Chapter 21 Seagrasses, Fish, and Fisheries
517
move over seagrass beds at night to feed, when reefs
are near seagrass beds.
B. Tidal Differences
Some species may move into seagrass with the flood
tide (Robertson, 1980), and other species are likely
to move away from seagrass beds, particularly at
extremely low tides when stranding may be a problem (Sogard et al., 1989c). Predators are expected
to move into seagrass habitats with the rising tide.
Therefore, predation rates should be higher at high
tide than low tide, although this may be dependent
on the tidal range. No significant differences in predation rates were found between tides (low vs. high)
at a temperate and a subtropical site (Peterson et al.,
2001). However, survival of soft crabs was greater
during low tide than high tide, and this result was
possibly caused by increased encounter rates between predatory crabs and molting individuals at
high tide (Ryer et al., 1997).
C. Intra-annual Variability
Many studies have found seasonal differences in assemblage structure (e.g. Connolly, 1994a) and have
often related these differences to environmental variables (e.g. temperature, salinity) or to characteristics
of the seagrass (e.g. changes in shoot density). Frequently, differences will reflect recruitment of juveniles at several times of the year and subsequent
movement of larger individuals to adjacent habitats
(see also “Temporary residents of seagrass beds”,
Section II.C). Temporal variation in recruitment may
also be influenced by coastal hydrodynamic processes which alter the delivery of larvae (Jenkins
and Black, 1994; Hamer and Jenkins, 1996).
D. Inter-annual Variability
Most studies are conducted on time scales of only
1–2 years at most, making them more amenable to
seasonal comparisons than to comparisons among
years. Few studies have focused on time scales
greater than 5 years, and most of these have considered estuaries as a whole rather than assemblages
within seagrass habitats. An exception was Jackson
and Jones (1999) who sampled some sites that supported seagrass over a 10-year period. They found
significant annual variation in structure of assemblages and attributed this variation to differences in
recruitment success. Changes in weather patterns
among years (e.g. El Nino southern oscillation) may
cause inter-annual variation in magnitude, timing,
and number of recruitment events (Jenkins et al.,
1997a). A few long-term studies have also followed the decline, loss, and recovery of seagrass and
associated fish assemblages (e.g. Anderson, 1989;
Hughes et al., 2002) (see also “Fate of fisheries when
seagrass habitat has been lost”, next and “Correlation between seagrass cover and fisheries catch”,
Section VI.C).
VI. Links Between Seagrass and Fisheries
A. Fate of Fisheries When Seagrass Habitats
Have Been Lost
Loss or changes in species composition of seagrass
may affect abundance and diversity of fish within
seagrass beds, and the habitat to which juvenile fish
from seagrass beds may move as adults. Numerous examples exist of widespread loss of seagrass
beds (Table 3). For example, more than 4,000 ha of
seagrass beds in Florida Bay were completely lost
between 1987 and 1991, and an additional 23,000
ha were affected to a lesser degree (Robblee et al.,
1991). Likewise, in the Galveston Bay system of
Texas, 80% of submerged aquatic vegetation (SAV)
has been lost since the 1970s (Pulich and White,
1991). Little information is available on how seagrass losses affect fisheries resources, and most of
it is anecdotal. For example, although there were
severe declines in soft shell and razor clams, lobsters, and mud crabs following the decline of seagrass at Cape Ann (Massachusetts, USA) in the
1930s, declines in fisheries of the North Atlantic
were not as drastic as predicted (Thayer and Phillips,
1977). Sheridan et al. (1997) also noted that despite widespread mortality of turtlegrass (Thalassia testudinum) in Florida Bay and recolonization of
mud patches by various algal species and shoal grass
(Halodule wrightii), fisheries did not seem to have
been affected as no long-term declines occurred in
landings of important fishery organisms. However,
in Western Port Bay, Australia, a decline of 70%
of the seagrass cover was paralleled by a decline of
about 40% in commercial fish catches (Jenkins et al.,
1993). Although the results are suggestive of a link
with seagrass decline, alternative explanations such
as overfishing, changes in fishing effort, or larval
input cannot be rejected (Jenkins et al., 1993).
517
move over seagrass beds at night to feed, when reefs
are near seagrass beds.
B. Tidal Differences
Some species may move into seagrass with the flood
tide (Robertson, 1980), and other species are likely
to move away from seagrass beds, particularly at
extremely low tides when stranding may be a problem (Sogard et al., 1989c). Predators are expected
to move into seagrass habitats with the rising tide.
Therefore, predation rates should be higher at high
tide than low tide, although this may be dependent
on the tidal range. No significant differences in predation rates were found between tides (low vs. high)
at a temperate and a subtropical site (Peterson et al.,
2001). However, survival of soft crabs was greater
during low tide than high tide, and this result was
possibly caused by increased encounter rates between predatory crabs and molting individuals at
high tide (Ryer et al., 1997).
C. Intra-annual Variability
Many studies have found seasonal differences in assemblage structure (e.g. Connolly, 1994a) and have
often related these differences to environmental variables (e.g. temperature, salinity) or to characteristics
of the seagrass (e.g. changes in shoot density). Frequently, differences will reflect recruitment of juveniles at several times of the year and subsequent
movement of larger individuals to adjacent habitats
(see also “Temporary residents of seagrass beds”,
Section II.C). Temporal variation in recruitment may
also be influenced by coastal hydrodynamic processes which alter the delivery of larvae (Jenkins
and Black, 1994; Hamer and Jenkins, 1996).
D. Inter-annual Variability
Most studies are conducted on time scales of only
1–2 years at most, making them more amenable to
seasonal comparisons than to comparisons among
years. Few studies have focused on time scales
greater than 5 years, and most of these have considered estuaries as a whole rather than assemblages
within seagrass habitats. An exception was Jackson
and Jones (1999) who sampled some sites that supported seagrass over a 10-year period. They found
significant annual variation in structure of assemblages and attributed this variation to differences in
recruitment success. Changes in weather patterns
among years (e.g. El Nino southern oscillation) may
cause inter-annual variation in magnitude, timing,
and number of recruitment events (Jenkins et al.,
1997a). A few long-term studies have also followed the decline, loss, and recovery of seagrass and
associated fish assemblages (e.g. Anderson, 1989;
Hughes et al., 2002) (see also “Fate of fisheries when
seagrass habitat has been lost”, next and “Correlation between seagrass cover and fisheries catch”,
Section VI.C).
VI. Links Between Seagrass and Fisheries
A. Fate of Fisheries When Seagrass Habitats
Have Been Lost
Loss or changes in species composition of seagrass
may affect abundance and diversity of fish within
seagrass beds, and the habitat to which juvenile fish
from seagrass beds may move as adults. Numerous examples exist of widespread loss of seagrass
beds (Table 3). For example, more than 4,000 ha of
seagrass beds in Florida Bay were completely lost
between 1987 and 1991, and an additional 23,000
ha were affected to a lesser degree (Robblee et al.,
1991). Likewise, in the Galveston Bay system of
Texas, 80% of submerged aquatic vegetation (SAV)
has been lost since the 1970s (Pulich and White,
1991). Little information is available on how seagrass losses affect fisheries resources, and most of
it is anecdotal. For example, although there were
severe declines in soft shell and razor clams, lobsters, and mud crabs following the decline of seagrass at Cape Ann (Massachusetts, USA) in the
1930s, declines in fisheries of the North Atlantic
were not as drastic as predicted (Thayer and Phillips,
1977). Sheridan et al. (1997) also noted that despite widespread mortality of turtlegrass (Thalassia testudinum) in Florida Bay and recolonization of
mud patches by various algal species and shoal grass
(Halodule wrightii), fisheries did not seem to have
been affected as no long-term declines occurred in
landings of important fishery organisms. However,
in Western Port Bay, Australia, a decline of 70%
of the seagrass cover was paralleled by a decline of
about 40% in commercial fish catches (Jenkins et al.,
1993). Although the results are suggestive of a link
with seagrass decline, alternative explanations such
as overfishing, changes in fishing effort, or larval
input cannot be rejected (Jenkins et al., 1993).