Effects of prey density on nocturnal zooplankton predation throughout the ontogeny of juvenile <Emphasis Type="Italic">Platax orbicularis</Emphasis> (Teleostei: Ephippidae)

Juvenile Platax orbicularis switches foraging tactics from diurnal herbivory to nocturnal zooplanktivory within a day. To examine how juvenile fish actively feed on zooplankton prey during nighttime, a field-recorded video analysis was conducted in the reefs off Kuchierabu-jima Island, southern Japan. Juveniles consistently showed fast and sudden attacks that were accurately directed at individual zooplankton prey, and changed feeding frequencies with different prey densities. A negative relationship was observed between feeding frequency and prey density, with higher feeding frequencies occurring at lower prey concentrations, implying a disturbance effect of clouded zooplankton prey on the juvenile fish. A clear transition from a ram-based to a suction-based feeding mode was observed with fish size, suggesting that changes in the feeding behaviors occur even in juveniles fishes, without drastic morphological changes.     

From autochthonous to allochthonous resources: seasonal shifts in food use by stream‐dwelling YOY Arctic charr Salvelinus alpinus through the ice‐free season

Substantial seasonal changes in resource use associated with enhanced water‐column use were revealed in stream‐living YOY Arctic charr Salvelinus alpinus during the ice‐free season. In July, YOY individuals showed a diet dominated by aquatic invertebrates (mainly Chironomidae larvae), but despite the small size of the fish, the abundance of terrestrial insects in their diet increased markedly from July to September (from 1·9 to 62·8%). Similarly, the frequency of surface drifting foragers, i.e. individuals feeding on allochthonous resources, increased from July to September (from 20·6 to 80%); allochthonous resources thus constituting an important energy subsidy for YOY S. alpinus during the late sub‐Arctic summer.     

Interspecific differences in drift behaviour between the native <Emphasis Type="Italic">Gammarus pulex</Emphasis> and the exotic <Emphasis Type="Italic">Gammarus roeseli</Emphasis> and possible implications for the invader’s success

“Drifting” is known to subject aquatic invertebrates to intense predation by drift feeding fish. Consequently, interspecific variations in drifting behaviour could lead to differences in predation pressure between coexisting prey species. Predation being an important factor determining the success of invaders, differences in drift patterns could advantage either native or exotic invertebrates through differential predation by native fish predators. The exotic freshwater amphipod (Gammarus roeseli) has now largely colonized Western Europe where it is often found in sympatry with a native species (Gammarus pulex). Here we documented interspecific differences in drifting behaviour that might have favored the invader’s success through differential predation. Benthic and drifting amphipods were sampled three times at the same site to compare the proportion of each species within and between sample types (benthos or drift) across time. Compared with the benthos, where the invader (G. roeseli) was significantly less abundant than the native (G. pulex), G. roeseli was proportionally overrepresented in the drift but displayed a very different drifting pattern. While G. pulex drift rates remained roughly constant over a 24 h period, G. roeseli showed a marked diel periodicity with low diurnal and high nocturnal drift rates. Such drifting behaviour could procure this species with a competitive advantage regarding predation as most drift feeding fish are diurnal. As a result, the native appears more disadvantaged with respect to drift. This may partly explain the ability of G. roeseli to coexist with G. pulex in a habitat more suitable to the native.     

Control of foraging behavior of individuals within an ecosystem context: the clam Macoma balthica,flow environment,and siphon-cropping fishes

Macoma balthica (L.), an abundant clam, ubiquitous in temperate estuaries across the North Atlantic, is known to practice both alternative basic modes of feeding available to seafloor invertebrates. It either holds its feeding organ, the siphon, at a fixed position just above the sediment surface to filter out food particles suspended in the overlying water or else extends and moves its siphon around to vacuum up deposited food particles on the sediment surface. Previous laboratory experiments have established an understanding of the role of current flow in dictating the choice of whether suspension or deposit feeding will be used by marine invertebrates with the facultative flexibility to choose. Faster flows imply greater fluxes of suspended particles so that the energetic rewards of suspension feeding are enhanced. Slower flows imply reduced renewal rates of suspended foods in the bottom boundary layers and enhanced deposition of food particles on the seafloor so that a switch to deposit feeding is favored. Like early optimal foraging theory, this understanding is based on energetic considerations alone without incorporation of broader implications of how population interactions such as predation and competition influence individual foraging behavior. Feeding behavior of Macoma balthica is influenced in the Neuse River estuary by both hydrodynamics and siphon-cropping by juvenile demersal fishes. Under conditions of identical concentrations of suspended particulates in the water column and organic contents of surface sediments, Macoma exhibited much higher levels of deposit feeding where currents were slower. In addition, exclosure and fish inclosure experiments demonstrated that juvenile demersal fishes influence feeding behavior of Macoma by cropping exposed siphons and inducing reduction in deposit-feeding activity. Effects of croppers were substantial in early to midsummer, when juvenile fish abundances were greatest in trawl samples from this estuarine nursery and before the growing fish exhibited ontogenetic changes in diet away from early concentration on bivalve siphons. Field experiments in which siphon-cropping fish were caged at varying distances off the bottom failed to detect any effective behavioral avoidance by Macoma of cropping in response to proximity of fish. One might have hypothesized that under high risk of cropping, Macoma would switch to suspension feeding and away from deposit feeding, the feeding method entailing more risk of losses to croppers because of greater siphon activity and greater extension of siphons on the sediment surface. Consequently, partial predation by siphon-cropping fishes greatly reduces deposit-feeding activity by Macoma balthica during summer as an apparent direct effect of disfigurement and reduction of siphons, the organ required for efficient deposit feeding. Information on current flows alone would not suffice to predict feeding behavior of this marine invertebrate: the influence of partial predation must also be included.     

High altitude mountain streams as a possible refuge habitat for the catfish <Emphasis Type="Italic">Amphilius uranoscopus</Emphasis>

Amphilius uranoscopus is a catfish species, restricted to rivers and streams in east, southern and central Africa. It is likely to be displaced due to both competition and predation by exotic trout and other introduced fish. In high altitude mountain streams it can be the only species occurring, which means that this habitat may act as a refuge for this species. Ecosystems like this are threatened by habitat alteration and are therefore in need of protection. The abiotic environment, population structure, behavior and feeding biology of Amphilius uranoscopus were studied in a small, high-altitude perennial tributary of the Limpopo River in the Soutpansberg mountain range, Limpopo Province, South Africa, during 2005–2006. Here A. uranoscopus showed nocturnal behavior. It used dark hollow crevices in rapids as shelters during the daytime. The rapids are characterized by a high flow rate, high dissolved oxygen content and coarse riverbed substrate consisting mainly of boulders without fallen leaves. In contrast to the adults, juveniles found shelter among the fallen leaves in pools. At night, A. uranoscopus moved out of the rapids into the open water of the pools. The main food of A. uranoscopus consisted of macroinvertebrates, mainly Trichoptera larvae. Amounts of algae and detritus in its diet were negligible. A. uranoscopus foraged mainly in rapids and on rock surfaces, spending less time foraging between the fallen leaves at the bottom of pools, in open water, at the surfaces of bottom and bank and near the water surface. At the water surface, it also fed opportunistically on terrestrial insects that dropped into the stream, like flying termites. A. uranoscopus was the top predator of the stream system and reached a high density (0.71 fish m⁻²). There was no competition from other fish species at the study site as they were lacking. A. uranoscopus showed a striking ability to climb and cling on to vertical substrates to conquer waterfalls and very shallow streams that sometimes become dry, using its fins, adhesive body and protruding mouth teeth.     

Pelagic to demersal transition in a coral‐reef fish,the orbicular batfish Platax orbicularis

Behavioural and ecological observations were made on young, reared Platax orbicularis in Opunohu Bay, Moorea, French Polynesia, during their transition from the pelagic, dispersive stage to the reef‐orientated demersal stage. Seventy‐two young P. orbicularis (17–75 mm standard length, L_S) were released in the pelagic zone and 20 (40–70 mm L_S) adjacent to the reefs. Swimming speed was slow (mean 5·2 cm s^?1) and independent of size. An ontogenetic descent was observed: the smallest P. orbicularis swam at the surface, medium‐sized P. orbicularis swam in midwater (mean 5–13 m) and the largest P. orbicularis swam to the bottom, where many lay on their sides. Platax orbicularis swam southerly on average, away from the ocean and into the bay. Smaller P. orbicularis were more likely to swim directionally than larger individuals. Young P. orbicularis released near reef edges swam at similar, but more variable speeds (mean 6·6 cm s^?1). About half of those released near reefs swam away, but fewer swam away from an inshore fringing reef than from a patch reef near the bay mouth. Many P. orbicularis swam up the slope onto the reef top, but the little settlement observed was near the reef base. Average, near‐reef swimming direction was also southerly. Some reef residents, in particular the triggerfish Balistapus undulatus, harassed young P. orbicularis.     

THE FEEDING HABITS OF THE WHITEWINGED BLACK TERN ON LAKE KARIBA

Bego, G. W. 1973. The feeding habits of the Whitewinged Black Tern on Lake Kariba. Ostrich 44: 149–153.

The stomach contents of 33 Whitewinged Black Terns were examined over the period 26 August 1971 to 6 January 1972. The most important single food item was the sardine, Limnothrissa miodon. This sheds new light on the habits of this fish. The terns fed primarily during calm weather. Sardines are caught by hovering and diving. Fishes of the genera Haplochromis and Synodonlis, also appeared in the stomach contents. Terns taken from marginal localities appear not to feed on fish. Of the insects eaten Odonata are the most important aquatic source of food, while many terrestrial insects are eaten (Orthoptera, Hemiptera, Coleoptera, Isoptera). Such insects could have been found drifting on mats of Salvinia on which the terns rest, otherwise insects are either hawked or skimmed from the surface of the water.     

Distribution and diet of the roughskin sculpin, <Emphasis Type="Italic">Trachidermus fasciatus</Emphasis>, larvae and juveniles in the Chikugo River estuary,Ariake Bay,Japan

Larvae and juveniles of the roughskin sculpin Trachidermus fasciatus were collected during four cruises in March and April 2001 to study the distribution and diet of the fish in the Chikugo estuary in the northern Ariake Bay, Japan. Sampling was conducted at seven stations covering an area approximately 30 km in length along the estuary, with salinity ranging from nearly 0 to about 30 PSU. Gut contents were analyzed by separating, identifying, and counting the prey organisms. Plankton samples were collected during each cruise to study the numerical composition and abundance of copepods in ambient water. A total of 1790 larvae and juveniles of T. fasciatus were collected; they were distributed in the five uppermost stations, covering approximately 20 km and ranging in salinity from 0.4 to 27.4 PSU. The fish fed almost exclusively on a single calanoid copepod species, Sinocalanus sinensis, which was numerically the most abundant in the ambient water at the upper part of the Chikugo River. At the lower part of the river, the fish positively selected S. sinensis while negatively selecting other species. T. fasciatus larvae experienced a high proportion of empty guts and low feeding intensity at around 9.0 mm body length, which appears to be associated with commencement of exogenous feeding. The proportion of empty guts reduced sharply at subsequent stages and was accompanied by an increase in feeding intensity. The oligohaline and mesohaline areas are important nursery grounds, and S. sinensis is an important prey copepod species for the larval and juvenile T. fasciatus in the Chikugo estuary.     

Juvenile Snooks (Centropomidae) as Mimics of Mojarras (Gerreidae), with a Review of Aggressive Mimicry in Fishes

Aggressive mimicry has been proposed for several unrelated fish species both in freshwater and marine environments. I describe herein a few additional examples, including the first ones from brackish water. In one well documented case, juvenile snooks, Centropomus mexicanus (Centropomidae) join bottom-foraging groups of the superficially similar mojarras, Eucinostomus melanopterus (Gerreidae) and prey on small fishes and crustaceans under such disguise. Two other snook species and two species of groupers (Serranidae), are here suggested as additional instances of aggressive mimicry. Furthermore, I review published examples of aggressive mimicry in fishes and indicate trends in the relationships between the mimics, their feeding tactics, and their putative models. Three large families, Serranidae, Cichlidae, and Blenniidae display most of the examples of aggressive mimicry, serranids being largely represented by the genus Hypoplectrus and blenniids by the tribe Nemophini only. Three major trends are here indicated for aggressive mimics: (1) fish species that feed on prey smaller than themselves tend to mimic and join fish species harmless to their prospective prey; (2) fish species that feed on prey larger than themselves tend to mimic mostly beneficial fish species (cleaners) or, less frequently, join species harmless to their prospective prey; (3) fish species that feed on prey about their own size tend to mimic their prospective prey species, the perfect wolf in a sheep's clothes disguise type. The latter deceit is recorded mostly for scale and fin-feeding freshwater fishes.     

Susceptibility to freshwater acidification by two species of loon: Red-throated Loon (Gavia stellata) and Arctic Loon (Gavia arctica) in southwest Sweden

In southwest Sweden, the two species of loon, Gavia stellata and G. arctica, have shown different trends in population size and production of young during the last decades. Both species fish in oligotrophic freshwaters, susceptible to acidification. The number of breeding sites occupied by G. stellata has been reduced by almost 50% during the last 40–50 years. For G. arctica, there are no indications of significant declines in population size or reproductive success during the last 20 years. The different trends in numbers and production of young might reflect different susceptibility to the ecological changes in acidified lakes. G. stellata prefer fishing lakes with high abundance of Acerina cernua and salmonid and cyprinid fish, such as Coregonus albula and Rutilus rutilus. They also feed their prefledged chicks almost entirely on cyprinid and salmonid fish. G. arctica prefer fishing lakes with high transparency and, when feeding in groups, high abundance of Perca fluviatilis. Their young can be fed on aquatic insects as a supplement to the fish diet. Thus, G. stellata to a higher degree than G. arctica relies on fish which are susceptible to low pH, and G. arctica may also benefit from the increased abundance of aquatic insects in lakes with reduced predation from fish. Furthermore, high water transparency is important for the selection of lakes by G. arctica but not by G. stellata. In G. stellata, high contents of mercury in eggs can be related to the intake of fish in lakes affected by acidification.     

Tidal activity pattern and feeding behaviour of the ophiuroid Ophiocoma scolopendrina on a Kenyan reef flat

Ophiocoma scolopendrina exhibits a distinctive pattern of feeding activity on intertidal reef platforms off Kenya. With the first wave of the flooding tide, dense aggregations of these ophiuroids (up to 320 m⁻²) engage in a 1–2 min burst of surface-film feeding, vigorously sweeping the air-water interface and associated sea foam with the ventral surface of 2–4 arms. Suspension feeding (with arms extended in the water column) is the primary feeding mode throughout the rest of the tidal cycle (involving 25–65% of the population at a time), while bottom feeding (with arms extended along the substratum) is infrequent (<10%). Field experiments showed that surface-film feeding is regulated by water depth and can be triggered by suspended particles. This feeding mode appears to be an adaptation to the intertidal habitat, enabling the ophiuroids to exploit a nutrient-rich surface film during a temporal refuge (low tide) from fish predation. Dense populations of O. scolopendrina may represent an important trophic link between producers of particulate organic material and higher-level consumers in coral reef environments.Tara Oak and Robert E. Scheibling contributed equally to this paper. The order of authorship is alphabetical     

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.     

The early life history of kurosoi,Sebastes schlegeli (Scorpaenidae), in the Sea of Japan

Larval and juvenile stages of kurosoi,Sebastes schlegeli, are described and illustrated from wild specimens. Some ecological aspects of larvae and juveniles are also described. Notochord flexion occurred between 5.6–7.5 mm SL. Transformation occurred between 13–20 mm SL. Preflexion and flexion larvae ofS. schlegeli can be distinguished from similar larvae by the pigmentation of the dorsal and ventral midlines of the tail and absence of pigmentation on the ventral portion of the rectum. After notochord flexion, the dorsal and lateral regions in both larvae and pelagic juveniles were heavily pigmented, suggesting adaptation for neustonic life style. Larvae and juveniles were caught at many coastal stations, but did not occur in cooler offshore waters. Larvae smaller than 20 mm SL inhabited surface waters. Until ca. 40 mm SL, juveniles inhabited mainly surface waters (without drifting seaweed), but also used other habitats, such as the drifting seaweed, and near the sea bed. Small larvae (<7 mm SL) fed mainly on copepod nauplii. Larger larvae fed on calanoid copepodites andEvadne nordmanni. Pelagic juveniles fed mainly on fish eggs, with fish larvae also being important food items for some individuals. Most food items taken by juveniles that were associated with drifting seaweed were eggs with attaching filaments (Cololabis saira andHyporhamphus sajori), suggesting that the high density of such food items both attracts and keeps juveniles around drifting seaweed.     

Effect of water velocity and temperature on energy use,behaviour and mortality of pallid sturgeon Scaphirhynchus albus larvae

Natural reproduction of pallid sturgeon Scaphirhynchus albus has been limited for decades and a recruitment bottleneck is hypothesized to occur during the larval stage of development. In this study, we evaluated the effects of water velocity and temperature on the swimming activity, energy use, settling behaviour and mortality of endogenously feeding larvae. The swimming activity of drifting sturgeon larvae (i.e., fish exhibiting negative rheotaxis) increased at low water velocity. In subsequent experiments, we observed greater energy depletion and resultant mortality of larvae in no-flow environments (0 cm s⁻¹) compared to tanks with water velocity ranging from 3.5 to 8.3 cm s⁻¹. The growth rate of drifting larvae was positively related to water temperature (18.7–23.3°C), but reduced growth rate at low water temperature (18.7°C) resulted in protracted development that extended average drift duration by ~4 days compared to larvae reared at 23.3°C. This study provides evidence that cooler summer water temperatures, characteristic of present-day conditions in the upper Missouri River, can reduce larval development and extend both the drift duration and distance requirements of S. albus. Moreover, if dispersed into low velocity environments, such as in reservoir headwaters, larvae may experience increased mortality owing to a mismatch between early life stage drift requirements and habitat conditions in the river. Manipulation of water releases to increase seasonal water temperature below dams may aid survival of S. albus larvae by shortening the time and distance spent drifting.     

Microhabitat use by the rainbowfish <Emphasis Type="Italic">Melanotaenia duboulayi</Emphasis> in a subtropical Australian stream

Microhabitat use and feeding behavior of the rainbowfish Melanotaenia duboulayi (Castelnau) were investigated in a slow-flowing stream adjacent to riparian forest in south-eastern Queensland, Australia. Fish were more abundant in vegetated areas, but did not enter dense Vallisneria beds, where predators were observed. In sunny conditions shoals of juveniles occurred near the water surface feeding floating material on the surface, but larger fish tended to occur at the bottom near submerged vegetation, often utilizing the overhanging aquatic plant community as a refuge and food source. In the middle of the day, juveniles and small fish seemed to show behavioral thermoregulation at the surface in the warmest site. Under cloudy conditions, however, fish of all sizes preferred deeper water. The present study suggests that in still and sunny pools thermal change caused by sunlight influences the microhabitat choice of small fish. A field experiment using a kingfisher model implies that fish swimming at the surface could escape from aerial predators in sunlit conditions by responding to moving shadows, but could not do so under cloudy conditions. Electronic Publication     

Assessment of Grey Heron predation on fish communities: the case of the largest European colony

The fish predation rate by Grey Heron Ardea cinerea was studied during two breeding seasons (1987–88) in the largest European colony at the Lake of Grand-Lieu (Loire-Atlantique, France). The herons' diet was compared to the available fish population of its main feeding area, the marsh of Bourgneuf (16000 ha) which is composed of former salt pans and meadows drained by a dense network of shallow ditches. This study is the first attempt to assess the predation exerted by an ardeid colony on a fish community over such an extensive natural environment. It also provides the first data about the abundance and the structure of fish communities in shallow coastal dyked marshes. For this purpose, two different sampling methods were used according to the water's salinity. In fresh waters, electrofishing was used as the removal method, and density estimates were calculated with Carle & Strub estimator (1978). Fish were caught in randomly selected stations (sections of ditches enclosed by two 5 mm mesh nets). In brackish waters, pools and ditches were drained. The distribution of the herons at the feeding areas was determined by direct observations, by counting flights from the colony, and by radio-tracking. The diet was investigated by observing adult herons on the foraging areas, and by analyzing the prey regurgitations of the young at the nests. The global food consumption was assessed from Marion (1988), according to the birds' activity determined during 5 years of radio-tracking. Altogether, at least 39 species of fish were available in the herons' feeding area (during the reference period, 87–88) and the mean fish biomass was 270 kg per ha of open water, or 30 kg per ha of marsh (open water = 11.2% of the marsh area). The fish community was dominated by eel Anguilla anguilla (145 kg ha^–1, 50,8% of the total biomass), and catfish Ictalurus melas (40 kg ha^–1, 14%). Except for small and inaccessible species (living in the deepest parts of the marsh), heron diet was very similar to fish species composition of the community occurring in the marsh. The catfish was the species captured most frequently by the heron (45% of the mass), the eel was second with 28% of the mass. The catfish was probably over represented in the diet considering that they are caught in catfish-dumps created by professional fishermen at Grand-Lieu lake, in order to reduce the density of this undesirable species. Inversely, small species such as Gasterosteus aculeatus were not found in the diet whereas they are very numerous in the marsh. On average herons of Grand-Lieu colony catch 1.92 kg of fish per ha of marsh (6% of the fish standing crops in the marsh) during the breeding season, the main predation period.     

Comparative Allometric Growth of the Mimetic Ephippid Reef Fishes Chaetodipterus faber and Platax orbicularis

Mimesis is a relatively widespread phenomenon among reef fish, but the ontogenetic processes relevant for mimetic associations in fish are still poorly understood. In the present study, the allometric growth of two allopatric leaf-mimetic species of ephippid fishes, Chaetodipterus faber from the Atlantic and Platax orbicularis from the Indo-Pacific, was analyzed using ten morphological variables. The development of fins was considered owing to the importance of these structures for mimetic behaviors during early life stages. Despite the anatomical and behavioral similarities in both juvenile and adult stages, C. faber and P. orbicularis showed distinct patterns of growth. The overall shape of C. faber transforms from a rounded-shape in mimetic juveniles to a lengthened profile in adults, while in P. orbicularis, juveniles present an oblong profile including dorsal and anal fins, with relative fin size diminishing while the overall profile grows rounder in adults. Although the two species are closely-related, the present results suggest that growth patterns in C. faber and P. orbicularis are different, and are probably independent events in ephippids that have resulted from similar selective processes.     

Diet partitioning in sympatric Atlantic salmon and brown trout in streams with contrasting riparian vegetation

Prey intake by Atlantic salmon Salmo salar and brown trout Salmo trutta was measured across different riparian vegetation types: grassland, open canopy deciduous and closed canopy deciduous, in upland streams in County Mayo, Western Ireland. Fishes were collected by electrofishing while invertebrates were sampled from the benthos using a Surber sampler and drifting invertebrates collected in drift traps. Aquatic invertebrates dominated prey numbers in the diets of 0+ year Atlantic salmon and brown trout and 1+ year Atlantic salmon, whereas terrestrial invertebrates were of greater importance for diets of 1+ and 2+ year brown trout. Terrestrial prey biomass was generally greater than aquatic prey for 1+ and 2+ year brown trout across seasons and riparian types. Prey intake was greatest in spring and summer and least in autumn apart from 2+ year brown trout that sustained feeding into autumn. Total prey numbers captured tended to be greater for all age classes in streams with deciduous riparian canopy. Atlantic salmon consumed more aquatic prey and brown trout more terrestrial prey with an ontogenetic increase in prey species richness and diversity. Atlantic salmon and brown trout diets were most similar in summer. Terrestrial invertebrates provided an important energy subsidy particularly for brown trout. In grassland streams, each fish age class was strongly associated with aquatic, mainly benthic invertebrates. In streams with deciduous riparian canopy cover, diet composition partitioned between conspecifics with older brown trout associated with surface drifting terrestrial invertebrates and older Atlantic salmon associated with aquatic invertebrates with a high drift propensity in the water column and 0+ year fish feeding on benthic aquatic invertebrates. Deciduous riparian canopy cover may therefore facilitate vertical partitioning of feeding position within the water column between sympatric Atlantic salmon and brown trout. Implications for riparian management are discussed.     

Quantitative field investigations of feeding and territorial behaviour of young-of-the-year brook charr,Salvelinus fontinalis

Synopsis Direct observations of young-of-the-year brook charr, Salvelinus fontinalis, in a second-order woodland stream indicated that most of their feeding effort was directed toward sub-surface, drifting prey (83% of feeding time). Feeding from the substrate and water surface was much less frequent (17% of feeding time). Comparisons of gut contents to drift net and substrate fauna samples corroborated that the most commonly consumed prey (chironomid and trichopteran larvae, ostracods, and ephemeropteran nymphs) were captured primarily from sub-surface, invertebrate drift. The disproportionate numbers of some prey species in the guts of several fish indicate that some prey selection occurred. Territories appeared to be cardioid-shaped, and were often contiguous, with dominance hierarchies evident among the residents. Agonistic interactions were frequent. Charges and chases predominated (91% of interactions) while lateral displays were infrequent (9% of interactions). Overall, these fish spent most of the daylight hours station-holding (77%) and feeding (18%). While only 3% of total time was spent in aggression, this amounted to 14% of the time a fish spent away from its station. There was some indication that territories were defended at a cost of feeding time.     

Feeding rhythm in relation to changing patterns of pH in the gut of Sarotherodon galilaeus (Artedi) of Lake Kainji,Nigeria

Different sizes of Sarotherodon galilaeus of Lake Kainji were caught at different hours to determine their feeding time and feeding intensity in relation to pH. It was found that the fish fed with different intensity during the day, and there was no feeding during the night. The pH values were found to vary with the amount of food in the gut.