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C H A P T E R F O U R
Social Aggregation in the PelagicZone with Special Reference to Fishand Invertebrates
David A. Ritz*,1, Alistair J. Hobday, John C. Montgomery
and Ashley J.W. Wardy
Contents1. Introduction 163
2. Aggregation Principles and Features in Pelagic Ecosystems 166
2.1. Origins of sociality 170
2.2. Significance and benefits of social aggregation 171
2.3. Structure and functions of social aggregations 176
2.4. Association patterns within aggregations 183
2.5. Sensing the behaviour of neighbours 184
2.6. Social networks 190
3. Technology Breakthroughs in Experimental and Observational
3.1. Video and motion analysis software 192
3.2. Optical plankton counters and holography 197
3.3. Acoustic technology 198
3.4. Electronic tags 203
3.5. Future technology challenges 204
4. Theoretical Developments in Social Aggregation 205
5. Social Aggregation, Climate Change and Ocean Management 208
6. Conclusion 211
6.1. Do reviews stimulate new work? 211
6.2. Future needs and synthesis 212
* 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
Advances in Marine Biology, Volume 60 2011 Elsevier LtdISSN: 0065-2881, DOI: 10.1016/B978-0-12-385529-9.00004-4 All rights reserved.
Aggregations of organisms, ranging from zooplankton to whales, are an
extremely common phenomenon in the pelagic zone; perhaps the best known
are fish schools. Social aggregation is a special category that refers to
groups that self-organize and maintain cohesion to exploit benefits such as
protection from predators, and location and capture of resources more effec-
tively and with greater energy efficiency than could a solitary individual. In
this review we explore general aggregation principles, with specific reference
to pelagic organisms; describe a range of new technologies either designed
for studying aggregations or that could potentially be exploited for this pur-
pose; report on the insights gained from theoretical modelling; discuss the
relationship between social aggregation and ocean management; and specu-
late on the impact of climate change. Examples of aggregation occur in all
animal phyla. Among pelagic organisms, it is possible that repeated co-
occurrence of stable pairs of individuals, which has been established for
some schooling fish, is the likely precursor leading to networks of social
interaction and more complex social behaviour. Social network analysis has
added new insights into social behaviour and allows us to dissect aggrega-
tions and to examine how the constituent individuals interact with each
other. This type of analysis is well advanced in pinnipeds and cetaceans, and
work on fish is progressing. Detailed three-dimensional analysis of schools
has proved to be difficult, especially at sea, but there has been some prog-
ress recently. The technological aids for studying social aggregation include
video and acoustics, and have benefited from advances in digitization, minia-
turization, motion analysis and computing power. New techniques permit
three-dimensional tracking of thousands of individual animals within a single
group which has allowed novel insights to within-group interactions.
Approaches using theoretical modelling of aggregations have a long history
but only recently have hypotheses been tested empirically. The lack of syn-
chrony between models and empirical data, and lack of a common framework
to schooling models have hitherto hampered progress; however, recent
developments in this field offer considerable promise. Further, we speculate
that climate change, already having effects on ecosystems, could have dra-
matic effects on aggregations through its influence on species composition
by altering distribution ranges, migration patterns, vertical migration, and
oceanic acidity. Because most major commercial fishing targets schooling
species, these changes could have important consequences for the depen-
Key Words: social aggregation; pelagic zone; marine; association; social
networks; technology; climate change; modelling
162 David A. Ritz et al.
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.
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).
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.
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
163Social Aggregation in the Pelagic Zone with Special Reference to Fish and Invertebrates
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).
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
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
Oxford University Press.
164 David A. Ritz et al.
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.