Functional role of fish in tropical freshwaters

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Functional role of fish in tropical freshwaters

• Phytoplankton consuming fish are common in the
  tropics
• As such, fish do not only consume zooplankton,
  they also compete with them for phytoplankton
• Result
• Zooplankton show adaptations to minimize
  exploitation by fish (small body size, spines)
• Phytoplankton show adaptations to minimize
  exploitation by fish (toxic strains, gelatinous
  sheaths)

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Temperate lakes
Tropical lakes
Source Nilssen 1984
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Why are fish so abundant in tropical freshwaters?

• Higher primary production and thus more energy
  available to support higher trophic levels.
• The fish-algae link in the food chain avoids the
  intermediate trophic level (e.g., zooplankton)
  and thus more efficient energy transfer.

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The role of fish in material cycling in tropical
flood plains
Source Horne and Goldman, 1994
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Fish diversity in tropical rivers
Amazon
Mekong
Mississippi
Source Horne and Goldman, 1994
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Why are fish (and other higher organisms) so
diverse intropical freshwaters?

• Productivity-stability hypothesis
• Structural hypothesis
• Competition-predation hypothesis
• Stability-time hypothesis
• Productivity-disturbance hypothesis

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Competitive exclusion
Extinction
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Some common tropical fish families

• Southern Asia Cyprinidae (minnows) and Siluridae
  (catfishes)
• Africa Cyprinidae, Siluridae, Characidea (incl.
  piranhas, tetras), and Cichlidae.
• Tropical South/Central America Characidea,
  Siluridae, Cichlidae.

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Cyprinidae Minnow family

• Large, very diverse family
• Usually 8-9 rays in single dorsal fin
• Absent from South America/Australia
• Lack teeth in mouth
• Cycloid scales on body

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Siluridae catfish family

• Whiskers (barbels) around the mouth
• No scales
• Stout spines at the dorsal and pectoral fin
  origins
• Adipose fin
• Broad, flat head
• Cosmopolitan

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Tetras
Characidae

• Closely related to minnows
• Adipose fin
• Teeth
• Often brightly colored,
• popular aquarium fishes
• Central/South America, Africa

Piranhas
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Some Amazonian highlights.
Red piranha
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Cichlidae Haplochromine cichlids

• Africa, Central and South America,
• some in Asia
• Very diverse
• Broken lateral line
• Often brightly colored,
• popular aquarium fishes
• Introduced in the U.S.

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Cichla ocellaris
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Size of Costa Rica 51,100 km2, or less than half
the size of Ohio (116,096 km2)
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Freshwater fishes of Costa Rica.

• Diversity
• 135 species of fishes (freshwater only) compared
  with some 160 species of fish in Ohio.
• 19 endemics
• Central America has more than 350 species of fish
  (especially rich in poeciliids and cichlids).
• Major threats
• habitat destruction
• application of agrochemicals
• mining of large rivers for sand and gravel
• pollution (sewage, pulp waste from coffee
  plantations, industrial waste, and sediment
  erosion
• exotic introductions (Tilapia, trout and guppy)
• Paleogeography
• Species dispersal between South and North America
  was facilitated when an intercontinental land
  bridge existed some 65-55 million years B.P.
• This land bridge disappeared later and was
  reformed during the Pliocene (5 million years
  B.P.) and persists today. Dispersal of fishes
  occurred primarily from the South to the North.
• In addition to dispersal, vicariance was
  important in producing the present-day diversity.
• Biotopes
• High precipitation, many rivers, but few large
  lakes.
• The largest, Lago Arenal, created by damming the
  Arenal River to create a hydroelectric reservoir.

Source Bussing, W.A. 2002.
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Schiemer, Fritz. 1996. Significance of
filter-feeding fish in tropical freshwaters.
Perspectives in Tropical Limnology.SPB Academic
Publishing, Amsterdam, The Netherlands 65-76.

• Top-down food chain effects seem more significant
  in the tropics than in temperate zone.
• water birds affecting fish and mollusks
  zooplanktivorous fish impact on zooplankton
• Cascade on phytoplankton community less clear but
  phyto-planktivorous fish (many cyprinids and
  cichlids) seem important.
• The role of phytoplanktivorous fish in the
  temperate zone is insignificant.
• Increased phytoplanktivorous fish
• increased small algae
• increased productivity of algae
• increased microbial activity
• All these effects are largely determined by the
  ability of the fish to disrupt and digest the
  consumed particles!

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Some excretion of viable algal cells
Excretion of DOM/nutrients
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