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
• Harvest regulations
• Managing fisheries & habitats
• Protecting populations & habitats
• Restoring populations & habitats
• Fisheries exploitation data
• Applied life history data
• Human dimensions: socio-economic data
• Physiology
• Behaviour
• Population ecology
• Ecosystem ecology
• Habitat data (limnology, oceanography)
• Life historyBasic science
Applied science
ManagementConservation
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
Cole 1983
or Pelagic zone
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
6
Open-water limnetic zone
Deep-water profundal zone
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
7
Open-water limnetic zone
Deep-water profundal zone
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
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De
pthTemperature
Epilimnion – homogeneous and warm
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
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Open-water limnetic zone
Deep-water profundal zone
Lake trophic (productivity) status• Two fundamental lake types at either end of the
ageing and productivity spectrum
– Oligotrophic and eutrophic
EutrophicOligotrophic
Oligotrophic lakes are:• 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:• 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
1st trophic level: producer
2nd trophic level: primary consumer
3rd trophic level: secondary consumer
4th trophic level: tertiary consumer
5th trophic level: quaternary consumer
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
Pelagicor
Limneticareas of
lakes
Profundal and
littoral(and
streams)
General aquatic food web
General aquatic food web
• Food webs
–Arrows show energy flow
–Complexities arise because the various sub-systems (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
Mar
k D
avid
Th
om
pso
n
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
Herbivorebiomass
Piscivore biomass
Phytoplanktonbiomass
Planktivorebiomass
piscivore
herbivores
planktivores
phytoplankton
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
otterbiomass
human Predation intensity
kelpbiomass
sea urchinbiomass
humans
otters
sea urchins
Kelp
Estes et al. 2011 Science
Absent Present
• 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
Trophic cascades: experimental results
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
• 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 high hatchery salmon 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)
– Other plankton abundances remained unchanged
Top-down vs bottom-upTrophic 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 higherones (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
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