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Functional role of fish in tropical Functional role of fish in tropical freshwatersfreshwaters
• Phytoplankton consuming fish are common in the tropicsPhytoplankton consuming fish are common in the tropics
• As such, fish do not only consume zooplankton, they also As such, fish do not only consume zooplankton, they also compete with them for phytoplanktoncompete with them for phytoplankton
• Result:Result:
– Zooplankton show adaptations to minimize Zooplankton show adaptations to minimize exploitation by fishexploitation by fish
• small body size, spinessmall body size, spines
– Phytoplankton show adaptations to minimize Phytoplankton show adaptations to minimize exploitation by fishexploitation by fish
• toxic strains, gelatinous sheaths, spinestoxic strains, gelatinous sheaths, spines
Peridinium, Lake Kinneret
Phyto-plankton
BacteriaPhyto-
planktonBacteria
CrustaceansCrustaceansA B Rotifers and ProtozoansRotifers and Protozoans
Temperate lakesTemperate lakes Tropical lakesTropical lakes
Source: Nilssen 1984Source: Nilssen 1984
BA BA
FishFish
Why are fish so abundant in tropical Why are fish so abundant in tropical freshwaters?freshwaters?
• Higher primary production and thus more Higher primary production and thus more energy available to support higher trophic energy available to support higher trophic levels.levels.
• The fish-algae link in the food chain avoids the The fish-algae link in the food chain avoids the intermediate trophic level (e.g., zooplankton) intermediate trophic level (e.g., zooplankton) and thus more efficient energy transfer.and thus more efficient energy transfer.
Source: Horne and Goldman, 1994
Fish diversity in tropical rivers
Fish Species Richness: Tropical and Non-Tropical Rivers
0
200
400
600
800
1000
1200
1400
0 1000 2000 3000 4000 5000 6000 7000
Drainage Basin Area (km2 X 1000)
No
. of
Fis
h S
pec
ies
Tropical
Non-Tropical
Amazon
Mississippi
Mekong
Orinoco
Geographical features of the Nile (left) and Amazon (below) basins. Only the larger tributaries of the Amazon are shown, but 20 of them are >1,000km long. Compare with the simple Nile system.
[Osborne 2000]
Why are fish (and other higher Why are fish (and other higher organisms) so diverse inorganisms) so diverse in
tropical freshwaters?tropical freshwaters?
• Productivity-stability hypothesisProductivity-stability hypothesis
• Structural hypothesisStructural hypothesis
• Competition-predation hypothesisCompetition-predation hypothesis
• Stability-time hypothesisStability-time hypothesis
• Productivity-disturbance hypothesisProductivity-disturbance hypothesis
SpeciesSpeciesDiversityDiversity
DisturbanceDisturbance
Low intensityLow intensity High intensityHigh intensity
CompetitiveCompetitiveexclusionexclusion
ExtinctionExtinction
Low frequencyLow frequency High frequencyHigh frequency
IntensityIntensity
FrequencyFrequency
LowLow
HighHigh
LowLow HighHigh
Maximum diversity
Maximum diversity
Some common tropical fish familiesSome common tropical fish families
• Southern Asia: Southern Asia: Cyprinidae (minnows) and Cyprinidae (minnows) and Siluridae (catfishes)Siluridae (catfishes)
• Africa: Africa: Cyprinidae, Siluridae, Characidea Cyprinidae, Siluridae, Characidea (incl. piranhas, tetras), and Cichlidae(incl. piranhas, tetras), and Cichlidae..
• Tropical South/Central America: Tropical South/Central America: Characidea, Characidea,
Siluridae, Cichlidae.Siluridae, Cichlidae.
Cyprinidae – Cyprinidae – Minnow familyMinnow 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
Cycloid scales:Smooth posterior margin
Ctenoid scales:Spiny posterior margin
Siluridae – catfish familySiluridae – catfish family
• Whiskers (barbels) around the mouth• No scales• Stout spines at the dorsal and
pectoral fin origins• Adipose fin• Broad, flat head• Cosmopolitan
CharacidaeCharacidae
TetrasTetras
PiranhasPiranhas
• Closely related to minnows• Adipose fin• Teeth• Often brightly colored,
popular aquarium fishes• Central/South America, Africa
CichlidaeCichlidaeHaplochromine 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.
Species RichnessCosta
RicaUSA
Vascular plants
12,119 19,473
Fish 234 1,101
Amphibians 184 283
Reptiles 258 360
Birds 878 508
Mammals 228 428
Source for Costa Rica data : INBIO – Instituto Nacional de Biodiversidad.Source for U.S.A. data: World Resources Institute
Species RichnessCosta
RicaUSA Costa
Rica /10,000km2
USA /10,000
km2
Vascular plants
12,119 19,473 2,372 210
Fish 234 1,101 46 12
Amphibians 184 283 36 3
Reptiles 258 360 50 4
Birds 878 508 172 5
Mammals 228 428 45 5
Source for Costa Rica data : INBIO – Instituto Nacional de Biodiversidad.Source for U.S.A. data: World Resources Institute
Protected areas and National Parks of Costa Rica
www.costaricabureau.com/nationalparks.htm
National system of parks and reserves - 34 protected areas, including 28 national parks, covering almost 1,5 M acres (573,000 ha) or 11% of the country’s land area (27% with forest reserves and wildlife refuges included).
Freshwater fishes of Costa Rica.• Diversity
– 234 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.
Gatún Lake, Panama(423 km², 5.2 km³)
• Part of Panama Canal, formed in 1912 by damming the Chagres River.
• Acts as water storage for the canal during the dry season.
• 202,000m ³ of water is released from the lake when a ship passes through to the Gatún locks.
• 14,000 vessels travel through the lake each year.
• Gatún dam is the primary source for Canal operations.
Cichla ocellaris (peacock bass)• Native to the Amazon River in Northern South America. • Introduced to Panama in the late 1960’s.
Zaret and Paine 1973. Species Introduction in a Tropical Lake. Science. 182, 449-455.
Danger of introducing top predators: The case of introduction of Cichla ocellaris to Lake Gatun (Panama)
(Osborn 2000, after Zaret & Paine 1973)
Generalized food web of common Lake Gatun populations before (top) and after (bottom) the introduction.
Thick arrow – food items of major importanceThin arrow – food items of minor importance
[A = tarpon; B = black tern; C = several species of herons and kingfishers; D, E, F, = small perch like fish, silversides and other characins; G = lifebearers; H = cichlasoma; I, J = zooplankton and insects; K, L = algae; M= adult Cichla ocellaris; N = young Cichla.]
Comparing a non-Cichla site in Trinidad Arm and a Cichla site near the shores of Barro Colorado Island
Other Management Problems
Mosquito
Common Black Tern (Chlidonias niger)
Atlantic Tarpon (Tarpon atlanticus)
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!
Cyprinids
No stomach; digest mostly already decaying algal cells
and detritus/bacteria
Some viable algal cells pass through gut
excreted after being enriched in the nutrient-rich digestive
system.
Cichlids
Have stomach and the ability to secrete acid.
Lyse algal cell walls
High assimilation efficiency for phytoplankton
High excretion of dissolved nutrients.
Hypothesis on the effects of different seston-filtering phytoplanktivorous fish(Schiemer 1996)
Cyprinid
Cichlid
Some excre
tion of
viable algal ce
lls
Excretio
n of
DOM/nutrients