摘要:

SummaryThe size of theTilapia niloticapopulation in Lake George was determined using purse seines. Production, mortality rate, size at maturity, and other parameters of the population were also determined. In Lake George, the factors that regulate the population ofT. niloticaare both natural and artificial. The restricted size of nursery zones, strong fish‐killing storms, predation and old age are some of the natural regulators. For the adult population, mortality due to these factors is low (natural mortality coefficient,M=0.1). However, commercial fishing appears to have a stronger controlling force. Since the intensification of commercial fishing in 1950, the size of the fish caught has been decreasing, whereas the number of fish caught has been increasing yearly, probably as a result of increasing effort and the use of small mesh nets. These two trends may be the cause forT. niloticato mature at a smaller size than they did 15 years ag

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00096.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SummaryImpala in southern and eastern Africa differ in the proportion of the year during which adult males are effectively territorial, and consequently in the age at which young males are driven from the female herds. A comparable difference in the age at which male mortality accelerates, relative to female's, suggests that this differential mortality is a product of male‐imposed social organization. In theory this produces enhanced resource flow to females in the population, leading to a greater natality per unit of resource than would be possible without these effects of social organization. It seems that the impala's kind of social organization can not have a density‐dependent regulatory funct

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00097.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

Summary(1) The scale and pattern of the elephant mortality occurring in Tsavo National Park during the dry years 1970 and 1971 was investigated.(2) Total numbers were estimated from an aerial stratified systematic sample, and a correction made for juvenile carcasses lost through disintegration. The overall total was estimated atc.5900 although this included a somewhat arbitrary addition for juveniles estimated to have disintegrated during the study period.(3) The aerial census provided the basis for an analysis of the spatial distribution of carcasses.(4) The age distribution of the mortality was determined from the collection of found jaws, after the correction made for collection bias and carcass disintegration. Sex composition was determined from a large sample of known‐age tusks.(5) Age and sex‐specific mortality were calculated, which provided interesting comparisons with the pattern of more normal years. There was good evidence to show: (a) a prolonged phase of high juvenile mortality; (b) raised overall rates; (c) preferential adult female mortality.(6) The spatial distribution of the mortality coincided with: (a) areas of high, recent immigration; (b) areas of low and erratic rainfall; (c) areas of profound vegetation change. Within these areas it was concentrated near permanent water. Its occurrence in time coincided with 2 years of low rainfall and prolonged dry seasons.(7) The report concludes with some possible implications of the mortality for Tsavo National P

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00098.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SummaryFollowing the 1970‐71 drought, which resulted in heavy elephant mortality, a long‐term study was initiated in Tsavo National Park to determine movements and home ranges of individual elephants and their relations to overall distribution patterns and environmental factors such as rainfall. The preliminary results presented show the post‐drought situation but it is recognized that they may not represent normality. Methods used were radio‐tracking and observations of visually identifiable individuals. Two aerial counts provided data on overall distribution.Two bulls and two cows were radio‐tagged in Tsavo West and two bulls and four cows in Tsavo East, providing home range and movement data. Home ranges in Tsavo West (mean = 350 km2) were considerably smaller than in Tsavo East (mean = 1580 km2).The differences between Tsavo West and Tsavo East are probably related to habitat quality, Tsavo East receiving less rainfall than Tsavo West. This view is supported by the fact that the 1970‐71 mortality was largely confined to Tsavo East.The movements of individuals were useful in interpreting relatively major shifts in elephant distribution that occurred within a 4‐month period.The following preliminary conclusions emerge from the results obtained so far:(1) Elephants in the Tsavo area, particularly Tsavo East, were more mobile during the study period than was assumed from earlier studies, undertaking long‐distance movements in fairly direct response to localized rainfall.(2) A subdivision of the overall population into locally distinct units may exist during the dry season but did not obtain after significant rainfall, when the elephants ranged over wide areas and came into contact with animals from other dry‐season units.(3) Circumstantial evidence suggests that food is the primary proximal factor governing movements and distribution of elephants in the area. Food availability, in turn, is determined largely by the spatial and temporal pa

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00099.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

摘要:

SummaryBlack rhinoceros were studied in the Masai Mara Game Reserve from May 1971 to August 1972. Data were gathered on their density, distribution, population structure, social organization, feeding habits and daily activities.The study area, its vegetation, and the study methods are described briefly. Rhino occupied thirteen separate distribution areas, their distribution being influenced by food availability, water, cover and the presence of human and domestic animals. The number of rhino in each distribution area were within the range of one to thirty‐one (densities 0–04 to 0–24/km2), giving a mean density in the rhino‐occupied area (749 km2) of 0–14/km2and in the whole study area (1530 km2) of 0–07/km2.Of the 108 rhino in the population, 73% were adult and 27% were immature. At least one adult and one immature were found in nearly all distribution areas. Adult rhinos formed one age class while the immatures were divided into three age classes. Sex ratio and mortality were investigated.Social units and their home range sizes in relation to density and distribution are discussed. Some aspects of social organization, feeding habits and daily activities will be discussed in a later paper.In conclusion, recommendations for rhino management based on preservation of habitat, particularly cover, more sources of water and human settlement are discussed. Suggestions for further lines of research are als

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00100.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00101.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00102.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY

ISSN:0141-6707    

DOI:10.1111/j.1365-2028.1973.tb00103.x

出版商:Blackwell Publishing Ltd    

年代:1973

数据来源: WILEY