[Advances in Marine Biology] Advances in Marine Biology Volume 60 Volume 60 || Social Aggregation in the Pelagic Zone with Special Reference to Fish and Invertebrates

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<ul><li><p>C H A P T E R F O U R</p><p>Social Aggregation in the PelagicZone with Special Reference to Fishand Invertebrates</p><p>David A. Ritz*,1, Alistair J. Hobday, John C. Montgomery</p><p>and Ashley J.W. Wardy</p><p>Contents1. Introduction 163</p><p>2. Aggregation Principles and Features in Pelagic Ecosystems 166</p><p>2.1. Origins of sociality 170</p><p>2.2. Significance and benefits of social aggregation 171</p><p>2.3. Structure and functions of social aggregations 176</p><p>2.4. Association patterns within aggregations 183</p><p>2.5. Sensing the behaviour of neighbours 184</p><p>2.6. Social networks 190</p><p>3. Technology Breakthroughs in Experimental and Observational</p><p>Methods 192</p><p>3.1. Video and motion analysis software 192</p><p>3.2. Optical plankton counters and holography 197</p><p>3.3. Acoustic technology 198</p><p>3.4. Electronic tags 203</p><p>3.5. Future technology challenges 204</p><p>4. Theoretical Developments in Social Aggregation 205</p><p>5. Social Aggregation, Climate Change and Ocean Management 208</p><p>6. Conclusion 211</p><p>6.1. Do reviews stimulate new work? 211</p><p>6.2. Future needs and synthesis 212</p><p>Acknowledgements 214</p><p>References 214</p><p>* School of Zoology, University of Tasmania, Hobart, Australia Wealth from Oceans Flagship, CSIRO Marine and Atmospheric Research, Hobart, Tasmania, Australia Leigh Marine Laboratory, University of Auckland, New Zealandy School of Biological Sciences, University of Sydney, Sydney, New South Wales, Australia1 Corresponding author: Email: David.Ritz@utas.edu.au</p><p>Advances in Marine Biology, Volume 60 2011 Elsevier LtdISSN: 0065-2881, DOI: 10.1016/B978-0-12-385529-9.00004-4 All rights reserved.</p><p>161</p></li><li><p>Abstract</p><p>Aggregations of organisms, ranging from zooplankton to whales, are an</p><p>extremely common phenomenon in the pelagic zone; perhaps the best known</p><p>are fish schools. Social aggregation is a special category that refers to</p><p>groups that self-organize and maintain cohesion to exploit benefits such as</p><p>protection from predators, and location and capture of resources more effec-</p><p>tively and with greater energy efficiency than could a solitary individual. In</p><p>this review we explore general aggregation principles, with specific reference</p><p>to pelagic organisms; describe a range of new technologies either designed</p><p>for studying aggregations or that could potentially be exploited for this pur-</p><p>pose; report on the insights gained from theoretical modelling; discuss the</p><p>relationship between social aggregation and ocean management; and specu-</p><p>late on the impact of climate change. Examples of aggregation occur in all</p><p>animal phyla. Among pelagic organisms, it is possible that repeated co-</p><p>occurrence of stable pairs of individuals, which has been established for</p><p>some schooling fish, is the likely precursor leading to networks of social</p><p>interaction and more complex social behaviour. Social network analysis has</p><p>added new insights into social behaviour and allows us to dissect aggrega-</p><p>tions and to examine how the constituent individuals interact with each</p><p>other. This type of analysis is well advanced in pinnipeds and cetaceans, and</p><p>work on fish is progressing. Detailed three-dimensional analysis of schools</p><p>has proved to be difficult, especially at sea, but there has been some prog-</p><p>ress recently. The technological aids for studying social aggregation include</p><p>video and acoustics, and have benefited from advances in digitization, minia-</p><p>turization, motion analysis and computing power. New techniques permit</p><p>three-dimensional tracking of thousands of individual animals within a single</p><p>group which has allowed novel insights to within-group interactions.</p><p>Approaches using theoretical modelling of aggregations have a long history</p><p>but only recently have hypotheses been tested empirically. The lack of syn-</p><p>chrony between models and empirical data, and lack of a common framework</p><p>to schooling models have hitherto hampered progress; however, recent</p><p>developments in this field offer considerable promise. Further, we speculate</p><p>that climate change, already having effects on ecosystems, could have dra-</p><p>matic effects on aggregations through its influence on species composition</p><p>by altering distribution ranges, migration patterns, vertical migration, and</p><p>oceanic acidity. Because most major commercial fishing targets schooling</p><p>species, these changes could have important consequences for the depen-</p><p>dent businesses.</p><p>Key Words: social aggregation; pelagic zone; marine; association; social</p><p>networks; technology; climate change; modelling</p><p>162 David A. Ritz et al.</p></li><li><p>1. Introduction</p><p>The marine pelagic zone is defined as the water column, usually in theopen sea. Further divisions of the water column into epipelagic and mesope-lagic can be made; however, here we use the term generally. It differs fromthe coastal marine domains with regard to ecological patterns; high alphadiversity, low beta diversity; apparent lack of keystone predators; few exam-ples of trophic cascades; and little apparent competition for space. Themarine pelagic environment represents 99% of the biosphere volume (Angel,1993). In addition to supplying more than 80% of the fish consumed byhumans (Pauly et al., 2002), pelagic ecosystems account for almost half of thephotosynthesis on Earth (Field et al., 1998). Just as productivity in the pelagicocean is not uniform, individuals are not distributed evenly, and clustering isthe norm. Because of the lack of geological substrate, as in coastal regions,many pelagic species are highly mobile as individuals or populations. In thisreview, we focus on examples from species living in the upper 200 m, whichis also known as the euphotic zone.</p><p>Animals need to eat to survive, and in mobile pelagic ecosystems thismeans finding prey. However, the average concentration of resources in theworlds oceans is insufficient for growth and survival of a variety of marinespecies, ranging from planktonic larvae to top predators (Steele, 1980; Levin,1992; Genin et al., 2005). Therefore, their survival depends on encounteringdense patches of prey that, in the case of zooplankton, form aggregationsthat vary in size along a continuum of spatial scales from 107 to 101 m(Fig. 4.1) (Haury et al., 1978; Mackas et al., 1985, Nicol, 2006).</p><p>Steeles (1980) analysis showed that the patchiness resulting from aggrega-tion increases with trophic level (Fig. 4.2). This seems to be a consequenceof the fact that the higher the trophic level, the less the response to thedetailed structure of the local environment, and a greater ability to use large-scale ocean features such as currents or fronts. The higher the trophic level,the less are the organisms dependent on short-term events such as storms,which markedly affect phytoplankton production, and active behaviour playsa more dominant role in generating patchiness.</p><p>This prey aggregation, in turn, aggregates their predators at the samelocations. But why is phytoplankton, the base of the food chain, patchy?The main limitations on primary production are physical and chemical (i.e.light and nutrient concentrations). Variations in the distribution of lightand nutrients occur both temporally and spatially in the ocean. The higherthe trophic level, the lower the physical environment plays in drivingspatial variability of standing stock, and the more behavioural processesassume importance (Steele, 1980; Folt and Burns, 1999). A challenge for thepredators then is first to locate these patchy prey aggregations and to remain</p><p>163Social Aggregation in the Pelagic Zone with Special Reference to Fish and Invertebrates</p></li><li><p>within them until it is no longer profitable to continue feeding. Area-restricted search patterns for food are widespread phenomena among pelagicpredators from copepods to whales indicating that many predators areadapted to find and exploit aggregated prey (Steele, 1980; Leising andFranks, 2000; Leising, 2001; De Robertis, 2002). While aggregation is ubiq-uitous at all scales in pelagic ecosystems, it is not simply a passive processwhere individuals gather together to exploit a food source and separate oncethe food has been eaten. The numerous additional benefits of group livingensure that groups of many different species remain cohesive for non-feedingperiods though membership may change. These benefits are usually reportedas protection from predators, facilitation of foraging and feeding, access tocentralized information, energy saving and facilitation of mate finding andreproduction (Wilson, 1975; Ritz, 1994; Hamner and Parrish, 1997;Heppner, 1997; Krause and Ruxton, 2002).</p><p>Persistent animal aggregation has been called a central problem in eco-logical and evolutionary theory (Levin, 1997; Flierl et al., 1999) because ofthe apparently conflicting requirements of short-term selfishness and longer-term group benefits. It may be that the study of the social histories ofgenetic aggregations and organelle symbioses can resolve this dilemma(Frank, 2007). We contribute to the analysis of social aggregation by</p><p>Figure 4.1 The Stommel diagram, overlain to show the scales that can be sampled withvarious platforms, and features such as fronts. From Kaiser et al. (2005), with permission from</p><p>Oxford University Press.</p><p>164 David A. Ritz et al.</p></li><li><p>reviewing the social behaviour of invertebrates and fish living in the upper200 m of the pelagic environment, but where appropriate, we use examplesfrom marine birds and mammals. This review builds on Ritz (1994), andthus we restricted the present review to post-1994 discoveries except wherereference to earlier papers is necessary for clarity or because of previousomission. Because the scope of this review has been expanded to includefish and, where appropriate, other vertebrates, relevant pre-1994 papers arealso included for these groups. We explore general aggregation principles(Section 2), describe a range of new technologies and provide examples ofthe insights gained from their use (Section 3), and from theoretical modelling(Section 4). In Section 5 we discuss the relationship between social aggrega-tion and ocean management and speculate on the possible impact of climatechange. Since this review complements Ritz (1994), we also examinewhether the post-1994 literature on the subject of social aggregation indi-cates if the earlier review stimulated research in directions identified as beingparticularly worthy of further study. We did this by using search terms associ-ated with the previously identified gaps for the subsequent period. We con-clude with areas ripe for further research to advance understanding of socialaggregation (Section 6).</p><p>We note that review papers offer an opportunity for synthesis, com-parison, gap analysis and identification of new areas for attention. Explicitguidelines to achieve these objectives in a repeatable and transparentfashion have been codified for medical reviews by Roberts et al. (2006),who also note that ecological reviews often fail to measure up to thesecriteria. Many of these criteria helped to shape this review, but in particu-lar, identification of the sources of evidence and how they were obtainedallows assessment as to whether the material included is likely to becomprehensive with respect to a topic of interest. Depending on the</p><p>Figure 4.2 Patchiness resulting from aggregation increases with trophic level. Modifiedfrom Steele (1980).</p><p>165Social Aggregation in the Pelagic Zone with Special Reference to Fish and Invertebrates</p></li><li><p>presentation of this material, this allows repeatability in future. We performeda comprehensive search for relevant material using several search engines: ISIWeb of Science, Google Scholar, Science Daily using the following terms:Social aggregation in pelagic environments; group dynamics; three-dimensional analysisof pelagic aggregations; modelling pelagic aggregations; pelagic aggregations and oceanmanagement; pelagic aggregations and climate change, and the contractions of thesewords. We did not to restrict our literature search to specific journals, as wewere concerned we might miss important insights and contributions injournals covering alternative disciplines. Additional materials were obtainedfrom reference lists in papers located using our search procedure, ourpersonal reference collections, and from discussion with expert colleagues. Inthis way we accessed relevant breakthroughs in the study of social insects andhumans. Grey literature is difficult to access with traditional search tools (e.g.Biological Abstracts), but increasing use of the Internet allows searching usingthe same keywords for posted grey literature.</p><p>2. Aggregation Principles and Features inPelagic Ecosystems</p><p>Before concentrating on social aggregation, some general pointsabout aggregation are relevant. For example, the importance of aggregationfor energy transfer is often ignored. This energy transfer can be trophic, orspatial, connecting habitats and allowing biological processes to be enhancedin non-productive areas. Hydrodynamic patterns can concentrate resources(Alldredge and Hamner, 1980) while migrating animals cause cross-habitatredistribution of carbon and nutrients (Young et al., 1996). Furthermore ithas been shown that schooling animals, by their swimming actions, are animportant source of fine-scale turbulence in the ocean (Huntley and Zhou,2004). They found that estimated rates of kinetic energy production by ani-mal schools are all of the same order, i.e. 1025 W kg21, irrespective of size(see Table 4.1).</p><p>Based on these data it appears that animal-induced turbulence is compara-ble in magnitude to energy dissipation resulting from major storms. In fact,according to Dewar et al. (2006), the biosphere generates enough power tostir the ocean. More recent work by Katija and Dabiri (2009) shows thatsuch fine-scale turbulence is primarily dissipated as heat. These authors high-light an alternative mechanism of mixing originally suggested by Darwin(1953), which depends on animal shape and drift volume, i.e. the volumeof fluid that migrates with the animal as it swims. Importantly, the drift vol-ume of adjacent animals in an aggregation may increase the effective size oftheir combined boundary layers, enhancing the possibility of vertical mixing.</p><p>166 David A. Ritz et al.</p></li><li><p>The disadvantage of group living includes predator attraction, local deple-tion of food resources, competition for food and spread of disease (Parrishand Edelstein-Keshet, 1999; Hoare and Krause, 2003) and the trade-offs havebeen examined using a range of evolutionary models. These studies oftenadvocate greater integration between empirical work, theoretical and model-ling approaches (see Parrish and Edelstein-Keshet, 1999).</p><p>Aggregations in the pelagic ecosystem may occur as a result of severalprocesses:</p><p>1. Passive aggregation including the concentrating effects of circulationsuch as fronts from river plumes, Langmuir circulation and internal waves(Flierl et al., 1999; Banas et al., 2004), and over abrupt topographies,such as the shelf break and seamounts (Boehlert and Genin, 1987), andcoral reefs (Genin et al., 1988, 1994).</p><p>2. Active and non-social aggregation including independent attrac...</p></li></ul>