Chapter 21 Seagrasses, Fish, and Fisheries
513
2. Degree of Heterogeneity or Patchiness
Few studies have investigated how the degree of
heterogeneity of seagrass may influence abundance,
growth, or survival of fish and invertebrates. The
abundance of fish was influenced only by continuity
of vegetation and not by morphology, suggesting
that continuous uninterrupted beds should be preferred to patchy beds due to greater protection from
predators (Salita et al., 2003). Rates of predation on
bay scallops (Argopecten irradians, Pectinidae) were
similar among very patchy, patchy, and continuous
seagrass habitat in short-term experiments (24 h),
but more scallops were taken from very patchy seagrass beds over longer time periods (Irlandi et al.,
1995). Increased predation in seagrass patches may
occur because non-vegetated habitats that surround
patches allow access to predators. Further studies are
needed to test the generality of these patterns.
3. Patch Isolation
Although habitat fragmentation may create a greater
number of habitat patches that are smaller in size
than the original contiguous tract of habitat, it may
also result in spatially isolated remnant patches. The
isolation of patches may affect hydrodynamic conditions with associated affects on benthic assemblages
(Turner et al., 1999). The effect of patch isolation
on abundance, growth, and survival of organisms in
seagrass systems remains largely unexplored (but
see Hovel and Lipcius, 2002). Crab (Callinectes
sapidus, Portunidae) survival was higher in connected patches than where seagrass patches were
isolated, but was not correlated with patch size or
seagrass structural complexity, suggesting that the
non-vegetated sediment surrounding patches may
have acted as a corridor allowing predators access
to seagrass patches (Hovel and Lipcius, 2002). Further empirical research using both fish and invertebrates is needed to determine how isolation may affect abundance, growth, and survival of organisms.
In addition, patch isolation may be particularly important in determining whether organisms can successfully move among patches and to adult habitats.
4. Position of Patches Within Landscape
The species composition and abundance of fish and
invertebrates can vary considerably among seagrass
beds within an estuary (Bell et al., 1988; Sogard
et al., 1989b; Jenkins et al., 1996; Hannan and
Williams, 1998). Frequently, one site within an estuary was found to have consistently high or low abundances of one or two species through time despite
sites being chosen to minimize differences (Ferrell
et al., 1993). For example, a single site in Botany Bay
(NSW, Australia) had consistently high recruitment
among a range of species compared to other sites
within Botany Bay and elsewhere (McNeill et al.,
1992).
The distance to the mouth of the estuary may affect
the distribution and abundance of fish and invertebrates in seagrass beds because larvae of different
species are often found in different parts of an estuary and the structure (height and density of seagrass
leaves), size, and shape of seagrass beds often varies
throughout the estuary (Bell et al., 1988). Because
the structure of seagrass beds may vary throughout
the estuary, differences in vegetational characteristics also may contribute to differences in abundances
(e.g. Sogard et al., 1989a). In addition, water circulation patterns and physical factors (e.g. temperature
and salinity) are likely to vary at this broader spatial scale. Species richness and abundances of common species varied among zones along an estuary
and among arms of the estuary, but these differences were not due to physical complexity of seagrass or to temperature and salinity gradients (Bell
et al., 1988). Differences among zones of the estuary
were attributed to the combined effects of spawning
location and nature of eggs and larvae; species that
spawned within the estuary and had demersal eggs
were most abundant furthest up the estuary, whereas
those that spawned at sea or near the mouth of the estuary and had pelagic eggs were found closest to the
mouth of the estuary (e.g. Bell et al., 1988; Hannan
and Williams, 1998). No studies have investigated
whether growth or survival of organisms may also
vary along this scale.
A conceptual model of factors that have the potential to influence assemblages and densities of fishes
in seagrass beds in Florida Bay was proposed by Sogard et al. (1989a) (Fig. 5). The model suggests that
initial densities of juveniles depend on recruitment
from the larval pool, which is dependent on the intensity of adult reproduction and passive immigration
from other areas. Survival of larvae is constrained
by both physical and biological interactions. Larval
duration, regional hydrodynamics, and larval behavior also may influence recruitment patterns (Jenkins
et al., 1998). Once recruited, juveniles and adults
513
2. Degree of Heterogeneity or Patchiness
Few studies have investigated how the degree of
heterogeneity of seagrass may influence abundance,
growth, or survival of fish and invertebrates. The
abundance of fish was influenced only by continuity
of vegetation and not by morphology, suggesting
that continuous uninterrupted beds should be preferred to patchy beds due to greater protection from
predators (Salita et al., 2003). Rates of predation on
bay scallops (Argopecten irradians, Pectinidae) were
similar among very patchy, patchy, and continuous
seagrass habitat in short-term experiments (24 h),
but more scallops were taken from very patchy seagrass beds over longer time periods (Irlandi et al.,
1995). Increased predation in seagrass patches may
occur because non-vegetated habitats that surround
patches allow access to predators. Further studies are
needed to test the generality of these patterns.
3. Patch Isolation
Although habitat fragmentation may create a greater
number of habitat patches that are smaller in size
than the original contiguous tract of habitat, it may
also result in spatially isolated remnant patches. The
isolation of patches may affect hydrodynamic conditions with associated affects on benthic assemblages
(Turner et al., 1999). The effect of patch isolation
on abundance, growth, and survival of organisms in
seagrass systems remains largely unexplored (but
see Hovel and Lipcius, 2002). Crab (Callinectes
sapidus, Portunidae) survival was higher in connected patches than where seagrass patches were
isolated, but was not correlated with patch size or
seagrass structural complexity, suggesting that the
non-vegetated sediment surrounding patches may
have acted as a corridor allowing predators access
to seagrass patches (Hovel and Lipcius, 2002). Further empirical research using both fish and invertebrates is needed to determine how isolation may affect abundance, growth, and survival of organisms.
In addition, patch isolation may be particularly important in determining whether organisms can successfully move among patches and to adult habitats.
4. Position of Patches Within Landscape
The species composition and abundance of fish and
invertebrates can vary considerably among seagrass
beds within an estuary (Bell et al., 1988; Sogard
et al., 1989b; Jenkins et al., 1996; Hannan and
Williams, 1998). Frequently, one site within an estuary was found to have consistently high or low abundances of one or two species through time despite
sites being chosen to minimize differences (Ferrell
et al., 1993). For example, a single site in Botany Bay
(NSW, Australia) had consistently high recruitment
among a range of species compared to other sites
within Botany Bay and elsewhere (McNeill et al.,
1992).
The distance to the mouth of the estuary may affect
the distribution and abundance of fish and invertebrates in seagrass beds because larvae of different
species are often found in different parts of an estuary and the structure (height and density of seagrass
leaves), size, and shape of seagrass beds often varies
throughout the estuary (Bell et al., 1988). Because
the structure of seagrass beds may vary throughout
the estuary, differences in vegetational characteristics also may contribute to differences in abundances
(e.g. Sogard et al., 1989a). In addition, water circulation patterns and physical factors (e.g. temperature
and salinity) are likely to vary at this broader spatial scale. Species richness and abundances of common species varied among zones along an estuary
and among arms of the estuary, but these differences were not due to physical complexity of seagrass or to temperature and salinity gradients (Bell
et al., 1988). Differences among zones of the estuary
were attributed to the combined effects of spawning
location and nature of eggs and larvae; species that
spawned within the estuary and had demersal eggs
were most abundant furthest up the estuary, whereas
those that spawned at sea or near the mouth of the estuary and had pelagic eggs were found closest to the
mouth of the estuary (e.g. Bell et al., 1988; Hannan
and Williams, 1998). No studies have investigated
whether growth or survival of organisms may also
vary along this scale.
A conceptual model of factors that have the potential to influence assemblages and densities of fishes
in seagrass beds in Florida Bay was proposed by Sogard et al. (1989a) (Fig. 5). The model suggests that
initial densities of juveniles depend on recruitment
from the larval pool, which is dependent on the intensity of adult reproduction and passive immigration
from other areas. Survival of larvae is constrained
by both physical and biological interactions. Larval
duration, regional hydrodynamics, and larval behavior also may influence recruitment patterns (Jenkins
et al., 1998). Once recruited, juveniles and adults