A general theory on fish aggregation to floating objects: An alternative to the meeting point hypothesis

An immense variety of fish may, on occasions, aggregate around or beassociated with floating structures such as drifting algae, jelliedzooplankton, whales, floats or anchored fish aggregating devices (ineffect, there are over 333 fish species belonging to 96 familiesrecorded in the literature).Several hypotheses have been advanced to explain this behaviour ofpelagic fish, although the most widely accepted theory is that fish usefloating materials, to some extent, to protect themselves frompredators. However, we think that aggregation under floats may be theresult of behaviour that has evolved to safeguard the survival of eggs,larvae and juvenile stages, during dispersion to other areas. Naturalfloating structures (e.g., algae, branches of trees) drift in seacurrents that originate in places where the floating objects arefrequently found (e.g., river estuaries, coastal areas). These same seacurrents also introduce some of the planktonic production generated inthese areas into the oligotrophic pelagic environment. Fish associatedwith drifting floating structures probably feed on invertebratesassociated with the structures. However, they may also benefit from theaccumulated plankton in the converging waters. Adult fish of somemigratory species (tuna, dolphinfish, etc.) have also developed similarassociative behaviour around drifting objects for other reasons (e.g.,resting places, presence of bait fish, geographical references andschool recomposition). In this context, the meeting point hypothesis isonly applicable to one specific case, the tuna and tuna-like species.Aggregative and associative behaviour, under and around floatingdevices, may be the result of convergent behaviors that result fromdifferent motivations. However, generally this behaviour can beexplained by the fact that drifting floating objects represent a meansof reaching relatively rich areas, where larvae and juvenile fish havean increased chance of survival.     

Further evidence for size-assortative schooling in sticklebacks

Using brook ( Culaea inconstans ) and 10-spined ( Pungitius pungitius ) sticklebacks we examined body-size related schooling behaviour. Small and large sticklebacks were allowed to choose between two test schools, of small and of large individuals, with and without a piscivorous fish ( Oncorhynchus mykiss ) visible. Sticklebacks of both species preferred the company of fish of matching body size: small fish associating with a school of small fish, large fish with a school of large fish. While no interspecific differences were found in responses to school selection, body size and predator presence did affect selection of school-type. In both species, small fish tended to show a stronger preference for matching schools. The preference was enhanced in small fish with presence of a predator, but not in large fish. These observations are further evidence for assortative schooling in sticklebacks.     

School size affects individual feeding success in three-spined sticklebacks (Gastevosteus aculeatus L.)

The effect of school size on the feeding success of individual three-spined sticklebacks was studied. We found that the proportion of fish feeding on benthic prey increased with school size and that fish in large schools tended to start feeding sooner than fish in small schools. The total number of strikes also increased in larger schools. Despite this evidence for a foraging benefit associated with school membership we propose that improved feeding returns do not alone explain stickleback schooling.     

Difference on reproductive trait of skipjack tuna <Emphasis Type="Italic">Katsuonus pelamis</Emphasis> female between schools (free vs FAD school) in the tropical western and central Pacific Ocean

In order to clarify the difference on reproductive traits of female skipjack tuna between fish aggregation device (FAD) and free-swimming (Free) school, we examined the relative condition factor and sex ratio, minimum size at first maturity and spawning fraction in tropical western and central Pacific Ocean. The relative condition factor in developing (FAD: 1.00 ± 0.06, Free: 1.05 ± 0.07) and spawning capable phase (FAD: 1.01 ± 0.06, Free: 1.04 ± 0.06) was higher in the free-swimming school than those in the fish aggregation device school (p < 0.01). The proportion of mature fish was significant difference between each schools (p < 0.05). The sex ratios in the schools were not biased (p > 0.05). Observed minimum size at first maturity were 40.0 cm FL in FAD and 42.5 cm FL in free-swimming school. Spawning capable fish were observed year around in FAD and for seven months in free-swimming school. The estimated total spawning fractions based on the postovulatory follicle method were 0.50 and 0.49 in the FAD and free-swimming schools, respectively. These results suggested that differences in the school type did not affect inherent spawning characteristics (i.e. sex ratio, minimum size at first maturity, or spawning season) and may shortly affect the oogenesis throughout the food availability.     

School fidelity in fishes: The yellow perch pattern

School fidelity concerns the cohesiveness of social aggregations over time, as measured by the regularity or repeatability with which known individuals re-occur as members of a school. School fidelity was measured by divers who observed yellow perch (Perca flavescens) marked with individually recognizable tags in Cazenovia Lake, New York. Yellow perch were facultative schoolers, with individual differences in schooling tendency (measured as percentage of time in schools) accounting for different numbers of observations of schooled versus solitary fish. The greatest co-occurrence was found between fish with strong schooling tendencies, but within this group co-occurrence was random. Home ranges overlapped greatly between individuals, yet co-occurrence was more a function of schooling tendency than of overlapping home ranges. Individuals with overlapping home ranges but with weak schooling tendencies seldom co-occurred. Home range size increased directly with schooling tendency: the distances that individuals moved apparently depended on the movements of the schools they joined, with strongly schooling fish remaining in schools longer. Individuals joined and left schools frequently; this also reflects a lack of fidelity between fish. Fish schools that form primarily for predator avoidance may show higher levels of fidelity than do schools that form for foraging or hydrodynamic efficiency.     

Species-specific schooling behaviour of fish in the freshwater pelagic habitat: an observational study

Social living of animals is a broadly occurring phenomenon, although poorly studied in freshwater systems, fish schooling behaviour is an excellent example. The composition of fish schools, species-specific schooling tendencies and preferences of adult fish were studied in the pelagic habitat of the Římov Reservoir, Czech Republic. Video recordings captured over a total of 34 days (16 h per day) in the clear water period of three seasons were analysed. From four species identified as school-forming species – bream, bleak, roach and perch, 40% of the individuals observed formed schools of 3–36 individuals. Although conspecific schools prevailed, 20% of individuals formed heterospecific schools, except bleak that schooled strictly with conspecifics. Schools were composed of individuals of similar body size and life strategy. Heterospecific schools were significantly larger than conspecific schools and showed uneven proportion among species, that is, one species being more abundant when the school dimension increased. Probability of encounter in bleak was lowest and proved highest inclination for schooling. Gregarianism levels depended on species morphology and body size, with larger and morphologically advanced fish tending less to sociability. This indicates that the antipredator function of schooling behaviour is intensified with increasing vulnerability of the species.     

Spatial and seasonal distribution of the tuna catch on floating objects in the eastern pacific ocean during 1992–1993

An analysis of the catch associated with floating objects by the Mexican tuna purse‐seine fleet in the eastern Pacific Ocean during 1992–1993 was made to determine the spatial and seasonal distribution. The information used was generated by observers of the Programa Nacional de Aprovechamiento del Atun y Protección a los Delfines (PNAAPD). There was no clear seasonal and spatial distribution of floating objects examined in this study, however there were areas where floating objects were more common; the mouth of the Gulf of California, waters offshore Peru, and in oceanic waters. The largest catch of yellowfin tuna was offshore of Peru in winter. Two areas with largest (length) yellowfin tuna were the mouth of the Gulf of California and offshore Peru. For skipjack tuna, the largest catch was offshore Peru in winter, but the largest skipjack were caught between 120° and 130°W along 10°N in spring. The largest yellowfin tuna were captured by sets on bamboo, fish aggregating devices (FADs), planks and boards, and logs (trees or parts). The largest skipjack were captured by sets on dead whales, kelp paddies, planks and boards, and pallets and crates. Most of the sets were made during the early hours of the day but an important number of log sets were made in the early afternoon. For the period analyzed, floating objects were more frequent during fall and winter with the area offshore of Peru the most important.     

The Large-Scale Deployment of Fish Aggregation Devices Alters Environmentally-Based Migratory Behavior of Skipjack Tuna in the Western Pacific Ocean

Fish aggregation devices (FADs) have been used extensively in the tuna purse seine fishery since the 1980s. This long-term modification of natural habitat has generated discussions as to whether FADs impact movement patterns of tuna species. We examined this question using data collected from the skipjack tuna (Katsuwonus pelamis) fishery. We used the longitudinal gravitational center of catch (G) to examine temporal variability in skipjack movement in the Western and Central Pacific Ocean, and related this to El Niño Southern Oscillation (ENSO) events. We found that in most cases G for free-swimming school sets changed with the onset of ENSO events, while G for floating-object-associated school sets remained relatively constant. This suggests that skipjack exhibit distinguishable behavioral strategies in response to ENSO events: they either react by moving long distances or they associate with floating objects. There has been no previous attempt to evaluate the interaction between FADs and the environmentally-determined movement of skipjack; this study shows evidence of an interaction, which should be considered when managing skipjack populations.     

Self-Organized Shape and Frontal Density of Fish Schools

Models of swarming (based on avoidance, alignment and attraction) produce patterns of behaviour also seen in schools of fish. However, the significance of such similarities has been questioned, because some model assumptions are unrealistic [e.g. speed in most models is constant with random error, the perception is global and the size of the schools that have been studied is small (up to 128 individuals)]. This criticism also applies to our former model, in which we demonstrated the emergence of two patterns of spatial organization, i.e. oblong school form and high frontal density, which are supposed to function as protection against predators. In our new model we respond to this criticism by making the following improvements: individuals have a preferred ‘cruise speed’ from which they can deviate in order to avoid others or to catch up with them. Their range of perception is inversely related to density, with which we take into account that high density limits the perception of others that are further away. Swarm sizes range from 10 to 2000 individuals. The model is three‐dimensional. Further, we show that the two spatial patterns (oblong shape and high frontal density) emerge by self‐organization as a side‐effect of coordination at two speeds (of two or four body lengths per second) for schools of sizes above 20. Our analysis of the model leads to the development of a new set of hypotheses. If empirical data confirm these hypotheses, then in a school of real fish these patterns may arise as a side‐effect of their coordination in the same way as in the model.     

Gear selectivity of functional traits in coral reef fisheries in Brazil

Small-scale reef fisheries are important commercial and subsistence activities that support the livelihoods of millions of people in tropical regions. Tropical marine fisheries typically target a diversity of species caught with a matching diversity of fishing gears and practices. Here, we explored how multiple fishing gears select for distinct functional traits of fish assemblages inside a large multiple use marine environmental protected area off northeastern Brazil. In 1833 landing interviews with local fishers, we identified 101 species, which were categorized according to six traits: body size, schooling behavior, mobility, position in the water column, diet and period of activity. Our research is the first to explore the broad patterns of gear selectivity with regards to fish functional traits for different habitat types. While gears used in reef habitats were highly selective of sedentary and benthic species that form schools with few individuals, gears used in coastal lagoons were selective of highly mobile pelagic species that form large schools. We found a low competitive interaction between different gear types, meaning there was a low overlap in trait selectivity between fishing gears. We also found direct associations between gears and fish functional traits: hooks and line targeted species that exhibit limited mobility capabilities, making these species more vulnerable to local levels of fishing effort. In contrast, nets and fish corrals targeted mobile species that exhibited a greater diversity of functional traits. Some of our results contrasted with the current literature on the topic, with differences highlighting the need for more research to clarify global patterns of trait selectivity by gear type. Our results have implications for fisheries management in northeastern Brazil: gear bans and effort caps are commonly used management measures that can foster fisheries sustainability by minimizing impacts to fish assemblage functions.

     

A general theory on fish aggregation to floating objects: An alternative to the meeting point hypothesis

Reviews in Fish Biology and Fisheries - An immense variety of fish may, on occasions, aggregate around or beassociated with floating structures such as drifting algae, jelliedzooplankton, whales,...     

人工集鱼装置对热带金枪鱼类行为模式的影响

金枪鱼类会聚集在漂浮物周围,人类依据该行为特性研制出人工集鱼装置(fish aggregation devices,FADs)诱集金枪鱼,从而使金枪鱼围网的产量和捕捞效率大幅提高.但目前科学界仍不确定FADs为何能够吸引金枪鱼以及其广泛使用是否会对金枪鱼资源和大洋中上层生态系统产生潜在影响.针对这些问题,国外学者展开了大量以FADs周围金枪鱼为研究对象的个体行为学试验.本文归纳并整理了过去30多年中较可靠和经典的相关试验,从金枪鱼的趋向性行为、集群行为、随附行为、摄食行为和垂直移动行为5个方面分别阐述了漂流FADs和锚泊FADs下金枪鱼的行为模式,并对国内今后开展相关研究的重点方向和注意事项进行了展望.     

Processes affecting size of fish schools in agent-based model

Fish school size varies with several factors, such as environmental conditions, individual size, time of day, and variations between individuals. In this study, we investigate the mean size of fish schools formed in a simple agent-based model, wherein each individual swims at a fixed speed with the direction modified by interactions with the neighbors. If alignment (tendency to swim in their neighbors' direction) is stronger than cohesion (tendency to swim toward the neighbors), then “marches” are formed, in which individuals swim in the same direction. If alignment is weaker than cohesion, then “circles” are formed, in which individuals chase one another and the entire group moves minimally. We analyzed various processes that control the shape and mean size of the schools. The school's shape is primarily affected by the magnitude of alignment, cohesion, and individual noises. In contrast, the mean size of the group is affected by several factors, including the spatial interaction range, initial configuration, individual differences in swimming speed, and sudden encounters with predators.     

Evidence for spatial limitation of the bluestripe snapper Lutjanus kasmira in French Polynesia from parasite and otolith shape analysis

The study focused on small‐scale location and movement patterns of the bluestripe snapper Lutjanus kasmira on the north coast of Moorea (Society Archipelago, French Polynesia, south‐central Pacific). Juveniles of this species occur in the estuary, and adults occur widely in the lagoon, the outer slope and in the intermediate channel, where fish aggregate in large schools during daytime. While fish were all sampled within a few hundred metres, they exhibited significantly different parasite fauna and otolith shapes according to their locality: estuary, lagoon, channel and outer slope. While juveniles did not exhibit any parasites in the estuary parasitological and otolith‐shape data suggest that adults rarely move between their three adjoining habitats, and more interestingly that large aggregations in the channel are formed predominantly by resident individuals with limited local movement, including at night. Besides giving some information on the host biology, such findings may have application in local fisheries management.     

Emergence of Oblong School Shape: Models and Empirical Data of Fish

The main benefit of the oblong shape of schools of fish is supposed to be the protection against predation. Models of self‐organised travelling groups have shown that this shape may arise as a side effect of the avoidance of collisions with group members. These models were developed for schools of fish in open water, whereas the oblong shape of schools of real fish has mostly been observed in schools in tanks. Therefore, it is not known how school shape in a tank originates neither in models nor in real fish. To find out what causes this shape, we use the combination of a theoretical and an empirical study. We test the predictions produced by our earlier models regarding the effect of school size on the school shape both in a model of self‐organised schooling in a tank and empirically. Empirically, we study the 3D positions of all individuals in the schools of 10–60 real mullets (Chelon labrosus). We calculate for each individual its distance to its nearest neighbour and its velocity and we measure per school its length and width. The relation between school shape and size in the model and in the real mullets supports our prediction and thus supports the hypothesis that school shape may be emergent from the avoidance of collisions during coordinated travelling.     

Quantifying the interplay between environmental and social effects on aggregated-fish dynamics

Demonstrating and quantifying the respective roles of social interactions and external stimuli governing fish dynamics is key to understanding fish spatial distribution. If seminal studies have contributed to our understanding of fish spatial organization in schools, little experimental information is available on fish in their natural environment, where aggregations often occur in the presence of spatial heterogeneities. Here, we applied novel modeling approaches coupled to accurate acoustic tracking for studying the dynamics of a group of gregarious fish in a heterogeneous environment. To this purpose, we acoustically tracked with submeter resolution the positions of twelve small pelagic fish (Selar crumenophthalmus) in the presence of an anchored floating object, constituting a point of attraction for several fish species. We constructed a field-based model for aggregated-fish dynamics, deriving effective interactions for both social and external stimuli from experiments. We tuned the model parameters that best fit the experimental data and quantified the importance of social interactions in the aggregation, providing an explanation for the spatial structure of fish aggregations found around floating objects. Our results can be generalized to other gregarious species and contexts as long as it is possible to observe the fine-scale movements of a subset of individuals.     

人工集鱼装置对热带金枪鱼类摄食模式的影响研究进展

热带金枪鱼类具有聚集在漂浮物体周围的行为特性,形成的集群表现稳定,可捕性较高。以此特性,金枪鱼围网渔业研制并投放了大量的人工集鱼装置(Fish Aggregation Device,FAD)用于聚集并捕捞金枪鱼。然而,大规模出现的FAD会使某些海域海面漂浮物的密度迅速增加,从而在一定程度上人为地改变了金枪鱼的表层栖息环境,对金枪鱼种群具有一系列可能的潜在负面影响,摄食模式的改变就是其中之一。归纳并综合了近年来国内外关于FAD对金枪鱼类摄食模式影响的相关研究,从摄食行为、日摄食量、饵料种类与组成以及生态位宽度4个方面对比了随附于FAD的金枪鱼和自由状态下同类的不同,发现大多情况下FAD的存在会使金枪鱼的摄食模式发生一定的改变。最后,归纳了过往实验存在的不足,对今后研究的发展方向进行了展望。