Functional role of fish in tropical freshwaters Phytoplankton consuming fish are common in the tropicsPhytoplankton consuming fish are common in the tropics.

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.

The role of fish in material cycling in tropical flood plains

Source: Horne and Goldman, 1994

Global freshwater fish species richness(Fernando, 1994)

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

Some Amazonian highlights….

Red piranha

Black pacu

Electric eel

Candiru

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.

Size of Costa Rica: 51,100 km2, or less than half the size of Ohio (116,096 km2)

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

Size of Costa Rica: 51,100 km2, or less than half the size of Ohio (116,096 km2)

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).

You will find temperate plants and birds in the highlands

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.

Belonesox belizanus (pike killifish)   Poeciliids

Cichlids

3 feet

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.

Spread of Cichla

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.]

Decrease in Native Fish Species

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

Tilapia Barbus Cyprinid

March 1980 August 1982

Zsecchi (cm) 100-130 50-80

Chl a (ug l-1) 14.2 43.9

Seston (mgC l-1 3.58 5.32

Production (mgO2/mg chl a hr-1) 25.5 17.8

Labile P/Chl a 0.60 0.24