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Digital electromagnetic telemetry system for studying behaviour of decapod crustaceans
No full textA telemetry system for studying decapod crustacean behaviour is described which uses low frequency, digitally encoded electromagnetic tags whose signals are detected with a grid of loop aerials on the seabed. Electromagnetic telemetry can be used to study short range movements of cryptic animals in topographically complex habitats that are not amenable to ultrasonic telemetry. Digital encoding allows many individuals to be monitored simultaneously and one or more behavioural, physiological or environmental variables to be telemetered. In the present system, tag signals convey identity and a measure of activity derived from an integral tilt switch. Translocational movements are indicated by detection of tags with different aerials. A central data logger located on the seabed decodes and records tag signals and environmental measurements. Design life of tags is >1 year and the receiving system batteries are replaced by divers at intervals of up to 4 weeks. In field tests, crab (Cancer pagurus L.) and lobster (Homarus gammarus (L.)) activity was monitored at an artificial reef for 14 months. Examples of the type of information acquired are presented to illustrate the capabilities of the system and potential applications are discussed. Limitations of digital electromagnetic telemetry stem mainly from the short range of detection, the need for cables on the seabed and the size and shape of the transmitting tag
Movement of wild European lobsters Homarus gammarus in natural habitat
No full textMost previous mark-recapture studies of the movement of European lobsters have used externally attached tags which were lost at ecdysis, restricting the range of possible recapture intervals. Published studies of this species with tags designed to be retained through ecdysis have focussed on artificially reared lobsters, or wild lobsters released in artificial habitat. Here, we report a mark-recapture study of the mobility of wild European lobsters conducted in 3 areas on the south coast of England, using persistent tags. Tagged lobsters were smaller than the fishery minimum landing size (50 to 84 mm carapace length) and were therefore likely to be predominantly sexually immature. The majority of these lobsters do not appear to undertake extensive alongshore or on-/offshore migrations. Recapture distances ranged from 0 to 45 km, but 95% of recaptured lobsters moved <3.8 km from their original release positions over periods of up to 862 d. Distance moved did not differ between males and females, but there was a weak positive relationship between recapture distance and body size. The directional distribution of movements >0.5 km differed between the 3 study areas and appeared to be related to the spatial configuration of local lobster habitat, but with a marked tendency for offshore movement in 2 of the areas. Movements >5 km from all 3 release areas, which comprised 5% of recaptures after >6 mo, were generally undertaken by larger lobsters and were almost exclusively to the west or southwest, against the general direction of tide- and wind-generated residual water movement in the English Channel
Modelling the trajectories of migrating Atlantic salmon (Salmo salar)
No full textThis paper describes a model for simulating the trajectories of migrating Atlantic salmon (Salmo salar) in the ocean. Surface current and temperature representations were used as boundary conditions for simulation of migration trajectories. Representations of surface currents were derived from a general circulation model forced by realistic winds and then tested through comparisons with observed trajectories of drifting buoys. Observed climatology data were used to represent sea surface temperature patterns. The model was used to simulate the trajectories of 15 individual salmon that were tagged in their home rivers and subsequently recaptured at sea. In contrast to a random swimming direction model, trajectories simulated using both rheotaxis and thermotaxis as direction-finding mechanisms passed close to the recapture locations of the salmon. The timings and positions of the trajectories simulated using rheotaxis corresponded more closely with the observed data than those simulated using thermotaxis. This work indicates that either rheotaxis or thermotaxis, or a combination of the two, are possible direction-finding mechanisms for migrating Atlantic salmon
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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