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Fish Conservation and
Management
CONS 486
Trophic pyramids, food webs, and trophic
cascades… oh my!
Ross Chapter 2, Diana Chapter 1
Trophic interactions
• Limnological classification review
• Trophic pyramids and productivity
– Food webs
• Trophic cascades
– Examples
Major theme: Linking science to
conservation & management
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Physiology
Behaviour
Population ecology
Ecosystem ecology
Habitat data
(limnology,
oceanography)
• Life history
• Protecting
populations &
habitats
• Restoring
populations &
habitats
Basic science
Applied
science
Conservation
Management
• Fisheries
exploitation data
• Applied life history
data
• Human
dimensions: socioeconomic data
• Harvest regulations
• Managing fisheries
& habitats
Introduction
• To conserve fish, it’s not enough to only understand
how individual species may compete or prey upon
– Must also take a larger view and consider how
communities (groups of species) interact
• Trophic level interactions can differ among different
aquatic systems
– E.g., epilimnetic vs hypolimnetic systems
– Low order vs high order stream systems, etc.
• Very exciting review of limnological terms and
locations!
Lake Zonation
or Pelagic zone
Cole 1983
Lake zonation: Littoral zone
• Shoreline areas extend to edge of rooted
vegetation
– High erosion due to wave and ice action
therefore relatively coarse sediments
• Subject to fluctuating temperatures, can be
very warm in summer
Open-water limnetic zone
Deep-water profundal zone
6
Lake zonation: Littoral zone
• Well lit, high plant growth, large inputs of LW
and leaf litter
– Due to wave action and gravity, eventually this
detritus moves out of littoral zone
• High production of aquatic invertebrates on
plants and substrate
• Macrophytes, rocks and large wood create
good rearing areas for fish
– Predominantly perciformes and some cypriniformes
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Lake zonation: Limnetic zone
• Open water, little influence of large wood or other
structures
• Plankton zone (phyto and zoo)
– Lots of sunlight, photosynthesis
– O2 production
• No macrophytes
• Rearing area for planktivorous fish
– Kokanee/sockeye fry, whitefish
Open-water limnetic zone
Deep-water profundal zone
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Temperature
Epilimnion – homogeneous and warm
Depth
Metalimnion - thermocline
Hypolimnion –
homogeneous and
cool
Lake zonation: Profundal zone
• Includes benthic zone (ecological region along substrate)
• Bottom sediments, soft and muddy, very little physical structure
– Most decomposition occurs here, sediments can get anoxic
• Supports inverts which often tolerate low oxygen
• LW, litter or sediment from riparian/hillslopes settle here
• If O2 is adequate, spawning habitat for bottom dwelling fish
– Suckers, burbot, lake trout and other salmonids
Open-water limnetic zone
Deep-water profundal zone
10
Lake trophic (productivity) status
• Two fundamental lake types at either end of the
ageing and productivity spectrum
– Oligotrophic and eutrophic
Oligotrophic
Eutrophic
Oligotrophic lakes are:
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Young and deep
Nutrient input from watershed is low
Small littoral area with few plants
Low levels of detritus and decomposition
Abundant oxygen throughout entire lake
Low phyto, zooplankton and fish production
Small epilimnion relative to hypolimnion
Hypolimnion well oxygenated all year therefore
good habitat for some fish (salmon!)
Oligotrophic lakes
Eutrophic lakes are:
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Old and shallow
Nutrient rich
High phytoplankton and plants
Large littoral and epilimnion
– Contributes to abundant warm water fish
• Hypolimnion small and anoxic/hypoxic
– Poor salmonid habitat
Eutrophic lakes
Trophic pyramids
• Trophic pyramids: display food structure of an
ecosystem
– Illustrates the productivity and types of organisms in
consecutive trophic levels
5th trophic level: quaternary consumer
4th trophic level: tertiary consumer
3rd trophic level: secondary consumer
2nd trophic level: primary consumer
1st trophic level: producer
Trophic pyramids: Lakes
• Different productivity pyramids for a typical lake within the
two stratified layers and in the littoral zone in both
eutrophic and oligotrophic lake
– Note different base of pyramid yet piscivores rule!
Diana Figures 1.2 and 1.3
Trophic pyramids: Streams
• River continuum concept: continuum between
narrow low order streams and wide high order
streams
Pyramids vs webs
• Trophic pyramids provide a simple way to examine
energy flow in a system
– But do not reveal the typical complexities and multiple
energy pathways that exist…
• Food webs!
VS
General aquatic food web
Pelagic
or
Limnetic
areas of
lakes
Profundal
and
littoral
(and
streams)
General aquatic food web
• Food webs
–Arrows show energy flow
–Complexities arise because the various subsystems (e.g. epilimnion, hypolimnion,
littoral) are linked in space so energy moves
between them
• E.g., in a lake, disparate areas like pelagic and
littoral areas get linked by detrital, bacterial
and nutrient cycles
–Especially once lakes start to mix
Mark David Thompson
Aquatic and terrestrial webs are linked
Trophic cascades
• Predators can cause changes to their prey
populations
– BUT predators can cause changes to populations in
trophic levels beyond those they feed on
• In these instances, top predators are considered a
keystone species
– Their presence affects total trophic structure
Trophic cascades
• Trophic cascades characterized by relatively simple
food webs
– The more “chain-like”, the more likely it is to occur
• Imagine a scenario with a single piscivore, a few
panktivores, herbivores, and phytoplankton
piscivore
Herbivore
biomass
herbivores
Planktivore
biomass
planktivores
Phytoplankton
biomass
phytoplankton
Piscivore
biomass
Trophic cascades
• Early 1900s, Alaskan coast had lush kelp communities with
thriving otter, seal and bald eagle pops
• Hunting reduced mammal pops and they had few kelp beds
• Sea otters legally protected 1911
– Habitat they occupied began to grow lush kelp communities
• Otters prey on sea urchins which graze on the kelp,
• Thus, humans kept otter populations down, which led to
high urchin populations, which led to low kelp populations
humans
otter
biomass
otters
sea urchin
biomass
sea urchins
kelp
biomass
Kelp
human
Predation intensity
Trophic cascades: experimental results
• Long Lake (Michigan) with largemouth bass present (right) and
experimentally removed (left)
• Bass decrease zooplantivorous fishes
– zooplankton have less predation & increase in abundance
– more zooplankton consumes phytoplankton (incl algae)
• Less algae/phytoplankton means clearer water
Absent
Estes et al. 2011 Science
Present
Applying Trophic-cascades: ‘Bio-manipulation’
• University of Wisconsin, Lake Mendota, Madison Wisconsin
• Nutrient run-off leading to algal bloom
creating odor and O2 issues in the lake
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Added 300 adult bass (piscivores)
1 year later other fishes (minnows, zooplanktivores) eliminated!
Zooplankton biomass doubled, phytoplankton biomass halved
– Water clarity improved and odor problem solved
Trophic Cascades can occur in large systems
• Lake Michigan started salmon hatchery programs in
1970s & 80s
• By mid-1980s the main pelagic prey of adult salmon
(alewife) had reached record low numbers
– Simultaneous to this was a large increase in daphnia (a
large bodied zooplankton)
– Otherdaphnia
plankton abundances remained unchanged
alewife
Top-down vs bottom-up
Trophic cascades illustrate ‘top-down’ influences:
Predation controls abundance at each successive lower trophic level
A ‘bottom-up’ phenomenon would involve lower trophic levels influencing
successively higher ones (eg. via nutrient or food availability – can be largely affected
by stochastic effects of climate)
‘Top-down’ patterns are largely affected by biotic processes whereas ‘Bottom-up’
patterns by abiotic processes.
Both processes can occur in aquatic ecosystems
– Bottom-up often influences lower trophic levels
– Top-down often influences higher trophic levels