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

10

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 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)

– Other plankton abundances remained unchangedalewife

daphnia

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