Large Shoals Research Articles

Using a multi-criteria approach to assess post-release recovery periods in behavioural studies: study of a fish telemetry project in the Seine Estuary

Biotelemetry has many advantages for monitoring fish behaviour. However, the accuracy of results can be impacted by changes in fish behaviour following tagging and other forms of human intervention. Different fish take different amounts of time to return to normal behaviour patterns. This recovery period is often difficult to assess. In many studies, it is simply ignored, while in others an assumed duration is used. This assumption is rarely based on objective criteria. To address this challenging aspect of fish telemetry, a multi-criteria stepwise approach was developed based on complementary criteria obtainable through prior knowledge of the normal behaviour of studied species (home range, diel rhythm, homing, shoaling, migration…). It was applied to the case study of an acoustic telemetry project in the Seine Estuary (France) for three estuarine species exhibiting contrasted ecological traits: European eel Anguilla anguilla (Linnaeus 1758), thin lipped grey mullet Liza ramada (Risso 1827) and bream Abramis brama (Linnaeus 1758). Taking into account the particular traits of the species studied, we used the following three criteria: time to return to core area of activity, time to return to rhythmic activity, and time to return to site of capture. Post-release periods of recovery varied greatly between species. The median value was 10 days for eel, 25 days for mullet, and 1 day for bream. During this period, eels moved very little and the schedule pattern presented a diel rhythm with most detections occurring at night. All mullet exhibited rapid downstream trajectories after release, with larger distances covered during the ebb. Only five individuals returned later to the study site. This behaviour turns out to be not only an effect of post-release stress, but also the result of normal shifts in feeding habitat use by large shoals of mullet. Common bream exhibit very short periods of recovery with strong site fidelity. Most of the individuals of the different species (72%) return to their site of capture. The approach allows the identification of individual periods of recovery specific to the species and environment being studied. It maximises the amount of conserved data representing normal behaviour and can be implemented with various types of tracking data. Analysis of this period provides additional information about the stress response of species and their associated behaviour.

Spatial structure and distribution of small pelagic fish in the northwestern Mediterranean Sea.

Understanding the ecological and anthropogenic drivers of population dynamics requires detailed studies on habitat selection and spatial distribution. Although small pelagic fish aggregate in large shoals and usually exhibit important spatial structure, their dynamics in time and space remain unpredictable and challenging. In the Gulf of Lions (north-western Mediterranean), sardine and anchovy biomasses have declined over the past 5 years causing an important fishery crisis while sprat abundance rose. Applying geostatistical tools on scientific acoustic surveys conducted in the Gulf of Lions, we investigated anchovy, sardine and sprat spatial distributions and structures over 10 years. Our results show that sardines and sprats were more coastal than anchovies. The spatial structure of the three species was fairly stable over time according to variogram outputs, while year-to-year variations in kriged maps highlighted substantial changes in their location. Support for the McCall's basin hypothesis (covariation of both population density and presence area with biomass) was found only in sprats, the most variable of the three species. An innovative method to investigate species collocation at different scales revealed that globally the three species strongly overlap. Although species often co-occurred in terms of presence/absence, their biomass density differed at local scale, suggesting potential interspecific avoidance or different sensitivity to local environmental characteristics. Persistent favourable areas were finally detected, but their environmental characteristics remain to be determined.

Towards of a firmer explanation of large shoal formation, maintenance and collective reactions in marine fish

Avoiding predation is generally seen as the most common explanation for why animals aggregate. However, it remains questionable whether the existing theory provides a complete explanation of the functions of large shoals formation in marine fishes. Here, we consider how well the mechanisms commonly proposed to explain enhanced safety of group living prey explain fish shoals reaching very large sizes. By conceptually re-examining these mechanisms for large marine shoals, we find little support from either empirical studies or classical models. We address first the importance of reassessing the functional theory with predator-dependent models and the need to consider factors other than predation to explain massive fish shoals. Second, we argue that taking into account the interplay between ultimate benefits and proximate perspectives is a key step in understanding large fish shoals in marine ecosystems. Third, we present the growing body of evidence from field studies that identify shoal internal structure as an important feature for how large shoals can form, maintain and react as a coordinated unit to external stimuli. In particular, we consider a mechanistic basis of local rules of interaction for group formation and collective dynamic properties that can account for groups reaching very large sizes. Recent research in collective animal behaviour has shifted focus from the importance of global properties (group size) to local properties (local density and information transfer). In contrast to studies of fish shoals in the laboratory, the difficulty in measuring behaviour in large shoals in marine systems remains a major constraint to further work. Advances in acoustical observation have shown the greatest potential to provide data that can link proximate mechanisms in, and ultimate functions of, large marine fish shoals.

Predator driven diel variation in abundance and behaviour of fish in deep and shallow habitats of an estuary

Our traditional understanding of the behaviour of large predatory fish and their smaller prey in estuarine ecosystems is often restricted by different gear types and visibility. In this study we determined the diel distribution and inferred movements of fish in an estuary in shallow and deep habitats (<1 m and 4 m deep respectively), using an unbaited acoustic camera (DIDSON). Baitfish (<100 mm TL) formed small and large shoals during the day in both shallow and deep habitats, compared to loose aggregations during the night or when they were inactive and not observed. Three larger size classes of fish (small, 100–300 mm Total Length (TL); medium, 301–500 mm TL and large >500 mm TL) were also more abundant during the day, likely due to general higher activity. This coincided with predatory activity with attacks by larger fish (301–500 mm and >500 mm) witnessed during the day but not at night. This heightened activity is the likely cause for changes in the schooling behaviour of the baitfish. The proportion of medium and large fish in the shallow habitat at night increased by over 50% as they moved from deeper areas of the estuary, showing the abundance of large predators in shallow water can be related to diel period. This highlights the pervasive top down influence even small numbers of predators can exert on the behaviour and distribution of estuarine fish assemblages.

Context-dependent group size preferences in large shoals of three-spined sticklebacks

Grouping behaviour is widespread in animals. One important reason for grouping is the reduction of individual predation risk; the larger a group, the greater the protection for the individual. Fishes, in particular, have become a model taxon in experimental research to study proximate and ultimate causes of grouping. Accordingly, numerous studies have so far demonstrated that fishes prefer to shoal with larger groups. Thus far these studies have usually examined small groups, with up to 20 individuals. However, in nature groups are often much bigger (up to several hundreds of individuals), and theory predicts that benefits, for example due to dilution effects, decline exponentially with increasing group size. Furthermore, discrimination might be absent because of limited cognitive ability. Thus, it is essential to test whether the findings from small groups also apply to large groups. Here, we examined group size preferences in the three-spined stickleback, Gasterosteus aculeatus, a small fish that forms large shoals in nature. In five experiments, subadult sticklebacks were given the choice between two shoals differing in group size (numerical contrasts: 15 versus 60, 20 versus 60, 30 versus 60, 40 versus 60 and 50 versus 60). Test fish on average preferred the larger group; this preference was stronger in the beginning of the respective trial and decreased over time. Moreover, preferences for the larger shoal decreased with decreasing group size differences, implying context-dependent preferences. We found significant discrimination up to numerical contrasts of 40:60. Our results are in accordance with the findings of shoal size discrimination in small groups and with optimality hypotheses, but might also reflect the impact of cognitive constraints.

Interspecific Aggressive Behaviour of Invasive Pumpkinseed Lepomis gibbosus in Iberian Fresh Waters

Pumpkinseed Lepomis gibbosus (L.) are successful invaders in Europe, where this species exerts multiple ecological effects, mainly through trophic interactions. Behavioural interference represents a potential impact for native fauna and this is of particular conservation concern in the Iberian Peninsula because of the highly valuable endemic fauna inhabiting streams of this region. However, aggressive interactions have not previously been examined under natural conditions in Iberian fresh waters. To address this gap in knowledge, the aim of the present study was to assess the effect of pumpkinseed aggression on endemic fauna of an Iberian stream, the River Bullaque (central Spain). In September 2009, we analysed the aggression and environmental contexts of these behavioural interactions by snorkelling: aggressor size, aggression type, shoal size, previous activity to aggression, recipient species, response to aggression, microhabitat structure and prey availability. Small pumpkinseed displayed more threat and fewer pursuit behaviours relative to medium and large individuals, reflecting an ontogenetic behavioural shift from low to high aggression intensity. Small aggressors came from large shoals, with bottom feeding being the most frequently observed activity prior to an aggressive interaction; whereas large pumpkinseed were less gregarious and they were mostly ambulating within the water column prior to aggression. Recipient species of aggression included non-native crayfish and fishes, and more importantly, endemic fishes and frogs. Retreat was the most common response to aggression, irrespective of aggressor size. Small pumpkinseed displayed aggressive behaviours over coarse substrata containing elevated macrobenthos biomass; whereas aggression by large individuals was observed in deeper waters. These findings suggest that small and large pumpkinseed exert a high impact on other stream residents through aggression in competition for food and territory defence, respectively. This study highlights the usefulness of direct observations in the wild for assessing the effects of behavioural interference of invasive fishes on Iberian aquatic communities.

High population density enhances recruitment and survival of a harvested coral reef fish

A negative relationship between population growth and population density (direct density dependence) is necessary for population regulation and is assumed in most models of harvested populations. Experimental tests for density dependence are lacking for large-bodied, harvested fish because of the difficulty of manipulating population density over large areas. We studied a harvested coral reef fish, Lutjanus apodus (schoolmaster snapper), using eight large, isolated natural reefs (0.4-1.6 ha) in the Bahamas as replicates. An initial observational test for density dependence was followed by a manipulation of population density. The manipulation weakened an association between density and shelter-providing habitat features and revealed a positive effect of population density on recruitment and survival (inverse density dependence), but no effect of density on somatic growth. The snappers on an individual reef were organized into a few shoals, and we hypothesize that large shoals on high-density reefs were less vulnerable to large piscivores (groupers and barracudas) than the small shoals on low-density reefs. Reductions in predation risk for individuals in large social groups are well documented, but because snapper shoals occupied reefs the size of small marine reserves, these ecological interactions may influence the outcome of management actions.

Extrinsic and intrinsic factors influence daily activity of a Mediterranean cyprinid

Factors shaping individual behavior remain poorly understood for European cyprinids. We examined the effects of extrinsic (temperature and year) and intrinsic factors (body size and shoal size) on the daily activity of chub Squalius torgalensis, a critically endangered cyprinid found in the Torgal stream, Portugal. We quantified chub behaviors while snorkeling, using instantaneous sampling of focal individuals, in spring 2009 and 2010. We used Principal Component Analysis to describe individual behavior and assessed competing models relating individual behavior to explanatory variables using Akaike's Information Criterion. Year had a strong influence on chub behavior, with individuals spending more time drift foraging and orienting in 2009, whereas in 2010 they spent more time benthic foraging and cruising. Shoal size did not influence behaviors in 2009, but in 2010 individuals in small shoals foraged benthically and searched with greater frequency than those in large shoals. Individuals increased cruising and decreased searching as they grew in length, but the opposite trend was observed as temperature increased. Our results indicate that both extrinsic and intrinsic factors influence daily activity of chub, with individual behaviors varying between years, and depending upon shoal size, body size and temperature. This suggests that chub display high levels of individual behavioral flexibility, which may be important for fish inhabiting harsh and highly variable Mediterranean streams. Conservation efforts for this species will benefit by integrating behavioral information into management strategies.

Number without language: comparative psychology and the evolution of numerical cognition

Number without language: comparative psychology and the evolution of numerical cognition

Quantification abilities in angelfish (Pterophyllum scalare): the influence of continuous variables

Previous studies investigating quantity discrimination have shown that angelfish are able to select the larger of two groups of conspecifics (shoals). The discrimination limits shown by angelfish were similar to those found for other vertebrates when large (≥4) and small quantities (<4) were presented. However, in these studies, no attempt was made to control for non-numerical features of the stimulus shoals and thus the question whether numerical or some quantitative attributes of the shoals were utilized for making the choices could not be answered. Here, we investigate whether angelfish can discriminate between shoals differing in numerical size using non-numerical attributes. We systematically manipulate density, inter-fish distance, and overall space occupied by the shoals, one factor at a time, and analyse the choices angelfish made between the contrasting stimulus shoals. The stimulus shoals consisted of contrasts between large (10 vs. 5) and small (3 vs. 2) number of conspecifics. We found density to be a sufficient condition for discrimination between large shoals as the test subjects preferred the more dense shoal. Manipulation of inter-fish distance indicated that this variable is not a necessary factor in discrimination at either shoal size contrast. Likewise, we found that the size of space occupied by the contrasted shoals also did not significantly influence discrimination. Sensitivity to the density of large shoals indicates that angelfish can discriminate shoal size using this non-numerical cue. Nevertheless, the factors we examined may represent only a subset of all possible non-numerical features upon which angelfish may base their discrimination. Thus, we suggest that further research is required to clarify whether and under what circumstances angelfish may use numerical or non-numerical features when discriminating between shoals of differing size.

Individual sociability and choosiness between shoal types

In social species, individual and group fitness and social dynamics in groups often depend on group size and on the group's social composition (e.g. the mix of personality types within groups). In turn, the size and social composition of groups is an emergent outcome of the grouping tendencies of individuals. While grouping behaviour has often been studied at a species level, within-species variation in grouping tendency has rarely been studied. We examined the role of personality type in shoaling preferences in a social fish, the western mosquitofish, Gambusia affinis. After scoring individuals for their social personality types (sociability), we gave individuals the choice to associate with shoals that differed in size and in the average sociability of individuals in the shoal. Shoal preferences depended on both the individual's sociability and on shoal size and composition in sociability types, and on the interaction between the individual's sociability and shoal size and composition. On average, fish preferred large to small shoals and groups made up of social individuals to groups made of asocial individuals at least when shoal size was small. Individual sociability types were linked to the time individuals spent far from any shoal and to consistent differences in choosiness about social contexts. Asocial individuals shoaled less and swam more between shoals than social ones, but preferred large shoals during the short time that they shoaled. These results can help explain patterns of group size and composition and individual and population-level personality-dependent dispersal.

Activity counts: the effect of swimming activity on quantity discrimination in fish.

Human infants and non-human animals can discriminate the larger of two sets of discrete items. This quantity discrimination may be based upon the number of items, or upon non-numerical variables of the sets that co-vary with number. We have demonstrated that angelfish select the larger of two shoals of conspecifics without using inter-fish distance or space occupied by the stimuli as cues. However, density appeared to influence the choice between large shoals. Here, we examine the role of another non-numerical cue, swimming activity of the stimulus fish, in quantity discrimination by angelfish. To control this variable, we varied the water temperature of the stimulus aquaria or restricted the space occupied by each fish in the stimulus shoals. We used the previously successfully discriminated contrasts consisting of large (10 vs. 5) and small (3 vs. 2) shoals. We also studied whether more active or less active shoals are preferred in case of equally sized shoals (10 vs. 10, 5 vs. 5, and 3 vs. 3). When differences in stimulus fish activity were minimized by temperature manipulation we found angelfish to prefer the larger shoal in the 3 vs. 2 comparison, but not in the 10 vs. 5 comparison. When activity was controlled by space restriction, angelfish preferred the larger shoal in both numerical contrasts. These results imply that the overall activity level of the contrasted shoals is not a necessary condition for small shoals discrimination in angelfish. On the other hand, the results obtained for the large shoals, together with results obtained in the control treatments (equal numerical contrasts and differing activity levels), suggest that activity is a sufficient condition for discrimination when large shoals are involved. Further experiments are needed to evaluate the influence of other continuous variables, and to assess whether the mechanisms underlying performance are comparable to those suggested for other animals.

Water mass and bathymetric characteristics of polar cod habitat along the continental shelf and slope of the Beaufort and Chukchi seas

Relatively little is known about the distribution of fish in deep water (>200 m) in the Beaufort Sea. Data collected by an Acoustic Doppler Current Profiler operated in the Chukchi and Beaufort seas in summer were examined for evidence of fish biomass detections between 18 and 400 m. The presence of fish in waters between 1 and 30 m was explored opportunistically with a non-scientific echo sounder. Evaluation of findings was enhanced by measurements of water column properties (temperature, salinity, fluorescence and transmissivity). Relatively small shoals of fish were detected on the Chukchi shelf and eastern Chukchi shelf break, and also on the Alaskan and Canadian Beaufort shelves in the upper 20 m (T = 2–5°C). Much larger shoals (putative polar cod) were detected within Atlantic Water along the Beaufort continental slope (250–350 m) and near the bottom of Barrow and Mackenzie canyons, where temperatures were above 0°C. A warm-water plume of Alaska Coastal Current water with high concentrations of phytoplankton, zooplankton, and fish was found extending along the shelf 300 km eastward of Barrow Canyon. In contrast to the warm surface and Atlantic Water layers, very few fish were found in colder, intermediate depth Pacific-origin water between them. The large biomass of fish in the Atlantic Water along the continental slope of the Chukchi and Beaufort seas represents previously undescribed polar cod habitat. It has important implications with regard to considerations of resource development in this area as well as understanding impacts of climate change.

Imaging and localizing fish and marine mammals with acoustics.

During the 2006 ocean acoustic waveguide remote sensing (OAWRS) experiment in the Gulf of Maine, large shoals of Atlantic herring were instantaneously imaged in the frequency range from 300 to 1200 Hz. Simultaneously, several thousand instances of marine mammal vocalizations were passively recorded on a high resolution towed horizontal receiving array. A vast majority of the vocalizations were from humback whales in the 300–600 Hz frequency range. Vocalizations from other marine mammals species ranging from 40 Hz to over 3 kHz were also recorded. Various approaches are employed to localize and track the whales both passively and actively. The bearing of calling whales can be found by beamforming their vocalizations measured on the receiving array. An efficient and robust matched filter is designed to enhance the signal‐to‐noise ratio of the whale vocalizations. The array invariant method [Lee and Makris, J. Acoust. Soc. Am. (2006)] is then applied for instantaneous whale range estimation. The array invar...

Monitoring of ship damage condition during stranding

This study proposes a new procedure for the estimation of stranding forces and their contact positions. The method is based on the measurement of a few characteristic on-site parameters, i.e., the draughts and bending moments acting on a stranded ship. A procedure that estimates penetration into the ship bottom based on knowledge of the resistance versus penetration relationship is also presented. The seabed topology is parameterized by a paraboloid. This geometry can, in principle, characterize a wide range of obstructions, from sharp rocks to large shoals. Nonlinear finite element analysis is used to simulate various stranding situations and to generate the corresponding resistance versus penetration curves. The present method provides insight into the identification of real stranding scenarios in terms of the location of obstructions, their possible shapes and the corresponding resistance-penetration curves. The proposed methodology represents a first step towards a tool for quick decision making during salvage operations. The ultimate goal is to allow near real-time prediction of the risk of penetration into cargo tanks and hull girder failure. To demonstrate the effectiveness of the proposed method, it is applied to a simulated trial stranding scenario.

Fish and sessile assemblages associated with wind-turbine constructions in the Baltic Sea

Offshore wind farms are being built at a high rate around the world to meet the demand for renewable energy. We studied fish and sessile communities on and around offshore wind-turbine foundations in the southern Baltic Sea, 7 years after construction, using visual census techniques to determine how fish, sessile-invertebrate and algal communities are affected by the introduction of such structures. Fish assemblages were dominated by two-spotted gobies (Gobiusculus flavescens) that were found in large shoals in close association with the vertical surface. At the seabed, close to the foundation, the black goby (Gobius niger) was recorded in large numbers. The most obvious difference in fish densities was found between wind-power foundations extending through the entire water column and the surrounding open waters. Fouling assemblages on the vertical foundation surfaces and at the seabed just below differed from those at the seabed further away by having higher coverage of blue mussel (Mytilus edulis) and less algal growth. The results from the present study suggest that the introduction of offshore wind turbines in marine waters could have a positive effect on fish numbers and the presence of sessile invertebrates.

The effects of isolation and experience on preference for large shoals in juvenile fish

Shoaling behaviour confers fitness benefits that depend on individual fish making specific choices of shoalmates. Some of these choices have been shown to be influenced by experience, raising questions as to whether all shoaling choices depend on experience. Here we show that juveniles of three fish species, zebrafish (Danio rerio), pearl danios (Danio albolineatus) and guppies (Poecilia reticulata), were all capable of discriminating between shoals of differing size, choosing to swim near the larger shoal, without any prior experience with other fish. Both zebrafish and the closely related pearl danio, reared in isolation until 40 days of age, spent significantly more time near a shoal of four conspecifics over a solitary fish. Guppies raised in isolation until 50 days of age spent significantly more time near a shoal of four conspecifics over a shoal of two conspecifics. These results demonstrate an innate ability in juveniles of these species to choose large shoals over small shoals. However, when guppies were raised in groups of three they did not demonstrate a significant preference for larger shoals, suggesting that experience can still play a role in size preference in shoaling behaviour.

Effect of multiple scattering and modal dispersion on population density imagery of Atlantic herring shoals with ocean acoustic waveguide remote sensing.

Wide area sonar such as ocean acoustic waveguide remote sensing (OAWRS) has been shown to be a useful tool for instantaneously imaging large shoals of fish distributed over continental shelf regions. Here, we show that the population density of fish groups can be accurately estimated by using incoherent averaging of the matched filtered returns. Numerical Monte-Carlo models are applied to simulate the active imaging system and determine the statistics of the received matched filtered intensity scattered off Atlantic herring shoals in the Gulf of Maine. The model includes multiple scattering of the dense fish groups and uses a range-dependent acoustic model to simulate modal dispersion in the fluctuating ocean environment. We illustrate the effects of modal dispersion on the sonar imagery for various seafloor types. We determine the conditions for when multiple scattering is significant for Atlantic herring shoals, which depends on the fish density, imaging frequency, and mean target strength. Results of the model are compared to OAWRS sonar imagery collected during the 2006 Gulf of Maine Experiment.

Spatial and temporal coherence of scattered field from large fish groups instantaneously imaged with ocean acoustics waveguide remote sensing (OAWRS).

Wide area sonar system, such as OAWRS (ocean acoustics waveguide remote sensing), can instantaneously image large shoals of fish spanning tens of kilometers from long ranges in an ocean waveguide. An analytic and numerical model is developed for the spatial and temporal coherence of the scattered field from a large group of fish in a shallow water waveguide. The temporal coherence of the received signal depends on both the temporal coherence of the random waveguide through the medium Green’s function and the correlation time scale for fish group distribution. The spatial coherence of the received signal depends on both the spatial correlation scale of the random waveguide inhomogeneities and that for the imaged fish group distribution. The temporal and spatial correlation scale for the fish group distribution are determined from experimental data acquired in the Gulf of Maine during the 2006 Ocean Acoustics Waveguide Remote Sensing (OAWRS) Experiment. The effect of temporal and spatial coherence of the sc...

Fish fauna destruction after the introduction of a non-native predator (Cichla kelberi) in a Neotropical reservoir

In South America, the introduction of peacock-bass (Cichla), a voracious predator fish, has been an underestimated threat for native fish communities. Although this predator is widespread in many reservoirs, few studies have explored its impact on biodiversity. To investigate the relationship between invasion and fish diversity, the present study followed a natural experiment in the Rosana Reservoir (Parana River basin), where Cichla kelberi were introduced in 2004. We monitored fish assemblages associated with submerged macrophytes between 2003 and 2007, using a 1 m2 throw trap. In the years following the introduction, fish diversity dramatically changed. For example, in March 2007, mean fish density and richness were reduced by ca. 95 and 80%, respectively, and many small-sized species had vanished. One aspect was the gradual change of biodiversity, which unfolded at two times during each year: (1) impacts during summer/autumn periods, which coincided with large shoals of young C. kelberi in the patches; and (2) assemblage recovery during the spring. The sequence of extinction-colonization events, however, might not be able to maintain fish assemblages due to the decrease in recovery intensity each spring; assuming a constant decline rate in the coming years, we predict complete assemblage extinction by the summer of 2010. Results from this natural experiment provided evidence supporting the collapse of fish assemblages soon after the introduction of C. kelberi. Such rapid destruction (2 years) reveals an important homogenizing force behind this predator and stresses the need for control measures that prevent new transferences among South American basins.