Dense Schools Research Articles

Acoustic identification and density estimate of Diaphus theta (myctophidae) off Hokkaido, Japan

Diaphus theta is the most abundant micronektonic fish in the northwestern Pacific. Field acoustic data and biological samples were obtained both day and night off of eastern Hokkaido, Japan in September 2006 to estimate the fish density and diel changes of its vertical distribution. Differences of acoustical target strength (TS) values between at 38 and 120 kHz were applied to identify D. theta echos using field echo data. Theoretical estimation using swimbladder acoustic scattering models showed that the TS difference (TS 120-38 kHz) of larger fishes (over 60 mm) was between −4 and 1 dB; that for smaller fishes (less 60 mm) was less than .4 dB. These values differed from those of other major component species of the deep-scattering layer, such as krill and pollock, suggesting that the echo from D. theta is acoustically identifiable. Diel vertical distributions of D. theta were estimated after identification. Relatively dense schools were observed at around 400-m depth during daytime, although the schools were scattered widely above 100-m depth during nighttime. These results matched those obtained by biological sampling. In the surface layer (above 100 m), the estimated fish densities were about tenfold higher at nighttime than in daytime.

ICELANDIC KILLER WHALES ORCINUS ORCA USE A PULSED CALL SUITABLE FOR MANIPULATING THE SCHOOLING BEHAVIOUR OF HERRING CLUPEA HARENGUS

ABSTRACT Icelandic and Norwegian killer whales feed on herring, after debilitating them with underwater tail slaps. We analysed sound recordings of Icelandic and Norwegian killer whales engaged in feeding and other behaviour. We describe a pulsed call made by Icelandic killer whales shortly before underwater tail slaps, which had an atypical low frequency (average peak frequency: 683 ±131 Hz), long duration (3.0 ±1.1 s) and high intensity (source level 169–192 dB pp re 1 μPa @ 1 m). The low-frequency emphasis of this call was below the most sensitive hearing range of killer whales, suggesting that the call may not be optimal for interspecific communication. However, herring could easily perceive the killer whale call since the frequency content is similar to the resonant frequency of their swim bladder as well as to the most sensitive frequency band of hearing in this species. Previous studies have shown that sound may cause schooling herring to cluster. A high density of herring in a school would increase the effectiveness of the underwater tail slaps. We suggest that some Icelandic killer whales use this low-frequency call to herd herring into dense schools immediately before delivering an underwater tail slap, thereby increasing their foraging success.

Stilt-Root Subsistence: Colonial Mangroves and Brazil’s Landless Poor

Stilt-Root Subsistence: Colonial Mangroves and Brazil’s Landless Poor

Star acoustic surveys of localized fish aggregations

“Stars” are an alternative design for acoustically acoustically surveying isolated fish aggregations in which transects cross at a centre point over an aggregation to form a star or wheel-spoke pattern. The method was devised particularly for aggregations of orange roughy ( Hoplostethus altanticus Collett) to cope with the practical difficulties of manoeuvring a vessel towing a transducer over a small, dense school of fish. Several methods of analysing star patterns were considered including kriging and a method that ignored the spatial arrangement of the transects. However, simulations of roughy aggregations showed that the best approach was to transform from Cartesian to polar coordinates and then use standard statistical methods (polar method). The polar method was robust to shifts in the transect centre off the aggregation centre and aggregation movement in a random way. Variance estimation was best with a polar version of transitive kriging. Stars using the polar method were also better than the usual parallel transect design when transect numbers were low but the results were similar when six or more transects were used. However, in all the cases considered parallel transects consistently overestimated the variance. We conclude that star transects offer a robust and effective way of estimating the biomass of small, localized aggregations of fish that minimizes vessel time, and yields good precision.

Characterizing uncertainty in target-strength measurements of a deepwater fish: orange roughy (Hoplostethus atlanticus)

Abstract The variability of ensemble 38 kHz, target-strength (TS38) estimates for orange roughy (Hoplostethus atlanticus) (4.9 dB, factor of 3.1) in deep water (>600 m) limits the use of echo integration for absolute-biomass estimates. Orange roughy are high in oil content, have a wax-ester swimbladder, and show an active-avoidance response to sampling gear. The interpretations of ensemble, in situ target strengths of orange roughy (range 〈TS38〉=−52.9 to −51.0 dB for standard fish length 〈SL〉 = 35 cm) are lower than previous model and surface-based measurements (〈TS38〉 = −48 dB, SL = 35 cm). In situ TS measurements from individuals on the periphery of dense schools were processed to minimize uncertainties from single-target selection criteria, species composition, and active avoidance. Video and acoustic-tracking data quantified the variability in TS measurements arising from the variability in fish orientation. Multi-frequency acoustics and fish tracking are used to quantify in situ TS variability due to species identification and fish density. The Kirchhoff-ray-mode backscatter model was used to illustrate the sensitivity of species-specific backscatter to assumptions of tilt-angle and material properties (density and sound-speed contrasts). We conclude that a remaining source of uncertainty for in situ TS measurements is the assumption that dispersed targets are representative of the survey population.

Diurnal changes in the acoustic-frequency characteristics of Japanese anchovy (Engraulis japonicus) post-larvae “shirasu” inferred from theoretical scattering models

Abstract Diurnal changes in the acoustic-frequency characteristics of “shirasu”, a post-larval stage of Japanese anchovy (Engraulis japonicus), were analysed based on theoretical scattering models. Since post-larval swimbladders contain gas at night but not during the day, the distorted-wave Born approximation (DWBA), based on the deformed-cylinder model, which assumes there is no gas in the swimbladder, was applied to daytime observations, and the fish-bladder resonance model, which assumes there is gas in the swimbladder, to those taken at night. The two sets of acoustic frequencies employed were 50 and 200 kHz, which are those used for shirasu commercial fishing, and 38 and 120 kHz, which are usually employed for acoustic surveys using a quantitative echosounder. During the daytime, differences in shirasu target strengths between the two frequencies were large, especially between 38 and 120 kHz, but at night there was little difference with frequency, suggesting that acoustic surveys for identifying and estimating the abundance of shirasu should be done during daytime, when differences occur between frequencies and the shirasu form dense schools.

Digging in the deep: killer whales' advanced hunting tactic

We document one of the most spectacular predator-prey interactions observed in a marine ecosystem, in which groups of killer whales force tens of tons of herrring out of their safe deep-water habitat to the surface where the whales are superior. Killer whales performing this hunting tactic congregate in large groups, dive to the limit of their capacity, lift herring vertically more than 150 m by coordinated action, and split large aggregations of fish into small, dense schools before attacking them.

Diel vertical and horizontal distribution of crustacean zooplankton and young of the year fish in a sub-alpine lake: an approach based on high frequency sampling

Understanding the spatial dynamics of predators and their prey is one of the most important goals in aquatic ecology. We studied spatial and temporal onshore-offshore distribution patterns in young of the year (YOY) Eurasian perch (Perca fluviatilis) and crustacean zooplankton (Daphnia hyalina, Cyclops prealpinus) along a transect in Lake Annecy (France). Our study represents a first attempt at coupling hydroacoustic fish survey and high frequency zooplankton recording to assess simultaneously the large-scale distribution patterns of YOY fish and their zooplankton prey over a diel cycle (day, dusk and night sampling). We hypothesized that the spatial distribution of zooplankton could be shaped by both anti-predator behaviour (horizontal and vertical migrations) and predation losses. Fish biomass, size structure and diet were assessed from split-beam echosounding and net trawling samples, whereas crustacean abundances were estimated with a small modified Longhurst-Hardy continuous plankton recorder. We evaluated the diel changes in the spatial distribution patterns of fish and zooplankton and determined the overlap between their distributions. Fish biomass was dominated by YOY perch in upper warmer layers and salmonids (Coregonus lavaretus and Salvelinus alpinus) in the colder and oxygenated deep layers. YOY perch were aggregated in dense schools in the epilimnion during the day and dispersed at night. Fish biomass was distributed along a strong increasing onshore-offshore gradient at night, whereas crustacean prey showed a decreasing gradient. This onshore-offshore negative gradient in crustacean distribution, expressed on a shorter scale during the day, shifted toward the surface waters at night. A distinct kinetic of diel vertical migration (DVM) patterns was exhibited by daphnid and cyclopoid populations and resulted in distinct vulnerability to perch predation. Spatio-temporal distribution of crustaceans in Lake Annecy during the diel cycle study was probably shaped both by predation loss to YOY perch and by anti-predator behaviour (DVM, DHM) by zooplankton. The implications for fine-scale studies of fish-zooplankton interactions are discussed.

Exploitation of small tunas by a purse-seine fishery with fish aggregating devices and their feeding ecology in an eastern tropical Atlantic ecosystem

We investigated the effects of a purse-seine fishery with drifting fish aggregating devices (FADs) in the South Sherbo area of the Equatorial Atlantic, located between 0–5N and 10–20W. There had been no surface fishing activity in the area until 1975. Since 1991, fishing operations on schools of tuna associated with FADs has become widespread and this offshore area has developed into a major fishing zone. Exploitation rates are high between November and January. The fishery exploits multispecies concentrations of skipjack (71%), bigeye (15%), and yellowfin (14%) tunas of similar size (mode: 46-cm forklength). The use of FADs increased the vulnerability of small tunas and induced changes in fishing patterns. The mean individual weight of skipjack caught has decreased since 1991, due either to overfishing or to a growth change. Data from scientific observers were used to estimate discards and by-catches generated by FAD fishing during 1998. Discards of tunas (including frigate and little tunas) represented 7.6% of the total catch. Other by-catch (dominated by wahoo, billfish, triggerfish, sharks, barracudas, and dolphinfish) represented 2.3%, including 0.4% discarded at sea. Stomach content analysis showed that a mesopelagic species, Vinciguerria nimbaria (Photichthyidae), which during daylight concentrated in the upper layers in dense schools, was the main prey of all small tunas. The South Sherbro area appears to have exceptional environmental conditions. It is suggested that the FAD fishery may have wide-ranging effects on the migration of tunas in general and on the productivity of the skipjack population in particular.

Trawl sampling of small pelagic fish off Angola: effects of avoidance, towing speed, tow duration, and time of day

Pelagic fish distributed near the surface oV Angola were sampled with a medium-sized pelagic trawl having a vertical opening of about 20 m. Numerous trawls targeting on dense sardinella schools (Sardinella maderensis and Sardinella aurita) failed to yield large catches. Schools that were at the trawl depth as the vessel passed over them were often observed far below the foot rope of the trawl, indicating vertical avoidance. The net sonde transducer often recorded schools in the net mouth for long periods: it was assumed that these fast-swimming fish were able to swim along with the trawl, and escaped when the towing speed was reduced during hauling. No correlations were found, however, between towing speed (2.7‐5.0 knots) and the total catch of sardinella. Increasing the two duration during the day to more than 2 h also failed to yield consistently higher catches. A negative correlation at night between tow duration (15‐60 min) and catch size arose simply because the trawl was towed for a longer time to obtain adequate samples in areas with scattered recordings. During the day, sardinella was caught both inshore and further oVshore, while horse mackerel (Trachurus trecae and Trachurus capensis) was caught further oVshore only. Both species were well represented in catches made at night. There were substantial day and night diVerences in the length distributions of sardinella, with small sardinella being caught only at night. ? 1999 International Council for the Exploration of the Sea

Size-related habitat use and schooling behavior in two species of surgeonfish (Acanthurus bahianus and A. coeruleus) on a fringing reef in Barbados, West Indies

Surgeonfish (Acanthuridae) are prominent, herbivorous members of coral reef communities that occur as dispersed individuals and small, loose groups ('non-schooling fish') or as members of large, highly aggregated, mixed-species schools ('schooling fish'). We examined the relationships among fish size, habitat use and schooling in two species of surgeonfish on a fringing reef in Barbados, West Indies. Both ocean surgeonfish, Acanthurus bahianus, and blue tangs, A. coeruleus, appeared to show ontogenetic habitat shifts. The density of juvenile ocean surgeonfish was highest in the back reef (inshore), lower on the reef crest (intermediate) and lowest in the spurs and grooves (offshore) zone, but schooling adults were most abundant in the spurs and grooves zone. In a multiple regression considering the effects of depth, algal cover, rugosity and distance from shore, the density of non-schooling ocean surgeonfish was positively associated with percent algal cover on the substratum and negatively with distance from shore. Newly settled blue tangs occurred only in the reef crest and spurs and grooves zones, but larger juveniles were more common in the back reef, while adults were more evenly distributed across zones. The density of non-schooling blue tang was positively associated with rugosity, distance from shore, and percent algal cover. In both species, schooling occurred primarily in adults; small juveniles never participated in the large, dense schools. The proportion of adults that were schooling increased from the back reef to the reef crest to the spurs and grooves zone. These results are consistent with the hypothesis that schooling permits adult surgeonfish access to higher quality food in the territories of damselfish (Pomacentridae) that predominate on the reef crest and spurs.

An integrated approch to the foraging ecology of marine birds and mammals

Birds and mammals are important components of pelagic marine ecosystems, but our knowledge of their foraging ecology is limited. We distinguish six distinct types of data that can be used in various combinations to understand their foraging behavior and ecology. We describe methods that combine concurrent dive recorder deployment, oceanographic sampling, and hydroacoustic surveys to generate hypotheses about interactions between the physical environment and the distribution, abundance, and behavior of pelagic predators and their prey. Our approach is to (1) map the distribution of whales in relation to the distribution of their prey and the physical features of the study area (bottom topography, temperature, and salinity); and (2) measure the foraging behavior and diet of instrumented whales in the context of the fine-scale distribution and composition of their prey and the physical environment. We use this approach to demonstrate a relationship between blue whale distribution, sea surface temperature, and concentrations of their euphausiid prey at different spatial scales offshore of the Channel Islands, California. Blue whale horizontal spatial distribution was correlated with regions of high acoustic backscatter. Blue whale dive depths closely tracked the depth distribution of krill. Net sampling and whale diet revealed that whales fed exclusively upon dense schools of Euphausia pacifica (between 100 and 200 m) and Thysanoessa spinifera (from the surface to 100 m). Whales concentrated foraging efforts upon those dense euphausiid schools that form downstream from an upwelling center in close proximity to regions of steep topographic relief. We propose that (1) the distribution of Balaenoptera whales in the coastal California Current region is defined by their attraction to areas of predictably high prey density; (2) the preferred prey of these whales are several species of euphausiids ( E. pacifica, T. spinifera, and N. simplex) that are abundant in the California Current region; (3) blue whales concentrate their foraging efforts on dense aggregations of euphausiids found at discrete depths in the water column; (4) these localized areas of high euphausiid densities are predictable and sustained by enhanced levels of primary productivity in regions which are located downstream from coastal upwelling centers (indicated by sea surface temperature); (5) topographic breaks in the continental shelf located downstream from these upwelling centers work in concert with euphausiid behavior to collect and maintain large concentrations of euphausiids swarms, and (6) despite seasonal and inter-annual variability, these processes are sufficiently consistent that the distribution of Balaenoptera whales can be predicted.

Population dynamics of a dense assemblage of marine detritivores

Accumulations of macrophyte detritus provide food and refuge for a dense (up to 3.5·10 6 ind. m −2) assemblage of amphipod and leptostracan crustaceans in a Southern California submarine canyon. The objective of this work was to determine how the physical environment and biological interactions limit these populations, that rarely, if ever, appear to be food limited. Preliminary observations suggested that the detritus-crustaceans would incur significant losses during the Winter, resulting from wave disturbance. Predation by dense schools of fishes associated with this habitat also appeared to be important. Laboratory and field experiments demonstrated that the detritus provided an effective refuge for the crustaceans. Predation rate of fishes upon detritus-associated crustaceans increased as the density of prey populations increased and/or as the thickness of the detritus mat was reduced (increasing number per m 3 but not per m 2). Density (m −2) of the mat crustaceans fluctuated seasonally, being greatest in the Winter and Spring following storms that eliminated large portions of the habitat. Few animals appeared to be directly eliminated by the storm disturbance. The important effect of storms was to reduce the quantity and quality of the detritus refuge by concentrating the crustaceans into smaller patches with less detrital cover. During the calm summer months the bacterium Beggiatoa sp. spread out over large portions of the detritus mat. Beneath the bacteria oxygen concentration was reduced, and infaunal density was two orders of magnitude lower than in unaffected portions of the mat. The summer increase in bacterial cover constituted a biological disturbance that functionally reduced the habitat area available to the mat fauna and left them more vulnerable to predation. Secondary production in the detrital mats is among the highest reported from natural environments. This is possible because the mat crustaceans are unable to graze down their food supply (detritus and associated microbes). The refuge provided by the detritus for large populations of invertebrates also is crucial; larger refugia translated into increased carrying capacity which allowed both greater population sizes and production, much of which became available to fishes. Most marine ecosystems are supported by allochthonous material that enters food webs via detritivores. Seafloor features that collect detritus constitute an important source of patchiness over a wide range of scales because detrital delivery is governed by physical processes. The La Jolla/Scripps Canyon system, by accumulating organic debris, provides a large food source from shallow to continental slope depths. This resource supports large numbers of fishes and presumably increases local production in higher trophic levels.

Bias in Hydroacoustic Estimates of Fish Abundance due to Acoustic Shadowing: Evidence from Day–Night Surveys of Vertically Migrating Fish

We investigated the hypothesis that fish schooling behavior leads to underestimation of fish abundance when assessed by acoustic methods. Current methods for acoustic quantification of fish abundance rely on the assumption that fish biomass and numerical fish abundance are linearly related to acoustic scattering under all natural fish densities. However, cage experiments as well as field observations have indicated that acoustic shadowing effects occur at very dense and large aggregations of fish. Acoustic surveys of the pelagic fish community of Lake Memphremagog, Quebec, were conducted when fish were aggregated and dispersed. These differences in aggregation were related to diel responses to light levels. We found that estimates of fish abundance, as measured by echo integration, were consistently and significantly lower when fish were aggregated in dense schools. This bias was not due to diel changes in average echo level per fish, which exhibited no relationship to diel changes in vertical and hortizontal distributions. We conclude that the reduced abundance estimates obtained when fish were aggregated resulted from acoustic shadowing. Our data suggest that this bias may be as large as 50%.

若狭湾西部海域におけるタチウオの漁業生物学的研究 ＶＩＩＩ 若狭湾西部海域におけるタチウオの日周期的鉛直移動

To clarify diurnal movements of a ribbon fish Trichiurus lepturus, exploratory fishing expedi-tions utilizing handline, longline and observations through fish finder were made at the mouth of Maixuru Bay, the western Wakasa Bay, on 5 and 8 June, 1985. Hourly angling and observations of echo traces of T. lepturus indicated that the fish signicantly showed diurnal vertical migration.T. lepturus mainly distributed from middle tosurface layers and made scattering school in nighttime. T. lepturus descended to bottom layer until just before sunrise, and made dense school at bottom layer in daytime. T. lepturus ascended and scattered from middle to surface layers in twilight. Feeding activity of T. lepturus was increased in twilights. Diurnal movements of T. lepturus synchronized with that of anchovy Engraulis japonicus which was thelr main food in this area. This sychronizing might brought T. lepturus to increase chances of encounter wih the food. Bottom longliners at this area also synchronized their managements with diurnal movomots of the fish to effeciently catch them.

Aspects of Schooling in Euphausia Superba

ABSTRACT Divers observed schools of Euphausia superba off the South Shetland Islands and in Gerlache Strait near the Antarctic Peninsula. Euphausiids almost always occurred within the confines of dense schools swimming in one direction. Schools formed an infinite variety of shapes but usually were narrow in at least one dimension, so that individuals within a school were never far from its edge. Schools moved at about 20 cm/s, primarily horizontally, and vertical displacement was always accompanied by much larger horizontal excursions. Opaque, presumably necrotic, euphausiids occurred within schools, indicating that schooling may have epidemiologic disadvantages. Schools exhibited a variety of behavioral responses to potential and actual predators (sea birds and whales). A hypothesis of ontogenetic differences in schooling behavior proposes that juveniles migrate long distances horizontally in search of food over both shallow and deep waters, but that schools of adults spawn in or near deep water so that rapidly sinking eggs are not deposited on the sea bed.

Effects of second-order scattering on high resolution sonars

To achieve simultaneous high volume coverage and high spatial resolution, sonars employing multiple narrow steered receiver beams in conjunction with a single wide transmitter beam are quite often used. The effects of second-order scattering on multibeam sonars have been calculated. The analysis is in terms of the backscattered energy (or ‘‘integrated echo’’) due to clouds of point scatterers. The work applies to scattering such as from clouds of bubbles or dense schools or ‘‘swarms’’ of swimbladder-bearing fish. Under certain conditions second-order scattering will increase the echo energy an appreciable amount, especially as the ratio of transmitter beamwidth to receiver beamwidth increases. The major conditions are (1) dense clouds of nondirectional scatterers are insonified, (2) the sonar signal suffers measurable attenuation due to the extinction cross section of the scatterers, and (3) the absorption cross section of the scatterers is small. The echo energy due to both first- and second-order scattering are calculated for several beamwidth ratios. It is shown that the backscattered energy is increased by up to 100% due to second-order scattering.

Behavior of Antarctic Krill, Euphausia superba: Chemoreception, Feeding, Schooling, and Molting

Krill do not feed by passive, continuous filtration but use area-intensive searching and various rapid feeding behaviors to exploit local high food concentrations. Chemicals alone at low concentrations, not particles, trigger feeding. Krill form dense schools that move rapidly and migrate primarily horizontally. Abrupt disruption of a school can trigger mass molting, and molts may act as decoys.

Evolution of electric communication channels of mormyrids

1. Field studies in the Ivindo River district of Gabon, West Africa, yielded information on the ecology and the electric discharge characteristics of 23 species of mormyrid fishes. The electric organ discharge (EOD) acts as an electric signature that defines the properties of an electric communication and electrolocation channel. Correlations between the properties of this signature and the ecological and social conditions of the fish in their natural environment suggest that signature properties are under natural selection, much as are social communication displays. 2. EOD duration is reduced in those species that aggregate in dense schools. Those with long-duration discharges are widely spaced in stream habitats. EOD waveforms are usually species-specific. EODs are highly divergent within some genera, but convergence to the same EOD pattern is also found among schooling species. Species producing monophasic EODs inhabit rivers rather than small streams. A river habitat is evenly represented by fish species with EODs at all frequencies in the available spectrum. Stream habitats are represented mainly by species with shortduration pulses that emphasize the high-frequency end of the available spectrum.

Aerial Observation of Feeding Behavior in Four Baleen Whales: Eubalaena glacialis, Balaenoptera borealis, Megaptera novaeangliae, and Balaenoptera physalus

Distinct behavioral differences were noted from aerial observations of four species of baleen whales (Eubalaena glacialis, right whale; Balaenoptera borealis, sei whale; Megaptera novaeangliae, humpback whale; Balaenoptera physalus, finback whale) feeding together on 30 April and 1 May 1975. The right and sei whales fed together on patches of plankton. Right whales fed steadily with mouths open in the densest areas, while the sei whale followed a faster but more erratic path through the patches, alternately opening and slowly closing its mouth with slight throat distension at each closing. Humpback and finback whales fed together on dense schools of fish associated with the patches of plankton. The humpback fed by rushing, generally from below the schools of fish, while finback feeding was by more horizontal passes sometimes characterized by sharp turns and rolls within the fish schools and often with enormous throat distension.