摘要:

We studied five Alabama reservoirs during 1992 through 1994 to explore the influence of both reservoir hydrology and larval shad (Dorosomaspp.) on young-of-year crappie (Pomoxisspp.) recruitment. Mean relative abundance of age-0 crappie was slightly and negatively correlated with larval gizzard shad (D. cepedianum) density but not with that of larval threadfin shad (D. petenense). Alternatively, age-0 crappie growth was positively correlated with threadfin shad density but not with gizzard shad density, which indicated that shad were not adversely affecting young crappie growth via competition for food. Neither crappie abundance nor growth was related to the dates of peak larval gizzard shad or threadfin shad occurrence. Age-0 crappie abundance was positively related to mean summer (June-August) retention time in four short retention time (< 15 d) reservoirs. However, when gizzard shad density was included into the analysis, this variable was no longer a significant predictor of crappie abundance, indicating that the relation between larval gizzard shad density and age-0 crappie abundance was overridden by the influence of mean summer retention time. Our findings indicate that the potential for interactions between young crappie and shad is certainly viable, but any impacts that larval shad might have had on age-0 crappie abundance and growth were not detected.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663594

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Short-term caging experiments were conducted to examine the direct effects that the crayfishOrconectes propinquushas on benthic communities. Stream-colonized tiles (precolonized within the stream) and uncolonized clay tiles were placed in eight elevated, hardware-cloth metal cages. Four cages had crayfish (~8 crayfish m−2); the other four were empty controls. Chlameasured on pre-colonized tiles five days after the commencement of the experiment indicated that crayfish had no significant effect on algal biomass.O. propinquusalso had no measurable impact on snail abundance from tiles sampled after five days. Samples taken after seventeen days from initially uncolonized tiles showed that crayfish had a direct negative effect on diatom abundance. Tiles with crayfish had 50% fewer diatoms than control cages, although there were no differences in the relative abundance of the 6 most abundant diatom genera. Insects collected from these tiles (day 17) did not differ in density, relative abundance (by order), or size between crayfish treatments. The results indicate that crayfish can have direct negative effects on the diatom portion of the algal community.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663595

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

We studied the effects of grazing by the mayflyStenonemasp. and the caddisflyPsilotretasp. on periphyton communities at two sites in the field, and we manipulated grazer densities in laboratory experiments to measure grazing effects on the periphyton community. In the field, periphyton abundance on unglazed clay tiles was compared between tiles elevated above the stream bottom to excludePsilotretasp. from the grazing community and tiles placed directly on the stream bottom allowing access by all grazers. There was no significant difference in ash free dry mass (AFDM) or chlorophyllabetween these two treatments at either site, suggesting thatPsilotretasp. had little affect on the periphyton community. In the laboratory, two sets of experimental stream chambers containing tiles with natural periphyton assemblages were used, and grazer densities within the chambers were varied to reflect the different densities of grazers at the field. After two weeks, neitherPsilotretasp. norStenonemasp. showed significant weight changes. However, survivorship ofStenonemasp. was inversely related to grazer density, suggesting that inter- or intra-specific competition may have existed at the highest grazer densities. Both AFDM and chlorophyllawere significantly higher in control chambers than chambers with grazers; also, there were no significant differences in AFDM or chlorophyllaconcentration among the different grazer density treatments. Thus, unlike the field exclusion studies which showed no impact ofPsilotretasp. grazing on periphyton, laboratory studies showed pronounced grazing effects. These results suggest that our laboratory stream chambers may have influenced biotic (predation/competition) and/or abiotic (flow regime, etc.) factors allowing grazers to show more dramatic impacts on periphyton than occur in the natural environment. These results suggest that caution should be taken when extrapolating grazing effects observed in the laboratory to a field situation.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663596

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The zooplankton community of Lake Livingston was examined in terms of composition, abundance, distribution, and appropriateness as a food resource for paddlefish,Polyodon spathula. By comparing the range of interraker distances of paddlefish (0.06–0.08 mm) to size measurements of different zooplankton taxa, we determined that paddlefish do not consume small rotifers and copepod nauplii as efficiently as they do larger, copepods and cladocerans. All zooplankton taxa (rotifers, cladocerans, and copepods) were comprised of relatively small organisms, and community abundance and biomass were dominated by small rotifers and copepod nauplii most of the year. In fall and winter, however, the biomass was dominated by copepods. A bioenergetics model predicted the paddlefish biomass that could be supported in Lake Livingston at a density of 1.17 kg ha−1, with a range of 0.06 to 10.45 kg ha−1.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663597

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Bulinus truncatus, the intermediate snail host ofSchistosoma haematobium, was widely distributed within the irrigation system at Tessaout-Amont area, located about 70 Km northeast of Marrakech. The lakes and the primary canals were not colonized, but secondary canals and especially various associated structures (syphon boxes, drop structures and weirs) were important habitats forB. truncatus. Bulinus truncatusis particularly abundant in tertiary canals, but it is there almost completely restricted to the syphon boxes as these canals, except for syphon boxes, dry out quickly when irrigation stops. The distribution ofB. truncatusseems to be governed by irrigation practices in terns of water management, rather than by water physicochemical parameters or occurrence of aquatic plants.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663598

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Larvae of the Asiatic clam,Corbicula fluminea, and their tissue products stimulated gluttonous feeding behavior in a predatory rhabdocoel turbellarian,Macrostomumsp. This gluttony resulted in death of the worms. I suggest that lethal gluttony may be induced by some prey species and serve as an altruistic mechanism sacrificing numerous individuals to eliminate predators and provide greater survival for the remaining prey. This mechanism may be advantageous especially to a species which is expanding its range into regions where it encounters potential predators for the first time. Lethal gluttony could eliminate the predator population before it adapts to the newly introduced prey.

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663599

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663600

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

ISSN:0270-5060    

DOI:10.1080/02705060.1998.9663583

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor