摘要:

Abstract–Experiments on the dispersal of O‐group salmon from a ‘point‐stocking’ were made in 1988, 1989, 1990 and 1991 in Bollihope Burn, Co. Durham. Experiments on ‘scatter‐stocking’ were made in 1992 and 1993, but the 1992 experiment was aborted for technical reasons. In each year the fish were stocked as fry in spring and their distribution in September was ascertained by electrofishing. Some fish from point‐stocking dispersed up to 50 m upstream or 500 m downstream of the stocking point, but the majority remained close to the stocking point. This gave rise to considerable spatial variation in September population density and fish weight, which could be modelled by semi‐logarithmic curves. Survival from point‐stocking was 19% and 14% in 1988 and 1989, respectively. Approximately 20 to 25% of the September survivors from point‐stocking were found upstream of the release point. Scatter‐stocking gave relatively little variation in population density or fish weight in September, and e

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1995.tb00021.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract–Habitat selection theory assumes that individuals either know the quality of habitatsa priorior can learn it through experience. This work tests the effects of total food supply and of water velocity on the ability of drift‐feeding stream fish (Rhinicthys atratulus) to discriminate between two patches in a laboratory flow‐tank that differ only in the amount of food in the patch. If the fish can discriminate between the patches, their distributions should be predicted by ideal free distribution models; otherwise, their distributions should be random. The results show that at high total food supply levels, fish distributions differ from random in general agreement with ideal free distribution models; total food supply decreases, fish distributions become random, indicating that total food supply levels affect the ability of fish to discriminate between habitats that differ only in food supply. Increased water velocity was expected to affect fish distributions similarly, but its effect was not significant. The results suggest that habitat selection theory should consider both differences between habitats and the ability of fish to perceive those differ

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1995.tb00022.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract–The size, density and production rate of eel were determined at 16 contrasting sites of 4 streams along the course of a Cantabrian river over the years 1990–1993. In addition, the diel cycles of feeding activity were determined monthly in another tributary over the year 1991–1992. Eels in the Esva were small, short‐lived and mostly males (>99%). The seasonal patterns of eel density, feeding activity and condition were tightly fitted with each other, size‐independent and regulated mainly by water temperature. Mean eel size increased with distance from the river mouth, but at each site, it remained similar between seasons and years. In contrast, density decreased upstream and showed marked seasonal and annual fluctuations. For all the sites, the number of eels increased in spring and summer and peaked by late autumn, coincident with higher water temperature and lower discharge. Mortality and migration rates were correlated with eel numbers for each stream, suggesting a densitydependent regulation. Production rates were correlated with distance from the river mouth, which explained only 10.2% of the variation, but they were also correlated with the initial and mean numbers, suggesting that site‐specific factors acting upon density also influence production. Average production for the first year was 158.6 kg–ha–1. year–1(range 56.5–378.0) but decreased to 104.2 and 89.7 in the next 2 years. The reduction in the numbers of eels rather than the later reduction in mean size was responsible for this decrease. Monthly water temperature, distance from the river mouth and the initial number and size of eels for each site explain most of the variation observed in the population parameters along the cours

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1995.tb00023.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract–We examined the growth of individual age‐0 smallmouth bass collected from the North Anna River, Virginia (USA), to determine whether growth during early life stages was most affected by temperature, duration of growing season, growth during previous stages, or whether growth was a stochastic process. Spawning was delayed in 1990, all individuals grew in warm temperatures, and their growth rates during early life stages were not correlated with prevailing temperatures or with growth during previous stages. By the end of the summer, swim‐up date was not correlated with size at the time of collection. In 1991, individuals that swam‐up earliest consistently grew slower (and in colder temperatures) during early life stages. Late swim‐ups (that were exposed to warmer temperatures) grew faster during early life stages but had a shorter growing season. By the middle of the summer, all individuals had an equal chance of being among the largest members of the cohort. The size distribution of the age‐0 cohort at the end of the summer did not differ between years, despite the fact that spawning occurred 3–5 weeks later in 1990 (when temperatures were maximized) than in 1991 (when temperatures were

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1995.tb00024.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

Abstract–The role of predation risk and structural complexity in determining the habitat use and activity patterns of roach, rudd and perch was assessed using a series of video‐recorded laboratory trials. The time spent in open and structured habitats, vertical swimming heights and activity levels of each species were observed in the presence and absence of a potential, pike, predator. Habitat complexity varied between treatments with artificial stem densities of 200, 400 and 600 stems‘m−2. Predator free trials showed that roach and rudd spent significantly less time in structured habitats than perch. Increasing stem density had no significant effect on the habitat choice of perch but did affect the distribution of roach and rudd. Stem density influenced the vertical swimming height of rudd or perch but not roach, although the effects of habitat complexity on swimming activity were more complicated. Pike were themselves influenced by increases in stem density, only selecting structured habitats when stem densities were less than 600 stems‘m−2. In the presence of a predator, both roach and rudd increased the amount of time spent in structured areas. These observed differences were independent of stem density. Perch, however, decreased the amount of time spent in structural habitats at all stem densities. Predation risk also prompted significant changes in both vertical swimming height and activity levels of potential prey fish. A pike predation success hierarchy of rudd‐roach‐perch was also observed during the study. The reasons for this patter

ISSN:0906-6691    

DOI:10.1111/j.1600-0633.1995.tb00025.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY