Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 217

and eventually sink in deeper waters. Furthermore, critical situations are avoided
because oxygenated conditions are re-established by water exchange and mixing. 91

5.4 CHANGES IN LAGOON PROCESSES AND MANAGEMENT

OF LIVING RESOURCES

Because of their high biological productivity, lagoons are usually used for fisheries
and aquaculture exploitation.
Coastal lagoons constitute an integral part of marine fisheries and provide impor-
tant spawning and nursery grounds for many fish species. These lagoons and their
wetlands provide valuable products and services, which include supporting the
fisheries and protecting biodiversity. 125 The fishery yield is dependent upon a
lagoon's geographical location and morphometry. The significant environmental and
anthropogenic factors determining the yield are the exchanges of water between
lagoons and the ocean, the physico-chemical properties of the water, the extent of
aquatic vegetation, and the fishing pressure. The single influence of freshwater input
may be considered negligible while the influence of oceanic tide through the inlets
is significant. The single most influential factor of all is the fishing pressure. 126
Small, intermittently open lagoons present a complex management problem. These
water bodies are easily degraded in terms of water and sediment quality, and are often
opened mechanically because of flooding or health concerns. However, fish assem-
blages change after opening, with several species being recruited from the ocean. The
importance of these areas as fish habitats and the recorded changes in fish assemblages
after opening suggest that careful management is required. 127 Improvement of fisheries
implies enhancement of fish production by manipulation of the existing fish population,
and/or optimization of the aquatic environment for fish production. Regular stocking
with fish may be necessary to compensate low natural recruitment, or disruption of
recruitment in the case of land-locked lagoons.
High pollution levels in coastal lagoon waters caused significant changes in fish
species composition. For instance, in the Curonian Lagoon, for many commercial
migratory and predatory fish species such as salmon ( Salmo salar ), twaite shad
( Alosa fallax ), zanthe ( Vimba vimba ), eel, pikeperch, pike, and turbot harvests are
much lower than 40-50 years ago. 128
The aquaculture systems more suitable to lagoon environments include cages
for fish and shrimp and systems for rearing bivalve molluscs or seagrasses on
artificial substrates. 103 Other important systems are fish and shrimp culture in earth
ponds, a growing economic activity in Mediterranean countries, and bivalve culture
in tidal flats in leaky lagoons such as the Ria Formosa (Portugal). In this lagoon it
is estimated that 5 to 8% of the natural wet area is occupied by bivalve culture
(between 500 to 800 ha), with a production of clams of more than 2000 tons per
year. This is an important economic activity, representing more than 90% of income
of aquaculture products. 129 However, extreme and fluctuating environmental condi-
tions of choked and restricted lagoons make aquaculture activity infeasible because
of the low growth rates and fecundity of the individuals. 130 Furthermore, aquaculture
may be both a polluter of the littoral and a petitioner for clean water. 130,131 In addition,