Chapter 21 Seagrasses, Fish, and Fisheries
525
macroinvertebrates, and density of bay scallops (Argopecten irradians, Pectinidae) in both seagrass and
sand flat habitats (Peterson et al., 1987). Clam harvesting did not affect the density or species composition of benthic macroinvertebrates, but led to
substantial decreases in seagrass biomass, a potentially negative impact on recruitment of hard clams,
and a reduction in abundance of bay scallops. Recovery of seagrass biomass occurred within 1 year
in the raking and light clam-kicking treatments, but
seagrass biomass was still around 35% lower than
predicted from controls after 4 years in the intense
clam-kicking treatments (Peterson et al., 1987). Intense mechanical clam harvesting in vegetated areas
also may have impacts on commercial fisheries, as
seagrass biomass is likely to decline.
Trawling differs from dredging in that trawls
are dragged on or near the bottom (Dayton et al.,
1995). Although habitat damage by trawling is not
a major issue in estuarine or nearshore areas other
than in the tropics (Blaber et al., 2000), it has
been reported to destroy Zostera marina in Scotland (Northridge, 1991, cited in Dayton et al., 1995)
and Posidonia beds in the Mediterranean (Dayton
et al., 1995; Sanchez-Jerez and Ramos-Espla, 2000).
The predominant means by which trawling eliminates Zostera is by altering sediment type (Dayton
et al., 1995). In the southwest Mediterranean, boats
often illegally trawl over Posidonia oceanica meadows and cause physical degradation of the habitat.
Sanchez-Jerez and Ramos-Espla (1996) investigated
the effect of bottom trawling over seagrass habitat on seagrass characteristics, and fish and invertebrate assemblages. Although they compared two
sites 6 km apart (one trawled and one untrawled
area) and their results therefore could reflect spatial
variation between sites, they did find differences in
density and assemblage structure of fishes between
the trawled and untrawled areas. At coarse levels of
taxonomic resolution significant differences in assemblage structure of invertebrates could not be detected, but at finer levels (e.g. family and species)
amphipods and isopods showed significant differences between trawled and untrawled sites (SanchezJerez and Ramos-Espla, 1996). To reduce the impact
of trawling in the Mediterranean, anti-trawling artificial reefs have been deployed. Such reefs were
shown to change the assemblage structure of fishes
at a scale of tens to hundreds of meters, mainly
because reef-associated species congregate around
them (Sanchez-Jerez and Ramos-Espla, 2000).
Roller beam trawls are used to capture live shrimp
(primarily pink shrimp, Farfantepenaeus duorarum,
Penaeidae) in seagrass beds of Florida. These shrimp
are used as bait in the recreational fishery. Although
these trawls are designed to roll over the bottom, they
may break off and collect leaves of seagrass (Meyer
et al., 1999). No significant reduction in mean shoot
density, number of blades per shoot, longest blade
length per shoot, total blade length per shoot, and
above and below-ground biomass was detected with
four levels of trawling (1, 3, 9, and 18 trawls) (at nine
areas with Tampa Bay, Florida, USA) suggesting that
roller beam trawls have minimal impact on seagrass
habitat (Meyer et al., 1999). However, potential effects of fishing are likely to be highly dependent on
statistical power (Peterman, 1990). Despite minimal
impact on seagrass, 42 species of fish were caught by
the trawl, and these showed variable (0–100%) rates
of survival; small fish were significantly more susceptible to trawl-induced mortality than larger fish
(Meyer et al., 1999). Because trawling-induced mortality was found to differ among species, roller beam
trawling may alter species diversity and composition.
Commercial harvesting of bay scallops (Argopecten irradians, Pectinidae) is done principally
by using a boat to tow heavy epibenthic gear, whereas
recreational and small-scale commercial harvesters
use hand collecting, hand-pulled dredges, and raking to take scallops (1984). Scallop harvesting may
therefore impact the seagrass habitat itself. Fonseca
et al. (1984) found that as dredging effort increased,
the number of seagrass shoots and seagrass dry
weight per core also decreased. These impacts are
likely to have implications for the habitat itself, as
well as settlement of scallop larvae.
Many juvenile animals are taken as by-catch
by fishers in estuaries and shallow waters. Pink
shrimp (Farfantepenaeus notialis, Penaeidae) are intensively exploited using small-mesh nets in West
African lagoons and estuaries. Along with these
shrimp vast numbers of non-marketable juvenile
fishes also are captured, including juveniles of valuable fisheries (Blaber et al., 2000). Although large
numbers of juveniles are removed, the quantitative
impact on the estuarine fish community and of the
artisanal fishery for adults is unknown (Blaber et al.,
2000). Juvenile fish also may be collected from estuaries for export in the ornamental fish trade. For
example, Monodactylus argenteus (Monodactylidae) and Scatophagus argus (Scatophagidae) are
525
macroinvertebrates, and density of bay scallops (Argopecten irradians, Pectinidae) in both seagrass and
sand flat habitats (Peterson et al., 1987). Clam harvesting did not affect the density or species composition of benthic macroinvertebrates, but led to
substantial decreases in seagrass biomass, a potentially negative impact on recruitment of hard clams,
and a reduction in abundance of bay scallops. Recovery of seagrass biomass occurred within 1 year
in the raking and light clam-kicking treatments, but
seagrass biomass was still around 35% lower than
predicted from controls after 4 years in the intense
clam-kicking treatments (Peterson et al., 1987). Intense mechanical clam harvesting in vegetated areas
also may have impacts on commercial fisheries, as
seagrass biomass is likely to decline.
Trawling differs from dredging in that trawls
are dragged on or near the bottom (Dayton et al.,
1995). Although habitat damage by trawling is not
a major issue in estuarine or nearshore areas other
than in the tropics (Blaber et al., 2000), it has
been reported to destroy Zostera marina in Scotland (Northridge, 1991, cited in Dayton et al., 1995)
and Posidonia beds in the Mediterranean (Dayton
et al., 1995; Sanchez-Jerez and Ramos-Espla, 2000).
The predominant means by which trawling eliminates Zostera is by altering sediment type (Dayton
et al., 1995). In the southwest Mediterranean, boats
often illegally trawl over Posidonia oceanica meadows and cause physical degradation of the habitat.
Sanchez-Jerez and Ramos-Espla (1996) investigated
the effect of bottom trawling over seagrass habitat on seagrass characteristics, and fish and invertebrate assemblages. Although they compared two
sites 6 km apart (one trawled and one untrawled
area) and their results therefore could reflect spatial
variation between sites, they did find differences in
density and assemblage structure of fishes between
the trawled and untrawled areas. At coarse levels of
taxonomic resolution significant differences in assemblage structure of invertebrates could not be detected, but at finer levels (e.g. family and species)
amphipods and isopods showed significant differences between trawled and untrawled sites (SanchezJerez and Ramos-Espla, 1996). To reduce the impact
of trawling in the Mediterranean, anti-trawling artificial reefs have been deployed. Such reefs were
shown to change the assemblage structure of fishes
at a scale of tens to hundreds of meters, mainly
because reef-associated species congregate around
them (Sanchez-Jerez and Ramos-Espla, 2000).
Roller beam trawls are used to capture live shrimp
(primarily pink shrimp, Farfantepenaeus duorarum,
Penaeidae) in seagrass beds of Florida. These shrimp
are used as bait in the recreational fishery. Although
these trawls are designed to roll over the bottom, they
may break off and collect leaves of seagrass (Meyer
et al., 1999). No significant reduction in mean shoot
density, number of blades per shoot, longest blade
length per shoot, total blade length per shoot, and
above and below-ground biomass was detected with
four levels of trawling (1, 3, 9, and 18 trawls) (at nine
areas with Tampa Bay, Florida, USA) suggesting that
roller beam trawls have minimal impact on seagrass
habitat (Meyer et al., 1999). However, potential effects of fishing are likely to be highly dependent on
statistical power (Peterman, 1990). Despite minimal
impact on seagrass, 42 species of fish were caught by
the trawl, and these showed variable (0–100%) rates
of survival; small fish were significantly more susceptible to trawl-induced mortality than larger fish
(Meyer et al., 1999). Because trawling-induced mortality was found to differ among species, roller beam
trawling may alter species diversity and composition.
Commercial harvesting of bay scallops (Argopecten irradians, Pectinidae) is done principally
by using a boat to tow heavy epibenthic gear, whereas
recreational and small-scale commercial harvesters
use hand collecting, hand-pulled dredges, and raking to take scallops (1984). Scallop harvesting may
therefore impact the seagrass habitat itself. Fonseca
et al. (1984) found that as dredging effort increased,
the number of seagrass shoots and seagrass dry
weight per core also decreased. These impacts are
likely to have implications for the habitat itself, as
well as settlement of scallop larvae.
Many juvenile animals are taken as by-catch
by fishers in estuaries and shallow waters. Pink
shrimp (Farfantepenaeus notialis, Penaeidae) are intensively exploited using small-mesh nets in West
African lagoons and estuaries. Along with these
shrimp vast numbers of non-marketable juvenile
fishes also are captured, including juveniles of valuable fisheries (Blaber et al., 2000). Although large
numbers of juveniles are removed, the quantitative
impact on the estuarine fish community and of the
artisanal fishery for adults is unknown (Blaber et al.,
2000). Juvenile fish also may be collected from estuaries for export in the ornamental fish trade. For
example, Monodactylus argenteus (Monodactylidae) and Scatophagus argus (Scatophagidae) are