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Aquatic Ecology Course
Zoo 374
Freshwater Ecosystems
Only 3% of the world's water is
fresh. And 99% of this is either
frozen in glaciers and pack ice or is
buried in aquifers. The remainder
is found in lakes, ponds, rivers, and
streams.
Freshwater ecosystem
Freshwater ecosystems cover 0.8% of the
earth's surface. They generate nearly 3% of
its net primary production. Freshwater
ecosystems contain 41% of the world's known
fish sp.
There are 3 types of freshwater ecosystems:
Lentic: slow-moving water, including pools,
ponds and lakes.
Lotic: rapidly-moving water: streams & rivers.
Wetlands: areas where the soil is saturated or
inundated for at least part of the time.
Importance of fresh water
ecosystems
Fisheries
Aquaculture
Sources
Lakes
Rivers
Lakes
Some studies about distribution, origin
and morphometry of the world’s large
lakes were done. Natural lakes, fresh
and salt with a surface area greater
than 500 sq km were included.
They are 253 known lakes.
Large lakes occur on all continents
except Antarctica, but nearly half
of them are found in North America
These data show that the large lakes
of the world occupy a surface area
of 1,456,000 sq km and they have
an estimated volume of 202,000km3
Large lakes account for about 90%
of the total surface area and
volume of water held in all lakes
, . of the world
Fisheries management in Fresh
water ecosystem
Firstly, we have to know the difference between :
Natural aquatic ecosystems
Modified aquatic ecosystems
Natural aquatic ecosystems
* Fishes and other aquatic resources are captured from a great
variety of freshwater ecosystems, most of which are natural.
• Natural aquatic ecosystems include lakes, swamps and
floodplains, collectively called standing waters, and rivers and
streams, collectively called running waters.
*Distribution of natural waters
- The distribution pattern of natural freshwaters among the
continents is uneven which has important implications for
aquatic production, inland fisheries and aquaculture.
- For example, the greatest occurrence of standing waters is in the
relatively unproductive northern areas of the Northern
Hemisphere. In contrast, the distribution of running waters in
the form of perennial rivers is more homogeneous, with the
exceptions of the great deserts of North and Southwest Africa,
the Arabian Peninsula and Australia.
- The status and trends of aquatic ecosystems, both natural and
modified, are closely linked to the condition of adjacent
terrestrial ecosystems.
Modified aquatic ecosystems
• Nearly all inland water bodies have been modified to some
extent through human intervention. For instance, the enriching
effects of excess fertilizers and livestock wastes in the runoff
from farmland and lakes have caused biological environmental
impacts on rivers and streams.
* New ecosystems have been created by the physical modification
of natural aquatic ecosystems, such as rivers and streams that
have been dammed to form reservoirs and irrigation systems
for agriculture.
• Reservoirs themselves can be engineered in special ways to
facilitate, for example, fish passage and fish capture or
otherwise to stimulate increased production through habitat
enhancement for a specific species.
* Natural and modified ecosystems are further transformed in their
particular community structures through biodiversity - by the
introduction of new species - and by periodic stocking.
SO.,,,,,,,
* All modifications affect the fisheries potential of aquatic
ecosystems. Many are subtle, with long-term irreversible
effects and may eventually prove negative.
* Large reservoirs are among the most conspicuous man-made
aquatic ecosystems. Many small water bodies seem natural
but, in fact, are reservoirs that have been created primarily as
community water supplies, to water livestock or to irrigate
crops……….. Such water bodies can have multiple uses,
including fisheries and aquaculture but, to be successful, they
require well-planned integrated watershed management. Rice
paddies are a prime example of an agricultural crop system that
produces a second crop - fish - which in turn enhances the rice
production.
Fish resources in lakes
The degree of resource utilization differs greatly from
lake to lake and according to two main types of
fisheries: demersal/inshore and pelagic/offshore.
Currently, the demersal/inshore resources are heavily
exploited or overexploited. In Asia, there is a relatively
advanced national capacity for the development of
enhanced fisheries but less experience in social and
political issues. African large lakes are receiving
adequate biological attention through a number of
international activities, with research focusing
particularly on lakes Victoria ,Tanganyika and Malawi.
However, governmental support in some African
countries is still low, with little money allocated from
national budgets for their development.
Fish resources in lakes (Cont.)
Africa, especially East Africa, is endowed with
numerous lakes that support very important
fisheries, in turn providing a livelihood to millions of
people and contributing significantly to food supply.
In many of these lakes fisheries are reaching a state
of maturity and consequently management problems
are rising. For 11 lakes, shared by 11 countries of
eastern Africa, fisheries employs close to half a
million people, with perhaps three times as many
engaged in secondary activities and related
services, thus supporting about 4 percent of the
population of the region as a whole
The State of World Inland Fishery
Resources
Fishes, lampreys, amphibians, crustaceans and molluscs
constitute the broad groups of inland resources that
directly or indirectly support fisheries (see FAO
statistics). Some 11 500 fish species - 41 percent of all
fishes - are exclusively freshwater and about 1 percent
are diadromous .
Due to statistical problems, the state of inland water
resources must be implied from other information.
Trends in annual capture data provide one line of
evidence. Globally, the trend for capture has been for
modest annual increases of about 2 percent during the
last two decades of the twentieth century. From a
continental perspective, trends show increases in Asia,
Africa and Latin America, decreases in the
Commonwealth of Independent States and Baltic States
(former USSR), North America and Europe, and stability
in Oceania.
The State of World Inland Fishery
Resources (Cont.)
Another line of evidence on the state of inland resources
comes from the status and trends of ecosystems .
Generally, it can be inferred that aquatic ecosystems are
in a state of decline throughout most of the world. How
can this be reconciled with an apparent trend for
increases in capture output from inland resources?
One reason is a human-induced enriching of aquatic
systems from agriculture and urban sources to produce
more fish. Another is that the combined effects of fishing
and physical and chemical changes in inland waters
have caused a shift to species that are more productive
per unit of area, weight-wise, but may be of lesser
economic value.
Better governance -- with attention to aquatic ecosystem
management such as habitat enhancement and
stocking interventions -- along with broad approaches to
management,
such
as
integrated
watershed
management, also increase inland output.
Factors affecting the fisheries and fish
production in lakes
Natural factors
1) Peripheries erosion
2) Lake area reduction
3) Sea water level rise
4) Lagoon saltation
5) Sea lagoon connection siltation
Human related factors
1) Agriculture
2) Industrial sewage discharge
3) Land reclamation
4) Coastal constructions
5) Illegal fishing
6) Recreational activieties
7) Increasing population
8) Number of boats and fishermen
9) Pollution
These factors, some of them or even one of them may cause drastic
changes in the environmental conditions of lakes all over the world
affecting the fisheries of the lakes as well as the fish communities
inhabiting these lakes.
WHY…..
not
only
lakes
????
Fresh-water habitats are extremely diverse, and include both
still-water environments like lakes and ponds, and flowingwater environments like rivers and streams.
Still-Water Habitats
Lakes and ponds
• Like oceans, lakes have pelagic and benthic zones. The
temperature of lake water varies depending on depth, and
can also change dramatically over seasons. The epilimnion
is the topmost layer of lake water. It is significantly warmer
than deeper areas due to heating by sunlight. The
hypolimnion layer describes deeper, colder lake water. Many
of the nutrients in lakes collect at lake bottoms.
• Turnover occurs when all the water in a lake is nearly
thermally uniform and mixed, distributing nutrients
throughout the water. Turnover occurs twice a year in many
temperate lakes, but may occur only once in subtropical
environments, or not at all in permanently stratified lakes.
Still-Water Habitats (Cont.)
• Lakes also can be described as either oligotrophic or
eutrophic (or in between these two extremes).
•
Oligotrophic lakes have low levels of nutrients and low
productivity. They generally contain cold, highly
oxygenated water and support species adapted to these
conditions. Eutrophic lakes, on the other hand, have
plentiful nutrients and are highly productive. Species that
inhabit eutrophic lakes must be tolerant of low oxygen
levels and warm temperatures. In general, oxygen levels
in lakes depend on the amount of water circulation, the
surface area that is exposed to air, and levels of oxygen
consumption by living organisms.
Still-Water Habitats (Cont.)
Flowing-Water Habitats
Rivers and Streams
The habitats available in rivers and streams differ in several
ways from those in lakes and ponds .
1) Because of the current (flowing water), the water is usually
more oxygenated .
• River species generally have special adaptations for living in water
currents. Some species are sessile and live anchored to the river
bottom. Other species have evolved adaptations such as suckers or
hooks to keep themselves from being washed away. Still other
species are strong swimmers. Many of these have flattened bodies
that help them resist the pressure of the current.
• Compared to lakes, rivers tend to be well-oxygenated because of the
constant motion of the water. Temperatures can change quickly in
rivers, but do not span as great a range as in lakes or other still water.
Because there is less penetration of light in flowing water, plant
diversity is generally lower in.
2) Photosynthesizers play a minor role in the food chains
here; a large fraction of the energy available for
consumers is brought from the land; e.g., in falling leaves.
Thank You for your attention