Total Larval Abundance Research Articles

Interannual summer biodiversity changes in ichthyoplankton assemblages of the Strait of Sicily (Central Mediterranean) over the period 2001–2016

Interannual fluctuations in the structure and the composition of ichthyoplankton assemblages in the pelagic waters of the Strait of Sicily (SoS, Central Mediterranean) were investigated, trying to relate them to the observed variability in oceanographic conditions. Plankton data used in this study were from 16 summer surveys carried out in the SoS every year from 2001 to 2016, using oblique bongo plankton net (0–100 m) tows. Out of more than 12,000 fish larvae collected in the sampling stations included in the analysis, 9,519 of them were identified and regularly classified in 15 orders and 49 families. Ichthyoplankton assemblages, defined at the family level due to the uniform availability of this information along the time series, showed a decreasing trend over time in total larval abundance, along with taxonomic (family) richness and Shannon index (α diversity), more pronounced in the shelf area and in the slope area, respectively. Conversely, the relatively high levels of yearly compositional changes observed in the larval assemblage from both shelf and slope areas, as estimated by the Jaccard dissimilarity index (β diversity), did not show any significant linear trend. In addition, a biodiversity hotspot (both in terms of family richness and Shannon index) was evidenced in the frontal structure characterizing the southeastern part of the study area. Generalized additive models were used to evaluate the effect of oceanographic conditions on the temporal and spatial patterns of ichthyoplankton biodiversity. Results evidenced the role of salinity, surface temperature, and surface currents in modulating biodiversity indices, especially in the shelf area. Finally, the relevance of local frontal oceanographic structures in sustaining high biodiversity levels is postulated.

Seasonal Larval Abundance of the Blackfly, Simulium damnosum Complex and the Physico-chemical Properties of their Breeding Sites in Oji River System, Enugu State, Nigeria

Seasonal larval abundance of the blackfly, Simulium damnosum complex, and the physico-chemical properties of their breeding sites in Oji river system was carried out, with the aim of finding out the season that promotes the breeding of S. damnosum complex and the effect of the physico-chemical properties of the breeding sites of S. damnosum complex on larval abundance. Larvae of S. damnosum complex were collected monthly from forty-eight (48) breeding sites in four sampling units for three years, in dry and wet seasons. The physico-chemical properties of the breeding sites of S. damnosum complex studied were copper, dissolved oxygen, magnesium, nitrate, pH, phosphate, potassium, temperature, and water velocity. A total of six hundred and ninety one (691) larvae of S. damnosum complex were collected in the Oji river system in both seasons. Wet season catch was more and comprised 82% of the total catch, while dry season catch comprised 18% of the total catch. Dry and wet seasons catches in the sampling units were found to differ statistically. All the physico-chemical properties of the breeding sites of S. damnosum complex determined showed positive correlations with total larval abundance in both seasons, except copper which showed no correlation with total larval abundance in the dry season. Wet season promotes larval abundance of S. damnosum complex, and the physico-chemical properties of the breeding sites of S. damnosum complex play some roles (in varying degrees) in larval development.

Seasonal, interannual and spatial variability in the reproductive dynamics of Penaeus merguiensis

Penaeid prawns (shrimp) are short-lived and fecund, with a complicated life cycle that includes offshore spawning followed by a coastal or estuarine postlarval and juvenile phase. Factors affecting survival during the early life-history stages, and during movement between these stages, will affect variability in recruitment to the nursery ground, the offshore subadult and adult population, and, ultimately, catch. The inability to predict recruitment, and ultimately commercial offshore catch, has been complicated by an incomplete understanding of these factors. The reproductive dynamics of Penaeus (Fenneropenaeus) merguiensis were investigated by simultaneous adult and larval sampling on 66 lunar-monthly surveys from March 1986 to March 1992 in Albatross Bay, northeastern Gulf of Carpentaria, Australia. Egg production was seasonal, with the highest production from 6-mo-old newly recruited spawners, and another peak from 12mo-old spawners. Larval abundance (no. m-2) followed the same seasonal pattern as the abundance of eggs. However, interannual variation in egg and larval abundance was large, and there was a weak correlation between monthly egg and larval abundance. Larval abundance appeared to be further influenced by fluctuations in chlorophyll a concentration, a measure of food availability. There was evidence of a match/mismatch relationship between larval abundance and episodic chlorophyll increases. While there was no direct spawner (egg production)-fishery recruit relationship in P. merguiensis over the 6-yr study, there was a strong relationship between total larval abundance in spring and the size of the commercial catch 3 to 6 mo later. Therefore, factors affecting larval survival, including food availability, have significant implications for fishery production.

Larval fish assemblages of myctophids in the deep water region of the southern Gulf of Mexico linked to oceanographic conditions

In the Gulf of Mexico, the distribution of larval fish assemblages have been linked to continental shelf vs oceanic habitats and mesoscale structures such as cyclonic and anticyclonic eddies. Myctophids have a worldwide distribution and are one of the mesopelagic fish families with higher biomass, thus playing a key role in the trophic structure of oceanic communities. However, studies focused on their larval ecology and distribution within the gulf are limited. This study evaluated whether the formation of myctophid larval assemblages can be linked to local adult population spawning patterns and/or oceanographic conditions and surface transport. Ichthyoplankton samples were collected in August–September 2015 using standard bongo net tows. Stations extended throughout Mexico's Exclusive Economic Zone (19°N–25°N) deep water region, including the Yucatan Channel. Myctophid comprised 24.6% of the standardized total larval abundance and were present at all stations. Two dominant assemblages (out of six identified with a Bray–Curtis dissimilarity analysis) were found in stations of the central gulf and the Bay of Campeche. The high proportion of preflexion larvae coupled with differences in hydrographic parameters and surface circulation between the central gulf and the Bay of Campeche, indicated that local adult fish populations and their spawning patterns likely contribute more to the formation of these assemblages than oceanographic conditions and surface transport. Stations from the northern Yucatan Peninsula located within the region of influence of the Loop Current shared a common assemblage, which may reflect biogeographic differences in the myctophid species composition between the northern Caribbean and western gulf. These results highlight the importance of adult distribution and local spawning, providing insight into biogeographic patterns during the early life stages of mesopelagic fishes in the Gulf of Mexico.

Interannual variability in the abundance and composition of spring larval fish assemblages in the Strait of Georgia (British Columbia, Canada) from 2007 to 2010

AbstractSpring larval fish assemblages in the Strait of Georgia (British Columbia, Canada) were characterized for the first time based on three spatially extensive field surveys in late‐April of 2007, 2009 and 2010, a period which spanned both warm and cold environmental conditions. The abundance, diversity and community structure of the assemblages were examined to investigate interannual variability, and responses to environmental fluctuations. A total of 49 taxa from 23 families were identified. The dominant taxa were Clupea pallasi, Gadus chalcogrammus, Merluccius productus, Leuroglossus schmidti, Lyopsetta exilis and Sebastes spp. Total larval abundance was much lower in 2009 (c. 32 per 1000 m3) than in 2007 and 2010 (c. 200 per 1000 m3). However, the mean size of individuals from several species was largest in 2009. Assemblage structure varied dramatically; from dominance by M. productus in 2007, to dominance by C. pallasi and G. chalcogrammus in 2009, followed by a shift to dominance by benthic species including Sebastes spp. and several flatfishes in 2010. Variability in assemblage structure among the three study years was primarily related to water temperature, whereas within‐year patterns were more closely associated with salinity and chlorophyll, both of which were affected by estuarine circulation. This study provides baseline information about the status of the larval fish community in the Strait of Georgia in recent years, and offers a better understanding of their short‐term dynamics and response to environmental fluctuations.

Hydrographic and fish larvae distribution during the “Godzilla El Niño 2015–2016” in the northern end of the shallow oxygen minimum zone of the Eastern Tropical Pacific Ocean

AbstractBased on hydrographic data and vertical distributions of tropical species of fish larvae (Diogenichthys laternatus, Vinciguerria lucetia, Bregmaceros bathymaster, and Auxis spp.), effects of “Godzilla El Niño 2015–2016” in the shallow oxygen minimum zone off Mexico were analyzed. Zooplankton samples were collected during four cruises, before (February 2010 and April 2012) and during (June 2015 and March 2016) the warm event. Temporal series of sea surface temperature revealed that June 2015 was the warmest June of the last years. Conservative temperature was >2°C higher than normal in the surface mixed layer, and the suboxic layer (4.4 µmol/kg) reached as shallow as 100 m depth. Unexpected results were that larval abundances were relatively high during the warm event, unlike zooplankton volumes, which declined. Before the warm event, V. lucetia and Auxis spp. were more abundant in the surface mixed layer, while B. bathymaster and D. laternatus dominated in the thermocline and shallow hypoxic layer (44 µmol/kg). However, during the event in June 2015, all species were most abundant in the surface mixed layer, which implied that the species adapted to hypoxia had inverted their normal pattern of distribution, possibly as consequence of the rise of the suboxic layer; however, further observations are required to confirm this generality. Results showed no dramatic change in the total larval abundance during the warm event. Nevertheless, a differential response in their vertical distribution was evident in association with changes in the depth of the shallow hypoxic and suboxic layers. This might indicate adaptability of tropical species to prolonged periods of warming in the oceans.

Spatio-temporal variations in abundance and assemblage patterns of fish larvae and their relationships to environmental variables in Sirindhron Reservoir of the Lower Mekong Basin, Thailand

The spatio-temporal dynamics of fish larvae in Sirindhron Reservoir, north-east Thailand, were investigated from February 2008 to January 2009. The five most abundant species accounted for 53.6% of the total catch and comprised Clupeicthys aesarnensis, Rasbora borapetensis, Barbonymus gonionotus, Esomus metallicus and Oreochromis niloticus. Total larval abundance and species richness varied among sampling months but did not vary with zones in the reservoir. The abundance and species richness of fish larvae showed correlation with flooding, vegetation cover, water level, water temperature and turbidity. A self-organising map (SOM) was used to represent the larval fish assemblage patterns and three assemblage patterns were clearly distinguished primarily according to seasons. In conclusion, the fish larval abundance was greatest during the wet season and the most diverse assemblages were found in the zones adjacent to the connected river upstream during the wet season.

The influence of pre-settlement and early post-settlement processes on the adult distribution and relative dominance of two invasive mussel species

The structure of a community is governed by a complex combination of processes whose relative importance varies over time and space. Larval dynamics, settlement and recruitment are thought to be important processes limiting adult abundance and distribution of benthic invertebrates with planktonic larvae. Two invasive molluscs with similar morphology and resource needs, the Eurasian zebra mussel Dreissena polymorpha and the quagga mussel Dreissena rostriformis bugensis, co-occur in several North American lakes and rivers but often differ in their adult distribution over depth. Following establishment, the quagga mussel typically replaces the zebra mussel in abundance, particularly in deeper waters. A field sampling programme conducted over 3 years in a lotic system (the Soulanges Canal connected to the St. Lawrence River) examined the extent to which adult distribution and the differential dominance of these two species are determined by larval supply (i.e. larval abundance near settlement sites), settlement and recruitment. Total dreissenid larval abundance in the water column at two depths was determined weekly, and larval competence (size) and species-specific larval composition was estimated, during the main settlement period over three consecutive years. The pattern of total dreissenid settlement over the depth gradient was determined by deployment of settlement plates at both depths. Total abundance and proportional abundance of zebra mussel and quagga mussel juveniles and adults in each depth zone were determined monthly from July to September each year. Mean dreissenid larval size did not differ between depths and the supply of late-stage larvae was generally low, but total larval abundance was consistently greater in deeper water. This differential larval abundance established settlement and recruitment patterns in the canal but contrasted predictions based on total adult dreissenid abundance – which was higher in the shallow zone. Therefore, the significant factor dictating the abundance of adult mussels in these two depth zones must be post-recruitment mortality, rather than larval supply, settlement or recruitment. Despite a strong species-specific adult depth zonation, larvae and juveniles showed no consistent differences in species proportions over the depth gradient. In fact, zebra mussels dominated larval abundance at both depths for c. 50% of the sampling dates and dominated juvenile abundance at both depths throughout most of the sampling period. In contrast, the proportional abundance of zebra mussels in the adult dreissenid community was consistently 4–5 times higher in the shallow zone. These results indicate that larval supply, settlement and recruitment processes are not responsible for determining total adult dreissenid distribution or species dominance. Rather, these patterns appear to be governed by post-recruitment factors that manifest themselves in later stages of mussel development and growth.

Reactions of temperate reef fish larvae to boat sound

Anthropogenic sound as a stressor in aquatic systems is of increasing concern. The present study evaluated whether boat sound either deters larval fish or interferes with their ability to use biological sounds for navigating. Pre-metamorphic larval fishes were caught in a pair of light traps deployed off a shallow rocky reef in Port Phillip Bay, Australia, with one trap randomly assigned one of two boat sound treatments, at sound pressure levels experienced in situ, and the other without any sound treatment. The boat sound treatment included either (1) boat engine sound without any other sound, or (2) boat engine sound with biological and physical sound from a pier, a positive but weak cue indicative of much of the available benthic habitat in the vicinity. Total larval abundance and species composition of the assemblage were compared between the sound-exposed trap and the control trap. When only boat sound was broadcast, no significant differences between sound-exposed and control traps was observed. When boat sound was broadcast together with pier sound, significantly more fishes were caught in the sound-exposed trap (~300% of ‘control’ traps). The results of this study suggest that a) common boat sound is not readily avoided by larval fishes, and that b) larval fishes are attracted to biological sound in spite of the presence of boat sound. Together the results suggest that boat sound at levels experienced in situ might not have deleterious effects on recruitment of larval fishes, i.e. the capacity to alter recruitment patterns. Copyright © 2011 John Wiley & Sons, Ltd.

Vertical Distribution of Fish Larvae and Its Relation to Water Column Structure in the Southwestern Gulf of California1

ABSTRACT The seasonal evolution of vertical distribution of fish larvae and its relationship with seasonal stratification, as measured by a quantitative stability parameter, were analyzed for a region off Bahia de La Paz in the southwestern Gulf of California. Samples were obtained with an opening-closing net (505 μm) in 50-m depth strata from surface to 200-m depth in May, July, and October 2001 and February 2002. Significant differences in total larval abundance and in dominant species (mesopelagic and epipelagic) were found among strata from May to October. More larvae were found in maximum-stability strata (from 16 ±5 to 48 ±17 m depth) than below the pycnocline (from 100- to 150-m depth). In February, the 100-m-deep surface mixed layer had a weak pycnocline at its base, and no significant difference was found. Results show that vertical distribution of fish larvae in this area depends mainly on the seasonal evolution of the water column structure, with most fish larvae in the pycnocline, at the most ...

Vertical structure of very nearshore larval fish assemblages in a temperate rocky coast

Small-scale vertical patterns of larval distribution were studied at a very nearshore larval fish assemblage, during the spring–summer period of several years, at two depth strata (surface and bottom) using sub-surface and bottom trawls. A total of 4,589 larvae (2,016 from surface samples and 2,573 from bottom samples) belonging to 62 taxa included in 22 families were collected. Most larvae belonged to coastal species. Although inter-annual variations in larval density and diversity could be found, total larval abundance was always higher near the bottom whereas diversity was higher at the surface. A marked distinction between the structure of surface and bottom assemblages was found. Sixteen taxa explained 95% of the similarity among surface samples. Larvae which contributed most to this similarity included species like clupeiformes, sparids and serranids, and also blenniids, tripterygiids and some labrids. In the bottom samples, fewer species were present, with only six taxa, almost exclusively from species which lay demersal eggs, contributing to 95% of the similarity between samples. Larvae present at the surface were significantly smaller than at the bottom. For some of the most abundant species found at the bottom, only small larvae occurred at the surface while the whole range of sizes was present at the bottom, indicating that larvae may be completing the entire pelagic phase near the adults’ habitat. These results indicate that larval retention near the reefs probably occurs for these species, although for others dispersal seems to be the prevailing mechanism.

Horizontal spatial and temporal distribution patterns of nearshore larval fish assemblages at a temperate rocky shore

There have been no previous studies of the composition of nearshore larval fish assemblages along the coast of Portugal. We aimed to describe the composition and horizontal distribution patterns of larval fish assemblages and their temporal dynamics near a rocky reef at depths shallower than 13 m (inshore) and at two miles (3.70 km) from shore (offshore), as well as along transects perpendicular to the shoreline, from the reef to 10 miles offshore (18.52 km). Samples were taken using 5 min sub-surface trawls at the rocky shore of the Arrábida Marine Park (W Portugal). A total of 1021 larvae were collected, belonging to 61 taxa inshore and to 29 taxa offshore. Along transects, 626 larvae of 52 taxa were collected. Most larvae belonged to coastal species associated with rocky reefs. Total larval abundance and diversity were higher from May to July, which is consistent with the spawning activity of adults. Diversity and total larval abundance decreased significantly with increasing distance from shore, both in the inshore/offshore comparison and in the transects, where this decrease was evident at a very small spatial scale (within the first mile from the reef). Species assemblages differed in the pattern of distribution, with most species clearly associated to the extreme nearshore. The distribution patterns obtained were independent of the spawning mode of species. Results are discussed in the light of the possible physical mechanisms that can potentially act at the Arrábida Marine Park to facilitate larvae retention and the role of larval behaviour.

Monitoring native non-target Lepidoptera for three years following a high dose and volume application of Bacillus thuringiensis subsp. kurstaki

The operational use of Bacillus thuringiensis subsp. kurstaki (Btk) to control the western spruce budworm in the mountainous terrain of British Columbia, Canada, gives variable results at the registered dose of 30 BIU ha − 1 in 2.4 L ha − 1. Btk (Foray 48B®, 60 BIU ha − 1 in 4.8 L ha − 1) was applied experimentally in 1996 over 150-ha of western spruce budworm-infested Douglas-fir forest to determine if increasing both the dose and volume of Btk applications gives more uniform and effective control. We sampled native caterpillars on the shrub Ribes cereum Douglas in this and a similar, untreated area 2 km away from 1996 to 1998 to assess the impact of this high dose and volume Btk application on non-target Lepidoptera. The numerically dominant lepidopteran species were Euhyponomeutoides gracilariella Busck (52%) and Gelechia ribesella Chambers (32%). The remaining 16% of the guild was comprised of numerous sparsely distributed species. Total larval abundance was significantly lower on plants that were sprayed with Btk than on plants that were covered to exclude Btk within the treatment area in the year of application. Geometrids appeared to be less affected by the treatment than other taxa. Both G. ribesella and E. gracilariella appeared to make a full recovery by the second year post-treatment, but we did not find a full recovery of the sparsely distributed species.

Monitoring Nontarget Lepidoptera onRibes cereumto Investigate Side Effects of an Operational Application ofBacillus thuringiensissubsp.kurstaki

The abundance of nontarget Lepidoptera on the shrub Ribes cereum Douglas was monitored from 1997 to 2000 in an Interior Douglas-fir forest in British Columbia to assess potential side effects of an operational program to control the western spruce budworm, Choristoneura occidentalis Freeman. The treatment was a single application of Bacillus thuringiensis subsp. kurstaki (Btk) (Foray 48B) at the rate of 30 BIU/ha in 2.4 liter/ha. The guild of leaf-feeding Lepidoptera on R. cereum was dominated by Gelechia ribesella Chambers, and Euhyponomeutoides gracilariella Busck, which made up 24 and 62% of the guild, respectively. The remaining 14% of the guild comprised numerous sparsely distributed species (at least 45 different species based on morphotypes). Total larval abundance was significantly lower on plants that were sprayed with Btk than on plants that were covered to exclude Btk. Covering the plants was a novel approach that enabled us to replicate the treatment within a single spray area. G. ribesella and E. gracilariella were significantly reduced by Btk, but a modest reduction of 'sparsely distributed species' was not significant. Both G. ribesella and E. gracilariella appeared to make a full recovery within 2 yr of the Btk spray, but as a group the abundance of the sparsely distributed species was lowest in the year 2000 in both the Btk sprayed area and an untreated comparison area. This suggests a general decline independent of the treatment. Microscopic examination of cadavers of the two major nontarget species showed the presence of Btk in some of the larvae reared from the treated plot, but Btk was absent in larvae from the reference plot.

Displacement of diverse ichthyoplankton assemblages by a coastal upwelling event on the Sydney shelf

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 176:49-62 (1999) - doi:10.3354/meps176049 Displacement of diverse ichthyoplankton assemblages by a coastal upwelling event on the Sydney shelf Kimberley A. Smith*, Iain M. Suthers School of Biological Science, University of New South Wales, Sydney, New South Wales 2052, Australia *E-mail: marsci@unsw.edu.au ABSTRACT: The influences of upwelling-favourable winds and thermocline displacement on the distribution of temperate/tropical ichthyoplankton assemblages were determined from vertically stratified plankton hauls across the Sydney shelf, southeastern Australia. Five stations were sampled along a shore-normal transect off Sydney, Australia, on 3 or 4 consecutive nights during January and April 1994. High taxonomic diversity (111 ichthyoplankton families) was attributed to the convergence of temperate and tropical waters in this region. Total larval abundance was higher in January than in April, reflecting the spawning times of many coastal taxa. In both months, highest larval density and taxonomic diversity were associated with the interface between the mixed layer and the thermocline. In April, a period of upwelling-favourable winds resulted in the offshore displacement of the nearshore mixed layer, as well as numerous shelf-spawned larvae (e.g. Centroberyx affinis, Chromis hypsilepsis, Arripis trutta) from the nearshore region, and coincided with the injection of deeply distributed mesopelagic larvae into the nearshore zone (e.g. Scopelosaurus sp., Melamphaes sp.). Shelf-spawned larvae were displaced to the outer shelf, where they were still associated with the mixed layer/thermocline interface. Dynamic ichthyoplankton distributions are interpretable, at least at a scale of weeks, by considering larvae as tracers of the local hydrography. KEY WORDS: Ichthyoplankton assemblages · Hydrography · Upwelling · Thermocline Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 176. Publication date: January 18, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

Nearshore patterns of coral reef fish larval supply to Barbados, West Indies

Although larval supply is essential to the maintenance of many marine populations, relatively little is known about the processes influencing the supply of larvae to nearshore habitats. To measure the temporal and spatial patterns of larval supply to the island of Barbados, West lndies, an array of light traps was deployed at 3 sites along the west coast for 2 spring recruitment seasons (1991-92). Three replicate traps were deployed nightly a t a central site, and 3 more traps were situated at both a northern and a southern site on alternate weeks during a 60 d period in 1991. In 1992, 3 replicate light traps were deployed nightly at each of the 3 sites for 70 d . A total of 82 species from 31 farmlies was collected; collections from each year were dominated by species from several families: Atherinidae, Blennidae. Scaridae, Pomacentridae, and Acanthuridae. Temporal patterns of abundance were similar for the majority of the species collected: total larval abundance and diversity was greatest during the second half of the lunar cycle, peaking on the third quarter moon. Spatial patterns of larval supply were also distinct: the overall supply of larvae was typically lowest in the central reglon of the Island. Supply of individual species was usually higher at one or the other or both ends of the west coast compared to the central site, with only 2 exceptions. Variation in the temporal and spatial pattern of bdal currents may account for patterns In the supply of ichthyoplankton. Temporal patterns of north-south (along shore) tldal flow were similar at all 3 sites: maximum nightly transport to the south occurred durmg maxlmum amphtude t ~ d e s (during the new and full moons), while .maximum nightly transport to the north occurred dunng rmnmum amphtude tides (during the quarter moons). East-west (onshore-offshore) transport due to tidal flows occurred on a much smaller scale and dlffered among sites. At the northern and southern s~ te s , nightly transport by tidal currents was generally eastward, or onshore, while at the central slte transport was generally west, or offshore. At both the southern and central sites in 1992, the temporal pattern of larval supply was significantly correlated to east-west transport by tidal currents integrated over the prevlous night. Onshore transport of larvae by tidal currents may be reduced at the central region of the west coast, resulting in both lower abundances and an overall lower diversity of reef fish larvae there. Patterns in the nightly transport by large-scale, externally forced currents (periods greater than 39 h ) were generally similar across all sites. Transport by these currents in both the north-south and east-west direct~ons was signif~cantly correlated to the mean number of larvae collected by the light traps, suggesting some influence of external forcing such as wind or other oceanographic features. Thus, larval supply to Barbados appears to be Influenced by processes occurring at 2 scales. Behavioral or passive synchronization to the lunar and tidal amplitude cycles results in regular, predictable peaks in the supply of late-stage larvae. Superimposed on this pattern are less predictable, large-scale events which can also greatly influence the nearshore transport of larvae.

Seasonal settlement of shipworms (Bivalvia: Teredinidae) in Tauranga Harbour and Wellington Harbour

Abstract The seasons of settlement of shipworms in Tauranga and Wellington harbours in 1970–72 were studied using Pinus radiata test‐blocks. At Wellington the seasonal abundance of late‐stage shipworm larvae was monitored in the plankton. In Tauranga Harbour Bankia australis, Lyrodus medilobatus, Lyrodus pedicellatus, and Nototeredo edaxspawned and settled predominantly in summer, when water temperatures were above 17–18°C. Bankia neztalia had peak periods of settlement in winter and early summer and there was no correlation of settlement with water temperature. In Wellington Harbour B. neztalia spawned and settled mainly during winter when water temperatures were 11–15°C. Total larval abundance at the 2 test sites in Wellington Harbour was an approximate indication of the relative intensity of settlement, but seasonal larval abundance did not correlate well with seasonal settlement. B. neztalia larvae settled on air‐dried Pinus radiata wood within 6 days of the immersion of the wood. Annual variations in...

Lacustrine Seston and Other Factors Influencing the Blackflies (Diptera: Simuliidae) Inhabiting Lake Outlets in Swedish Lapland

The blackfly (Diptera: Simuliidae) assemblages of three localities in a river system in Swedish Lapland were examined. One locality was at a lake outlet, one further down in the same river and one in a nearby small forest stream. The three dominant species at the outlet were virtually absent from the other localities, where, conversely, most larvae belonged to species not present at the outlet. Total larval abundance was very much higher at the outlet, where larvae were present for a shorter period than elsewhere. Even within a few hundred metres from the outlet the relative growth rate of larvae was slower than at the outlet, and only a little further downstream the species prevailing at the outlet were replaced by other species. Large particles, such as phytoplankton and coarse detritus, occurred in similar numbers everywhere, and at all times, and therefore did not seem to control the abundance of blackfly larvae. After examining the role of temperature and substrate it is concluded that food, that is small particles possibly down to colloidal size, provide the resource maintaining the huge larval aggregations immediately below the lake outlet. These particles, produced on the lake bottom by decomposition processes during the winter, are washed out into the river at ice-melt. Changes in their availability and/or nutritive value limit their usefulness as food to the section of the river immediately downstream from the lake. Additional benefits may accrue to larvae growing up in these areas of extreme population density.

Abundance and Microdistribution of Larval Chironomidae in Stabilizaton Lagoons1, 2

Two different population structures of larval Chironomidae present in a 3-celled vs. a 1 cell 1 stabilization lagoon were studied from 1964 to 1966 in Oregon. Statistical analysis of dredging results from the 3-celled lagoon showed that total larval abundance was greater in winter, greater in the primary cell, and consistently greater in peripheral zones. Significant differences in larval population densities showed the following order of abundance: Glyptotendipes barbipes (Staeger) > Chironumus riparius Meigen > Anatopynia dyari (Coquillett). The established order of abundance resulted from greater tolerance on the part of G. barbipes than either C. riparius or A. dyari for physicochemical stress. Moreover larval A. dyari required more space for growth and development than did the tube-dwelling species. Dredging data from the 1-cell lagoon showed that total larval abundance was greater in the summer, greater for G. barbipes than for A. dyari, and greater for G. barbipes in both peripheral and central zones. The absence of C.riparius populations resulted from sustained sewage loading. The main factors affecting midge populations in both lagoons are considered to be: sewage influent quality and quantity, depth of water, degree of peripheral slope, and availability of dissolved oxygen at the soil-water interface.