Large Shoals Research Articles

Leadership in fish shoals

AbstractLeadership is not an inherent quality of animal groups that show directional locomotion. However, there are other factors that may be responsible for the occurrence of leadership in fish shoals, such as individual differences in nutritional state between group members. It appears that front fish have a strong influence on directional shoal movements and that individuals that occupy such positions are often characterised by larger body lengths and lower nutritional state. Potential interactions between the two factors and their importance for positioning within shoals need further attention. Initiation of directional movement in stationary shoals and position preferences in mobile shoals need to be addressed separately because they are potentially subject to different constraints. Individuals that initiate a swimming direction may not necessarily be capable of the sustained high swimming performance required to keep the front position or have the motivation to do so, for that matter. More empirical and theoretical work is necessary to look at the factors controlling positioning behaviour within shoals, as well as overall shoal shape and structure. Tracking of marked individuals whose positioning behaviour is monitored over extended time periods of hours or days would be useful. There is an indication that shoal positions are rotated by individuals according to their nutritional needs, with hungry fish occupying front positions only for as long as necessary to regain their nutritional balance. This suggests that shoal members effectively take turns at being leaders. There is a need for three‐dimensional recordings of shoaling behaviour using high‐speed video systems that allow a detailed analysis of information transfer in shoals of different size. The relationship between leadership and shoal size might provide an interesting field for future research. Most studies to date have been restricted to shoals of small and medium size and more information on larger shoals would be useful.

Spatial association between murres (Uria spp.), puffins (Fratercula spp.) and fish shoals near Pribilof Islands, Alaska

Feeding by piscivorous marine seabirds necessitates some spatial association between them and their prey. Many attempts have been made to establish spatial associations between seabirds and their prey, yet small-scale ( 10 birds) of piscivorous murres (Uria spp.) and puffins (Fratercula spp.) are con- sistently associated with their major prey in this area, large shoals of Age-0 pollock Theragra chalcogramma. Furthermore, the number of birds in a bird cluster is usually proportional to the bio- mass of fish under that cluster.

Variation in structure and distribution of pre‐spawning Pacific herring shoals in two regions of British Columbia

In two regions of British Columbia, small and medium‐sized shoals of pre‐spawning Pacific herring Clupea harengus pallasi were most numerous and occurred as neighbours more frequently than would be expected by chance. Herring displayed a typical diurnal vertical migration, although shoals in the Central Coast were more evenly distributed through the water column during the day than those in the Strait of Georgia. As maturation progressed, shoals were found in shallower water, more densely packed and vertically extended in shape. In the Strait of Georgia, shoals were considerably larger, and positioned relatively deeper in the water column. Very large (>5000 t) shoals were found at specific locations characterized by high water exchange. Large shoals may act as an anti‐predator strategy to the abundant sealions and birds. Moreover, staying deep in the water column, and associating with dips in the sea bed, reduces predation risk and confers energy savings through hydrodynamic advantage. Smaller, denser, vertically extended, travelling shoals, prevalent during the later stages of gonad maturation, were similar in nature to labile spherical shoals observed in the Central Coast at different locations each day. Both these shoal types may benefit from synchrony and co‐ordination when evading predators that is not possible for larger shoals. The more highly clustered distribution of shoals in the Central Coast is presumed to be linked to their tight association with rock outcroppings typical of this region.

Variation in structure and distribution of pre-spawning Pacific herring shoals in two regions of British Columbia

In two regions of British Columbia, small and medium-sized shoals of pre-spawning Pacific herring Clupea harengus pallasi were most numerous and occurred as neighbours more frequently than would be expected by chance. Herring displayed a typical diurnal vertical migration, although shoals in the Central Coast were more evenly distributed through the water column during the day than those in the Strait of Georgia. As maturation progressed, shoals were found in shallower water, more densely packed and vertically extended in shape. In the Strait of Georgia, shoals were considerably larger, and positioned relatively deeper in the water column. Very large (>5000 t) shoals were found at specific locations characterized by high water exchange. Large shoals may act as an anti-predator strategy to the abundant sealions and birds. Moreover, staying deep in the water column, and associating with dips in the sea bed, reduces predation risk and confers energy savings through hydrodynamic advantage. Smaller, denser, vertically extended, travelling shoals, prevalent during the later stages of gonad maturation, were similar in nature to labile spherical shoals observed in the Central Coast at different locations each day. Both these shoal types may benefit from synchrony and co-ordination when evading predators that is not possible for larger shoals. The more highly clustered distribution of shoals in the Central Coast is presumed to be linked to their tight association with rock outcroppings typical of this region.

Parasitism and Shoal Size in Juvenile Sticklebacks: Conflicting Selection Pressures from Different Ectoparasites?

In gregarious animals, group size correlates negatively with infection levels by some kinds of parasites and positively with infection by others. Conflicting selection pressures can be exerted simultaneously on a host species by different parasite species. Among stationary, mixed‐species shoals of juvenile threespine sticklebacks,Gasterosteus aculeatus, and blackspotted sticklebacks,Gasterosteus wheatlandi, shoal size correlated differently with levels of infection by two species of ectoparasites. Stickleback shoal size correlated positively with infection levels by the copepodThersitina gasterostei, which is transmitted among fish by short‐lived planktonic larvae. In contrast, infection levels by the highly mobile crustacean parasiteArgulus fundulidid not decrease as shoal size increased, as predicted from an earlier laboratory experiment. The species composition of the different stickleback shoals also had an influence on some aspects of infection by these two parasite species. The contrasting mode of transmission of the two parasites results in one parasite species having a higher transmission rate among fish within large shoals, whereas the success of the other parasite species is independent of fish shoal size. The two ectoparasites may thus exert different selection pressures on stickleback shoal sizes.

Is there always an influence of shoal size on predator hunting success?

Theoretical and empirical studies predict that there should be a decrease in hunting success of predators with increasing prey group size. Most of these studies investigated situations in which predator and prey were in full view of each other before, during and after an attack. In this study, single rock bass Ambloplites rupestris were given an opportunity to launch surprise attacks at shoals of creek chub Semotilus atromaculatus that ranged in size from two to 13 fish. There was no significant influence of either shoal size or attack distance on predator success rate and no significant relationship between attack distance and shoal size. Furthermore, it was found that the leading fish of a shoal was attacked significantly more often than fish in other shoal positions, indicating that predation risk was not shared equally among shoal members. Also, leading fish in larger shoals (eight to 13 fish) were not more likely to survive a predator attack than ones in small shoals (two to seven fish).The consequences of these results are discussed in the general context of antipredator benefits of grouping.

Is there always an influence of shoal size on predator hunting success?

Theoretical and empirical studies predict that there should be a decrease in hunting success of predators with increasing prey group size. Most of these studies investigated situations in which predator and prey were in full view of each other before, during and after an attack. In this study, single rock bass Ambloplites rupestris were given an opportunity to launch surprise attacks at shoals of creek chub Semotilus atromaculatus that ranged in size from two to 13 fish. There was no significant influence of either shoal size or attack distance on predator success rate and no significant relationship between attack distance and shoal size. Furthermore, it was found that the leading fish of a shoal was attacked significantly more often than fish in other shoal positions, indicating that predation risk was not shared equally among shoal members. Also, leading fish in larger shoals (eight to 13 fish) were not more likely to survive a predator attack than ones in small shoals (two to seven fish).The consequences of these results are discussed in the general context of antipredator benefits of grouping.

Henslow's swimming crab (Polybius henslowii) as an important food for yellow-legged gulls (Larus cachinnans) in NW Spain

An analysis of the contents of 2562 pellets sampled from 1987 to 1993 at breeding colonies and roosting sites showed that Henslow's swimming crabs (Polybius henslowii) are by far the most important marine prey for yellow-legged gulls (Larus cachinnans) on the coasts of Galicia (north-western Spain), occurring in 36.4% of pellets. The results also suggest that yellow-legged gulls in Galicia are to a great extent marine foragers. Galicia has one of the largest yellow-legged gull populations in western Europe, largely dominating the seabird community. Polybius henslowii is the most abundant decapod crab over the continental shelf of Galicia. It enters coastal waters in large shoals and frequently stays close to the sea surface. Compared with the diets of other yellow-legged gull populations or any of the other closely related gull species, such as the herring (L. argentatus) and the lesser black-backed gull (L. fuscus), Polybius henslowii appears as a characteristic and even exclusive prey of yellow-legged gull populations in the Iberian Atlantic. There is also some evidence that the regular irruption of large Polybius henslowii shoals is a phenomenon peculiar to Iberian Atlantic waters.

Shoal Choice Behaviour in Fish: the Relationship Between Assessment Time and Assessment Quality

Abstract In this study, we investigated the role of assessment time in group size discrimination and in particular the trade-off between the time cost involved in gathering information and the potential benefits derived from the acquired information. In a first experiment, we presented individual chub, Semotilus atromaculatus, with a choice between 4 vs 4 and 8 vs 8 conspecific stimulus fish. After release we recorded the time taken by test fish to make a choice between the two stimulus shoals in the presence and absence of a fright stimulus. Test fish significantly reduced their response time in the presence of a fright stimulus and larger shoals (8 vs 8) were more quickly approached than smaller ones (4 vs 4). In a control experiment, chub were given a choice between an empty cylinder and a shoal (of 4 or 8 fish). By subtracting the response time in the control treatment from that in the choice treatment, we estimated the time test fish spent choosing between stimulus shoals to be 24-55% of the overall response time. These results indicate that choosing between different groups is associated with a significant time cost. In a second experiment, we presented test fish with stimulus shoals that differed in size: 4 vs 5, 4 vs 6, 4 vs 7 and 4 vs 8, to investigate how the response time of fish and their ability to distinguish between shoals of different size were affected by the magnitude of the shoal size difference and the presence and absence of a fright stimulus. The ability to discriminate between shoals of different size increased with increasing shoal size difference whereas response time decreased. Both response time and discrimination ability were significantly reduced in the presence of a fright stimulus. The latter suggests that the benefits derived from group size discrimination were increasingly outweighed by the time costs of making group size assessments in the presence of potential danger; i.e. making fast assessments became relatively more important than making correct ones.

Two examples of the interplay between field observations and laboratory experiments from 35 years of research with planktonic organisms

Two studies of complicated ecological phenomena in Lake Maarsseveen (The Netherlands) are presented to illustrate that a combination of field and laboratory analysis might be a successful approach. In the first one, the yearly varying ratio of population abundance of two diatoms, Asterionella formosa and Fragilaria crotonensis during early spring is explained by a combination of abiotic and biotic factors. A fungal parasite blocks population development of the first species at temperatures higher than 3–5 °C . The second species then competes successfully for phosphate and develops a large population. The interaction between Asterionella and the fungus proved to be intricate. Several physiological mechanisms operate together and combined with variable climatic factors makes the prediction of what species will be dominant, impossible. In the second example diel vertical migration of the hybrid Daphnia galeata × hyalina is analysed in the field and the laboratory. Migration is confined to six to seven weeks in June–July when large shoals of juvenile perch are present in the pelagic zone of the lake. These fish exudate a kairomone that enhances the reaction of Daphnia to relative increases in light intensity of dawn. Especially accelerations in rate of relative increases enhance swimming velocity. The extent of enhancement depends, however, on the concentration of the kairomone and on food concentration. A ’Decision Making Mechanism‘ is introduced and the relation between the response mechanism and the adaptive relevance of diel vertical migration is made.Finally considerations about research strategies and the importance of fundamental research are made. It is concluded that the development of limnology and aquatic ecology might be in danger when the present tendency to give prevalence to applied research continues. [KEYWORDS: strategies of plankton research, field observations, laboratory experiments, phytoplankton succession, fungus epidemics, diel vertical migration]

Density dependence and numerosity in fright stimulated aggregation behaviour of shoaling fish

Single three-spined sticklebacks, Gasterosteus aculeatus , were frightened with a light stimulus simulating an aerial predator while facing a choice between two conspecific display shoals of different membership sizes. We observed which shoal the test fish approached. Initially, both display shoals were equidistant from the test fish. The smaller shoal was then moved gradually closer whereas the larger shoal stayed at a constant distance. This experiment modelled an early stage of the aggregation behaviour of sticklebacks in response to perceived imminent predation risk. When the two display shoals were equidistant from the test fish, we found that the test animal preferred approaching the larger display shoal, and the magnitude of this preference increased with increasing display shoal size difference. This demonstrates that the aggregation behaviour of frightened sticklebacks is density dependent. Further, we found that sticklebacks made a trade-off between the distance to a display shoal and its membership size. In particular, for a given ratio of display shoal sizes, there was a critical distance at which half of all tested animals turned to one and the other half to the other display shoal. This demonstrates that the observed aggregation behaviour is also distance dependent. We introduce several elementary models which formalize individual predation risk and explore how distance and display shoal size contribute to total risk. In particular, we distinguish between total risk as a product or as a sum of the risk components associated with swimming distance and display shoal size, respectively. All models follow the ‘partial preferences’ paradigm of McNamara & Houston ( Anim. Behav . 35, 1084-1099 (1987)). We compare how closely these models match the observed data and how well they predict the empirical critical distances. We find a consistent discrepancy between theory and data, which we resolve by invoking a fundamental perceptual limit (numerosity) for shoal size discrimination.

Predator preferences for attacking particular prey group sizes: consequences for predator hunting success and prey predation risk

Models of group living assume that the hunting success of predators and per capita predation risk of prey decrease with increasing prey group size. In contrast to numerous studies documenting anti-predator advantages of group living, little is known about the choice of predators for particular prey groups. Two shoals of guppies,Poecilia reticulata, which differed in size (2 versus 5 fish, 5 versus 10, 2 versus 10, 10 versus 16), were simultaneously presented to individual blue acara cichlid fish,Aequidens pulcher, and the shoal that was attacked first, the number of subsequent attacks and the total time cichlids spent near each shoal during a 3-min period were recorded. For all three criteria, the cichlids showed a significant preference for the larger of the two shoals, resulting in a higher per capita risk in terms of first attack and attack duration for individuals in larger shoals. Further trials, in which guppy activity was manipulated by using different water temperatures, showed that the cichlids preferred a smaller over a larger shoal if the larger shoal was kept in cold water (i.e. exhibited less movement). This result demonstrated that the visual conspicuousness of the shoal, and not its size per se, was the key factor for the cichlids' choice of a prey shoal. In a final experiment, wherein cichlids were given an opportunity to attack free-swimming shoals (comprised 1-8 individuals), predator hunting success decreased with increasing shoal size. We conclude that, despite the over-proportionately high attack risk for individuals in large shoals, per capita predation risk is lower than in small shoals, and joining larger shoals should be advantageous for individual guppies. Cichlids, however, would do better in terms of hunting success by attacking small rather than large shoals. Potential implications of such predator preference for larger shoals are discussed.

Life-cycle biomagnification study in fish

Models of group living assume that the hunting success of predators and per capita predation risk of prey decrease with increasing prey group size. In contrast to numerous studies documenting anti-predator advantages of group living, little is known about the choice of predators for particular prey groups. Two shoals of guppies,Poecilia reticulata, which differed in size (2 versus 5 fish, 5 versus 10, 2 versus 10, 10 versus 16), were simultaneously presented to individual blue acara cichlid fish,Aequidens pulcher, and the shoal that was attacked first, the number of subsequent attacks and the total time cichlids spent near each shoal during a 3-min period were recorded. For all three criteria, the cichlids showed a significant preference for the larger of the two shoals, resulting in a higher per capita risk in terms of first attack and attack duration for individuals in larger shoals. Further trials, in which guppy activity was manipulated by using different water temperatures, showed that the cichlids preferred a smaller over a larger shoal if the larger shoal was kept in cold water (i.e. exhibited less movement). This result demonstrated that the visual conspicuousness of the shoal, and not its size per se, was the key factor for the cichlids' choice of a prey shoal. In a final experiment, wherein cichlids were given an opportunity to attack free-swimming shoals (comprised 1-8 individuals), predator hunting success decreased with increasing shoal size. We conclude that, despite the over-proportionately high attack risk for individuals in large shoals, per capita predation risk is lower than in small shoals, and joining larger shoals should be advantageous for individual guppies. Cichlids, however, would do better in terms of hunting success by attacking small rather than large shoals. Potential implications of such predator preference for larger shoals are discussed.

The influence of hunger on shoal size choice by three‐spined sticklebacks, Gasterosteus aculeatus

Single sticklebacks (Gasterosteus aculeatus) were presented with a choice between (a) a shoal and an empty compartment, and (b) two different sized shoals of conspecifics. The choice of hungry test fish (food‐deprived for 24 h and 72 h, respectively) was compared to that of fish which were fed to satiation once a day. It was hypothesized that due to the increased level of competition for food in large shoals, hungry fish would spend less time with the larger of two shoals than their well‐fed conspecifics. The results showed that there was indeed a significant trend for test fish to spend a decreasing amount of time near the compartment with the larger number of fish with increasing duration of food deprivation.

The influence of hunger on shoal size choice by three-spined sticklebacks, Gasterosteus aculeatus

Single sticklebacks (Gasterosteus aculeatus) were presented with a choice between (a) a shoal and an empty compartment, and (b) two different sized shoals of conspecifics. The choice of hungry test fish (food-deprived for 24 h and 72 h, respectively) was compared to that of fish which were fed to satiation once a day. It was hypothesized that due to the increased level of competition for food in large shoals, hungry fish would spend less time with the larger of two shoals than their well-fed conspecifics. The results showed that there was indeed a significant trend for test fish to spend a decreasing amount of time near the compartment with the larger number of fish with increasing duration of food deprivation.

Pack‐hunting in two species of catfish, Clavias gariepinus and C. ngamensis, in the Okavango Delta, Botswana

During the annual drawdown in the Okavango Delta, Botswana, large shoals of catfish migrate upstream in the main river channels. The examination of samples of the sharptooth catfish Clarias gariepinus and blunttooth catfish C. ngamensis revealed that they were pack‐hunting and selectively preying on two species of mormyrids, the bulldog Marcusenius macrolepidotus and the churchill Petrocephalus catostoma. The predation on mormyrids may be related to prey size and abundance. In addition, catfish may sense the electrical discharge given off by the mormyrids when disturbed. The catfish feed intensively for a few months and build up body reserves and reproductive condition prior to spawning on shallow floodplains with the onset of the annual floods from Angola.

Increased Recapture Rate of Adult Atlantic Salmon Released as Smolts into Large Shoals of Wild Smolts in the River Orkla, Norway

Abstract Carlin-tagged, hatchery-reared smolts of Atlantic salmon Salmo salar were released in the regulated River Orkla at the same time of year and same site from 1984 through 1988. Recapture rates of adult Atlantic salmon varied considerably (from 0.7 to 6.8% of number of smolts released). Survival of released hatchery-reared smolts increased when they were released into large shoals of migrating wild smolts. Wild Atlantic salmon smolts generally aggregate into large schools when migrating out of a river, although the size of schools varies between years. When the hatchery-reared smolts were stocked within lesser shoals, survival was lower than in years with larger shoals of wild smolts. The magnitude of water discharge, as well as the direction of change in the magnitude, greatly influences the initiation of smolt migration and shoal formation in the Orkla. Artificial manipulation of water discharges throughout May can influence formation of large shoals of wild smolts and is a means of achieving opti...

Predator shoaling moderates the confusion effect in blue-green chromis, Chromis viridis

In experiments, blue-green chromis [Chromis viridis (Cuvier 1830)] were fed on either scattered or aggregated swarms of brine shrimp (Artemia sp.). Ten runs with each prey dispersion treatment were performed with shoals of one, two, five and ten chromis. The mean lag in reaching peak feeding rate for fish fed on aggregated prey was significantly shorter in the larger chromis shoals. In contrast, with the scattered treatment all such lags were similar and very short. As foraging proceeded, higher feeding rates were observed in the larger feeding shoals, regardless of prey dispersion. Prey capture success (i.e. the rate of retention of intercepted prey) declined with time, but was significantly higher in groups of ten fish. Two main conclusions emerge. Firstly, grouping facilitated initiation of feeding by individuals preying on concentrated swarms and reduced the delay in reaching a maximum feeding level. This may have been due to a suppression of the confusion effect through reduced reliance upon vigilance. Secondly, reduced vigilance allowed larger shoals of chromis to feed effectively over more extended periods. Trends of increasing shoal cohesion and decreasing prey retention rate with time were consistent with a postulated increase in antipredator vigilance with declining feeding motivation.

Prehistoric nearshore and littoral fishing in the eastern Tropical Pacific: An ichthyological evaluation

Marine fish bones are abundant components of human food remains at prehistoric sites in the eastern Tropical Pacific. Their quantification and interpretation in terms of human procurement strategies have been hampered by the use of large-meshed sieves and by the obstacles presented to faunal experts by fish taxonomic diversity. Since many important food fish families are speciose, this paper emphasizes the importance of species-level identifications. Making extensive use of Spanish-language articles on fish distribution and ecology, and the commercial and artisanal fisheries literature, it identifies groups of nearshore and littoral fish species that show greater and lesser tolerance for hard substrates (reefs and rocks), estuaries and coastal lagoons, and deeper clearer waters farther offshore. Most of the regional archaeological sites were located near estuarine habitats. The drab-colored fish they attract are generally poorly known. Hence, a particularly detailed analysis is offered of fish behavior within low salinity coastal inlets. Fourteen archaeofaunal fish samples from Costa Rica, Panama, and Ecuador are evaluated in the light of the environmental information generated by the biological and fisheries surveys. It is concluded that most sites have a broadly estuarine orientation and that the input of hard substrates is everywhere <10%. Two sites, Salango, in Ecuador, and Vidor, in Costa Rica, exploited epipelagic fish that swim in large shoals and deep water demersal predators. Sites situated more than 10 km from the coast exploited a wide variety of marine fish. In Parita Bay, Panama, a comparison of fish faunas from Cerro Mangote (6000 B.P.) and Sitio Sierra (1800 B.P.) elicits the hypothesis that regional fishing strategies shifted between these dates from a shore-based, netless strategy to a more complex one that incorporated fine-meshed gill-nets and watercraft.

Habitat scale, female cannibalism, and male reproductive success in three-spined sticklebacks (Gasterosteus aculeatus)

We studied the effects of habitat scale (pool size) on the risk of egg cannibalism and the breeding behavior of three-spined sticklebacks (Gasterosteus aculeatus). Laboratory experiments showed that in pools with similar population density, a larger fraction of males nested in pools of 4.8-m2 surface area than in 15-m2 pools. Breeding males in the 4.8-m2 pools also had bigger territories, obtained more eggs, and were more aggressive toward intruders than those in the 15-m2 pools. The numbers of raids attempted on nests were similar in the two pool sizes. However, raiding-shoal sizes were larger, and hence individuals in these larger shoals were more succesful at overwhelming male territorial defenses in the 15-m2 pools than in the 4.8-m2 pools. At the field site, a salt marsh containing hundreds of tide pools of different sizes, sticklebacks settled at similar densities in small (<30 m2 surface area) and large (>355 m2) pools. This resulted in significantly more fish being present in large pools than in small pools, a concomitant increase in the amount of nest raiding, and a decrease in individual male reproductive success in large compared to small pools. A photogrammetry analysis of the salt-marsh pools suggests that the majority of fish breeding at the site will be subjected to a high risk of cannibalism because most fish nest in large pools. We conclude that spatial scale has a major influence on this species' breeding behavior and reproductive success.