Mass fishing by Cormorants <i>Phalacrocorax carbo sinensis</i> at lake IJsselmeer, The Netherlands:A recent and successful adaptation to a turbid environment

The habit of mass fishing by Cormorants at lake IJsselmeer, The Netherlands, is a recent phenomenon. During the first half of the 1970s the birds changed behaviour probably as a result of the deteriorating under water visibility in the lake (3-4 m water depth). The behavioural switch coincided with years of high numbers of Smelt Osmerus eperlanus and Ruffe Gymnocephalus cernuus present in the southeastern part of lake Markermeer, the birds' main fishing area at that time. Social fishing by Cormorants is directed towards the catch of relatively small, pelagically dwelling fish. It is argued that for a large water system where social fishing is the rule, a minimum colony size of c. 1000 pairs is required. Typically each colony had one socially fishing group (4000-5000 birds) that slowly changed position through the course of the day. Depending on the direction of the wind the flock's position could greatly change between days. Hunting speed was measured and coincided with maximum swimming speed of medium sized fish prey (15-25 cm). Hunting speed increased during the season probably as a result of the greater swimming speeds of the fish at higher temperatures. Intake rate was closely linked to the birds' position within the flock indicating local depletion of the fished water layer. Mass fishing was especially rewarding at intermediate light intensities under water (50-80 cm Secchi depth, or 300-500 mu E.m(-2)s(-1) at 40 cm depth). The habit of pushing up the fish against the light back-ground of the clear top water layer was only possible when wind caused no greater turbidity than 40 cm Secchi depth (100 mu E.m(-2)s(-1)) which is considered a breakpoint for this kind of behaviour. Adapting the habit of mass fishing effectively enabled the birds to exploit the turbid, rapidly changing environment which resulted in the extension of the foraging range thus maximising colony size relative to the resources available.</p

Solitary foraging in sand pits by breeding Cormorants <i>Phalacrocorax carbo sinensis</i>:Does specialised knowledge about fishing sites and fish behaviour pay off?

Though most Cormorants from the colonies exploiting the lake IJsselmeer area in The Netherlands nowadays have taken up the habit of fishing in large groups, some individuals are still observed fishing solitarily. This phenomenon occurs predominantly during the coldest months at well-defined patches, often located in sand pit regions. A case study in a sand pit in the early spring of 1982 revealed that Cormorants from Oostvaardersplassen took advantage of the fish located at their winter refuges inside the pit by specifically diving along the pit's steepest slopes. Thus, they were able to achieve a more than average daily ration of about 475 g of fish within a mere 15 minutes of foraging in the second half of March. Later on, the site became less profitable and the birds gradually left. The importance of high intake rates in early spring is stressed, in view of time consuming activities like nest site occupation, pair formation and accumulation of body reserves for egg-laying, incubation and chick feeding. It is suggested that only the 'higher quality' birds may have the experience and skill required to use solitary fishing techniques as a profitable alternative to social fishing in early spring.</p

Opvang van Ganzen op de Klei: evaluatie van experimenten in drie winters

Conform het Beleidskader Faunabeheer zijn opvanggebieden voor overwinterende ganzen aangewezen. In akkerbouwgebieden met zware klei zijn de mogelijkheden voor beheerspakketten voor opvang beperkt. Gedurende drie winters (2005-'08) zijn in Zeeuws- Vlaanderen en West Brabant een vijftal experimentele beheerspakketten op hun geschiktheid onderzocht. Het pakketgebruik door ganzen is gemonitord door ganzentellingen en keuteltellingen. Resultaten van onderzoek van: CLM, SOVON en Alterra, op verzoek van LNV, Faunafonds en LT

Meer zoogdieren bij minder vaak maaien van slootkanten

Meer natuur langs perceelranden hoeft dus niet altijd te leiden tot een verminderde agrarische productie op de percelen

Ganzen: geliefd, maar met mate

In het boerenland zijn ganzen een alledaagse verschijning geworden, niet alleen in de winter, maar ook in de zomerperiode zijn er grote aantallen. Vanuit het natuurbeleid wordt de toename toegejuicht, maar er zijn ook negatieve aspecten : de schade aan landbouwgewassen. Meer recent zijn er berichten dat de royale aanwezigheid van ganzen ook schade aan bepaalde vormen van natuur veroorzaakt. Daarmee is het tweeslachtige beeld van de ganzen in onze maatschappij gekenschetst : ze zijn welkom, maar met mate. Een overzicht van het aantal ontwikkelingen, de belangrijkste beleidsmaatregelen en de nieuwste bevindingen uit recent onderzoek

Fossil AGN jets as ultra high energy particle accelerators

Remnants of AGN jets and their surrounding cocoons leave colossal magnetohydrodynamic (MHD) fossil structures storing total energies ~10^{60} erg. The original active galacic nucleus (AGN) may be dead but the fossil will retain its stable configuration resembling the reversed-field pinch (RFP) encountered in laboratory MHD experiments. In an RFP the longitudinal magnetic field changes direction at a critical distance from the axis, leading to magnetic re-connection there, and to slow decay of the large-scale RFP field. We show that this field decay induces large-scale electric fields which can accelerate cosmic rays with an E^{-2} power-law up to ultra-high energies with a cut-off depending on the fossil parameters. The cut-off is expected to be rigidity dependent, implying the observed composition would change from light to heavy close to the cut-off if one or two nearby AGN fossils dominate. Given that several percent of the universe's volume may house such slowly decaying structures, these fossils may even re-energize ultra-high energy cosmic rays from distant/old sources, offsetting the ``GZK-losses'' due to interactions with photons of the cosmic microwave background radiation and giving evidence of otherwise undetectable fossils. In this case the composition would remain light to the highest energies if distant sources or fossils dominated, but otherwise would be mixed. It is hoped the new generation of cosmic ray experiments such as the Pierre Auger Observatory and ultra-high energy neutrino telescopes such as ANITA and lunar Cherenkov experiments will clarify this.Comment: 11 pages, 6 figures, additional references and explanations. Accepted for publication in MNRA

Vision and Foraging in Cormorants: More like Herons than Hawks?

Background Great cormorants (Phalacrocorax carbo L.) show the highest known foraging yield for a marine predator and they are often perceived to be in conflict with human economic interests. They are generally regarded as visually-guided, pursuit-dive foragers, so it would be expected that cormorants have excellent vision much like aerial predators, such as hawks which detect and pursue prey from a distance. Indeed cormorant eyes appear to show some specific adaptations to the amphibious life style. They are reported to have a highly pliable lens and powerful intraocular muscles which are thought to accommodate for the loss of corneal refractive power that accompanies immersion and ensures a well focussed image on the retina. However, nothing is known of the visual performance of these birds and how this might influence their prey capture technique. Methodology/Principal Findings We measured the aquatic visual acuity of great cormorants under a range of viewing conditions (illuminance, target contrast, viewing distance) and found it to be unexpectedly poor. Cormorant visual acuity under a range of viewing conditions is in fact comparable to unaided humans under water, and very inferior to that of aerial predators. We present a prey detectability model based upon the known acuity of cormorants at different illuminances, target contrasts and viewing distances. This shows that cormorants are able to detect individual prey only at close range (less than 1 m). Conclusions/Significance We conclude that cormorants are not the aquatic equivalent of hawks. Their efficient hunting involves the use of specialised foraging techniques which employ brief short-distance pursuit and/or rapid neck extension to capture prey that is visually detected or flushed only at short range. This technique appears to be driven proximately by the cormorant's limited visual capacities, and is analogous to the foraging techniques employed by herons

The Magnus expansion and some of its applications

Approximate resolution of linear systems of differential equations with varying coefficients is a recurrent problem shared by a number of scientific and engineering areas, ranging from Quantum Mechanics to Control Theory. When formulated in operator or matrix form, the Magnus expansion furnishes an elegant setting to built up approximate exponential representations of the solution of the system. It provides a power series expansion for the corresponding exponent and is sometimes referred to as Time-Dependent Exponential Perturbation Theory. Every Magnus approximant corresponds in Perturbation Theory to a partial re-summation of infinite terms with the important additional property of preserving at any order certain symmetries of the exact solution. The goal of this review is threefold. First, to collect a number of developments scattered through half a century of scientific literature on Magnus expansion. They concern the methods for the generation of terms in the expansion, estimates of the radius of convergence of the series, generalizations and related non-perturbative expansions. Second, to provide a bridge with its implementation as generator of especial purpose numerical integration methods, a field of intense activity during the last decade. Third, to illustrate with examples the kind of results one can expect from Magnus expansion in comparison with those from both perturbative schemes and standard numerical integrators. We buttress this issue with a revision of the wide range of physical applications found by Magnus expansion in the literature.Comment: Report on the Magnus expansion for differential equations and its applications to several physical problem