Fish Conservation and Management

Transcription

1 Fish Conservation and Management CONS 486 Trophic pyramids, food webs, and trophic cascades oh my! Ross Chapter 2, Diana Chapter 1

2 Trophic interactions Limnological classification review Trophic pyramids and productivity Food webs Trophic cascades Examples

3 Major theme: Linking science to conservation & management Physiology Behaviour Population ecology Ecosystem ecology Habitat data (limnology, oceanography) Life history Basic science Applied science Fisheries exploitation data Applied life history data Human dimensions: socioeconomic data Protecting populations & habitats Restoring populations & habitats Conservation Management Harvest regulations Managing fisheries & habitats

4 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!

5 Lake Zonation or Pelagic zone Cole 1983

6 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

7 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

8 Lake zonation: Limnetic zone Open water, little influence of large wood or other structures Plankton zone (phyto and zoo) Lots of sunlight, photosynthesis O 2 production No macrophytes Rearing area for planktivorous fish Kokanee/sockeye fry, whitefish Open-water limnetic zone Deep-water profundal zone 8

9 Depth Temperature Epilimnion homogeneous and warm Metalimnion - thermocline Hypolimnion homogeneous and cool

10 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 O 2 is adequate, spawning habitat for bottom dwelling fish Suckers, burbot, lake trout and other salmonids Open-water limnetic zone Deep-water profundal zone 10

11 Lake trophic (productivity) status Two fundamental lake types at either end of the ageing and productivity spectrum Oligotrophic and eutrophic Oligotrophic Eutrophic

12 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!)

13 Oligotrophic lakes

14 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

15 Eutrophic lakes

16 Trophic pyramids Trophic pyramids: display food structure of an ecosystem Illustrates the productivity and types of organisms in consecutive trophic levels 5 th trophic level: quaternary consumer 4 th trophic level: tertiary consumer 3 rd trophic level: secondary consumer 2 nd trophic level: primary consumer 1 st trophic level: producer

17 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

18 Trophic pyramids: Streams River continuum concept: continuum between narrow low order streams and wide high order streams

19 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

20 General aquatic food web Pelagic or Limnetic areas of lakes Profundal and littoral (and streams)

21 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

22 Aquatic and terrestrial webs are linked Mark David Thompson

23 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

24 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

25 piscivore Herbivore biomass herbivores Planktivore biomass planktivores Phytoplankton biomass phytoplankton Piscivore biomass

26 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

27 humans otter biomass otters sea urchin biomass sea urchins kelp biomass Kelp human Predation intensity

28 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 Present Estes et al Science

29 Applying Trophic-cascades: Bio-manipulation University of Wisconsin, Lake Mendota, Madison Wisconsin Nutrient run-off leading to algal bloom creating odor and O 2 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

30 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 unchanged daphnia alewife

31 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