SOME OBSERVATIONS ON THE FISH LIFE IN A SEASONAL SAND RIVER

Summary

Five fish species were collected in a seasonal sand river that had migrated upstream more than 150 km from the permanent Limpopo River. The population structure indicated that mainly juveniles had taken part in this colonisation. No fish were collected in the winter when the river was dry. Information was obtained of fish surviving underground in the dry river bed.     

Colonization, riparian habitat selection and home range size in a reintroduced population of European beavers in the Loire

1. Colonization in a reintroduced population of European beavers in the Loire Valley was studied between 1974 and 1999. It followed a discontinuous remoteness model and a scattered distribution, beavers occupying only 25% of the river system over the 2800 km explored.
2. After 5 years, the colonization rate reached 104.2% year^–1 of new sites occupied (SD 75% year^–1), before dropping over the next 20 years. Nevertheless, the number of new colonies per km (0.125) remained stable throughout the years.
3.  Populus nigra , Salix alba and Fraxinus angustifolia were the dominant woody species in beaver sites, often associated with some herbaceous species.
4. The length of willow grove dominated by S. alba and P. nigra ( x ) was the best predictor of beaver home range ( y ), fitting the equation y =−0.742 x  + 5.9. Long-term maintenance of the population requires a minimum of 1.79-km of willows per colony.
5. In cutting tree trunks, beavers stimulate shoot development from the remaining stumps. They rejuvenate riparian forests, increase the number of tree stems and help stabilize the banks. Their effect on woody plant morphogenesis may have consequences for the helophyte communities used as food or habitat by other aquatic species.     

Colonial breeding in Mute swans (Cygnus olor) associated with an allozyme of lactate dehydrogenase

The Mute swan ( Cygnus olor Gmelin (Anatidae)) is a common water bird of lowland freshwaters and coastal shallows. Its typical breeding system involves lifelong monogamous pairs which vigorously defend large breeding territories, sometimes killing intruding swans that are unable to escape. However, in some unusual circumstances (superabundant food coupled with limited nesting sites) Mute swans may nest colonially. At the only two colonies in southern England a rare allele for lactate dehydrogenase (LDH) was found to be unusually common and colony swans carrying this allele were shown to breed more successfully. This isolated finding could, however, have originated either from a chance 'founder' effect or from human management of the main colony. We now show that this allele is also significantly commoner at two recently formed colonies in Denmark, implying that the association between the allele and colonial breeding may be widespread and longstanding.     

Mute swan (Cygnus olor) winter distribution and numerical trends over a 16-year period (1987/1988–2002/2003) in France

The numbers of mute swans (Cygnus olor) at 98 wetland locations in France were monitored monthly during the winter (December–February) for 16 years by a national network of observers as part of a broader national wildfowl monitoring scheme. Log-linear Poisson regressions with TRIM software were used to estimate missing counts and produce national numbers and indices. These corrected indices were in turn used to calculate an average annual rate of change and the associated confidence interval that subsequently enabled the computation of a cumulated (global) rate of change and associated confidence interval. The latter were interpreted to classify the numerical trends over the whole period. For the six wetlands with the largest numbers of mute swans, average numbers of swans were also compared between wetlands and months. General Linear Models were then used to test simultaneously for (1) differences between years and between wetlands for each month and (2) between years and between months separately for each of the six wetlands. The mute swan average annual rate of change was >7% for each month. The global rate of variation corresponded to a “strong increase” after TRIM analyses. This result strongly supports the necessity of maintaining both national and international monitoring schemes in order to be able to quickly detect major increases in swan numbers at specific sites, particularly where increasing numbers may result in a conflict with farmers or in inter-specific competition with other waterbirds and/or represent a possible sanitary/public health hazard due to the potential for swans to carry avian influenza viruses.     

Do mute swan (Cygnus olor) grazing, swan residence and fishpond nutrient availability interactively control macrophyte communities?

The mute swan (Cygnus olor Gmelin) is one of the largest herbivorous waterbirds in the world. Its population increased dramatically over the last decades in Western Europe, leading to concerns about its potential impact on aquatic ecosystems. Indeed, swan consequences on fishponds remain poorly investigated, although fishpond animal communities and economic value both largely depend on aquatic macrophytes. We carried out an experiment in the Dombes region (Eastern France) with 96 exclosures on 24 fishponds. Our aim was to assess the impact of swan grazing on aquatic macrophyte presence, abundance and community structure (diversity and evenness) during the growing season (April to July). We also considered the potential effect of swan stay (i.e. number of swan days ha⁻¹) and nutrient availability on macrophyte depletion. Swan grazing negatively affected the presence and abundance (% cover) of macrophyte beds, particularly at high swan density. No significant effect on dry biomass was found. Furthermore, swan grazing negatively affected community structure, suggesting that mute swan promoted the dominance of a few species in macrophyte communities. Whatever the macrophyte variable considered, nutrient availability in fishponds did not affect macrophyte depletion rate. It is speculated that both the repeated use of the same fishponds by birds and their expansion within the landscape may lead to more acute and broader consequences for macrophyte beds over the longer term.     

基于卫星追踪技术分析崇明东滩越冬小天鹅迁徙路线及特征

崇明东滩近年来已成为小天鹅在我国境内一处重要的越冬地;掌握该种群的迁徙路线和模式对于候鸟的跨区域联合保护具有重要意义。2022年2—12月;对崇明东滩越冬的10只小天鹅的春季和秋季迁徙行为进行了卫星追踪;收集了1336688条定位数据。本项研究识别了崇明东滩越冬小天鹅种群向西、中、东3个方向迁徙的路线;其中新发现了一条前往俄罗斯克拉斯诺亚尔斯克边疆区的喀拉海南部沿岸繁殖的迁徙路线;并且发现小天鹅秋季迁徙后的越冬地较为广泛;包括安徽、江西、上海等地。崇明东滩越冬小天鹅春季迁徙时间、停歇次数均显著高于秋季;迁徙速度显著低于秋季;春季迁徙进程整体慢于秋季。小天鹅春季的平均迁徙距离为5805.7±648.2 km;平均迁徙时间为107.9±31.8 d;平均停歇次数为13.7±3.7次;平均飞行速度为1256.2±385.3 km·d^-1;秋季的平均迁徙距离为5348.4±705.9 km;平均迁徙时间为61.3±15.3 d;平均停歇次数为6.0±1.9次;平均飞行速度1014.3±335.7 km·d^-1。本研究还更新了小天鹅迁徙路线上的重要停歇地;中国境内包括内蒙古的黄河湿地以及河北张家口的滦河流域。本研究有助于更新和判断中国越冬小天鹅种群的现状和栖息地生境状况;进而采取合理的监测和管理措施保护小天鹅种群及栖息地环境。     

The cost of migration: spoonbills suffer higher mortality during trans-Saharan spring migrations only

Explanations for the wide variety of seasonal migration patterns of animals all carry the assumption that migration is costly and that this cost increases with migration distance. Although in some studies, the relationships between migration distance and breeding success or annual survival are established, none has investigated whether mortality during the actual migration increases with migration distance. Here, we compared seasonal survival between Eurasian spoonbills (Platalea leucorodia leucorodia) that breed in The Netherlands and migrate different distances (ca 1000, 2000 and 4500 km) to winter in France, Iberia and Mauritania, respectively. On the basis of resightings of individually marked birds throughout the year between 2005 and 2012, we show that summer, autumn and winter survival were very high and independent of migration distance, whereas mortality during spring migration was much higher (18%) for the birds that wintered in Mauritania, compared with those flying only as far as France (5%) or Iberia (6%). As such, this study is the first to show empirical evidence for increased mortality during some long migrations, likely driven by the presence of a physical barrier (the Sahara desert) in combination with suboptimal fuelling and unfavourable weather conditions en route.