144
P.L. Colin
Palau, while other species did so throughout the day depending on the time of high
tide (Chap. 12.21 , PLC unpublished data).
5.8.2 Lunar Aspects of Aggregations
Specifi c lunar phases are associated with many life history events in reef fi shes and
delineating the lunar components of aggregation and spawning is of special importance. Tidal amplitudes are greatest (spring tides) and tidal currents strongest during
full and new moon periods, but these periods differ greatly in the ambient light
levels at night. The full moon provides signifi cant nocturnal light in the shallow reef
environment and most fi shes (and even humans) are able to see quite well with it.
Just prior to the full moon, the moon will already be above the horizon at dusk and
remain in the sky most of the night. In the days after the full moon, moonrise occurs
an hour or two after sunset and it is initially dark. After the moon has risen it is still
nearly full and provides light for the remainder of the night. Those fi shes making
long (multi-day) migrations to aggregations might, if they travelling at night, can
presumably migrate more easily around the time of the full moon with its illumination; while species with shorter migrations (which may not be moving at night)
would not be tied to a particular lunar phase. In the TWA some species known to
have lengthy migrations (Nassau grouper, red hind) aggregate on the full moon, but
additional information is needed for other species before this can be considered a
possible general principle. Conversely, on the new moon, it is dark all night long.
The days around the new moon provide essentially no lunar illumination.
The length of the lunar (synodic) month is approximately 29.5 days, and differs
from the length of Julian calendar months (28–31 days). Because of this disparity,
averaging about 0.9 days per month, the dates of the lunar phases change each year
on the Julian calendar. This makes direct comparison of aggregation/spawning times
on specifi c lunar phases of a given month between years problematic. The dates of
the lunar phases are about 10–11 days earlier in each successive year (Fig. 5.19 ) so
every 3 years the dates of lunar phases in a calendar month will have shifted back to
near the same position relative to the calendar month. Hence in Palau the full moon
of January 2001 was on the 10th, that of 2004 on 8th. The full moon in January 2002
was the 28th, but the full moon of December 2001 (30 December) is actually closer
to the 10 January 2001 moon. Whaylen et al. ( 2006 ) attempted to compensate for
full moons that occur earlier or later in a calendar month by using the winter solstice
(Dec 21 or 22) as a benchmark which changes little year to year. This is useful and
this (or similar) measure(s) should be used when comparing times of aggregation
and spawning between years.
The lunar timing of spawning may also determine the timing of pelagic phase
recruitment to the bottom. For Nassau grouper in the Bahamas, settlement of pelagic
juveniles occurred about 37–42 days after spawning and juveniles are believed to
recruit at night (Shenker et al. 1993 ; Colin et al. 1997 ) . Based on the larval life
length, this will occur near the time of the new moon, when moonlight is minimal,
and may benefi t recruits in reducing predation as they settle. If a species has a
P.L. Colin
Palau, while other species did so throughout the day depending on the time of high
tide (Chap. 12.21 , PLC unpublished data).
5.8.2 Lunar Aspects of Aggregations
Specifi c lunar phases are associated with many life history events in reef fi shes and
delineating the lunar components of aggregation and spawning is of special importance. Tidal amplitudes are greatest (spring tides) and tidal currents strongest during
full and new moon periods, but these periods differ greatly in the ambient light
levels at night. The full moon provides signifi cant nocturnal light in the shallow reef
environment and most fi shes (and even humans) are able to see quite well with it.
Just prior to the full moon, the moon will already be above the horizon at dusk and
remain in the sky most of the night. In the days after the full moon, moonrise occurs
an hour or two after sunset and it is initially dark. After the moon has risen it is still
nearly full and provides light for the remainder of the night. Those fi shes making
long (multi-day) migrations to aggregations might, if they travelling at night, can
presumably migrate more easily around the time of the full moon with its illumination; while species with shorter migrations (which may not be moving at night)
would not be tied to a particular lunar phase. In the TWA some species known to
have lengthy migrations (Nassau grouper, red hind) aggregate on the full moon, but
additional information is needed for other species before this can be considered a
possible general principle. Conversely, on the new moon, it is dark all night long.
The days around the new moon provide essentially no lunar illumination.
The length of the lunar (synodic) month is approximately 29.5 days, and differs
from the length of Julian calendar months (28–31 days). Because of this disparity,
averaging about 0.9 days per month, the dates of the lunar phases change each year
on the Julian calendar. This makes direct comparison of aggregation/spawning times
on specifi c lunar phases of a given month between years problematic. The dates of
the lunar phases are about 10–11 days earlier in each successive year (Fig. 5.19 ) so
every 3 years the dates of lunar phases in a calendar month will have shifted back to
near the same position relative to the calendar month. Hence in Palau the full moon
of January 2001 was on the 10th, that of 2004 on 8th. The full moon in January 2002
was the 28th, but the full moon of December 2001 (30 December) is actually closer
to the 10 January 2001 moon. Whaylen et al. ( 2006 ) attempted to compensate for
full moons that occur earlier or later in a calendar month by using the winter solstice
(Dec 21 or 22) as a benchmark which changes little year to year. This is useful and
this (or similar) measure(s) should be used when comparing times of aggregation
and spawning between years.
The lunar timing of spawning may also determine the timing of pelagic phase
recruitment to the bottom. For Nassau grouper in the Bahamas, settlement of pelagic
juveniles occurred about 37–42 days after spawning and juveniles are believed to
recruit at night (Shenker et al. 1993 ; Colin et al. 1997 ) . Based on the larval life
length, this will occur near the time of the new moon, when moonlight is minimal,
and may benefi t recruits in reducing predation as they settle. If a species has a