136
P.L. Colin
Sites might be benefi cial for adults, perhaps easy to locate during migrations and
traditionally used, or for propagules, through reduced predation on released eggs or
increased survival of eggs and larvae to the recruitment stage. Our knowledge of
these factors is only preliminary and any hypotheses should be tested empirically.
A major geographic difference exists between the TWA and IWP with the former
generally lacking major tidally driven currents while they are common in the latter.
This comes largely from differences in tidal amplitudes. Most IWP reef areas have
ranges of 1–2 m, inducing strong tidal currents between closely spaced islands and
reefs, while TWA locations have only about one half metre range and broad deep
(15–20 m) insular shelves without nearby shallow areas and strong tidal currents
(see also Chap. 4). A few areas in the TWA, such as the Exuma chain (Bahamas)
and the Florida Keys, have large shallow bank areas where strong tidal currents
course through gaps between islands, however these are exceptions.
Most offshore currents will run parallel to an island shore or relatively straight
barrier reef, but where a physical structure, such as a promontory, interrupts the parallel fl ow the current will be forced to deviate in direction (Chap. 6 ). This produces
eddying in the offshore fl ow which may help to retain or disperse larvae. Tidal channels have currents as tidal jets extending out to sea while shear fl ow on their edges
will produce eddies on the sides of the channel. Where barrier reefs exist water fl ows
from lagoon to ocean across the reef on falling tides as a lens, and can extend a short
distance out to sea from the reef as a discrete water mass (see Fig. 6.12).
The question of whether the oceanography of spawning sites promotes the dispersal and/or retention (or neither) of eggs and resulting larvae can be approached
in many ways. Currents measured at sites and times of aggregation spawning provide robust data on chances of dispersal versus retention. The concept of dispersal
kernals (Chap. 6 ) is useful in visualizing the distribution and movement of masses
of propagules. Recent studies of self-recruitment have clearly established that some
larvae can settle near to where they were spawned; “near” being a relative term
(Swearer et al. 1999 ; Jones et al. 1999, 2005 ) , but dispersal should ensure that most
larvae settle variable distances from where they were spawned (Cowen 2002 ; Mora
and Sale 2002 ) .
In the TWA minimal currents (“quiet currents” of Whaylen et al. 2006 ) have
been measured or observed at large TA sites during dusk spawning. For Nassau
grouper, Whaylen et al. ( 2006 ) reported spawning bursts to occur when currents
“were slack or negligible”. For red hind, Nemeth et al. ( 2007 ) found currents at a
shelf edge aggregation site of red hinds at their annual and lunar minima during the
winter spawning season with the week preceding the full moon phase when aggregation and spawning is occurring having currents of only 2.5–3.5 cm s
−1 during
spawning. Heyman and Kjerfve ( 2008 ) reported variable currents at a snapper-grouper TA site, with a mean modest speed of only 8 cm s
−1 . A new analysis of the
current data at a Nassau grouper site (Colin 1992 ) indicates during the late afternoon and early evening period (means 3–5 cm s
−1 ) currents were minimal on the
full moons of December and January (Fig. 5.15 ) and currents milder at the aggregation site than at two sites 5 and 6.5 km distant in opposite directions. There are
also a number of qualitative reports of mild currents at spawning sites and times
P.L. Colin
Sites might be benefi cial for adults, perhaps easy to locate during migrations and
traditionally used, or for propagules, through reduced predation on released eggs or
increased survival of eggs and larvae to the recruitment stage. Our knowledge of
these factors is only preliminary and any hypotheses should be tested empirically.
A major geographic difference exists between the TWA and IWP with the former
generally lacking major tidally driven currents while they are common in the latter.
This comes largely from differences in tidal amplitudes. Most IWP reef areas have
ranges of 1–2 m, inducing strong tidal currents between closely spaced islands and
reefs, while TWA locations have only about one half metre range and broad deep
(15–20 m) insular shelves without nearby shallow areas and strong tidal currents
(see also Chap. 4). A few areas in the TWA, such as the Exuma chain (Bahamas)
and the Florida Keys, have large shallow bank areas where strong tidal currents
course through gaps between islands, however these are exceptions.
Most offshore currents will run parallel to an island shore or relatively straight
barrier reef, but where a physical structure, such as a promontory, interrupts the parallel fl ow the current will be forced to deviate in direction (Chap. 6 ). This produces
eddying in the offshore fl ow which may help to retain or disperse larvae. Tidal channels have currents as tidal jets extending out to sea while shear fl ow on their edges
will produce eddies on the sides of the channel. Where barrier reefs exist water fl ows
from lagoon to ocean across the reef on falling tides as a lens, and can extend a short
distance out to sea from the reef as a discrete water mass (see Fig. 6.12).
The question of whether the oceanography of spawning sites promotes the dispersal and/or retention (or neither) of eggs and resulting larvae can be approached
in many ways. Currents measured at sites and times of aggregation spawning provide robust data on chances of dispersal versus retention. The concept of dispersal
kernals (Chap. 6 ) is useful in visualizing the distribution and movement of masses
of propagules. Recent studies of self-recruitment have clearly established that some
larvae can settle near to where they were spawned; “near” being a relative term
(Swearer et al. 1999 ; Jones et al. 1999, 2005 ) , but dispersal should ensure that most
larvae settle variable distances from where they were spawned (Cowen 2002 ; Mora
and Sale 2002 ) .
In the TWA minimal currents (“quiet currents” of Whaylen et al. 2006 ) have
been measured or observed at large TA sites during dusk spawning. For Nassau
grouper, Whaylen et al. ( 2006 ) reported spawning bursts to occur when currents
“were slack or negligible”. For red hind, Nemeth et al. ( 2007 ) found currents at a
shelf edge aggregation site of red hinds at their annual and lunar minima during the
winter spawning season with the week preceding the full moon phase when aggregation and spawning is occurring having currents of only 2.5–3.5 cm s
−1 during
spawning. Heyman and Kjerfve ( 2008 ) reported variable currents at a snapper-grouper TA site, with a mean modest speed of only 8 cm s
−1 . A new analysis of the
current data at a Nassau grouper site (Colin 1992 ) indicates during the late afternoon and early evening period (means 3–5 cm s
−1 ) currents were minimal on the
full moons of December and January (Fig. 5.15 ) and currents milder at the aggregation site than at two sites 5 and 6.5 km distant in opposite directions. There are
also a number of qualitative reports of mild currents at spawning sites and times