Relationships between fish predators and prey
Bottom up
•Richer systems have higher productivity at all trophic levels
•Enrichment usually increases the biomass of the top trophic level in the web
and their prey’s prey.
•(example: Fertilization to enhance sockeye salmon)
Top down
•Predators usually reduce the biomass of their prey
•And cause changes in the structure of prey communities
•Lake Michigan example
Bottom-up effect: Reductions in fish biomass usually accompany reductions in nutrient loading
Top-down effects of zooplanktivorous fish on zooplankton communities
Hrbacek (1964)
Brooks and Dodson (1966)
•Generally in lakes where
zooplanktivorous fish are
the top trophic level there is
•reduced zooplankton
biomass, &
•shift in community
compositon toward smaller
species and species with
more effective defenses
•Similar effects have been
noted in benthic
invertebrate communities.
Original Lake Michigan Food web
Lake trout Trophic position 4-4.5
“Once upon a time”
Benthos& zooplankton
sedimentation
Phtoplankton
Offshore food chain
Benthic algae
Aquatic
macrophytes
&detritus
Inshore food chain
Changes in the Lake Michigan Food web during the 60’s
Top-down cascade
Lake trout Trophic position 4-4.5
Lamprey wipes out lake trout
Alewife invades and outcompetes other zooplanktivores; becomes very abundant
Mysis very abundant
Benthos& zooplankton
Large zooplankton decimated
sedimentation
Phtoplankton
Algal blooms
Transparency drops
Offshore food chain
Benthic algae
Aquatic
macrophytes
&detritus
Inshore food chain
Reduction of littoral zone
Phylum Chordata—animals with a notochord, gill clefts and a dorsal nerve cord at some
stage of their life, most species with sexual reproduction (dioecious)
SubPhylum Vertebrata—animals with a vertebral column
Class Agnatha—jawless vertebrates
Family Petromyzontidae—lampreys
•Naked (no scales) eel-like body, no bones
•Jawless mouth with a circular sucking disc with rasping teeth
•Dorsal and caudlal fins but no paired fins,
•7 gill openings, no gill covers, single median nasal opening between eyes.
Adult lampreys 0.3-0.8 m long
Life cycle—many species anadromous and parasitic—spawn in freshwater, develop into a blind
and toothless ammocoetes larva that burrows into mud in streams and lakes--detritivore.
Larva 5-30 cm
Metamorphosis into a an adult with 1-7 yrs, followed by migration to sea. Adult of most species parastic on
marine salmonids—ruptures skin and sucks blood and fluids. Some freshwater species that are not parasitic
spawn soon after metamorphosis and die. All adults die after spawning.
Lake trout with parasitic marine lampreys—Petromyzon marinus
http://www.aquaticcommunity.com/news/Lamprey-fish.jpg
Test of the top-down cascade theory: introduce pacific salmon
Biomanipulation experiment
Alewife declines
Benthos& zooplankton
sedimentation
Large zooplankton recover
Phtoplankton
Benthic algae
Aquatic
macrophytes
Algal blooms stop
Transparency increases&detritus
Offshore food chain
Inshore food chain
Littoral zone expands
Zebra mussel invading a compartmentalized food web:
a combination of top-down & bottom-up effects
Prior to the
zebra mussel
invasion, the
rich nutrient
regime
allowed the
phytoplankton
to shade out
the littoral
zone
vegetation
A
H1
A2
H3
H2
As water
clears
light
reaches the
bottom and
plants
& benthic
algae
grow
F1
F2
P1
P2
Light is a key physical factor—determines the boundaries within which
photosynthesis (primary production) can take place
Rooted plants cannot grow at depths beyond the light limit.
In offshore regions where the bottom is below the photic zone suspended
phytoplankton are the main photosynthetic organisms
Photic zone
Light limit
Phytoplankton compete for light with littoral vegetation (macrophytes, epiphytic,
and benthic algae) and enrichment by nutrients usually leads to a reduction in
the extent of the littoral zone community.
The zebra mussel is a mytilid mussel that has no competitors nor
efficient predators in North American freshwater communities.
Zebra mussels quickly covered all hard substrates including the
posterior siphonal region of nearly every native unionid clam in the
river, and are presently endangering the whole NA unionid fauna.
The establishment of zebra mussels in North America
“…the occurrence of its veliger larvae in the plankton…greatly
enhances its potential for introduction to the Great Lakes
through ballast water. If introduced Dreissena could establish
itself in North America.” Bio-Environmental services
consulting report 1981
The Beaches of
Lake Erie in 1988—
2 years following its
initial discovery in
Lake St. Clair.
Screens and pipes clogged
with zebra mussels
An expensive problem
In 2 yrs clogged pipes
and screens cost the
town of Monroe
Michigan (pop
45,000) $300,000
Clogged screens shut down power plants
Hydroblasting
zebra mussels
off the intake
screens of
power stations
and water
treatment plants
Young mussels
will clog these
screens again in
about 2 months.
Jobs, Jobs, Jobs !!
Zebra mussels are good for the economy!!
They’re providing lots of work.
The effect of sewage addition on the
food web of a downstream community
Most general prediction was that enrichment
by a combination of nutrients and sewage
derived organic matter would enrich benthic
communities, and this would lead to a general
increase in biomass of the whole food web.
Stable isotope
signatures
indicate that
benthic
invertebrate
biomass in the
sewage plume
was
>60%
originated from
sewage derived
organic matter
even though no
long-term
deposition was
occurring.
No shifts in community composition toward “anoxia tolerant” forms
were observed--the assimilation capacity was not being
exceeded by the loadings.
deBruyn and Rasmussen 2002, Ecol. Appl.
Trophodynamic model of biomass response
Outside weed beds
Inside weed beds
+
+
_
+
Epiphytic algae & POM +
Epiphytic invertebrates
Benthic
Invertebrates
SDOM
+
Particulate organic matter (POM)
And benthic algae
SDN
SDOM
SDN
How does sewage enrichment
affect the size-structure of fish
communities?
a) empirical size structure
models, bottom-up
b.) food-web dynamics
bottom up enrichment
coupled to a top-down cascade
The data clearly favoured the predictions of the food web model.
Large fish were the most enhanced relative to reference, the topdown cascade suppressed the biomass response in the small fish
size classes.
The impact of stocking bass
(warm-water piscivores) on lake food webs
Smallmouth bass
rockbass
Largemouth bass
•Most Canadian lakes now present thermal regimes
that are easily suitable for warm-water species, but
dispersal barriers have over most of the landscape
prevented invasions by bass, sunfish, catfish, and
most of the cyprinid family
•Human activities—stocking by fisheries agencies,
fishing with live bait, and drainage alterations have
caused a large scale expansion of the range of such
species into Canadian lakes occupied previously by
cold and cool water fish communities.
•Ontario Ministry people argued that bass would
coexist well with native lake trout since the latter
was a pelagic offshore predator, and the bass
would occupy the littoral. Thus the two piscivores
would be separated by a thermal barrier.
•That is, autecological considerations led to the
predictions that because the two species were
very different they would live in separate worlds
and not affect each other.
Food web considerations suggest that small lake food
webs would be dramatically altered by bass stocking
Lake trout
Bass
Trophic position
3.0-3.5
Trophic position
3.5-4.0
Benthos& zooplankton
Sedimentation
Phtoplankton
Benthic algae
Aquatic
macrophytes
&detritus
Food web considerations suggest that small lake food
webs would be dramatically altered by bass stocking
Lake trout
Bass
_
Trophic position
3.0-3.2
+
+
Trophic position
3.5-4.0
_
+
_
_
_
+
Benthos& zooplankton
_
Sedimentation
+
Phtoplankton
Benthic algae
Aquatic
macrophytes
&detritus
+
+
_
What changes did we actually see in our study lakes?
•Rapid increases in bass populations
•Rapid declines in lake trout biomass, growth rate , and use of
littoral habitat
•Reduction in lake trout trophic position from 3.5-3.8 down to
3.0-3.2. (a mixed diet of fish and invertebrates was replaced
by a nearly complete dependence on invertebrates)
•10-20 fold reduction in forage fish abundance, and
reductions in community diversity from 5-11 species to 1-3
species.
•Increased abundance of phytoplankton (blooms) and littoral
invertebrate abundance and species diversity.
Top-down effects.
Predators selectively remove vulnerable prey, and make it
possible for species and varieties that have better defense
mechanisms to win out over faster growing competitors that
lack defenses.
Prey defense mechanisms
•Reduced detectability
Smaller size, transparency, less turbulence
•Defensive behaviour
Vertical migration and night time activity, and avoidance
responses
•Unpalatability
Spines, toxicity
•Altered life-cycle
Diapause and speeding up life-history
Small size can be an effective defense
Example
Brooks and Dodson study
•Shifts toward smaller size
in predation impacted
communities.
Why do large herbivorous
zooplankton dominate
communities when there are no
zooplanktivores?
The size efficiency
hypothesis
Which Daphnia can deplete its
food supply the most and still
survive on it?
Why are larger Daphnia more
efficient than smaller Daphnia at
filtering even tiny algae?
Reduced visibility/ less pigmentation also works
In fishless lakes zooplankton are strongly pigmented,
mostly with carotenoid pigments that they obtain from
algae
In lakes with zooplanktivorous fish, zooplankton are
usually nearly transparent and thus very hard for fish to
see
Why do you think that pigmented zooplankton species and
varieties win out over transparent ones in fishless lakes?
Defensive behaviour
In fishless lakes many invertebrates swim about freely in
the water column of both lakes and streams during the
daytime
Where fish are present, they usually confine such
behaviour to the night hours and hide in the bottom during
the day.
Defensive behaviour: vertical migration
The effect of zooplanktivorous fish onvertical migration of herbivorous zooplankton
Defensive behaviouir: escape responses
McPeek’s studies on the escape response
of damselflies
Damselflies in fishless lakes are preyed
on heavily by dragonflies
The species that live in lakes with fish
usually respond to a nearby fish by
remaining motionless
The species that live in lakes without fish
respond to dragonflies and other
invertebrate predators by rapidly moving
a short distance.
Morphological Defenses
Spines and other extensions of the body
are a good defense against
zooplanktivorous fish
Daphnia with and without helments
Successful species invasions often involve unpalatable species
Fish predators generally avoid zooplankton with large spines
Sticklebacks are small fish that are extremely well defended against
piscivorous fish—large dorsal spines, pelvic spines, and armoured plates
Sticklebacks in fishless lakes have much smaller spines and much fewer
Armoured plates
Sunfish have both spines and deep body shape that can exceed most predator’s
gape..
As a result, most pumpkinseeds older than 1 or 2 years are rarely preyed upon by
pike or bass.
Top-down effects.
Predators selectively remove vulnerable prey, and make it
possible for species and varieties that have better defense
mechanisms to win out over faster growing competitors that
lack defenses.
Prey defense mechanisms
•Reduced detectability
Smaller size, transparency, less turbulence
•Defensive behaviour
Vertical migration and night time activity, and avoidance
responses
•Unpalatability
Spines, toxicity
•Altered life-cycle
Diapause and speeding up life-history
Defensive behaviour: night-time drifting in streams
Effect of brook trout on the drift response of benthic invertebrates
response = drift density (#/m3) / abundance (#/m2)
no f ish
0.030
f ish (0.5/m2)
Drift response
0.025
0.020
0.015
0.010
0.005
09-10
13-14
16-17
18-19
20-21
21-22
23-24
Time of day
•In completely fishless streams there is usually no difference between day and night
drift of invertebrates, but where drift feeding fish are present there is usually a sharp
increase in drift at night.
•The differences seen here (fish/no fish) are a result of consumption depleting the #/m3
of drifting inverts.
Drift net in a small Creek
Invertebrates that commonly occur in the drift
Some common mayfly
larvae (Ephemeroptera)
Net-spinning caddis larvae
(Trichoptera)