Spatial Positioning within a group and Predation

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Spatial Positioning within a group and Predation
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Benefit of grouping

• Reduced risk of attack
• Dilution effect
• BUT there are costs within the group.

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Whos at risk?

• Parrish (1989) Atlantic silversides.
• Found that stragglers when available were
  attacked more often.
• BUT central fish were the most at risk from
  attack compared to fish at the periphery.
• Contradicts the selfish herd model, Hamilton
  (1971)

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Whos at risk?

• In three studies by Krause (and others), they
  found that central fish were less likely to be
  attacked than fish at the leading edge of the
  shoal.
• In agreement with the selfish herd model.

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Reproduction

• Reproductive success can be an important factor
  in group living
• e.g. Amphiprion akallopisos

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Anemonefish / Skunk Clownfish
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Social Organisation

• Breeding pair
• Primary Helpers (related)
• Secondary Helpers
• (unrelated)
• Helpers are peripheral, helping breeding pairs

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Reproductive Benefits and Costs for peripheral
helpers

• BENEFITS
• Inclusive fitness (primary helpers only)
• Chance of becoming breeder when one of breeding
  pair dies

• COSTS
• Could possibly be breeding itself

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Bluegill Sunfish Mating Techniques

• Large male in central territory attracts females
  but
• Sneaky matings where satellite males make a quick
  dash between the spawning pair or
• Pose as female and get between other male and
  female
• This means there are advantages to being on the
  outside

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• In groups of male, being on edge means that may
  reach passing female quick enough to allow quick
  mating
• Sperm guarding mounted males cannot easily be
  displaced
• Indonesian Reef Fish

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• Males that are in the middle of groups, generally
  stronger, healthier etc., thus advantage to being
  in the middle for mate choice
• Males on outside generally smaller, thus faster
  swimmers and being on outside gives them sneaking
  opportunities without having to compete against
  larger fish

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The effects of spatial distribution on feeding in
fish schools
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• Food availability higher at edge
• Higher net energy pay-off
• Higher rates at front
• Hungrier towards front/edge
• Trade-off with predation risks
• Restricted competition due to uniformity within
  groups rotation?

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Mixed shoal of roach and chub (Jens Krause, 1993)

• Roach at front had highest feeding rates
• Also ate more plankton (others ate bottom food)
• Some were starved and reintroduced
• Hungry fish went to front

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Migrating shoals (Elisabeth DeBlois George A.
Rose, 1996)

• Atlantic cod
• School over 10km wide
• Differing positions ate different food types
• Lead fish (scouts) were larger
• Ate more
• Greater variation
• Preferred food types

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Living on the edge

• As previously discussed, greatest risk of
  predation at edge of school
• Feeding rates higher at edge
• Predation selects for uniformity (Krause, 1994)
• Restricted potential for resource competition
  lack of dominant individuals
• So generally there is a rotation, constant
  trade-off influenced by level of hunger

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Differential Costs and Benefits associated with
spatial position within a group

• Fish

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• Diamond Formation
• The undulatory movements of a swimming fish
  generate thrust type vortices
• These vortices may affect the locomotor
  efficiency of a trailing fish as a possible
  energy conserving mechanism.
• Diagram. Fish C benefits from the flow (straight
  arrows) induced by the vortices produced by fish
  A and B. (Weihs 1973)

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V z
A
C
B
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• Therefore Fish C has an increased benefit in
  relation to its hydrodynamics, in that it doesnt
  have to utilize as much energy as fish at the
  front of the group.
• On the other hand this position in the shoal can
  prove to be costly especially if it is regarding
  planktivorous fish, as the fish at the front of
  the shoal are first to encounter particles of
  food.

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Parasitism and Positioning Within the group

• Individuals found at the front and periphery of
  the shoal more likely to be parasitized
• More vulnerable?
• Greater need to be at the front?
• May encounter parasites first.
• However no difference in parasite load
• The effects of parasitism and body length on
  positioning within wild fish shoals
• A.J.W. Ward et al 2002

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Parasitism and Positioning Within the group

• May be the parasitism effecting the position
• Parasites may alter the hosts feeding motivation
  and thus seek positions at the front and
  periphery C. bulboglossa
• This could also make the host more vulnerable to
  predation
• The effects of parasitism and body length on
  positioning within wild fish shoals A.J.W. Ward
  et al 2002

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Effect on the Group Shape

• A streamlined elliptical shape to the group
• Long axis pointing in the direction of movement
• Conserves overall energy and decreases overall
  predation risk

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Effect on the Group Shape

• In an infected group, more fish taking a more
  frontal and peripheral position
• Results in the longest axis perpindicular to the
  direction of movement

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After an Attack

• Minnows had an increased nearest neighbour
  distance (NND).
• After a predator scare both groups decreased
  their NND but the infected group NND was still
  greater
• Suggested an inability to shoal tightly rather
  than a decision due to nutritional demands etc
• Other fish may deliberately avoid non identical
  (infected) individuals
• Parasite infection alters schooling behaviour
  deviant positioning of helminth infected minnows
  in conspecific groups

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Dominance and spatial positioning within a group

• Different than previous examples
• Positioning depends on dominance status rather
  than status depending on positioning
• Other factors are a result of dominance

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Fish

• In shoals of chub more dominant fish found in
  centre of group (Krause 1994)
• Subordinates on periphery
• Much more beneficial to be in centre

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Conclusion

• Spatial positioning is important for many aspects
  of the lives of fish as we have seen.
• Yet another cost associated with living in a
  group
• Get in the optimum position or you may not make
  it.

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References

• Fricke, H.W, 1979 Mating system, resource defence
  and sex change in the anemonefish, Amphiprion
  akallopisos Z. Tierpsych 50 313 -326
• Casselman, S.J, and Mongomerie, R., 2004 Sperm
  traits in relation to male colonial spawning
  Bluegill. Journal of Fish Biology 64 1700 1711
• Andersson, M, 1984 Sexual Selection Princeton,
  NJ, Princeton University Press

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References

• Rosen, (1959), Water Flow about a Swimming Fish.
  Station Technical Publication U.S. Naval Ordnance
  Test Station. NOTS TP 2298. 1-4, 1-94.
• Muller et al. (1997). Fish Foot Prints
  morphology and energetics of the wake behind a
  continuously swimming mullet. Journal of
  Experimental Biology. 200-289.

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• Breder. (1965). Vortices and fish schools.
  Zoologica 50. 97-114.

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References

• Differential fitness returns in relation to
  spatial positions in groups
• Krause, J., Biol Rev Camb Philos Soc 1994 May
  69 (2) 187-206
• The relationship between foraging and shoal
  position in a mixed shoal of roach and chub a
  field study
• Krause, J., Oecologia, Vol. 93, No. 3, 1993
  March 356-359
• Cross-shoal variabilty in the feeding habits of
  migrating Atlantic cod
• DeBlois, E., Rose, G.A., Oecologia, Vol. 108,
  No. 1, 1996 October 192-196