摘要:

AbstractFrom July 1989 to December 1994, an echo sounder provided monthly estimates, usually for both day and night, of pelagic salmonid densities in the North and South Basins of Windermere, the largest natural lake in England. Sampling was along contiguous transects, three in the North Basin and five in the South Basin. Records for Arctic charr (Salvelinus alpinus) could not be separated from those for brown trout (Salmo trutta), but previous sampling by gill‐nets and anglers showed that charr formed over 90% of this mixed population in the North Basin and about 60–75% in the South Basin. Associated with the increasing eutrophication of the lake, there has been a decline in anglers' catches of charr and, since 1984, an increase in brown trout taken in the pelagic zone of the South Basin. The echo‐sounder data showed that pelagic salmonid density in the North Basin was about two to five times that in the more eutrophic South Basin in 1989, 1990 and 1991. Since the start, in April 1992, of the reduction of phosphorus discharged from sewage works, this ratio has decreased, especially at night when the highest densities were recorded. This improvement was chiefly due to a significant (P<0.001) increase in the density of small fish (length<20 cm), in both the upper (depth20 m) water layers. Although a similar improvement has still to be shown in the upper water layer by larger fish above the size limit for removal by angling (20 cm), there has been a significant increase (P20 cm at water depths<20 m) should increase in the next few years, especially in the South Basin. It is therefore important to continue the monitoring program and thus ensure that there is advance warning of any marked changes in charr s

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00128.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractLaboratory studies were conducted to determine the effects of suboptimal prey density and length of prey‐deprivation period on swimming, feeding, and social behavior in larval American shad.Alosa sapidissima.Replicated prey‐density treatments of 1,000, 500, and 0Artemianauplii/liter and deprivation periods of 0,2, and 4 days were established for an 8‐day period. The duration or frequency of 11 behavior patterns was quantified with an event recorder during the experiment. Exposure to suboptimal prey densities affected three categories of larval behavior: swimming activities (pivot and dart), interaction with other larvae (escape or avoid), and stereotypical feeding responses (sigmoid and lunge). Location of a food patch, simulated by the sudden introduction of prey to aquaria, affected the frequency of feeding responses more than other categories of behavior. The patch model was supported as a foraging strategy in larvae. The ontogeny of prey deprivation was evidenced primarily by changes in swimming activity (reduced pivot and dart frequencies), though feeding responses (particularly fixate) were also diminished. Deprivation‐induced loss of pivot and fixate was an irreversible, pathological effect of starvation. Deprivation also resulted in greater vertical orientation (head up, 42°) of larvae than non‐deprived larvae (21–29°). These changes in behavior may result in less effective escape from predators, location of food patches, or pursuit and capture of prey items in rive

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00129.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractFrom 1986 to 1994, we collected 3,098 Gulf of Mexico sturgeon (Acipenser oxyrinchus de sotoi) during their spring migration from the Gulf of Mexico to the Suwannee River, Florida. We estimated population size and described migratory and seasonal growth patterns of sturgeon in the Suwannee River, using data from tag returns, movement of ultrasonic‐lagged fish and weight records. Population size estimates for 3.0‐ to 81.0‐kg Gulf of Mexico sturgeon in the Suwannee River ranged from 1,504 to 3,066 fish. Sturgeon migrated from the Gulf of Mexico into the Suwannee River from mid‐February to early May. Fish moved upstream and remained within a 2‐km upstreamdownstream “home” area. These sites were closely associated with undenvater springs that discharge into the river. Sturgeon migrated downstream and into the Gulf of Mexico in October and November. In the Gulf of Mexico, ultrasonic‐tagged fish were located in the Suwannee Sound area just north of Cedar Key, Florida. On an annual basis, fish gained an average of 20% of their body weight while in the Gulf of Mexico and lost about 12% of their weight during their stay in the Suwannee River. We observed movement of tagged sturgeon between the Suwannee and Apal

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00130.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractFeeding and habitat use behaviour of juvenile O+ fishes, principallyRutilus rutilus(L.) andLeuciscus cephalus(L.) was analysed at 4‐h intervals for a period of 24 h on 29 June, 27 July, and 13 September 1994 at a regulated site on the River Great Ouse, UK. Twenty‐five random point samples were taken by electrofishing during each sampling period from a 250 m reach of the river. Catch per unit effort decreased at night for bothR. rutilusandL. cephalus. The number of prey items in the gut and the number of taxa found also decreased at night except forR. rutiluson one occassion. Habitat use remained near constant, with only slight increases in depth and distance from the bank at night. Use ofNuphar luteaby roach increased at night. Chub tended to avoidNuphar luteaand were positively associated with emergent macrophyte and algal cover both during the day and ni

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00131.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractThe diets of six species of fish occurring in seasonally inundated waters of the southern acid peat flats of southwestern Australia were studied over a 23 month period. Fishes could be allocated to a benthos/lower water column guild or a midwater column/water surface guild. Segregation on the basis of prey size was noted for one species only, and this species had a larger mouth than the remaining species. Seasonal variation in within‐ and between‐guild overlap was observed. Highest overlap occurred in the mid‐wet season when inundation was greatest and all species foraged in extensive areas of very shallow water. Little segregation based on feeding position within the water column was possible, and some prey species (e. g. simuliid larvae) were particularly abundant at this time. Dietary overlap decreased as the dry season progressed and fish became concentrated in smaller areas of habitat. As fish demonstrated lower levels of food acquisition, this change in overlap was suggested to be in response to increased competition for limited food reso

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00132.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractGeographical, macro‐ and microhabitat distribution of and mate recognition among color morphs of a complex of Lake Victoria cichlids was studied. The distribution of most morphs was mutually exclusive but two morphs overlapped extensively and differed only in microhabitat requirements. Coloration differences between them were inherited, and wild fishes were homozygous in this respect. Positive morph assortative female mate choice was demonstrated. Over the largest part of their common geographical range intermediate phenotypes are absent and the two morphs are to be considered two biological species. However, in populations at one end of the range intermediate phenotypes with continuously varying male coloration make up the majority of the individuals. It is suggested that the status of color morphs as phenotypes of one species or as separate biological species is a function of environmental conditions that vary considerably between the localities. The data support the hypothesis that polymorphisms can be a step preceding speciation in cichlid fis

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00133.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractIn a controlled experiment, elvers (Anguilla anguillaL.) were reared for 137 weeks at 17, 20 and 26°C. Most eels with macroscopically sexable gonads were males. The proportions of females were 14% of sexable individuals at 26°C and 7–8% at 17 and 20°C. During the first 15 weeks, 20–50% of initial numbers were lost, but this early mortality was not significantly size selective. In spite of a long term experiment, a significant number of the survivors never attained a sexable size. Nongrowers (7–9 cm) were still alive nearly three years after being caught as glass eels. Among the earliest fast growers, 5% females, or less, were found. The proportion of females increased in later grading groups, but the pattern of increase did not indicate that males and females have separated mean ranks in a fixed growth hierarchy. Water temperature influenced observed sex ratios, but probably not through direct influence on the sex differentiation. Some alternative hypotheses, dealing with population density and individual growth status, are d

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00134.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

AbstractData on catches of O+ elvers from nine countries in Europe and of juvenile eels ascending the St. Lawrence River in Canada are presented and compared with previous records. Consideration of a parallel decline in recruitment in Canada supports the theory that oceanic conditions are responsible for the observed fall in catches. A substantial increase in the Loire, to 95 tons in 1994, may indicate an overall improvement in stocks.

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1996.tb00135.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY