Effects of Changing Environmental Conditions on Lagoon Ecology - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 215

coastal fishes, allowing a high standing stock. Three main groups of fishes may
occur in coastal lagoons:

1. Sedentary species—Those that spend their entire life cycle within coastal

lagoons. This group is very limited, especially when species with plank-
tonic stages are considered. Most lagoon fishes spawn outside of the
lagoon.

2. Seasonal migrants—Those that enter the lagoon during a more or less

well-defined season. This group dominates the fauna of coastal lagoons.

3. Occasional visitors—Those that enter and leave a lagoon without a clear

pattern within the same years and from year to year.

The relative level of recruitment into coastal lagoons is determined by the facility
with which fish can penetrate through the channels and by the total number of
potential recruits along the coast. This implies that recruitment to coastal lagoons
can be artificially increased by keeping the inlets of lagoons open during periods
when juveniles of preferred species occur along the coast, or by deepening the sill
of lagoons with very shallow mouths. 120
Some fisheries management practices are based, in general, on taking advantage
of fish migration by opening or digging outlets to increase the water renewal rate,
increasing at the same time the chance of fishes entering into the lagoon. These
practices, however, can drastically change the composition and structure of natural
assemblages, especially in choked lagoons, inducing high mortality rates of naturally
occurring organisms and replacing them by others from the open sea. There is
evidence showing that in lagoons the increase in renewal rates with the open sea
has involved an increase in the number of species and diversity, accompanied by a
dramatic decrease in the harvest of traditional fish species (mainly Mugilidae and
Sparidae). 65 Further, when the artificial temporary openings are later closed, there
are usually high mortality rates of marine organisms that had entered, 121 especially
if the environment inside the lagoon becomes brackish or hypersaline.
The low migration rates in choked lagoons lead to the populations inside them
potentially remaining isolated for long periods of time with loss of some genotypes.
Under such isolated conditions, populations usually show low genetic diversity and
high homocigosys, although in coastal lagoons the extreme environmental variability
can induce high genetic diversity and heterocigosys. 122

5.3.2 R ESTRICTED L AGOONS

Restricted lagoons have two or more entrance channels or inlets, and flushing times
are considerably shorter than for choked coastal lagoons. As a result, they have a
well-defined tidal circulation and exhibit salinity gradients from open sea to brackish
or hypersaline water. Restricted lagoons are also influenced by winds and are ver-
tically well mixed. Because of their productivity and shelter facilities, restricted
lagoons are used as nursery areas by nektonic crustaceans and fishes.
Restricted lagoons exhibit a well-defined pattern of horizontal zonation of biolog-
ical assemblages according to the model proposed by Guelorget et al. 109 Biological
diversity shows a gradient from maximum to minimum, from the communication