Biology of epiphytic Chironomidae (Diptera:Nematocera) in chalk streams

Submerged macrophytes are a conspicuous feature of chalk stream ecosystems, supporting large populations of diatoms and invertebrates, including larvae of the nematocerous family Chironomidae. Close temporal and spatial association is evident between chironomid larvae and diatoms, and diatoms are frequently a major component of the food of midge larvae. Larvae provided with food that is rich in diatoms survive better and grow and develop more quickly than larvae that are supplied with food that is predominantly fine organic detritus. First instar larvae selectively feed on diatoms but it is likely that selection is on the basis of particle size, rather than for diatoms per se. Three species of epiphytic larvae (2 Cricotopus spp. and Eukiefferiella ilkleyensis) transfer to a diet that is predominantly of fresh plant material (Ranunculus calcareus leaves) in the third and fourth instar. The life cycle strategies of many species of Orthocladiinae equip them to exploit temporarily favourable environmental conditions very rapidly and effectively. This fact helps to explain the coexistence of species with similar strategies of resource utilization, as part of the normal epiphytic fauna of chalk streams.     

FIRST RECORD OF THE SNAKE CATFISH CLARIAS THEODORAE (WEBER, 1897) (CLARIIDAE: PISCES) FROM THE TRANSVAAL,SOUTH AFRICA

SUMMARY

Two new distributional records of the snake catfish Clarias theodorae (Clariidae: Pisces) from Transvaal tributaries of the Limpopo River are documented. This is also the first record of the snake catfish from the Transvaal. The habitat preference of this species is briefly discussed and brief comment is made on probable migration routes.     

Genetic relationships between Marico barb (Barbus motebensis, Steindacher 1894), Redtail barb (B. gurneyi, Günther 1868) and Amatola barb (B. amatolicus, Skelton 1990)

Starch gel-electrophoresis was used to assess genetic differences between two populations of Barbus motebensis and two other taxonomic related and morphologically similar barb species. Ten of the 29 protein coding loci studied were polymorphic. In spite of the small bio-metric and meristic differences between the species studied, the extent of genetic differences suggests four distinct species. The evidence in support of a new barb species in the Ohrigstad River further adds to the current conservation importance of the rivers of the eastern Transvaal escarpment as a ‘hotspot’ for endemic fish species.     

THE SEASONAL MOVEMENTS OF GURNEY'S SUGARBIRD PROMEROPS GURNEYI IN THE LYDENBURG AREA,TRANSVAAL

De Swardt, D. H. 1991. The seasonal movements of Gurney's Sugarbird Promerops gurneyi in the Ly-denburg area, Transvaal. Ostrich 62: 40–44.

The seasonal movements of Gurney's Sugarbird Promerops gurneyi were studied in the Lydenburg area, eastern Transvaal, South Africa. Influx into the suburban habitat occurred durin the winter when the birds left the mountains at the end of the flowering period of Protea roupelliae. Birifs were mistnetted and each adult colour-ringed with a unique combination for individual recognition. In all, 256 Sugarbirds were ed in the mountain habitat and 50 in the suburban habitat. Of the unringed individuals 22,2 % were ted on cliffs with aloes and 79,3 % around P. roupelliae clumps late in the season. Of the ringed birds 25 (9,7%) were retrapped at sites in the mountain and 10 (3.9%) showed seasonal movements; 6 (8,3%) were recaptured in the same town localities where originally ringed. Mean distance moved was 7 km. An overall recapture rate of 7,5% was recorded during this study. When nectar became scarce the birds started moving away and utilized other food sources. Protea roupelliae showed a flowering peak from October to February which coincided with the sugarbirds' breeding season.     

Effects of acidification on the breeding ecology of a stream‐dependent songbird,the Louisiana waterthrush (Seiurus motacilla)

1. We compared breeding ecology of the Louisiana waterthrush (Seiurus motacilla) on acidified and circumneutral streams in the Appalachian Highlands of Southwestern Pennsylvania from 1996 to 2005. 2. Headwater streams impacted by acid mine drainage and/or acidic precipitation showed reduced pH (range 4.5–5.5) compared to four circumneutral streams (pH c. 7). Acid‐sensitive taxa, including most mayflies (Ephemeroptera), were almost completely absent from acidified streams, whereas several acid‐tolerant taxa, especially stonefly (Plecoptera) genera Leuctra and Amphinemura, were abundant. 3. Louisiana waterthrush breeding density (c. 1 territory km^?1) was significantly reduced on acidified streams compared to circumneutral streams (>2 territories km^?1). Territories on acidified streams were almost twice as long as on circumneutral streams. Territories usually were contiguous on circumneutral streams, but they were often disjunct on acidified streams. Breeding density declined on one acidified stream that we studied over a 10‐year period. 4. Clutch initiation was significantly delayed on acidified streams, on average by 9 days in comparison to circumneutral streams, and first‐egg dates were inversely related to breeding density. Birds nesting along acidified streams laid smaller clutches, and nestlings had shorter age‐adjusted wing lengths. Stream acidity had no effect on nest success or annual fecundity (fledglings/female). However, the number of young fledged km^?1 was nearly twice as high on circumneutral streams as on acidified streams. 5. Acidified streams were characterized by a younger, less site‐faithful breeding population. Individuals were less likely to return multiple years to breed, allowing inexperienced breeders to settle on acidified streams. Pairing success was lower on acidified streams, and we observed four cases of waterthrushes emigrating from territories on acidified streams to nearby circumneutral streams in the following year. 6. We conclude that acidified headwaters constitute lower quality habitat for breeding Louisiana waterthrush. However, breeding birds can apparently compensate for reduced prey resources to fledge young on acidified streams by increasing territory size, foraging in peripheral non‐acidified areas, and by provisioning young with novel prey.     

HABITAT DURATION,LENGTH OF LARVAL PERIOD,AND THE EVOLUTION OF A COMPLEX LIFE CYCLE OF A SALAMANDER,AMBYSTOMA TEXANUM

In many organisms, genotypic selection may be a less effective means of adapting to unpredictable environments than is selection for phenotypic plasticity. To determine whether genotypic selection is important in the evolution of complex life cycles of amphibians that breed in seasonally ephemeral habitats, we examined whether mortality risk from habitat drying in natural populations of small-mouthed salamanders (Ambystoma texanum) corresponded to length of larval period when larvae from the same populations were grown in a common laboratory environment. Comparisons were made at two levels of organization within the species: 1) among geographic races that are under strongly divergent selection regimes associated with the use of pond and stream habitats and 2) among populations within races that use the same types of breeding habitats. Morphological evidence indicates that stream-breeding A. texanum evolved from pond-breeding populations that recently colonized streams. Larvae in streams incur heavy mortality from stream drying, so the upper bound on length of larval period is currently set by the seasonal duration of breeding sites. We hypothesized that selection would reduce length of larval period of pond-breeders that colonize streams if their larval periods are inherently longer than those of stream-breeders. The results of laboratory experiments support this hypothesis. When grown individually in a common environment, larvae from stream populations had significantly shorter larval periods than larvae from pond populations. Within races, however, length of larval period did not correlate significantly with seasonal duration of breeding sites. When males of both races were crossed to a single pond female, offspring of stream males had significantly shorter larval periods than offspring of pond males. Collectively, these data suggest that differences in complex life cycles among pond and stream-breeders are due to genotypic selection related to mortality from habitat drying. Stream larvae in the common-environment experiment were significantly smaller at metamorphosis than pond larvae. Yet, the evolution of metamorphic size cannot be explained readily by direct selection: there are no intuitively obvious advantages of being relatively small at metamorphosis in streams. A positive phenotypic correlation was observed between size at metamorphosis and length of larval period in most laboratory populations. A positive additive genetic correlation between these traits was demonstrated recently in another amphibian. Thus, we suspect that metamorphic size of stream-breeders evolved indirectly as a consequence of selection to shorten length of larval period.     

MINUTES OF THE ANNUAL GENERAL MEETING OF THE SOUTH AFRICAN ORNITHOLOGICAL SOCIETY,HELD AT CAPE TOWN ON 17 May 1969

Earle, R.A. &; Oatley, T.B. 1983. Populations, ecology and breeding of the Orange Thrush at two sites in eastern South Africa. Ostrich 54:205-212.

The two populations of the Orange Thrush Turdus gurneyi studied represented two races, the nominate gurneyi in Natal and disruptans in Transvaal. In the Transvaal there was a large influx of individuals during the winter but numbers stayed constant during the breeding season with one pair/ha. Natal birds were more sedentary. Food seemed to be a major factor in regulating the winter population size of both the Orange Thrush and the Olive Thrush T. olivaceus in the Transvaal forest as the number of earthworms showed a peak just before the bird numbers peaked. In Natal the Orange Thrush showed a higher degree of specialization in diet than the Olive Thrush which his a mixed diet. Clutches consisted of two eggs and incubation lasted IS days. Chicks were brooded frequently, virtually to the time of fledging, thus protecting them from the cool, wet conditions of the forest. The nestling period was 15–18 days. The Orange Thrush preferred areas in the forest where little vegetation occurred at 1,0-1,5 m and where a good layer of leaf litter was present.     

SOME NOTES ON THE BIRDS OF THE ISOKA DISTRICT OF THE NORTHERN PROVINCE OF NORTHERN RHODESIA

Dean, W. R. J. 1978. Moult seasons of some Anatidae in the western Transvaal. Ostrich 49:76-84.

Spurwinged Geese Plectropterus gambensis, Egyptian Geese Alopochen aegyptiacus, Yellow-billed Ducks Anas undulata, Redbilled Teal A. erythrorhyncha and Southern Pochard Netta erythrophthalma have a flightless moult mainly during the dry season, from April to August, in the western Transvaal. South African Shelduck Tadorna cana moult during October to February after breeding during July and August. The Cape Shoveller Anas smithii has two main flightless periods, April-May and October-January. Cape Teal A. capensis have been recorded in flightless moult in October, December and January.

The duration of the flightless period correlates with wing length; larger and longer winged Anatidae require proportionally more time for wing moult than do smaller and shorter winged Anatidae.

Geese and shelducks moult on large open lakes with an open shore. Ducks have been recorded flightless on lakes and dams, with or without emergent vegetation.     

Relationships Between Environmental Variables and Benthic Diatom Assemblages in California Central Valley Streams (USA)

This study examines distributional patterns of benthic diatom assemblages in relation to environmental characteristics in streams and rivers in the California Central Valley ecoregion. Benthic diatoms, water quality, and physical habitat conditions were characterized from 53 randomly selected sites. The stream sites were characterized by low mid-channel canopy cover and high channel substrate embeddedness. The waters at these sites were enriched with minerals and turbidity varied from 1.3 to 185.0 NTU with an average of 13.5 NTU. A total of 249 diatom taxa were identified. Average taxa richness was 41 with a range of 7–76. The assemblages were dominated by Staurosira construens (11%), Epithemia sorex (8%), Cocconeis placentula (7%), and Nitzschia amphibia (6%). Multivariate analyses (cluster analysis, classification tree analysis, and canonical correspondence analysis) all showed that benthic diatom assemblages were mainly affected by channel morphology, in-stream habitat, and riparian conditions. The 1st CCA axis negatively correlated with mean wetted channel width (r = −0.66) and thalweg depth (r = −0.65) (Table 4). The 2nd axis correlated with % coarse substrates (r=0.60). Our results suggest that benthic diatoms can be used for assessing physical habitat alterations in streams.     

Selective Feeding of age-0 Arctic Grayling in Lake-Outlet Streams of the Northwest Territories, Canada

A paucity of information exists on the diet of Arctic grayling, Thymallus arcticus, particularly for young-of-the-year (YOY). We examined the diet of YOY Arctic grayling in relation to food availability, in the Barrenlands region of the Northwest Territories, Canada, where lake-outlet streams serve as nursery habitat for these fish. Given the small size of YOY grayling and the abundance of lake-derived microcrustacea in the drift of these lake-outlet streams, we anticipated that these prey would make up a major component of the YOY's diet. Food selectivity by YOY grayling, however, was strongly sized-biased; although microcrustacea dominated the drift, YOY primarily consumed larger taxa, especially Chironomidae and Simuliidae. Even among these taxa, grayling tended to select the larger individuals. As they grew, YOY grayling took larger numbers of both large and small prey, particularly the larger invertebrates, although prey size range did not change after mid-July. Selection of pupae and avoidance of Ephemeroptera suggest that prey characteristics other than size also contribute to selectivity by YOY grayling. The relatively limited consumption of terrestrial invertebrates and other large prey may reflect the small sizes of fish in this arctic study, as well as differences in prey availability. Despite the abundance of lake-derived prey, instream production of invertebrates should largely determine the productive capacity of Barrenlands streams as fish habitat.     

THE NEST OF THE KNYSNA SCRUB WARBLER (BRADYPTERUS SYLVATICUS) AND SOME NOTES ON PARENTAL BEHAVIOUR

Von Maltitz, F., Schmitt, M. B., &; Biggs, H. C. 1984. Measurements, moult and abundance of the Lizard Buzzard in the Transvaal. Ostrich 55: 177–181.

During a 12-year study 51 Lizard Buzzards Kaupifalco monogrammicus were captured in the Transvaal. South Africa. They occurred sporadically, with peaks in the years 1972, 1975, 1979 and 1983 and were significantly more common in winter. Moult was completed between January and June. Mass is given and recaptures documented.     

The trophic interactions of young Arctic grayling (Thymallus arcticus) in an Arctic tundra stream

1. Young (0+) Arctic grayling (Thymallus arcticus) have the potential to control the trophic structure of Arctic tundra streams through consumption, nutrient excretion and the modification of prey behaviour. The effect of young grayling on three trophic levels (algae, invertebrates and fish) was investigated by manipulating fish density and by fertilizing the river with phosphorus (P). 2. Nutrients, epilithic chlorophyll a, benthic invertebrates and fish biomass were measured within each fish density treatment (0, 4, and 40 fish m^–2) within the P-limited reference zone and the P-enriched fertilized zone of the Kuparuk River, Alaska. 3. Epilithic chlorophyll a increased with increased fish density in both reference and fertilized zones, while mayfly density decreased with increased fish density in the fertilized zone only. Final mean mass of young grayling in the 40 fish m^–2 cages was lower than mean mass in the 4 fish m^–2 cages. 4. Young grayling may produce a top-down cascading trophic effect in areas where nutrients are not limited. 5. River nutrient status and river discharge may modify the strength of top-down control by young grayling.     

NEW DISTRIBUTION RECORDS FOR CLARIAS THEODORAE WEBER 1897, BARBUS EUTAENIA BOULENGER 1904, BARBUS POLYLEPIS BOULENGER 1907 AND OPSARIDIUM ZAMBEZENSE (PETERS 1852) FROM THE WATERBERG,TRANSVAAL, SOUTH AFRICA.

Summary

New distribution records for four fish species in the Waterberg mountains of the Transvaal were established during surveys since 1987–92. Clarias theodorae was previously reported from one locality each in the Waterberg and Soutpansberg. The current survey disclosed its presence at an additional 11 localities. In the Transvaal Opsaridium zambezense was previously only known from the eastern parts of the Province. The current survey revealed its presence at one locality in the Waterberg. Clarias theodorae and Opsaridium zambezense are regarded as threatened in South Africa. Barbus eutaenia (two populations in the Waterberg) and B. polylepis (one population in the Waterberg) have relatively wide distributions in the Transvaal but their presence in the Waterberg represent comparatively unique, isolated populations. All four species were only found in relatively small numbers. Droughts during the nineteen eighties, and a concurrent increase in farm weir construction and water abstraction, jeopardise the survival of these small fish populations. Extreme drought conditions during 1991/92 have exacerbated adverse conditions for fish survival even further due to a general cessation of stream flow.     

Life on the edge: hydrogen sulfide and the fish communities of a Mexican cave and surrounding waters

Most eucaryotic organisms classified as living in an extreme habitat are invertebrates. Here we report of a fish living in a Mexican cave (Cueva del Azufre) that is rich in highly toxic H₂S. We compared the water chemistry and fish communities of the cave and several nearby surface streams. Our study revealed high concentrations of H₂S in the cave and its outflow (El Azufre). The concentrations of H₂S reach more than 300 μM inside the cave, which are acutely toxic for most fishes. In both sulfidic habitats, the diversity of fishes was heavily reduced, and Poecilia mexicana was the dominant species indicating that the presence of H₂S has an all-or-none effect, permitting only few species to survive in sulfidic habitats. Compared to habitats without H₂S, P. mexicana from the cave and the outflow have a significantly lower body condition. Although there are microhabitats with varying concentrations of H₂S within the cave, we could not find a higher fish density in areas with lower concentrations of H₂S. We discuss that P. mexicana is one of the few extremophile vertebrates. Our study supports the idea that extreme habitats lead to an impoverished species diversity.     

THE INFLUENCE OF PREDATION ON THE COMPOSITION OF FRESH-WATER ANIMAL COMMUNITIES

1. The introduction of Salmo trutta into an artificial pond was followed by great reduction in the numbers of tadpoles, certain beetles and Notonecta, all species to be seen in the open water. Of the species that sheltered more securely in the plant cover, the effect on some was a curtailment of their range; on others there was little reduction of range or numbers, particularly the numbers at the end of a generation. This was attributed to self-regulation of numbers and the creation of a reserve from which losses due to predation by fish on larger specimens could be replenished. 2. Changes in the number of fish in stretches of a stony stream exerted little effect on the Ephemeroptera, but records indicate that Gammarus and fish are rarely numerous in the same stretch. 3. The most abundant invertebrate carnivores in the fishpond wait for prey to come to them; of two others, a leech swims well but has poor seizing organs, a caddis-larva the reverse. The amount consumed by such predators falls rapidly as the prey becomes scarce. Moreover the main source of prey for the common predators is from the small Crustacea which are abundant only in summer and reproduce quickly. This prey thus has properties that prevent much reduction of numbers by predation. 4. On the stony substrata of Lake District lakes, Asellus and Planaria are numerous where conditions are productive, Ephemeroptera and Plecoptera elsewhere. The absence of these insects from productive places is attributed to predation. 5. Planaria cannot move fast and have no efficient seizing organs, but compensate to some extent by laying trails of slime in which prey becomes entangled. As Asellus grows, its chances of being overpowered by Planaria decrease. Planaria, therefore, feed regularly only when prey is abundant. When it is scarce they rest, and they are able to withstand starvation for a long period. 6. Planaria are preyed upon extensively by Odonata, newts and Plecoptera, and the first two keep them out of weedy ponds. The last may keep their numbers low in streams and perhaps also on stony shores of unproductive lakes, though here scarcity of food is important too. In productive lakes predation on flatworms is slight. 7. Protozoa exhibit three relationships between predator and prey, two of which have been seen in larger organisms. Prey avoid predation in cover. Predators cease activity when prey becomes scarce. Prey occurs in isolated colonies which when found are destroyed by the predator, but generally not before some individuals have dispersed and founded new colonies. 8. Only small invertebrates can survive predation by fish in the open water. Many also reproduce rapidly as long as conditions are favourable and enter a resting stage when they are not. Intense predation may eliminate large species. In some ponds in the Colorado mountains salamanders eliminate a large carnivorous copepod, which enables small Cladocera to survive. Absence of the copepod and presence of Cladocera of suitable size for it to feed on enable a Chaoborus larva to co-exist with the salamander. 9. Small planktivorous fish occur in the open water of some African lakes. The great size of these lakes probably makes possible the co-existence of small fish and their predators, but also makes investigation of the relations between the two difficult. 10. When species not previously present have gained access to lakes, the numbers of the native species of fish have often been greatly reduced. The exact nature of the relationship between newcomer and native has, however, not been established because other factors have been varying, observations have been scanty, or records have not been made for long enough. 11. In temporary and very small bodies of water predation is mainly by invading individuals that were reared somewhere else. Characteristic organisms are phyllopods in impermanent pools and mosquito larvae in both types of water, two groups that feed in the open and away from cover, an activity possible only where predation is slight.     

What predicts the use by brook trout (Salvelinus fontinalis) of terrestrial invertebrate subsidies in headwater streams?

相似文献   

NOTE ON THE LONGEVITY OF THE VULTURINE SEA.EAGLE,CYPOHIERAX ANGOLENSIS (Gm.)

Herremans, M. &; Herremans-Tonnoeyr, D. 1994. Seasonal patterns in abundance of Lilacbreasted Rollers Coracias caudata and Purple Rollers Coracias naevia inferred from roadside counts in eastern and northern Botswana. Ostrich 65:66-73

The relative abundance of rollers (Coraciidae) in 45933 km of roadside counts made in eastern and northern Botswana between May 1991 and April 1994 was assessed by season and biome. Lilacbreasted Rollers appeared to move from breeding grounds in the northern Botswana woodlands into the northern Kalahari, and north-westward from breeding grounds in the Transvaal into the Limpopo drainage system and adjacent eastern Kalahari, during winter. Purple Rollers showed an influx during autumn and winter over most of eastern Botswana, but birds did not remain during the drought winter of 1992. The pattern indicated a large scale west-east or southwest-northeast migratory movement, from breeding grounds in the drier parts of the Kalahari to wintering grounds in the more humid periphery of the Kalahari.     

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow ( Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of ¹⁵N and ¹³C isotopes and a whole stream ¹⁵N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The ¹⁵N and ¹³C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for ¹⁵N and ¹³C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a ¹⁵N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.     

Response of grazing impacts of an algivorous fish (Pseudogastromyzon myersi: Balitoridae) to seasonal disturbance in Hong Kong streams

1. Manipulative experiments were carried out in four Hong Kong streams (two shaded, two unshaded) to investigate the impact of grazing by an algivorous fish, Pseudogastromyzon myersi, on benthic algal biomass and assemblage composition. Experiments were conducted and repeated during both the dry and wet seasons to determine whether spate‐induced disturbance modified any grazing effect. Treatments comprised fish exclusion and inclusion via closed and open cages, with a no‐cage treatment used as a control for the cage effect. Treatments were maintained for 4 weeks in each experimental run. 2. Grazing by P. myersi reduced benthic algal biomass and the organic matter content of periphyton in open cages and the no‐cage treatment relative to closed cages. The similarity between open‐cage and no‐cage treatments was evidence that the overall difference among treatments was caused by limiting fish access to closed cages and not merely an artifact of caging. Grazing effects were broadly similar in all streams, but there was a significant statistical interaction between treatments and seasons. 3. Analysis of dry‐season data matched the overall trend in inter‐treatment differences, confirming the effects of grazing by P. myersi on algal biomass and periphyton organic matter. Significant differences in algal assemblage composition between closed‐cage and no‐cage treatments during the dry season reflected reductions in the abundance of erect, stalked diatoms (Gomphonema) and filamentous cyanobacteria (Homeothrix). Removal of these vulnerable overstorey algae by P. myersi resulted in greater abundance of understorey diatoms (Achnanthes and Cocconeis) in the no‐cage treatment in all streams during the dry season. The composition of algal assemblages in open cages was intermediate between the other two treatments. 4. Although fish densities were greater in all streams during the wet season, spate‐induced disturbance obscured grazing effects and there were no significant differences among treatments attributable to fish grazing. Seasonal variation in impacts of P. myersi grazing provides support for the harsh‐benign hypothesis, and confirms that biotic factors are less important controls of stream algal biomass and assemblage structure during periods (i.e. the wet season in Hong Kong) when abiotic disturbances are frequent or intense.     

Breeding location choice in salmon: causes (habitat, competition, body size, energy stores) and consequences (life span, energy stores)

Breeding location choice provides a mechanism by which individuals can directly influence their reproductive success. Location choice should therefore reflect individual condition, habitat features, and the intensity of competition; with these factors then influencing reproductive success. To test whether such patterns were detectable in the wild, we tagged 705 sockeye salmon (Oncorhynchus nerka) in a natural population, and monitored them from when they started breeding until they died. We evaluated the role of individual condition (size, secondary sexual traits, energy stores) in the acquisition of breeding locations that differed in the intensity of competition (female density, sex ratio) and habitat features (water depth, water velocity). We then evaluated the influence of breeding location on reproductive life span and energy stores. At a coarse level (20‐m stream sections), females consistently settled in certain locations, and these locations sustained high densities and held larger females. At a fine scale (0.5‐m breeding sites), (1) larger fish occupied deeper water (males, r²=0.072; females, r²=0.199), (2) higher levels of competition reduced reproductive life span for males (r²=0.139) but not females, and (3) fish with shorter reproductive life spans died with more energy remaining in their muscle tissue (males, r²=0.414; females, r²=0.440). These patterns were nested within a tendency for late breeding fish to have shorter reproductive life spans. Energy stores and secondary sexual traits did not influence breeding location choice, and larger fish did not acquire locations of higher intrinsic quality (i.e., those sections settled first and sustaining higher competition). Our study provides evidence that some aspects of individual condition influence breeding location choice, which then influences components of reproductive success.