Acoustic evaluation of the vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in Lake Biwa

SUMMARY: The vertical distribution of dwarf ayu Plecoglossus altivelis altivelis in the pelagic waters of Lake Biwa was evaluated from June to September in 1995–97 from eight acoustic surveys using a quantitative echosounder. In each survey, echoes from a depth range of 3 m to the sea bottom were collected at a station every 2.7 s for 24 h together with measurements of vertical profiles of water temperature and chlorophyll-a. The ayu's echoes were identified using an underwater video camera. The ayu were observed near the maximum chlorophyll-a depth and above the thermocline. Their density was highest at depths of 4–11 m with 2–4 individuals/m ³ and was almost zero below 20 m. Echo signs were recorded as having a frequent duration of more than 1 h at night, whereas were of a shorter duration in the day. The fish stay in the epilimnion during the day without any clear vertical migration, but are distributed more uniformly at night. The advantages of remaining in the epilimnion are discussed in terms of food availability and predator avoidance.     

Length-based Beverton and Holt spawning stock biomass per-recruit-models, with application to the Lates stappersii (Boulenger) stock in Lake Tanganyika

Two kinds of length-based Beverton and Holt spawning stock biomass per-recruit ( SSB/R )-models are presented. The first allows the computation of the absolute value of SSB/R . The second estimates the index of SSB/R , also called 'relative SSB/R '. Both models are extensions of the original age-based Beverton and Holt SSB/R -models. The SSB/R and its index can be assessed with reference to variations in the exploitation characteristics. These models were applied to the Lates stappersii (Boulenger) stock in Lake Tanganyika using fishing mortality as the independent variable. They produce an absolute value of SSB/R of 12.95 g, representing 22.4% of that of the 'virgin' stock for the 1993–1994 equilibrium-fishing regime. This level is indicative of an unsafe state of the stock in question. Moreover, the SSB for L. stappersii seems to be fragile under increased fishing pressure as, for example, doubling or tripling the current fishing regime would reduce the current SSB/R by 57.3 and 77.4%, respectively. In general, the use of length-based Beverton and Holt SSB/R -models is handicapped by underlying hypotheses and sub-models which are restrictive.     

Downstream migration of European lake trout, Salmo trutta f. lacustris L., and resident brown trout, Salmo trutta f. fario L., progeny in a Lake Constance affluent river

Abstract The local migratory behaviour of lake trout, Salmo trutta L., and resident brown trout, S. trutta L., progeny from an inflowing stream of Lake Constance was compared. No differences were detected with respect to emigration rate (3.5% and 4.2% for lake trout and resident trout, respectively), rate of residency (6.8% and 8.2% for lake trout and resident trout, respectively) and rate of survival (12.3% and 12.1% for lake trout and resident trout, respectively) one year after stocking. Some of the resident brown trout offspring became migrants and vice versa . The results indicate that the progeny of riverine brown trout contribute considerable numbers to lake trout stock.     

Australia‘s Lake Eacham rainbow fish: lessons and outlook

Recent DNA comparisons confirm that the endangeredLake Eacham rainbow fish (Melanotaenia eachamensis Allen andCross, 1982) is a separate species, distinct from othermelanotaeniids in the area. Although the large populationformerly in Lake Eacham has not been re-established, other smallnatural populations of M. eachamensis have been identified fromLake Euramoo and Dirran Creek.Commencing with 23 captive individuals (3 14 6 I), representing three original stocks, Project Eachamensis (initiated by the Australia New Guinea Fishes Association in July 1987) has stimulated and actively encouraged the establishment of new captive populations. Approximately 15 captive breeding populations are now reported in eastern Australia -- numbers of individuals are estimated to exceed 1000; additional Bowman and Tappin populations are being maintained in North America and Europe.A recent review of this conservation programme (involving individual aquarists, hobbyist organizations and scientists as well as the aquarium industry and government) has now recommended: an alteration to the conservati on status category from endangered to vulnerable; maintenance of existing captive populations; formulation of a comprehensive recovery plan with the long-term objective of re-establishing and increasing natural populations; and encouraging public education on the risks of translocation.     

The status of habitat of great white and pink-backed pelicans in northeastern KwaZulu-Natal,South Africa: a review

Habitat change, mainly through the actions of humans, poses a threat to great white (Pelecanus onocrotalus) and pink-backed (P. rufescens) pelicans in northeastern KwaZulu-Natal, the southernmost distributions of these species on Africa’s eastern seaboard. This study assessed the relative importance and state of the potential pelican habitat in the northeastern KwaZulu-Natal region, focusing particularly on Lake St Lucia and the Phongolo River floodplain. Great white pelicans breed on islands in Lake St Lucia. Should these islands be lost through falling water levels or their becoming joined to mainland by deposits of silt, or by flooding, no suitable habitat for their breeding will remain in the region. By contrast, the pink-backed pelican nests in trees, and there appears to be plentiful alternative habitat. Lake St Lucia and, to a lesser extent, the Phongolo River floodplain, are important foraging areas for both pelican species. The hydrology of both these systems has been affected by human activities.     

鄱阳湖生态经济区农民工回流决策与回流农民工职业选择的经济分析

基于货币成本收益和非货币成本收益,对鄱阳湖生态经济区农民工回流决策及回流农民工的职业选择进行了经济分析。结果显示,考虑机会成本因素后的转移纯收益小于0时,转移农民工就会主动回流,而回流纯收益与转移纯收益趋于平衡时,非货币因素也会成为影响农民工回流决策的重要因素。回流农民工职业纯收益大于0,农民工就会放弃原有职业选择新职业。     

青海湖流域生态环境保护与治理对策

通过论述青海湖区生态环境现状和生态环境恶化的原因,说明了青海湖区生态环境的重要地位和作用,生态环境保护、治理的意义,并提出了具体的治理措施。     

鄱阳湖水位特征与湿地生态保护

从分析江湖关系入手,研究鄱阳湖的水位及湿地生态特征,并提出人工控湖等加强湿地生态保护的对策建议。     

关于渝北区宝圣湖长效管理机制及城市湖库保护措施的探讨

通过对渝北区宝圣湖整治措施、整治效果及存在的问题进行分析,结合城市湖库污染成因,探讨了宝圣湖水环境保护长效管理机制及城市湖库环境保护措施,为城市湖库及其他地表水的保护提供依据。