseurs acoustiques ont récolté les sons en continu. Le rythme des appels à basse fréquence est un indice utile de l'abondance des éléphants que l'on a pu établir à un point d'eau en Namibie et dans une clairière d'une forêt d'Afrique centrale. Ici, nous estimons la taille d'une population d'éléphants en appliquant un modèle d'indice d'abondance et une fonction de détection mis au point en Afrique centrale à des données provenant des senseurs de Kakum pour des périodes d'échantillonnage simultanées. La batterie de senseurs a enregistré une moyenne de 1.81 appel par 20 min, pour une superficie de détection réelle de 10.27 km² en moyenne. L'estimation qui en résulte, qui est de 294 éléphants (95% IC 259,329), tombe dans les limites de confiance de récentes études basées sur les crottes. Un modèle acoustique étendu, qui estime aussi la fréquence à laquelle les éléphants sont silencieux pendant leur présence, donne une estimation de 350 éléphants (95% IC 315,384). Les intervalles de confiance d'une étude acoustique sont au moins la moitié de ceux des études par les crottes. Cette étude montre que l'étude acoustique est un outil intéressant pour estimer l'abondance des éléphants et aussi pour détecter d'autres espèces bruyantes et les bruits d'origine anthropique qui pourraient être associés au braconnage. [source]

SEASONAL OCCURRENCE OF SPERM WHALE (PHYSETER MACROCEPHALUS) SOUNDS IN THE GULF OF ALASKA, 1999,2001

MARINE MAMMAL SCIENCE, Issue 1 2004
David K. Mellinger
Abstract An acoustic survey for sperm whales was conducted in the Gulf of Alaska. Six autonomous hydrophones continuously recorded sound signals below 500 Hz from October 1999 to May 2001. After recovery, recordings were processed using an automatic process to detect usual clicks of sperm whales. The detection algorithm equalized background noise, summed the data in a frequency band, and then used autocorrelation to detect the whales' highly regular clicks. Detections were checked manually, revealing that 98% of detections did contain clicks. Results indicate that sperm whales are present in the Gulf of Alaska year-round; this result extends what is known from whaling data, which were gathered principally in summer. Sperm whales were more common in summer than winter by a factor of roughly two, and occurred less often at the westernmost site surveyed (52°N, 157°W) than elsewhere in the Gulf. This is the first study of sperm whales based exclusively on remote acoustic sensing. This methodology is feasible because sperm whale clicks extend to frequencies (,100 Hz) low enough to be recorded by low-sample-rate instruments that operate continuously, and because the detection algorithm has a low false-detection rate. The methodology may be replicated to facilitate comparisons between different time periods and geographic regions. [source]

From small-scale habitat loopholes to decadal cycles: a habitat-based hypothesis explaining fluctuation in pelagic fish populations off Peru

FISH AND FISHERIES, Issue 4 2004
Arnaud Bertrand
Abstract The Peru-Humboldt Current system (HCS) supports the world's largest pelagic fisheries. Among the world's eastern boundary current systems, it is the most exposed to high climatic stress and is directly affected by El Niño and La Niña events. In this volatile ecosystem, fish have been led to develop adaptive strategies in space and time. In this paper, we attempt to understand the mechanisms underlying such strategies, focusing on the El Niño 1997,98 in Peru from which an extensive set of hydrographic, capture and acoustic survey data are available. An integrated analysis of the data is crucial, as each has substantial shortcomings individually; for example, both catch data and acoustic surveys may easily lead to wrong conclusions. Existing hypotheses on anchovy and sardine alternations lead us to a ,habitat-based' synthetic hypothesis. Using our data, an integrated approach evaluated how fish responded to habitat variation, and determined the consequences in terms of fish-population variability. Various factors occurring at a range of different spatio-temporal scales were considered: interdecadal regime (warm ,El Viejo'/cool ,La Vieja' decadal scale); strength and the duration of the El Niño Southern Oscillation event (interannual scale); population condition before the event (interannual scale); fishing pressure and other predation (annual scale); changes in reproductive behaviour (intra-annual scale); presence of local upwelling (local scale). During El Niño 1997,98, anchovy was able to exploit a small-scale temporal and spatial ,loophole' inside the general unfavourable conditions. Moreover, sardine did not do better than anchovy during this El Niño and was not able to take advantage of the ,loophole' opened by this short-term event. Our results question the traditional view that El Niño is bad for anchovy and good for sardine. [source]

Acoustic estimation of wildlife abundance: methodology for vocal mammals in forested habitats

AFRICAN JOURNAL OF ECOLOGY, Issue 3 2010
Mya E. Thompson
Abstract Habitat loss and hunting pressure threaten mammal populations worldwide, generating critical time constraints on trend assessment. This study introduces a new survey method that samples continuously and non-invasively over long time periods, obtaining estimates of abundance from vocalization rates. We present feasibility assessment methods for acoustic surveys and develop equations for estimating population size. As an illustration, we demonstrate the feasibility of acoustic surveys for African forest elephants (Loxodonta africana cyclotis). Visual surveys and vocalizations from a forest clearing in the Central African Republic were used to establish that low-frequency elephant calling rate is a useful index of elephant numbers (linear regression P < 0.001, radj.2 = 0.58). The effective sampling area was 3.22 km2 per acoustic sensor, a dramatic increase in coverage over dung survey transects. These results support the use of acoustic surveys for estimating elephant abundance over large remote areas and in diverse habitats, using a distributed network of acoustic sensors. The abundance estimation methods presented can be applied in surveys of any species for which an acoustic abundance index and detection function have been established. This acoustic survey technique provides an opportunity to improve management and conservation of many acoustically-active taxa whose populations are currently under-monitored. Résumé La perte d'habitat et la pression de la chasse menacent des populations de mammifères dans le monde entier, ce qui entraîne des contraintes de temps critiques pour l'évaluation des tendances. Cette étude présente une nouvelle méthode de recherche qui échantillonne sur de longues périodes de façon constante et non intrusive et qui permet d'obtenir des estimations d'abondance à partir des taux de vocalisation. Nous présentons les méthodes d'évaluation de la faisabilité d'études acoustiques et nous développons des équations pour estimer la taille des populations. Pour illustrer ceci, nous montrons la faisabilité d'études acoustiques chez les éléphants de forêts africains Loxodonta africana cyclotis. Nous avons utilisé des études visuelles et des vocalisations d'une clairière forestière de République Centrafricaine pour établir que le taux d'appel à basse fréquence des éléphants est un indice intéressant du nombre d'éléphants (régression linéaire P < 0,001, radj² = 0,58). La superficie d'échantillonnage effective était de 3,22 km² par senseur acoustique, ce qui est une augmentation spectaculaire par rapport à la couverture des transects d'études par comptage des crottes. Ces résultats appuient le recours aux études acoustiques pour estimer l'abondance des éléphants dans de vastes zones retirées et dans des habitats variés, en utilisant un réseau de senseurs acoustiques bien répartis. Les méthodes d'estimation de l'abondance présentées peuvent être appliquées à toute espèce pour laquelle on a établi un indice d'abondance acoustique et une fonction de détection. Cette technique d'étude acoustique donne la possibilité d'améliorer la gestion et la conservation de nombreux taxons actifs au point de vue acoustique et dont les populations sont actuellement trop peu suivies. [source]