Reached Peak Abundance Research Articles

Canopy microclimate and arboreal lichen loading in subalpine spruce-fir forest

Hair lichen communities in Engelmann spruce (Picea engelmannii) – subalpine fir (Abies lasiocarpa) forests of the northern Cariboo Mountains (British Columbia) show distinct vertical zonation. Alectoria sarmentosa reaches peak abundance in the lower canopy (over 35 kg/ha) whereas Bryoria spp. lichens reach peak abundance in the upper canopy (over 250 kg/ha). These distribution patterns are accentuated by stand structure with trees growing in clumps retaining significantly higher lichen loading on a per branch basis compared to solitary trees. The vertical zonation of lichen communities is accompanied by distinct trends in canopy microclimate. Snowmelt events account for the largest proportion of observed thallus hydration in both Alectoria and Bryoria. Although canopy microclimate is surprisingly isothermal during rainfall events, the attenuation of thallus hydration after wetting is typically greater for lower canopy exposures. An important exception to this pattern is seen under midwinter conditions, when solar insolation is insufficient to sustain prolonged lower canopy snowmelt. Our data support the hypothesis that ventilation in upper canopy exposures is a contributing factor in the vertical zonation of Alectoria and Bryoria communities. Upper canopy Bryoria rely more heavily on snowmelt events to sustain thallus hydration, whereas lower canopy Alectoria utilize summer rainfall events to a greater extent. We hypothesize that physiological mechanisms, through which these patterns of canopy microclimate influence lichen zonation, may include an intolerance to prolonged wetting by Bryoria and higher resaturation respiration costs in Alectoria, which would limit it to more mesic canopy exposures. We believe that the observed distribution of canopy lichens ultimately reflects the long-term interaction of both physiological and successional processes (lichen colonization and dispersal) within the canopy.Key words: canopy, epiphytes, lichens, microclimate.

Impact of Bemisia whiteflies (Homoptera: Aleyrodidae) on alfalfa growth, forage yield, and quality.

A 3-yr project was initiated in 1993 to examine the effects of insecticides and sustained whitefly, Bemisia argentifolii Bellows & Perring [aka. B tabaci Gennadius (Strain B)], feeding on alfalfa plant growth and vigor in greenhouse cage studies, and to determine the impact of natural Bemisia whitefly populations on alfalfa forage yields and quality in a large-plot field experiment. Alfalfa plant growth and vigor after exposure to imidacloprid and a mixture of fenpropathrin and acephate insecticides did not differ from untreated plants in the greenhouse. Consequently, foliar and soil applied insecticides were used to manipulate whitefly densities on alfalfa plants to measure whitefly feeding effects on plant growth and forage yield. Heavy whitefly densities on untreated alfalfa plants in the greenhouse resulted in significant reductions in relative growth rates and net assimilation rates as compared with imidacloprid-treated plants that were maintained relatively whitefly-free. Reductions in alfalfa plant growth measured between infested and treated plants were proportional to whitefly densities. Field plot results derived from three crop seasons were relatively consistent with our greenhouse trials. Both experimental approaches clearly showed that alfalfa plants exposed to high densities of whitefly immatures and adults grew at a significantly slower rate and produced less foliage. As a result of reduced growth rates, alfalfa maturity in the naturally infested plots was estimated to be approximately 7-10 d behind managed plots. Delays in maturity resulted in significant reductions in forage yields of 13-18% during August-September harvests when whitefly populations reached peak abundance. Whitefly feeding stresses also effected hay quality through the reduction of crude protein content and contamination of foliage with honeydew and sooty mold. The status of the Bemisia whiteflies as an economic pest to alfalfa is clearly evident from these studies, but the damage potential of whiteflies in the southwestern United States appears to be restricted to one or two harvest periods during the summer coinciding with peak adult populations and their dispersal from alternate host crops.

Competition between Brachionus calyciflorus Pallas and Brachionus patulus (Müller) (Rotifera) in relation to algal food concentration and initial population density

Competitive laboratory experiments between Brachionus calyciflorus and B. patulus were conducted at low (1×106 cells ml−1) and high (3×106 cells ml−1) densities of Chlorella vulgaris and four initial inoculation densities (numerically, 100% B. calyciflorus; 75% B. calyciflorus + 25% B. patulus; 50% each of the two species; 25% B. calyciflorus + 75% B. patulus and 100% B. patulus). Population densities were enumerated and the medium was changed daily for 20 days. B. patulus was a superior competitor in low food density regardless of inoculation density. At high food density, B. calyciflorus showed higher population growth in the first week but thereafter was outcompeted by B. patulus regardless of initial density. When grown alone, B. calyciflorus reached peak abundances (mean ± standard error) of 31±3 and 81±7 individuals ml−1 at low and high food densities, respectively. The corresponding values for B. patulus were 130±2 and 306±13. The adverse effects of B. patulus on the peak abundances of B. calyciflorus were higher at low food concentration. Data on egg ratios (eggs female−1) revealed an inverse relation with population abundance of both tested rotifer species. Our results indicated that the rate of population increase of a species was not a good indicator of its competitive ability. Instead, the ability to reproduce under continuously diminishing food resources (until a threshold level) was responsible for the competitive edge of B. patulus over B. calyciflorus. This was further influenced by the relative inoculation densities of the tested rotifer species and the offered food densities.

The insect fauna of four weedy Hieracium (Asteraceae) species in New Zealand

Hieracium pilosella, H. praealtum, H. caespitosum, and H. lepidulum (hawkweeds) are serious weeds of New Zealand grasslands. A programme to develop biological control of hawkweeds with insects and a pathogen began in 1992. It first required a survey of the four weed species to determine the identity and seasonal distribution of insects occurring naturally on them. Seventy‐six phytophagous species were recorded, including native and exotic polyphagous and oligophagous insects. Insect species that were widely distributed and numerous on all four plant species included Nysius huttoni and several Rhypodes spp. (Lygaeidae), Carystoterpa fingens and Philaenus spumarius (Cercopidae), Aulacorthum solani, Macrosiphum euphorbiae and Myzus persicae (Aphididae), and Zygina zelandica (Cicadellidae). Different groups of insects reached peak abundance at different times between mid December and mid March. From the damage observed it seems unlikely that insects already present in New Zealand can control the plants. None of the specialised European Hieracium fauna was found.

Spatial distribution of littoral fish species in a large European lake, Lake Constance, Germany fig: 9 tab: 5

With 9 figures and 5 tables in the text Abstract: The spatial distribution and patterns of habitat use of littoral fish species in Lake Constance were studied using electric fishing and trammel net sampling to assess fish abundance and biomass in three depth strata at six different sampling sites within the littoral zone. Distribution differences among depth strata were found to be often more pronounced than among different sampling sites. Juvenile chub tLeuciscus cephalus i and dace tLeuciscus leuciscusi were most abundant in the shallow areas of <50 cm water depth with maxima of 54.9 and 29.2 ind . 100 m- 2. Juvenile bream (Abramis brama i were collected mainly in the deeper littoral parts within areas of dense submerged aquatic vegetation with maximal abundances of 63.3 ind 100m,2. Distribution of European eel (Anguilla angui//a) was found to be strongly dependent on the availability of adequate sheller. Dense accumu­ lations of up to 61.7 ind and 40,000 g-wet· 100m -2 were recorded within single sam­ ples at a boulder site. The benthic burbot (Lota Iota) and stone loach tNocmachciius barbatulusi also reached peak abundances in the shallow areas with up to 18.7 and 13.1 ind . 100m:'. The distribution of both species was strongly correlated with the availability of gravel substrate and shelter provided by larger stones. All percid species were found mainly in deeper littoral areas. Juvenile perch and ruffe used areas of submerged aquatic vegetation as preferred habitat. Older individu­ als of both species seemed to avoid these areas and were caught in greater abundances and biomasses in open, unvegetated habitats. The study shows a remarkably high temporal and spatial diversity in the littoral fish species of Lake Constance compared to many small North American lake ecosys­ tems. The results also clearly demonstrate the importance of sampling with high spa­ tial resolution for studies of littoral fish communities. Especially in large lakes, depth and size separation of fish species within single sampling sites may be as important for littoral habitat partitioning as distribution differences among different sampling sites.

Recruitment of the brown mussel Perna perna onto natural substrata: a refutation of the primary/secondary settlement hypothesis

The pattern of recruitment of the brown mussel Perna perna on an exposed rocky shore on the southeast coast of South Africa between March 1991 and March 1992 is reported. The densities and population size structures of plantigrades associated with 2 natural substrata, filamentous algae and established mussel clumps, are contrasted. Mussels of c500 pm shell length were present throughout the study period, but peaked in abundance dunng the winter months (June to September). Although the density of these early plantigrades vaned with both samphng date and site, type of substratum had no effect. Late plantigrades, mussels of >0.5 to 3.5 mm shell length, reached peak abundance amidst mussel clumps in winter as opposed to spring/early summer on algal turf. Sampling date, site and type of substratum all influenced the dens~ty of late plantigrades. The size distributions of the recruits found in association with the 2 substrata dlffered s~gn~ficantly on 4 of the 11 sampling dates. The recruitment pattern of P perna does not, therefore, conform with the generally accepted primary/secondary settlement hypothesis. The observed pattern lndlcates direct settlement of larvae from the plankton onto adult beds in addition to temporary attachment on filamentous algae. The relatively low intensities of recruitment observed, compared with those of other mytilids, may explain such anomalous attributes of P. perna as its lack of competitive dominance and low resilience to disturbance.

Comparing nekton assemblages of subtidal habitats in pipeline canals traversing brackish and saline marshes in coastal Louisiana

Subtidal habitats of pipeline canals in Louisiana brackish and saline marshes were sampled seasonally (fall, spring, and summer) between October 1991 and March 1993 with a 2-m2 throw trap to identify dominant natant species and test hypotheses relating habitat selection to water depth. Densities of nekton were compared among canals and between shallow ( -1m) areas within canals to test two null hypotheses: H1: Densities of nekton in pipeline canals are not related to maximum canal depth and H2: Densities of nekton in shallow and deep subtidal areas of canals are equal. Daggerblade grass shrimpPalaemonetes pugio, bay anchovyAnchoa mitchilli, blue crabCallinectes sapidus, brown shrimpPenaeous aztecus, and gulf menhadenBrevoortia patronus numericallly dominated nekton assemblages in both brackish and saline canals. Naked gobyGobiosomabosc, rainwater killifishLucania parva, and gulf pipefishSyngnathus scovelli were dominant only in brackish canals, whereas white shrimpPelaeus setiferus and Atlantic croakerMicropogonias undulatus were abundant in saline canals only. Although variation in the abundance of most numerically dominant species could not be related to maximum canal depth, the distribution of several species within pipeline canals was influenced by habitat depth and other interrelated factors. The degree of habitat segregation with depth was largely influenced by submerged aquatic vegetation (SAV) and salinity as well as water depth. Habitat segregation with depth was most pronounced in brackish canals during late spring (May) when SAV was present. Naked goby, rainwater killifish, blue crabs, and daggerblade grass shrimp were significantly more abundant in shallow water (<1 m) at this time. In saline canals, most blue crabs and daggerblade grass shrimp occupied shallow habitats in March when small juveniles of these species reached peak abundance. Bay anchovy exhibited a pattern opposite that of other species. In March, bay anchovy abundance was positively related to maximum canal depth in brackish canals, and densities were greater in deep than shallow areas of saline canals in June. Salinity may have affected the distribution of freshwater species (e.g., centrarchids) and limited their occurrence in saline canals. Increasing shallow subtidal habitat by backfilling canals may enhance the nursery habitat for some species, especially in brackish canals where the area of subtidal habitat capable of supporting SAV would be expanded.

Three stages of seasonal succession on the Savin Hill Cove mudflat, Boston Harbor

The benthic infaunal mudflat community of Boston Harbor’s Savin Hill Cove was sampled every other week from January though December 1986. Eighty replicate samples per date allowed precise estimates of the abundances of juvenile and adult stages of all macrofaunal taxa and many meiofaunal taxa. We describe the multivariate structure of%the seasonal succession of this community with a faunal distance metric approach. There were three groups of species that produced a three-stage or triangular succession pattern. Stage 1 is defined by a March bloom of harpacticoid copepods that closely follows a benthic diatom bloom. Harpacticoid copepod abundance rapidly declines in late spring and is followed by the recruitment of four opportunistic annelids, marking the break between stages 1 and 2. Stage 2 is a dense assemblage of four surface-deposit feeding and shallow subsurface-deposit-feeding annelids that reach peak abundance in June and decline in late summer, marking the break between stages 2 and 3. Stage 3 populations are more diverse than stage 2, reach peak abundance in fall, and decline in late fall. The infaunal community structure of December resembles that of the community the previous January. Succession on this mudflat is a fast-paced and dynamic process affected by epipelic diatom production, the timing and duration of juvenile recruitment, and the ability of the infauna to survive in dense assemblages of tube builders.

Cortical granule biogenesis is active throughout oogenesis in sea urchins.

Cortical granules are secretory vesicles formed in the eggs of most animals and are essential for the prevention of polyspermy in these organisms. We have studied the biogenesis of cortical granules in sea urchin oocytes by identifying cDNA clones that encode proteins targeted selectively to the cortical granules. These cDNA clones were identified by an immunoscreen of a cDNA library using antibodies to proteins of the fertilization envelope. Four different mRNAs were identified, ranging from 4 kb to 13 kb in length, that encoded proteins targeted specifically to cortical granules. Accumulation of these mRNAs began very early in oogenesis, in oocytes approximately 10-15 microns in diameter, and continued throughout oogenesis. The mRNAs reached peak abundance (on a per cell basis) in germinal vesicle stage oocytes, and the accumulation of each mRNA was linear with respect to oocyte growth. During breakdown of the germinal vesicle these mRNAs were degraded so that in eggs the mRNA signals were at background levels. Antibodies generated to recombinant proteins made from each of these cDNA clones showed that in the oocyte each cognate protein appeared early in oogenesis. These proteins accumulated only in cortical granules: no accumulation was seen in the cytoplasm, in Golgi, or in other vesicles, and no heterogeneity of the contents was seen within the population of cortical granules. Using these antibodies we show that cortical granules accumulated linearly throughout oogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

The distribution, seasonality and trends in abundance of southern right whalesEubalaena australisoff De Hoop Nature Reserve, South Africa

Between September 1984 and January 1993, monthly counts of southern right whales were made from up to four vantage points in the De Hoop Nature Reserve, South Africa. Evidence from aerial surveys indicates that these vantage points are well situated in relation to the main concentration of right whales off De Hoop. Right whales arrive in June, reach peak abundance in September and depart in December or January. The incidence of calves increases from zero in June to 15–22% between August and October (reflecting the season of births), before increasing again to 33% in November and 42% in December (reflecting the earlier departure of animals without calves). The numbers of right whales seen off De Hoop on aerial surveys from 1971 to 1992 have increased at a rate faster than that of the inshore population as a whole, indicating a degree of net immigration into the area. The rate of this immigration may have slowed down during the past 10 years. Shore counts for adults show a positive correlation with aerial c...

Major Temporal and Spatial Patterns of Gene Expression during Differentiation of the Sea Urchin Embryo

We have investigated the temporal and spatial patterns of accumulation of mRNAs randomly selected from the sea urchin gastrula polyadenylated RNA population. Three different assays show that the predominant temporal pattern of expression, exhibited by about three-fourths of these messages, consists of a large (mean 80-fold) increase in mRNA abundance between egg and gastrula stages. Most mRNAs are present in the maternal population and are detectable on blots as single mature-sized messages; however, a large number of high-molecular-weight, heterodisperse transcripts containing these same sequences also exist in the egg cytoplasm. The majority of gastrula messages are not embryo specific but are present in total adult urchin RNA at concentrations similar to those in embryos. Fine-scale RNA blot analysis indicates that the majority of mRNAs begin to accumulate at very early blastula stages, although there is considerable diversity in the time when these messages reach peak abundance. Most gastrula mRNAs are also spatially regulated during development. The observed distributions can be categorized into three major functional or regulatory classes: (1) Forty percent of mRNAs accumulate in cells which are cycling or preparing for growth. (2) About one-third of the messages accumulate in one or more differentiating cell types. (3) Only slightly more than one-fourth of the messages are present in all cell types throughout development. Most tissue-specific messages are relatively abundant, indicating that the differentiated functions of cells are executed through mRNAs operating at the level of hundreds of copies per cell. In contrast, most rare messages are expressed in most or all cell types, in which they function at only a few copies per cell. All messages which begin to accumulate before hatching blastula stage are initially distributed broadly, and their distribution becomes progressively restricted during embryogenesis. In contrast, all messages which begin to accumulate after the onset of gastrulation accumulate only in discrete subsets of cells. The results presented here illustrate much more extensive temporal regulation of gene expression during sea urchin embryogenesis than previously detected. This is accompanied by spatial regulation of expression of most genes which is itself temporally modulated as the cellular requirements for cell division and differentiation change during development.

Short-Term Seasonal Changes in Composition and Abundance of Fish in Sagehen Creek, California

Abstract Direct underwater observation was used to quantify numbers and locations of eight species of fish in 100-m sections along a 1,200-m reach of Sagehen Creek, California, at biweekly intervals between 5 July and 22 September 1983. Most of the eight species exhibited significant short-term changes in abundance and reached peak abundance at different times. Total fish abundance was highest in mid-July; the Margalefindex ofdiversity was highest in mid-August. Lahontan redside Richardsonius egregius, Tahoe sucker Catostomus tahoensis, and mountain sucker C. platyrhynchus appeared to migrate from a downstream reservoir into the stream during the spawning season. In addition, fish were not distributed evenly in the study reach. All species tended to prefer pool habitat and pools were not uniformly distributed. Relative fish composition determined at long-term permanent sections by electrofishing was not significantly associated with most underwater counts made at different times or stations. This study il...

Fine mapping of transcripts expressed from the US6 gene family of human cytomegalovirus strain AD169

By sequence analysis (K. Weston and B. G. Barrell, J. Mol. Biol. 192:177-208, 1986), the human cytomegalovirus (HCMV) strain AD169 HindIII X DNA fragment contains six open reading frames (US6 through US11; called the US6 family) which may encode glycoproteins. Sense transcripts from the US6 family were mapped. The kinetics of appearance of steady-state cytoplasmic RNA was different for each transcription unit. The 1.5-kb US11-US10 and the 1.7-kb US9-US8 transcripts belonged to the early kinetic class. The former reached peak abundance by 8 h postinfection, while the latter peaked at 24 h postinfection. These RNAs greatly decreased in abundance by 48 to 72 h after infection, unlike transcripts from other HCMV early transcription units reported previously. US6 and US7 messages were most abundant at late times postinfection. US6 transcripts utilized different initiation sites at early or late times postinfection. There was evidence for both spliced and unspliced messages from this family. In a transient expression assay, chimeric plasmids containing the regions upstream of the mapped transcription initiation sites were active in promoting indicator gene expression in HCMV-infected, but not uninfected, human foreskin fibroblast cells.

Effects of conversion to organic agricultural practices on soil biota

AbstractIn the fifth year of an agricultural conversion experiment in Pennsylvania, we studied the soil biological community under three treatment regimes planted with corn: organic-manure, organic-legume, and a conventional system. The organic treatments consisted of complex crop rotations, cultivations, and organic matter inputs to control pests and maintain soil fertility. The conventional system consisted of a simple corn/soybean rotation with synthetic fertilizer and pesticide inputs. High rates of CO2evolution (a measure of potential microbial activity) in the organic plots corresponded with high levels of organic matter input. Soil nematodes were most abundant in organic plots, although seasonal patterns differed between the two organic treatments. Soil microarthropods were dominated by fungivorous Prostigmata mites, which reached peak abundance in organic plots two to five months after organic matter incorporation. Oribatid mites, which were rare throughout the study, followed the same pattern of abundance in each treatment and were probably most influenced by tillage disturbances. Predatory Mesostigmata were generally more abundant in organic plots. Surface-dwelling Collembola were abundant briefly in the spring, but soil-dwelling species dominated numerically throughout the cropping season. Spring tillage appeared to have a strong negative effect on earthworm populations in all plots. Small earthworm species became abundant in organic-manure plots during the summer. Larger earthworm species were abundant in organic-legume and conventional plots after the autumn harvest, when crop residues covered the undisturbed soil The systems-level nature of the Conversion Project experiment makes it difficult to identify cause-effect relationships. The data do suggest that organic amendments tend to enhance soil biological activity, while tillage disturbances tend to disrupt the biotic community.

Bionomics and Seasonal Abundance of the Banded Sunflower Moth (Lepidoptera: Cochylidae) on Cultivated Sunflower in the Northern Great Plains

The banded sunflower moth, Cochylis hospes Walsingham, is a pest of cultivated sunflower, Helianthus annuus L., in the northern Great Plains. Adults were first detected between 7 and 11 July in each year between 1983 and 1986. Adult population peaks aldo showed little variation among years, appearing on 22-25 July each year. Oviposition on the heads (capitula) occurred when plants were in the early-bud stage through anthesis (R2-6). Sunflower plants were in the early bud stage (R2-3) when larvae were initially found between 14 and 23 July each year. Larvae were present in sunflower heads for 6-10 wk and reached peak abundance in early to mid-August. After eclosion, larvae fed on the inner surface of the head bracts or florets. The entire immature seed may be consumed by the developing larva. Older larvae (third to fifth instars) entered and fed on mature seeds after chewing a hole in the apical end of the hull (pericarp). After consuming the majority of the kernel, larvae moved to a new seed. Mature larvae dropped to the soil and constructed silken cocoons to overwinter. Linear regression analysis of measurements of head capsule width and body length substantiated that the larvae develop through five instars. Seasonal patterns of the life stages of banded sunflower moth were similar, but population densities varied among years. There was one generation per year. Each larva consumed more than six mature sunflower seeds.

Seasonal Abundance of Musca vitripennis (Diptera: Muscidae) on Terceira, Azores

Data from pyramid and electrocuting traps as well as records of Musca vitripennis Meigen interceptions on cargo aircraft indicate that M. vitripennis adults reach peak abundance on Lajes Field, Azores, from mid-August through mid-October. Disinsection of military cargo aircraft and other quarantine actions to prevent accidental movement of flies from the Azores to airports in the United States are required during this period.

Growth of Old Field Herbs on a Nitrogen Gradient

Eight species of herbs that reach peak abundance at different times during the first 50 years of secondary succession on a nitrogen impoverished sand plain were grown at both low and high density for one and two field seasons along an experimental nitrogen gradient. The data demonstrate that intraspecific competition is strong even on extremely nitrogen poor soils. The dependence of relative growth rates (RGR), biomass per plant, yield per pot, root:shoot ratios and seed:shoot ratios on soil total nitrogen were determined. These nitrogen-dependent characteristics were compared with the known successional status of the species. Species most abundant on early successional, nitrogen poor soils tend to have higher RGRmaX, lower root:shoot ratio, higher yield per pot and higher seed:shoot ratio than late successional species of more nitrogen rich soils. This suggests that early successional species may be inferior nitrogen competitors but have faster growth rates and better colonization abilities than later successional species. This supports the hypothesis that the successional dynamics of these herbs are the transient dynamics of colonization and competitive displacement, but tends to refute the equilibrial version of the resource ratio hypothesis of succession (i.e. succession caused by changing nitrogen to light ratios). Key-words: Successional herbs, nitrogen-limited growth, root:shoot, seed:shoot, succession on a sand plain, relative growth rates, transient dynamics, successional trade-offs

The Spatial Distribution and Ecology of Zoochlorellae‐Bearing Ciliates in a Productive Pond1

ABSTRACT. The spatial distributions of five zoochlorellae‐bearing ciliate species (Euplotes daidaleos, Frontonia vernalis, Acaryophrya sp., Disematostoma bütschlii, and Stokesia vernalis) were investigated in a productive freshwater pond. The vertical profiles of all species were compared to the levels of O2, CO2, light, temperature, dissolved inorganic nitrogen, and the availability of particulate food sources. In the stratified water column, Stokesia remained close to the water surface: all other species reached peak abundance close to the oxic‐anoxic boundary. The latter behavior probably accommodated the ciliates’requirements for aerobic respiration and particulate food and their dependance on the essential resources of light and dissolved nutrients, which came from opposite directions. With the collapse of stratification, Euplotes and Frontonia returned to the sediment where they reverted to a heterotrophic nutrition although they retained some zoochlorellae. Acaryophrya and Disematostoma also became heterotrophic, but they remained evenly distributed in the water column and they lost their zoochlorellae. Stokesia disappeared, presumably because it encysted. There was some evidence for vertical spatial separation of the five species in the water column.

Seasonal activity and abundance of the arbovirus vector Culex annulirostris skuse near Echuca, Victoria, in the Murray Valley of Southeastern Australia 1979-1985.

Abundance of Culex annulirostris in a state forest near Echuca, northern Victoria, was sampled with a dry ice-baited EVS trap from 1979 to 1985, generally on a fortnightly basis. The species regularly became active from October, the population reached peak abundance in mid summer and then declined to become undetectable from late May. Abundance varied between years, related to availability of larval habitats. The results confirm a pattern of seasonal activity in the Murray Valley that is relevant to arbovirus epidemiology and to vector control.

Movements, abundance, age composition and growth of bass, Dicentrarchus labrax, in the Severn Estuary and inner Bristol Channel

The movements, abundance, age composition and growth of bass, Dicentrarchus labrax (L.), in the Severn Estuary and inner Bristol Channel have been described from regular samples offish taken from power-station intake screens between 1972 and 1977. Bass began to move into the estuary during late August or September and reached peak abundance between September and early November. The tendency for fish to move seawards in the late autumn and winter may be related to declining water temperatures in the estuary. Likewise, differences in abundance between years appear to be related to annual differences in temperature. Although the populations were dominated by the 0 + age class, successively decreasing numbers of the next four age classes and occasional 5 + fish were also present. Respective mean standard lengths of bass in the estuary at the end of their first to fifth years of life were approximately 65 mm (≡ 4·5 g), 130 mm (≡ 37·9 g), 190 mm (≡ 121·1 g), 250 mm (≡ 280·4 g) and 290 mm (≡ 441.5 g). Growth occurred predominantly between May and September, with the mean standard length of 1 + fish increasing from approximately 65 mm (≡ 4·5 g) to 130 mm (≡ 37·9 g).