The Patos Lagoon Estuary, Brazil
181
matter to coastal waters (Abreu and Castello 1997). Natural runoff also
controls the quality of water in the estuary. High nutrient loads are added
to the lagoon during overflow of deficient sewage treatment facilities
and from large-scale agricultural activities. The eutrophication of limnic
lagoon areas induces changes in the phytoplankton composition and causes blooms {10 9 cells 1- 1 ) of the toxic blue-green alga Microcystis aeruginosa. Sporadic increases of suspended copper and lead concentrations in the
estuary may reflect metal input from industrial effluents and mining
activities in the drainage basin, though metal concentrations generally
correspond to natural background levels. Furthermore, natural runoff
introduces large amounts of sediments into the estuary. Although dredging
(approx. 30,000 m 3 month- 1 ) may solve immediate navigation problems in
the inlet and main shipping channel, the accumulative effect of dredging
has modified circulation patterns and deposition processes in the estuary
and in coastal nearshore areas. Over the last two centuries estuarine water
areas have decreased by more than 11 %, as a result of increased sediment
deposition after the construction of the inlet jetties in 1917 and the disposal of berth material from the expansion of the Rio Grande port. Dredging
and filling has also destroyed as much as 10% of valuable estuarine salt
marshes and has impacted vital Ruppia maritima habitats, owing to direct
burial or to reduced depth distribution following increased water turbidity
(Seeliger and Costa 1997).
For more than a century, trammel-, gill-, and channel net fisheries provided the socio-economic basis for many artisanal fishermen who benefited from the migration of crustaceans and fish in and out of the estuary.
The introduction of synthetic fiber nets, motor-powered boats and
modern means of transport and storage after 1945 significantly increased
fishing efforts, and heavy exploitation during the 1970s caused the collapse
of estuarine stocks of the black drum Pogonias cromis, the white croacker
Micropogonias furnieri and the catfish Netuma spp. In contrast, annual
mean landings (3,106 t) of Farfantepenaeus paulensis over a 20-year period
indicate that the excessive catch of juvenile shrimps does not lead to the
collapse of their fisheries because estuarine stocks depend principally on
recruitment and growth conditions rather than on fishing pressure.
Regrettably, prerequisites for the effective management of fishing activities
in the estuary, such as a clear understanding of the migration of stocks and
reliable landing statistics, have generally been neglected in favor of immediate financial profits and fishing efforts have not decreased (Haimovici et
al. 1998).
Historically, socio-economic interests tend to collide with ecological
needs. Since the present use of the estuary already conflicts with existing
management recommendations, long-term research and multi-faceted
181
matter to coastal waters (Abreu and Castello 1997). Natural runoff also
controls the quality of water in the estuary. High nutrient loads are added
to the lagoon during overflow of deficient sewage treatment facilities
and from large-scale agricultural activities. The eutrophication of limnic
lagoon areas induces changes in the phytoplankton composition and causes blooms {10 9 cells 1- 1 ) of the toxic blue-green alga Microcystis aeruginosa. Sporadic increases of suspended copper and lead concentrations in the
estuary may reflect metal input from industrial effluents and mining
activities in the drainage basin, though metal concentrations generally
correspond to natural background levels. Furthermore, natural runoff
introduces large amounts of sediments into the estuary. Although dredging
(approx. 30,000 m 3 month- 1 ) may solve immediate navigation problems in
the inlet and main shipping channel, the accumulative effect of dredging
has modified circulation patterns and deposition processes in the estuary
and in coastal nearshore areas. Over the last two centuries estuarine water
areas have decreased by more than 11 %, as a result of increased sediment
deposition after the construction of the inlet jetties in 1917 and the disposal of berth material from the expansion of the Rio Grande port. Dredging
and filling has also destroyed as much as 10% of valuable estuarine salt
marshes and has impacted vital Ruppia maritima habitats, owing to direct
burial or to reduced depth distribution following increased water turbidity
(Seeliger and Costa 1997).
For more than a century, trammel-, gill-, and channel net fisheries provided the socio-economic basis for many artisanal fishermen who benefited from the migration of crustaceans and fish in and out of the estuary.
The introduction of synthetic fiber nets, motor-powered boats and
modern means of transport and storage after 1945 significantly increased
fishing efforts, and heavy exploitation during the 1970s caused the collapse
of estuarine stocks of the black drum Pogonias cromis, the white croacker
Micropogonias furnieri and the catfish Netuma spp. In contrast, annual
mean landings (3,106 t) of Farfantepenaeus paulensis over a 20-year period
indicate that the excessive catch of juvenile shrimps does not lead to the
collapse of their fisheries because estuarine stocks depend principally on
recruitment and growth conditions rather than on fishing pressure.
Regrettably, prerequisites for the effective management of fishing activities
in the estuary, such as a clear understanding of the migration of stocks and
reliable landing statistics, have generally been neglected in favor of immediate financial profits and fishing efforts have not decreased (Haimovici et
al. 1998).
Historically, socio-economic interests tend to collide with ecological
needs. Since the present use of the estuary already conflicts with existing
management recommendations, long-term research and multi-faceted