524
Bronwyn M. Gillanders
isotope approach was possible because the majority
of rearing streams differed in their δ
87 Sr signatures
(Kennedy et al., 1997). Fish take up Sr isotopes in
direct proportion to their availability in water and
substitute Sr for Ca in calcified tissues during growth
(Hobson, 1999). The Sr isotopic composition of fish
otoliths is about the same as that of stream water
because Sr does not fractionate during assimilation
(Hobson, 1999). A similar approach may be possible for determining origins of fish from seagrass
beds in different regions or to determine the relative
contribution of different habitats to adult populations. It will, however, be necessary to determine
the spatial and temporal scale of isotopic differences
prior to determining links between habitats.
Elemental signatures (e.g. Sr, Mn, Ba) in the
otoliths of fish have also been used to evaluate contributions of larval/juvenile habitats to adult stocks
(e.g. Gillanders and Kingsford, 1996; Swearer et al.,
1999; Thorrold et al., 2001; Gillanders, 2002). Because of the acellular and metabolically inert nature
of otoliths, many of the elements that are used as
a natural tag and accreted onto the growing surface
of the otolith are permanently retained (see review:
Campana, 1999). Otoliths continue to grow through
time ensuring that the entire lifetime of a fish has
been recorded, and differences in chemistry between
layers of an otolith can be resolved to within days
or years of their deposition. The calcium carbonate
and trace elements that make up 90% of the otolith
are derived primarily from the water (Milton and
Chenery, 2001), although in practice physiological
filters likely ensure that there is not a simple linear relationship between the water and the otolith
(Campana, 1999). Similar approaches have recently
been investigated using calcified structures of crustaceans (e.g. Dibacco and Levin, 2000) and molluscs
(e.g. Zacherl et al., 2003).
The only study that has used elements in otoliths to
determine links between seagrass habitats and other
habitats was that of Gillanders and Kingsford (1996).
Significant differences were found in the elemental
signatures of otoliths of juvenile blue groper (Achoerodus viridis; Labridae) collected from two habitats, seagrass, and rocky reef. The central region of
adult otoliths corresponding to that laid down when
the fish was a juvenile was then analyzed to determine the relative importance of the two juvenile
habitats in supporting adult stocks and to demonstrate links between juvenile and adult habitats. Their
results suggested that while seagrass habitats contributed to adult populations in terms of numbers of
fish, rocky reef habitats were also important. A similar approach should be possible for other species.
VII. Conservation and Management Issues
A. General Observations
Interest in conservation biology within the marine
area is expanding rapidly as the effects of overfishing, pollution, habitat destruction and fragmentation, invasive species, and global climate change impact marine systems (Ruckelshaus and Hays, 1998).
Dramatic losses of seagrass habitats have occurred
worldwide due to direct impacts, but frequently indirect effects on fish and invertebrates within seagrass
habitats (see Section VII.C of this chapter) also are
common. In order to mitigate against loss of seagrasses, many restoration projects have been undertaken and the information obtained on the use of
restored seagrass by fish (see Section VII.C of this
chapter) is now available. A broad-scale approach
to management of seagrass is necessary, but no detailed studies exist that identify suitable indicators of
seagrass health (see Seagrass “health” and fisheries,
Section VII.D).
B. Impacts Due to Fisheries
The impacts of fishing gear (e.g. mechanical
dredges, hydraulic dredges) on seagrass systems
have been described as resembling ‘the effects of
terrestrial ploughs in that plants are uprooted and
dispersed or buried’ (Blaber et al., 2000). Direct impacts on seagrass beds are likely to vary depending
on the type of gear used. The mechanical harvesting of clams is particularly devastating. Mechanical
clam harvesting in seagrass beds is done by modifying boat engines so that the propeller wash is directed downwards (as opposed to backwards), and
in shallow water the action of the propeller suspends
bottom sediments and clams into a plume in the water column (Peterson et al., 1987). The suspended
clams (Mercenaria mercenaria, Veneridae) are then
collected by a trawl net towed behind the boat. A
field experiment was used to examine the effects of
clam raking (hand raking while walking across the
seagrass) and two different intensities of mechanical
harvesting of clams (‘clam kicking’) on recruitment
of clams, biomass of seagrass, density of benthic
Bronwyn M. Gillanders
isotope approach was possible because the majority
of rearing streams differed in their δ
87 Sr signatures
(Kennedy et al., 1997). Fish take up Sr isotopes in
direct proportion to their availability in water and
substitute Sr for Ca in calcified tissues during growth
(Hobson, 1999). The Sr isotopic composition of fish
otoliths is about the same as that of stream water
because Sr does not fractionate during assimilation
(Hobson, 1999). A similar approach may be possible for determining origins of fish from seagrass
beds in different regions or to determine the relative
contribution of different habitats to adult populations. It will, however, be necessary to determine
the spatial and temporal scale of isotopic differences
prior to determining links between habitats.
Elemental signatures (e.g. Sr, Mn, Ba) in the
otoliths of fish have also been used to evaluate contributions of larval/juvenile habitats to adult stocks
(e.g. Gillanders and Kingsford, 1996; Swearer et al.,
1999; Thorrold et al., 2001; Gillanders, 2002). Because of the acellular and metabolically inert nature
of otoliths, many of the elements that are used as
a natural tag and accreted onto the growing surface
of the otolith are permanently retained (see review:
Campana, 1999). Otoliths continue to grow through
time ensuring that the entire lifetime of a fish has
been recorded, and differences in chemistry between
layers of an otolith can be resolved to within days
or years of their deposition. The calcium carbonate
and trace elements that make up 90% of the otolith
are derived primarily from the water (Milton and
Chenery, 2001), although in practice physiological
filters likely ensure that there is not a simple linear relationship between the water and the otolith
(Campana, 1999). Similar approaches have recently
been investigated using calcified structures of crustaceans (e.g. Dibacco and Levin, 2000) and molluscs
(e.g. Zacherl et al., 2003).
The only study that has used elements in otoliths to
determine links between seagrass habitats and other
habitats was that of Gillanders and Kingsford (1996).
Significant differences were found in the elemental
signatures of otoliths of juvenile blue groper (Achoerodus viridis; Labridae) collected from two habitats, seagrass, and rocky reef. The central region of
adult otoliths corresponding to that laid down when
the fish was a juvenile was then analyzed to determine the relative importance of the two juvenile
habitats in supporting adult stocks and to demonstrate links between juvenile and adult habitats. Their
results suggested that while seagrass habitats contributed to adult populations in terms of numbers of
fish, rocky reef habitats were also important. A similar approach should be possible for other species.
VII. Conservation and Management Issues
A. General Observations
Interest in conservation biology within the marine
area is expanding rapidly as the effects of overfishing, pollution, habitat destruction and fragmentation, invasive species, and global climate change impact marine systems (Ruckelshaus and Hays, 1998).
Dramatic losses of seagrass habitats have occurred
worldwide due to direct impacts, but frequently indirect effects on fish and invertebrates within seagrass
habitats (see Section VII.C of this chapter) also are
common. In order to mitigate against loss of seagrasses, many restoration projects have been undertaken and the information obtained on the use of
restored seagrass by fish (see Section VII.C of this
chapter) is now available. A broad-scale approach
to management of seagrass is necessary, but no detailed studies exist that identify suitable indicators of
seagrass health (see Seagrass “health” and fisheries,
Section VII.D).
B. Impacts Due to Fisheries
The impacts of fishing gear (e.g. mechanical
dredges, hydraulic dredges) on seagrass systems
have been described as resembling ‘the effects of
terrestrial ploughs in that plants are uprooted and
dispersed or buried’ (Blaber et al., 2000). Direct impacts on seagrass beds are likely to vary depending
on the type of gear used. The mechanical harvesting of clams is particularly devastating. Mechanical
clam harvesting in seagrass beds is done by modifying boat engines so that the propeller wash is directed downwards (as opposed to backwards), and
in shallow water the action of the propeller suspends
bottom sediments and clams into a plume in the water column (Peterson et al., 1987). The suspended
clams (Mercenaria mercenaria, Veneridae) are then
collected by a trawl net towed behind the boat. A
field experiment was used to examine the effects of
clam raking (hand raking while walking across the
seagrass) and two different intensities of mechanical
harvesting of clams (‘clam kicking’) on recruitment
of clams, biomass of seagrass, density of benthic