ISSN:1052-7613    

DOI:10.1002/aqc.3270030202

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

Abstract1.Prior to 1983 the Danish legislation concerning streams gave priority to drainage of water. The revision of The Watercourse Act gave balanced priority to drainage of water and environmental quality, focusing on an ecologically more appropriate maintenance practice and giving special provisions for stream restoration activities.2.Different measures of single structure restoration have been used, the most common being replacement of weirs, dams or other obstacles by rapids, establishment of salmonid spawning grounds, and installation of new or improved fish ladders.3.The most common stream channel restoration method is integrated use of a number of single structure measures. Establishment of a two‐stage channel and re‐opening of small piped streams have also been used. Stream valley restoration includes restoration of old meanders or establishment of a new sinuous channel and involves the adjacent riparian areas.4.The changed stream maintenance practice involves a new strategy for dredging and cutting of weeds and bank vegetation in order to minimize the ecological damage caused by keeping a reasonable discharge capacity. In 1990, environmentally acceptable weed‐cutting was performed in 37% of all municipal streams and the bank vegetation was left uncut in a third of the streams. Similarly, more than half of the county streams were maintained using hand scythes and in 74% of the streams the bank vegetation was left uncut.5.Quantitatively, stream restoration has contributed little to the general improvement of Danish streams compared with changed maintenance practice. Stream restoration projects create public interest in the environmental quality of streams, but major improvements in the physical properties of Danish streams depend on future maintenance practice.6.Due to major changes predicted in Danish agriculture many riparian areas and wetlands will reappear and the natural or semi‐natural physical properties of streams will be re‐established by natural processes or changed in maintenance practice. However, there will still be large areas with intensive agriculture, where environmental and agricultural interests must be balanced. The Danish experience has shown that this is

ISSN:1052-7613    

DOI:10.1002/aqc.3270030203

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

Abstract1.Incised stream channel aquatic habitats typically are severely degraded. After the primary knickpoints or knickzones have passed, base flows are limited to shallow channels flanked by sandy berms within the enlarged high‐flow channel. Riparian vegetation, woody debris and pool habitat are in short supply, and stream systems become disengaged from their floodplains.2.We hypothesized that habitat recovery might be accelerated in channels that have incised and are regaining equilibrium through deposition of sandy berms by placing rock spurs in the channel and by planting woody vegetation on the berms. On the basis of literature review and a pilot study, planting designs were developed for a large‐scale field experiment: 2550 1.5 m long cuttings of native willow (Salixspp.) 2–25 cm in diameter were planted 1–1.2 m deep along the base‐flow channel of an incised stream. A ridge of stone was placed on the water side of the plantings, and 17 rock spurs were constructed by extending existing spur dikes from the opposite bank.3.Woody cover along the treated bank increased from 38% to 66% of bankline after one growing season. Survival of individual plantings was reduced from an estimated 60% to an observed 34% by competition from the exotic kudzu vine,Pueraria lobata.Mean depth and mean scour hole depth, corrected for stage variation, increased 44% and 82%, respectively. Mean scour hole width increased 130%. The mean length of fish and the number of fish species approximately doubled, while the total weight of fish captured by a unit of sampling effort increased by an order of

ISSN:1052-7613    

DOI:10.1002/aqc.3270030204

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

Abstract1.This paper provides a detailed review of the status and distribution of wintering seaducks (Scaup, Eider, Long‐tailed Duck, Common Scoter, Velvet Scoter and Red‐breasted Merganser) in Britain and Ireland. It is based on a literature review plus analysis of National Waterfowl Counts (NWC) and other data. A summary of current knowledge of wintering seaducks is provided together with information on threats to populations, conservation goals and research needs.2.Each species is examined in turn, with the emphasis on aspects of the population, habitat use, current distribution, trends and history. Important areas for individual species are highlighted.3.Overall, seaduck populations have changed considerably, those for Eider and Red‐breasted Merganser having expanded greatly, whilst those for Scaup, Common and Velvet Scoter have declined during historical and/or recent times. The trend for Long‐tailed Ducks is unclear. British population estimates are considered to be out of date and need to be revised. The premier areas for seaduck communities are highlighted, thus providing basic information for conservation action and future monitoring.4.Seaduck populations are threatened in a number of ways and the main factors are discussed. Oil pollution and the actions of commercial fisheries are the most important current threats. Conservation goals are suggested. These include an urgent need to develop appropriate domestic legislation to facilitate the protection of seaduck populations. Such legislation is currently almost entirely lacking.5.Some priorities for research, particularly for better population monitoring of seaducks in British and Irish waters, are provided. These include: alterations to the NWC scheme to allow the collation of opportunistic counts, made when the conditions are amenable for censusing seaducks; extension of the scheme into areas not currently surveyed (e.g. most parts of Ireland); and the establishment of intensive survey programmes to monitor seaducks in the key areas of Britain and Ireland. It is hoped that this paper will serve to stimulate interest and commitment from all who play a part in safeguarding our seaduck h

ISSN:1052-7613    

DOI:10.1002/aqc.3270030205

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

Abstract1.Annual rate of increase (r) was estimated for the Hooker's sea lion (Phocarctos hookeri) using demographic models that incorporated age‐specific estimates of reproductive rate (mx) and survival (lx).2.In the absence of empirical estimates of natural mortality (nx), survival schedules were derived by combining a range of incidental mortality rates (hx) in commercial fisheries with estimates of nxfor two species that exhibit similar life histories to Hooker's sea lions: northern fur seals (Callorhinus ursinus), model A, and Himalayan thar (Hemitragus jemlahicus), model B. Model A represented a population that would have a limited capacity for population increase; incidental mortality rates of mature females above about lo would result in a population decline. Model B represented a population that, even in the absence of incidental mortalities, would just remain stable.3.If either of these models accurately represents the population demography of Hooker's sea lions, then these animals have limited capacity for population increase. They are constrained by relatively delayed maturity, moderate reproductive rates and short life spans.4.While absolute estimates ofrfrom our models must be interpreted with caution, it is the responses ofrto incidental mortalities and to increased natural mortality of pups that are most informative. Both models showrdecreasing by 0.007–0.008 with a 0.01 increase in incidental mortalities of mature females. Both models also indicate that an increase in the natural mortality of pups by 0.027 will reducerby 0.0055 or more.5.Given the paucity of available empirical data, our results suggest that incidental mortalities of Hooker's sea lions in the trawl fishery for arrow squid (Nototodarus sloanii) should be minimized if a population decline is to be avoi

ISSN:1052-7613    

DOI:10.1002/aqc.3270030206

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

摘要:

Abstract1.Seals that attacked fish on 15 marine fish‐farms in south‐eastern Tasmania were large subadult and adult male Australian fur sealsArctocephalus pusillus doriferus.From July to November 1988 there were 235 attacks on fish‐farms where Atlantic salmonSalmo salarand rainbow troutS. gairdneriwere held in pens. Single animals usually attacked pens at night, irrespective of the size and species of fish in the pens. The seals damaged both pens and fish, and sometimes fish escaped as a result of the attacks.2.The vulnerability of fish‐farms was influenced by their proximity to seal haul‐out sites. Proximity to fishing ports and size of the fish‐farm had little influence.3.The use of shooting as a protection method was inefficient and ineffective because seals usually entered fish‐farms at night and showed no fear of shooters. There were many attacks in the presence of underwater acoustic seal scarers. Deterrents such as pursuit with boats, lights, seal crackers and emetics helped reduce the number of seal attacks.4.The only way totally to prevent seals from attacking fish‐farms is to exclude them from the vicinity of the fish pens with physical barriers that they cannot penetrate. These are currently in use, and include perimeter fences and protection nets made of steel mesh set around

ISSN:1052-7613    

DOI:10.1002/aqc.3270030207

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:1052-7613    

DOI:10.1002/aqc.3270030208

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:1052-7613    

DOI:10.1002/aqc.3270030209

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:1052-7613    

DOI:10.1002/aqc.3270030210

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY

ISSN:1052-7613    

DOI:10.1002/aqc.3270030211

出版商:John Wiley&Sons, Ltd    

年代:1993

数据来源: WILEY