177
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
aggregate, and they generally spawn at dusk at the shelf edge near topographic
projections of the reef adjacent to deep water. Transient spawning aggregations
form only several days a year at a particular phase of the lunar cycle, often on consecutive months (Colin 1992 ; Heyman et al. 2005 ) . After spawning, the fertilized
eggs may initially drift along the reef in the alongshore coastal boundary current,
but may also be quickly entrained into topographic eddies generated by nearby reef
promontories. Tidal eddies that form downstream of headlands, may be shed from
the headland source and move offshore or they may be reconstituted with reversed
rotation on the opposite side of the promontory when the tide reverses. Because
these tidal eddies persist briefl y, they have been called transient eddies (Signell and
Geyer 1991 ; Geyer and Signell 1992 ) . Retention of both water and particles (eggs/
larvae) in eddies depends on vorticity dynamics. Vorticity may lead to tidal eddies
dissipating over the subsequent tidal cycle, reforming on the other side of the headland, or detaching and drifting into deeper offshore waters, carrying a discrete body
of entrained water and materials such as sediments (Hamner and Hauri 1977 ) and
fi sh larvae (Kingsford et al. 1991 ; Burgess et al. 2007 ) . In contrast, eddies due to
persistent mean fl ow will remain attached to the headland and may accumulate and
retain larvae for as long as the fl ow and thus the eddy persist.
Two transient aggregations of the Nassau grouper provide an example of the
diversity of infl uence of the coastal boundary layer and island eddies on eggs and
early larvae. Drifters started at the Bahamas Nassau grouper spawning aggregation
sites on Long Island indicated that eggs and early larvae were retained close to the
island and over the shelf for many days. Drifters followed tidal trajectories with
directional reversals consistent with the behaviour of transient eddies formed by
promontories along the edge of Long Island (Fig. 5.9, Chap. 5 ; Colin 1992 ) . On the
other hand, at Little Cayman Island (a much smaller island with a narrow island
shelf) drifters released at Nassau grouper spawning sites indicated entrainment of
larvae within a few hours into the general Caribbean circulation (“Grouper Moon
project” – unpublished data). These larvae would thereafter spend much of their
time in the large, offshore, mesoscale eddies which predominate south of Cuba.
These two studies point out the divergent prospects of aggregations of the same
species, assuming that drifters reasonably mimic the general movement of eggs and
larvae. At one site larvae were retained by small transient, tidal eddies, while at a
second site eggs were advected rapidly away from shore into meso-scale eddies that
circulated hundreds of kilometres over deep oceanic water.
Sometimes divergent sites have similar potential larval fates. For example,
Nemeth et al. ( 2008 ) , using stationary current meters, found that the average current
direction for three red hind aggregation sites separated by 200 km all exhibited
transport of fertilized eggs onto the island shelf or banks (St. Thomas, US Virgin
Islands – island shelf; St. Croix, USVI – bank reef; Saba Bank, Netherlands Antilles –
bank reef). In this study, broad-scale conditions appeared to foster general retention
of eggs and larvae.
Clearly an understanding of small-scale and mesoscale eddies is exceptionally
important for evaluating the oceanographic processes that affect larval survivorship
for both resident and transient aggregators. The locations where transient aggregators
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
aggregate, and they generally spawn at dusk at the shelf edge near topographic
projections of the reef adjacent to deep water. Transient spawning aggregations
form only several days a year at a particular phase of the lunar cycle, often on consecutive months (Colin 1992 ; Heyman et al. 2005 ) . After spawning, the fertilized
eggs may initially drift along the reef in the alongshore coastal boundary current,
but may also be quickly entrained into topographic eddies generated by nearby reef
promontories. Tidal eddies that form downstream of headlands, may be shed from
the headland source and move offshore or they may be reconstituted with reversed
rotation on the opposite side of the promontory when the tide reverses. Because
these tidal eddies persist briefl y, they have been called transient eddies (Signell and
Geyer 1991 ; Geyer and Signell 1992 ) . Retention of both water and particles (eggs/
larvae) in eddies depends on vorticity dynamics. Vorticity may lead to tidal eddies
dissipating over the subsequent tidal cycle, reforming on the other side of the headland, or detaching and drifting into deeper offshore waters, carrying a discrete body
of entrained water and materials such as sediments (Hamner and Hauri 1977 ) and
fi sh larvae (Kingsford et al. 1991 ; Burgess et al. 2007 ) . In contrast, eddies due to
persistent mean fl ow will remain attached to the headland and may accumulate and
retain larvae for as long as the fl ow and thus the eddy persist.
Two transient aggregations of the Nassau grouper provide an example of the
diversity of infl uence of the coastal boundary layer and island eddies on eggs and
early larvae. Drifters started at the Bahamas Nassau grouper spawning aggregation
sites on Long Island indicated that eggs and early larvae were retained close to the
island and over the shelf for many days. Drifters followed tidal trajectories with
directional reversals consistent with the behaviour of transient eddies formed by
promontories along the edge of Long Island (Fig. 5.9, Chap. 5 ; Colin 1992 ) . On the
other hand, at Little Cayman Island (a much smaller island with a narrow island
shelf) drifters released at Nassau grouper spawning sites indicated entrainment of
larvae within a few hours into the general Caribbean circulation (“Grouper Moon
project” – unpublished data). These larvae would thereafter spend much of their
time in the large, offshore, mesoscale eddies which predominate south of Cuba.
These two studies point out the divergent prospects of aggregations of the same
species, assuming that drifters reasonably mimic the general movement of eggs and
larvae. At one site larvae were retained by small transient, tidal eddies, while at a
second site eggs were advected rapidly away from shore into meso-scale eddies that
circulated hundreds of kilometres over deep oceanic water.
Sometimes divergent sites have similar potential larval fates. For example,
Nemeth et al. ( 2008 ) , using stationary current meters, found that the average current
direction for three red hind aggregation sites separated by 200 km all exhibited
transport of fertilized eggs onto the island shelf or banks (St. Thomas, US Virgin
Islands – island shelf; St. Croix, USVI – bank reef; Saba Bank, Netherlands Antilles –
bank reef). In this study, broad-scale conditions appeared to foster general retention
of eggs and larvae.
Clearly an understanding of small-scale and mesoscale eddies is exceptionally
important for evaluating the oceanographic processes that affect larval survivorship
for both resident and transient aggregators. The locations where transient aggregators