Modeling spatiotemporal dynamics of krill aggregations: size,intensity, persistence,and coherence with seabirds

Understanding aggregation dynamics of forage species is important for evaluating biophysical scaling in marine ecosystems and heterogeneity of trophic interactions. In particular, zooplankton aggregations are fundamental units of many pelagic systems, but are difficult to observe continuously through space and time. Using an established modeling framework that encompasses a coupled regional oceanographic and individual‐based modeling system, we test the hypothesis that persistence (duration) of krill aggregations is dependent on their size, intensity, and location of formation within the coastal upwelling region of the California Current. In support of this hypothesis, we found that aggregation size is positively related to intensity, whereas persistence has a parabolic response to aggregation size and intensity, indicating the likelihood that large and highly persistent aggregations are rare. Persistence of krill aggregations also depends on formation location within coastal upwelling areas. We found that krill aggregations were more likely to form near a major seabird colony and that some coastal upwelling areas act as sources of aggregations for other areas. Observations of seabird aggregations were used to evaluate the potential structural realism of predicted krill aggregations. Seabird aggregations displayed marked coherence with predicted krill aggregations in space, providing important criteria on the scaling and availability of krill aggregations to breeding and migratory species. Predicting scales of krill aggregation dynamics will benefit ecosystem assessments, and numerical modeling of predator foraging and marine spatial management aimed at ensuring protection of ecologically important areas.     

Diel vertical migrations ofMeganyctiphanes norvegica in the Kattegat: Comparison of net catches and measurements with Acoustic Doppler Current Profilers

Diel vertical migration of a stable and well-defined population of Nordic krill,Meganyctiphanes norvegica (Crustacea, Euphausiacea) was investigated during eight days in August 1989, in the Läsö-Deep, East of the Danish island Läsö. Net catches with a multi-net (MOCNESS) and measurements with a moored and a shipboard Acoustic Doppler Current Profiler (ADCP) were compared. Backscattered energy as a measure for biomass gave good correlations to the dry weight ofM. norvegica and smaller zooplankton from net catches. Diel migratory patterns matched well, as determined, parallel with both methods. Migratory vertical velocity was determined with ADCP at 2–3 cm sec^?1. The potential for the use of ADCPs for biological investigation is discussed. Vertical migration was dependent on environmental parameters. The krill did not cross a temperature barrier of 14°C, although rich food sources were situated beyond it. Differences in salinity did not play a role. Currents were involved in plankton distribution. Light was an important Zeitgeber (synchronizer) and determined the density of the krill aggregations. Feeding behaviour did not interfere with the light-induced migratory pattern of Nordic krill at the Läsö-Deep.     

Winter aggregations of marine mammals and birds in the north-eastern Weddell Sea pack ice

Summary A seabird and mammal census was carried out in the north-eastern Weddell Sea during the austral winter of 1986. The German research icebreaker Polarstern operated in heavy pack ice along the Greenwich Meridian between the northern sea ice boundary and the Antarctic coast. Crabeater seals (Lobodon carcinophagus), minke whales (Balaenoptera acutorostrata), Adélie penguins (Pygoscelis adeliae), Antarctic petrels (Thalassoica antarctica) and snow petrels (Pagodroma nivea) were found to be more abundant in the vicinity of the submarine Maud Rise, about 700 km north of the continental margin, than in other areas of substantial ice cover traversed during that cruise. The aggregations of birds and mammals are expected to reflect aggregations of their principal food, krill (Euphausia superba) wintering underneath the ice cover. The distribution pattern of krill predators coincides with the course of a warm water belt upwelling near Maud Rise. This upwelling could induce local ice melting which in turn may result in an increased release of sea ice algae.     

2018年夏秋季南设得兰群岛周边水域南极磷虾集群类型及其影响因素

南极磷虾是一种典型的集群性海洋生物,其集群特征为行为生态学研究领域的重要内容之一。南极磷虾在南设得兰群岛周围高度密集分布,然而磷虾集群形状和大小的机制解释仍存在较大的争议。基于南设得兰群岛周边水域收集的Simrad EK80声学数据,本研究利用Echoview V6.16软件,对声学数据进行了分析,并对磷虾集群特征进行了划分。通过主坐标分析(PCoA)检验了环境因素(表温和海况)以及时空因素对各类型磷虾集群产生的影响。结果表明:海况对磷虾集群影响较大,光照强度次之;块状小型集群的时空分布较广,夜间与白天均占有较高比例(>30%);小型集群更易出现在白天,而大型集群则更多出现在深夜; 2月,磷虾集群与海况及纬度显著相关; 3月,集群与时段显著相关; 4月,集群与时段及海况显著相关。     

Variability of Euphausia superba populations near Elephant Island and the South Shetlands: 1981 vs. 1984

Summary March 1981 and 1984 Euphausia superba populations were compared using acoustics and net catches near Elephant Island, the South Shetlands, and in the Bransfield Strait. In 1981, krill tended to form large, thick swarms and in 1984, smaller, more dispersed, shallower patches. March body lengths of juvenile-adult krill were 22–59 mm in 1981 and 13–59 mm in 1984. Near Elephant I. in 1981, krill >45 mm were most numerous; in 1984 sizes <45 mm were dominant. In March 1984, the larger (>45 mm) body-size group was prereproductive and occurred from just west of Elephant I. westward into waters north of the South Shetlands; in 1981 the larger krill were postreproductive and more widely distributed in the Elephant I. area. Overall, netted postlarval krill, 1981 vs. 1984, averaged 73 vs. 48 individuals/m², or 54 vs. 16 g/m²; acoustic biomass estimates were 229 vs. 134–201 g/m². Larvae near Elephant I. averaged >2000/m² in 1981 vs. <1/m² in 1984—compatible with respective March reproductive states. Net-type comparisons revealed short-term (15 min to 6h) variability of a similar scale in both MOCNESS and bongo net catches, but bongo abundances averaged greater. Variation in maturity composition across 1981 swarms, patches, and random transects was like variation among the random 1984 tows; spatial distributions were more heterogeneous in 1984. The March 1984 krill of 20–44 mm (Year-2, mode 34 mm) relate to November 1983 krill of 9–30 mm (mode 21 mm), indicating growth averaging 12 mm during the season. Body-lengths and size-frequency modes of Year-2 and combined Years-3,3+ krill from comparable Feb-Mar data collected since 1968 suggest trends between times when (1) Year-2 krill average small and peak reproduction seems to be late in the season and/or weak (1979, 1982–1984), and (2) Year-2 krill are larger, and reproduction is possibly earlier and more successful (1976, 1980, 1981).     

南极半岛北部南极磷虾渔业空间点格局特征

南极磷虾作为南极生态系统中的关键物种,在空间分布上常表现出集群特征.这也反映到磷虾渔业生产的空间格局特征上.为了探讨捕捞能力有明显差异的船队在高/低单位捕捞努力量渔获量(CPUE)的情况下空间点分布格局特征及其生态学效应,基于南极半岛北部海域的两艘中国南极磷虾渔船(船A为专业南极磷虾渔船,船B为在智利竹筴鱼渔场与南极磷虾渔场转换的兼作渔船)的磷虾渔业数据,从空间点格局的角度出发,分别从两船的高、低CPUE的空间点格局在不同尺度上聚集特征,高、低CPUE在不同尺度上的二元点格局相关关系,以及CPUE点标记格局下的相关性关系等3个方面进行了分析.Ripley的L函数和标记相关函数分析结果表明: 研究对象在空间窗口所有尺度上的空间格局均表现为聚集性,高、低CPUE下均有聚集发生;在15 km尺度上,聚集强度近最大,在15~50 km尺度下,聚集程度稳定;总体上点格局分布的聚集强度依次为:船A高CPUE>船B低CPUE>船B高CPUE>船A低CPUE.船A高、低CPUE在0~75 km尺度上为正相关关系,在大于75 km尺度上为随机关系;船B在所有尺度上的高、低CPUE均为正相关,说明了低CPUE点事件伴随高CPUE的点事件同步发生,两者在大部分尺度下均显著相关.这是磷虾集群模式的动态性和复杂性造成.船A各点的CPUE值在0~44 km尺度上呈正相关,在44~80 km尺度上呈负相关;船B各点的CPUE值在50~70 km尺度上呈负相关,在其他尺度上无显著相关性;正相关反映了磷虾密集集群的种群分布特性,而负相关表明了磷虾群间由于食物和空间原因存在一定的竞争关系.捕捞能力强的船A和捕捞能力较弱的船B在点格局分布上存在较大差异.专业南极磷虾渔船更适于开展磷虾作业空间点格局分析及相关科学调查工作.     

Underwater acrobatics by the world's largest predator: 360° rolling manoeuvres by lunge-feeding blue whales

The extreme body size of blue whales requires a high energy intake and therefore demands efficient foraging strategies. As an obligate lunge feeder on aggregations of small zooplankton, blue whales engulf a large volume of prey-laden water in a single, rapid gulp. The efficiency of this feeding mechanism is strongly dependent on the amount of prey that can be captured during each lunge, yet food resources tend to be patchily distributed in both space and time. Here, we measured the three-dimensional kinematics and foraging behaviour of blue whales feeding on krill, using suction-cup attached multi-sensor tags. Our analyses revealed 360° rolling lunge-feeding manoeuvres that reorient the body and position the lower jaws so that a krill patch can be engulfed with the whale''s body inverted. We also recorded these rolling behaviours when whales were in a searching mode in between lunges, suggesting that this behaviour also enables the whale to visually process the prey field and maximize foraging efficiency by surveying for the densest prey aggregations. These results reveal the complex manoeuvrability that is required for large rorqual whales to exploit prey patches and highlight the need to fully understand the three-dimensional interactions between predator and prey in the natural environment.     

Genetic variation and population structure of krill (Euphausia superba dana) from the Prydz Bay region of Antarctic waters

Summary Samples of Antarctic krill E. superba from six locations near Prydz Bay were analysed electrophoretically to detect genetically-based protein variation. Analyses of allele distributions at four polymorphic loci indicate no evidence of significant heterogeneity, a result consistent with the hypothesis that all samples were derived from a single breeding population of krill. The results of this study agree closely with genetic data from other studies on krill in the Atlantic sector of the Southern Ocean, suggesting that krill stocks over at least 6000 km of Antarctic waters are derived from a single interbreeding population.     

A comparison of Antarctic petrel (Thalassoica antarctica) diets with net samples of Antarctic krill (Euphausia superba) taken from the Prydz Bay region

Samples of the stomach contents of Antarctic petrels (Thalassoica antarctica) were obtained on board ship in the Prydz Bay region of Antarctica from birds which had spontaneously regurgitated. The weight of food and the species composition of the stomach contents were measured. Antarctic krill,Euphausia superba, the sole prey item taken, were compared to krill obtained by nets in the same region as part of a large-scale krill survey. Krill from petrel stomach samples were larger in mean size than those sampled by nets. This may be attributed to selection of the larger sized krill by the petrels, it may be caused by the nets sampling different populations of krill or it may be due to net avoidance by the larger krill.     

Super-aggregations of krill and humpback whales in Wilhelmina Bay, Antarctic Peninsula

Ecological relationships of krill and whales have not been explored in the Western Antarctic Peninsula (WAP), and have only rarely been studied elsewhere in the Southern Ocean. In the austral autumn we observed an extremely high density (5.1 whales per km(2)) of humpback whales (Megaptera novaeangliae) feeding on a super-aggregation of Antarctic krill (Euphausia superba) in Wilhelmina Bay. The krill biomass was approximately 2 million tons, distributed over an area of 100 km(2) at densities of up to 2000 individuals m(-3); reports of such 'super-aggregations' of krill have been absent in the scientific literature for >20 years. Retentive circulation patterns in the Bay entrained phytoplankton and meso-zooplankton that were grazed by the krill. Tagged whales rested during daylight hours and fed intensively throughout the night as krill migrated toward the surface. We infer that the previously unstudied WAP embayments are important foraging areas for whales during autumn and, furthermore, that meso-scale variation in the distribution of whales and their prey are important features of this system. Recent decreases in the abundance of Antarctic krill around the WAP have been linked to reductions in sea ice, mediated by rapid climate change in this area. At the same time, baleen whale populations in the Southern Ocean, which feed primarily on krill, are recovering from past exploitation. Consideration of these features and the effects of climate change on krill dynamics are critical to managing both krill harvests and the recovery of baleen whales in the Southern Ocean.     

Interannual variation in the diet of non-breeding male Antarctic fur seals, <Emphasis Type="Italic">Arctocephalus gazella</Emphasis>, at Isla 25 de Mayo/King George Island

The diet of non-breeding male Antarctic fur seals, Arctocephalus gazella, was investigated at Stranger Point, King George Island, through the analysis of scats during three consecutive summer seasons (1996, 1997, 1998). Overall, fish and krill were the most frequent prey occurring, respectively, in an average of 82.9% and 78.8% of samples (n = 131), followed by penguins (22.8%) and cephalopods (17.8%). Myctophids constituted almost 90% of the fish predated, with Electrona antarctica and Gymnoscopelus nicholsi being the most abundant and frequent species consumed. All fish taxa identified were krill feeding species suggesting that seals foraged mainly on a krill and a fish community associated with krill aggregations. However, a seasonal change was observed in the relative proportions of the different prey taxa, with a progressive decrease with time in the occurrence of krill and a concomitant increase of fish, penguins and squid. Possible influence of the strong 1997/98 ENSO event is discussed.     

南极Gerlache海峡海鸟的取食集群及食性

1997年12月至1998年2月,我们对南极半岛席尔瓦角善于飞翔海鸟与企鹅的取食关联性进行了研究,同时调查了取食集团中主要鸟种的食性.发现每个取食集团中有35.6-37.0只善于飞翔的海鸟,其中几乎都有纹颊企鹅群 (Pygoscelis antarctica),黑背鸥(Larus dominicanus)、灰贼鸥(Catharacta maccormicki)、花斑鹱(Daption capensis)和巨鹱(Macronectes giganteus)是各集团中最常见的鸟类.各取样单元内有相关性的种数随季节变化而减少,一些种类的减少与特定的物候期有关.南极磷虾(Euphausia superba)是绝大部分飞翔海鸟的主要食物,研究发现黑背鸥与纹颊企鹅所捕食的南极磷虾的大小最为接近.飞翔海鸟的觅食行为表明:在海面上短时停留的飞翔海鸟也能够成功捕捉到磷虾,这可能与磷虾躲避企鹅的捕食有关[动物学报 53(3):425-430,2007].     

Spatial variance of mobile aquatic organisms: capelin and cod in Newfoundland coastal waters

Spatial variance in the distribution of aquatic mobile organisms differs from that of passive tracers such as phytoplankton or water temperature. On average, spatial variance of phytoplankton scales with sample unit as $L^2$ or equivalently with frequency as $f^{-2}$. Limited evidence suggests that spatial variance in the distribution of mobile organisms is concentrated at relatively small scales, with little increase over larger scales: spatial variance scales as $f^{-1}$ or less. We investigated whether spatial variance in distributions of a mobile predator, Atlantic cod (Gadus morhua), and a schooling prey, capelin (Mallotus villosus), also scale with frequency as $f^{-1}$. Acoustic surveys showed that at short time scales spatial variance in cod and capelin densities, as measured by spectral density, peaked at various scales ranging from 20 m to 10 km. At longer time scales, spatial variance of cod scaled as $f^{-1.08}$ at resolutions finer than 90 m, while scaling as $f^{-0.18}$ at coarser scales. Spatial variance of capelin scaled as $f^{-1.1}$ at resolutions finer than 400 m, while scaling as $f^{-0.21}$ at coarser scales. Spatial variance plots of krill and marine birds showed similar transitions from shallow to steep scaling. Shoaling, schooling and the aggregative response by predators to concentrations of prey were three processes hypothesized to influence spatial variance in distributions of mobile organisms. Numerical experiments showed that shoaling injects variance at large to intermediate scales, resulting in scalings flatter than $f^{-1}$. Additional experiments showed that schooling produces a transition from shallow to steep scaling as frequency increases. Spatial variance patterns in cod density were not due to aggregative responses by the predator to concentrations of capelin: there was no association, on average, at resolution scales from 20 m to 10 km. Exponent values for aquatic or terrestrial mobile organisms are predicted to be approximately two at the scale of an individual organism, 0.2 at scales that contain aggregations, and two at scales larger than that of populations. These findings suggest that relations between mobile organisms and large scale habitat variables will be difficult to detect, that stratified survey designs used to estimate commercial population sizes will be inefficient, and that rates of interaction between predator and prey will be underestimated if local observations are averaged over the spatial scale of the population.     

Long-term changes in Krill biomass and distribution in the Barents Sea: are the changes mainly related to capelin stock size and temperature conditions?

Krill plays a significant role in the Barents Sea ecosystem, providing energy transport between different trophic levels. The current paper presents the results of a long-term study (1980–2009) based on pelagic trawl catches from August to September. Our investigations show that the krill species were distributed widely in the Barents Sea and that the largest krill concentrations were restricted to the west-central and eastern parts of the Barents Sea. The current paper presents the relative biomass indices, and the estimates must be interpreted as minimum biomass. The mean annual krill biomass was estimated to be 22 million tonnes in wet weight, with the highest values being as much as 48 million tonnes. Capelin is the largest pelagic stock, and in some years, their biomass can amount to 4–7 million tonnes, which can impose high predation pressure on krill. When their biomass is high, capelin may consume close to 26 million tonnes annually. The predation from pelagic (herring and blue whiting) and bottom (cod and haddock) fish species was much lower, being 9 and 1 million tonnes, respectively. A negative relationship between krill biomass and capelin stock size above 74°N was observed during the study period. However, during the last decade, the krill biomass has increased despite heavy predation from capelin in some years. A positive significant linear relationship between the mean annual Kola temperature and the krill biomass seems to indicate that the recent warming conditions have favourable impacts on the krill populations in the Barents Sea.     

Biochemistry of the autolytic processes in Antarctic krill post mortem. Autoproteolysis.

1. Autoproteolysis post mortem was examined at 0 degree C by following the changes in the major classes of krill (Euphausia superba and Euphausia crystallorophias) proteins and by liberation of peptides and free amino acids, and was based on experiments conducted on board expedition vessels in the Antarctic. 2. Primarily salt-soluble proteins were broken down during the first week of incubation, whereas water-soluble and insoluble proteins were degraded to a much smaller extent. The enzymes responsible for the hydrolysis presumably originate primarily from the digestive apparatus of the krill. 3. In general, the individual amino acids were released at rates corresponding to their relative occurrence in the bulk protein of the krill. Alanine was liberated in larger amounts than would be expected from the composition of the krill protein, and was evidently formed also by reactions other than proteolysis. Glutamic acid, and certain amino acids which presumably occur with high frequency adjacent to glumatic acid residues in the krill protein, were liberated only to a limited extent, and accumulated in smaller peptides. 4. During proteolysis, arginine seemed to be converted to some degree into ornithine, and on prolonged incubation conversion of arginine and lysine into their corresponding decarboxylation products, agmatine and cadaverine, appeared to take place.     

Predicting krill swarm characteristics important for marine predators foraging off East Antarctica

Open ocean predator‐prey interactions are often difficult to interpret because of a lack of information on prey fields at scales relevant to predator behaviour. Hence, there is strong interest in identifying the biological and physical factors influencing the distribution and abundance of prey species, which may be of broad predictive use for conservation planning and evaluating effects of environmental change. This study focuses on a key Southern Ocean prey species, Antarctic krill Euphausia superba, using acoustic observations of individual swarms (aggregations) from a large‐scale survey off East Antarctica. We developed two sets of statistical models describing swarm characteristics, one set using underway survey data for the explanatory variables, and the other using their satellite remotely sensed analogues. While survey data are in situ and contemporaneous with the swarm data, remotely sensed data are all that is available for prediction and inference about prey distribution in other areas or at other times. The fitted models showed that the primary biophysical influences on krill swarm characteristics included daylight (solar elevation/radiation) and proximity to the Antarctic continental slope, but there were also complex relationships with current velocities and gradients. Overall model performance was similar regardless of whether underway or remotely sensed predictors were used. We applied the latter models to generate regional‐scale spatial predictions using a 10‐yr remotely‐sensed time series. This retrospective modelling identified areas off east Antarctica where relatively dense krill swarms were consistently predicted during austral mid‐summers, which may underpin key foraging areas for marine predators. Spatiotemporal predictions along Antarctic predator satellite tracks, from independent studies, illustrate the potential for uptake into further quantitative modelling of predator movements and foraging. The approach is widely applicable to other krill‐dependent ecosystems, and our findings are relevant to similar efforts examining biophysical linkages elsewhere in the Southern Ocean and beyond.     

Abundance, sizes and developmental stages of larval krill, Euphausia superba, during winter in ice-covered seas west of the Antarctic Peninsula

Larval krill were sampled west of the Antarctic Peninsula duringthree winter cruises: September 1991, June 1993 and September1993. Larval abundances were estimated from net catches andcompared directly to visual counts (made by a SCUBA diver) oflarvae occupying the ice habitat at the same sampling stations.The number of larvae per square meter sampled with nets wasmore often greater than that observed by the diver, irrespectiveof the sampling period. However, comparisons of larval abundancewithin sampling periods were not statistically significant.Larval krill collected by divers were significantly larger thanthose collected with nets for each of the three cruises. Thestage composition of larval krill also depended on the collectionmethod: net-collected samples contained a disproportionatelyhigh number of early furcilia larvae in June 1993 (early winter),and a disproportionately low number of early juveniles duringSeptember 1991 and 1993 (late winter). These results lead usto suggest that larval/juvenile krill occupy both the watercolumn and sea ice habitat during the austral winter, and thatthere are often differences in the sizes and developmental stagesof the two groups. For larval krill that occupied the sea icehabitat, aggregations were larger and more numerous during latewinter than in early winter. In addition, larvae within aggregationsoccupied structurally complex microhabitats, provided by over-raftedice floes, more often than they occupied smooth, downward-facingice surfaces where ice was not over-rafted.     

Evidence of resource partitioning between humpback and minke whales around the western Antarctic Peninsula

For closely related sympatric species to coexist, they must differ to some degree in their ecological requirements or niches ( e.g. , diets) to avoid interspecific competition. Baleen whales in the Antarctic feed primarily on krill, and the large sympatric prewhaling community suggests resource partitioning among these species or a nonlimiting prey resource. In order to examine ecological differences between sympatric humpback and minke whales around the Western Antarctic Peninsula, we made measurements of the physical environment, observations of whale distribution, and concurrent acoustic measurements of krill aggregations. Mantel's tests and classification and regression tree models indicate both similarities and differences in the spatial associations between humpback and minke whales, environmental features, and prey. The data suggest (1) similarities (proximity to shore) and differences (prey abundance versus deep water temperatures) in horizontal spatial distribution patterns, (2) unambiguous vertical resource partitioning with minke whales associating with deeper krill aggregations across a range of spatial scales, and (3) that interference competition between these two species is unlikely. These results add to the paucity of ecological knowledge relating baleen whales and their prey in the Antarctic and should be considered in conservation and management efforts for Southern Ocean cetaceans and ecosystems.     

Hierarchical patch dynamics and animal movement pattern

In hierarchical patch systems, small-scale patches of high density are nested within large-scale patches of low density. The organization of multiple-scale hierarchical systems makes non-random strategies for dispersal and movement particularly important. Here, we apply a new method based on first-passage time on the pathway of a foraging seabird, the Antarctic petrel (Thalassoica antarctica), to quantify its foraging pattern and the spatial dynamics of its foraging areas. Our results suggest that Antarctic petrels used a nested search strategy to track a highly dynamic hierarchical patch system where small-scale patches were congregated within patches at larger scales. The birds searched for large-scale patches by traveling fast and over long distances. Once within a large-scale patch, the birds concentrated their search to find smaller scale patches. By comparing the pathway of different birds we were able to quantify the spatial scale and turnover of their foraging areas. On the largest scale we found foraging areas with a characteristic scale of about 400 km. Nested within these areas we found foraging areas with a characteristic scale of about 100 km. The large-scale areas disappeared or moved within a time frame of weeks while the nested small-scale areas disappeared or moved within days. Antarctic krill (Euphausia superba) is the dominant food item of Antarctic petrels and we suggest that our findings reflect the spatial dynamics of krill in the area.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Asymmetric resource competition in a filter-feeding stream insect (Hydropsyche siltalai; Trichoptera)

SUMMARY. 1. Field experiments in a lake outlet stream were carried out on filter-feeding larvae of the caddisfly Hydropsyche siltalai Döhler, to assess whether microhabitat quality, establishment and growth rate is affected by position (upstream—downstream) within a dense aggregation of larvae.
2. Artificial substrates holding high-density aggregations of hydropsychid larvae were used. Marked III-instar H. siltalai larvae were implanted either in the upstream end or in the downstream end of an aggregation and recovered after 17 days.
3. Larvae recovered from the downstream end of an aggregation were smaller than those that had been implanted in the upstream end.
4. Fewer larvae had settled in the downstream end of aggregations compared to the exposed upstream part.
5. Naturally colonizing H. sitalai larvae were, in contrast to implanted larvae, more numerous behind aggregations. Small (first and second instar) Hydropsyche angustipennis (Curtis) and/or H. pellucidula (Curtis) larvae were more numerous in front of aggregations.
6. Current velocities, measured with a microcurrent velocity meter, and density of drifting prey, were significantly lower behind aggregations of hydropsychid larvae. Food availability decreased by about 50% behind aggregations.