522
Bronwyn M. Gillanders
and control sites. After deployment, the density of
crabs was reduced by one-half to 43-fold, whereas
densities of Nassau grouper (Epinephelus striatus, Serranidae) increased. Predators associated with
patch reefs, thus, had a significant impact on nocturnally foraging crabs in adjacent seagrass meadows. The results of this study highlight an important
trophic link between seagrass habitats and tropical
patch reefs.
D. Tracing the Contribution of Seagrass
Production to Fisheries Species Elsewhere
Using Stable Isotopes
Seagrass beds may indirectly support species in other
areas. Studies tracing the contribution of seagrass to
fisheries species outside of seagrass habitats have
focused on analysis of stable isotopes. Initial studies used a single isotope (e.g. δ
13 C), but later work
used two or more isotopes. First-feeding larval blue
grenadier (Macruronus novaezelandiae, Merlucciidae) were found to be part of a microbial planktonic
food chain that was based on seagrass detritus, which
is exported offshore during storm events (Thresher
et al., 1992). Seagrass detritus also may be important
in the food chain of prawns collected from offshore
waters, although based on the δ
13 C values the primary food source could also be benthic microalgae
or a mixture of seagrass and plankton (Loneragan
et al., 1997).
Using multiple stable isotopes may be particularly powerful for tracing trophic relationships in
coastal waters (Moncreiff and Sullivan, 2001). Surprisingly, few studies have used a multiple stable isotope approach (e.g. δ
13 C, δ
15 N, δ
32 S) to
study food webs in seagrass systems (see Mateo
et al., Chapter 7). The trophic relationships of fish
and invertebrates on the continental shelf of southeastern Australia were examined using stable isotopes of carbon and nitrogen (Davenport and Bax,
2002). Although the results of this study showed
that the main source of primary production was
oceanic phytoplankton, there was some evidence
that seagrass contributed to the benthos because
benthic invertebrates had δ
13 C values intermediate
between particulate organic matter and seagrasses
(Davenport and Bax, 2002). Multiple stable isotopes
have also been used to show that seagrass material
contributes substantially to fisheries species found
over adjacent non-vegetated habitats (Melville and
Connolly, 2003).
E. Juveniles Moving from Seagrass
to Other Habitats
Fish may move between habitats either through the
dispersal of pelagic larvae or by the movements of juvenile or adult fish between habitats. After settling in
seagrass habitats, the juveniles of reef species must
move from seagrass to reach coastal reefs. Where juveniles are found in seagrass as well as other habitats,
the relative contribution of different habitats to adult
populations needs to be determined. A recent paper
reviewed movement from juvenile to adult habitats
and the usefulness of different methods for determining movement (Gillanders et al., 2003). Most
studies have used either changes in abundance of
organisms among separate habitats or a progression
of size classes among the separate habitats to infer movement from juvenile to adult habitats (e.g.
Gunderson et al., 1990; Gillanders, 1997).
A few studies have used mark-recapture methods (e.g. Ruello, 1975, 1977; Koutsikopoulos et al.,
1995), although in many cases such studies examined movement of juveniles from estuaries rather
than seagrass per se. Artificial tagging methods, although providing good evidence for movement, are
difficult to implement in many cases due to the small
size of fish in seagrass beds, the high mortality rates
of juveniles (e.g. Hixon, 1991) and low rates of recapture of tagged fish (e.g. Sheaves, 1995). Large
numbers of organisms must be tagged to produce
even low numbers of recaptures. Recent developments with internal artificial tags (e.g. coded wire
tags, CWT; visible implant fluorescent elastomer,
VIF) show promise as tools for determining movement. Fish as small as 8–11 mm standard length
have been tagged on coral reefs using internal tags
(Beukers et al., 1995); the tags show relatively long
retention times (several months) and few biological side effects. The feasibility of using these tags
may depend on the visibility of the tags over long
time periods (e.g. 1–2 years). Juvenile organisms
grow fairly rapidly, and therefore it is possible that
growth of surrounding tissue may limit visibility of
tags. In addition, use of CWT and VIF tags will require individual handling of fish, which may be difficult for some species due to high mortality rates.
I am not aware of any studies that have used CWT
or VIF tags to track movement of organisms from
juvenile seagrass to adult habitats. Remote sensing
capabilities that do not rely on recapture per se are
an advance over mark-recapture techniques, but are
Bronwyn M. Gillanders
and control sites. After deployment, the density of
crabs was reduced by one-half to 43-fold, whereas
densities of Nassau grouper (Epinephelus striatus, Serranidae) increased. Predators associated with
patch reefs, thus, had a significant impact on nocturnally foraging crabs in adjacent seagrass meadows. The results of this study highlight an important
trophic link between seagrass habitats and tropical
patch reefs.
D. Tracing the Contribution of Seagrass
Production to Fisheries Species Elsewhere
Using Stable Isotopes
Seagrass beds may indirectly support species in other
areas. Studies tracing the contribution of seagrass to
fisheries species outside of seagrass habitats have
focused on analysis of stable isotopes. Initial studies used a single isotope (e.g. δ
13 C), but later work
used two or more isotopes. First-feeding larval blue
grenadier (Macruronus novaezelandiae, Merlucciidae) were found to be part of a microbial planktonic
food chain that was based on seagrass detritus, which
is exported offshore during storm events (Thresher
et al., 1992). Seagrass detritus also may be important
in the food chain of prawns collected from offshore
waters, although based on the δ
13 C values the primary food source could also be benthic microalgae
or a mixture of seagrass and plankton (Loneragan
et al., 1997).
Using multiple stable isotopes may be particularly powerful for tracing trophic relationships in
coastal waters (Moncreiff and Sullivan, 2001). Surprisingly, few studies have used a multiple stable isotope approach (e.g. δ
13 C, δ
15 N, δ
32 S) to
study food webs in seagrass systems (see Mateo
et al., Chapter 7). The trophic relationships of fish
and invertebrates on the continental shelf of southeastern Australia were examined using stable isotopes of carbon and nitrogen (Davenport and Bax,
2002). Although the results of this study showed
that the main source of primary production was
oceanic phytoplankton, there was some evidence
that seagrass contributed to the benthos because
benthic invertebrates had δ
13 C values intermediate
between particulate organic matter and seagrasses
(Davenport and Bax, 2002). Multiple stable isotopes
have also been used to show that seagrass material
contributes substantially to fisheries species found
over adjacent non-vegetated habitats (Melville and
Connolly, 2003).
E. Juveniles Moving from Seagrass
to Other Habitats
Fish may move between habitats either through the
dispersal of pelagic larvae or by the movements of juvenile or adult fish between habitats. After settling in
seagrass habitats, the juveniles of reef species must
move from seagrass to reach coastal reefs. Where juveniles are found in seagrass as well as other habitats,
the relative contribution of different habitats to adult
populations needs to be determined. A recent paper
reviewed movement from juvenile to adult habitats
and the usefulness of different methods for determining movement (Gillanders et al., 2003). Most
studies have used either changes in abundance of
organisms among separate habitats or a progression
of size classes among the separate habitats to infer movement from juvenile to adult habitats (e.g.
Gunderson et al., 1990; Gillanders, 1997).
A few studies have used mark-recapture methods (e.g. Ruello, 1975, 1977; Koutsikopoulos et al.,
1995), although in many cases such studies examined movement of juveniles from estuaries rather
than seagrass per se. Artificial tagging methods, although providing good evidence for movement, are
difficult to implement in many cases due to the small
size of fish in seagrass beds, the high mortality rates
of juveniles (e.g. Hixon, 1991) and low rates of recapture of tagged fish (e.g. Sheaves, 1995). Large
numbers of organisms must be tagged to produce
even low numbers of recaptures. Recent developments with internal artificial tags (e.g. coded wire
tags, CWT; visible implant fluorescent elastomer,
VIF) show promise as tools for determining movement. Fish as small as 8–11 mm standard length
have been tagged on coral reefs using internal tags
(Beukers et al., 1995); the tags show relatively long
retention times (several months) and few biological side effects. The feasibility of using these tags
may depend on the visibility of the tags over long
time periods (e.g. 1–2 years). Juvenile organisms
grow fairly rapidly, and therefore it is possible that
growth of surrounding tissue may limit visibility of
tags. In addition, use of CWT and VIF tags will require individual handling of fish, which may be difficult for some species due to high mortality rates.
I am not aware of any studies that have used CWT
or VIF tags to track movement of organisms from
juvenile seagrass to adult habitats. Remote sensing
capabilities that do not rely on recapture per se are
an advance over mark-recapture techniques, but are