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P.L. Colin
distance covered each minute, and numbers of fi shes counted on a per minute basis.
This allowed for an average density to be calculated for each species allowing
for statistical treatment of data and better comparisons over time (Yvonne Sadovy
de Mitcheson, Asap Bukurrou, Scott Kiefer and PC unpublished data, Palau
Conservation Society 2010 ) .
Permanent survey areas can also be set up as quadrats of equal area, which are
each surveyed independently. Kadison et al. ( 2006 ) delineated an area 1.5 km
long by 100 m wide of Grammanik Bank, an elongate shelf edge reef feature in
the US Virgin Islands, into six equal segments each about 160 m in length. Divers
swam or used Diver Propulsion Vehicles 5 m off the bottom of 30–40 m depth to
survey each section of the reef in a 20 m wide transect, counting species and
numbers below them. Short narrow (30 m by 2 m) transects (Nemeth 2005 ) were
carried out by a diver swimming at a constant speed with the tape rolling out
behind and using a 1 m piece of PVC pipe marked at 5 cm intervals to assist in
estimating fi sh lengths. Shapiro et al. ( 1993 ) divided up a red hind aggregation
site into large square sections using ropes and four divers simultaneously swam
the entire grid.
If fi sh are aggregated across a broad area and not in a single tight cluster, the
“GPS density” method mentioned earlier can be used to construct maps of fi sh density
and allow repeatable surveys. A GPS receiver in a waterproof fl oat is towed by a
single or multiple observer(s) swimming tracks across the aggregation area including areas where no fi sh occur. The GPS records its position every 15 s to 1 min while
data on numbers of fi sh seen within a given swath width by the observer are recorded
for each minute (using a watch synchronized with the GPS time). Later the downloaded position data provide the distance surveyed in each minute, the swath width
provides an area (of a rectangle) and fi sh numbers observed produces a density
measurement within the rectangle. The central position measurement of the rectangle
(at 30 s point for each min) gives the geographic centre of the rectangle. The series
of geolocated density measurements, as well as the locations where no fi sh occur, is
used to construct a map of the distribution of fi sh during a given survey (Fig. 9.1 ).
This method will be described in greater detail elsewhere (Colin et al. in prep), but
the basic method produces high quality information on distribution of fi sh within a
spawning aggregation, and estimates of the total number of fi sh within an aggregation. The outer limits of an aggregation can also be delineated, if distinct, (Fig. 9.10a ).
The area occupied by an aggregation day to day can be shown as well as change
occurring over time (Fig. 9.10b ). This is, perhaps, the closest to an archival method
for documenting the status of spawning aggregations yet available. Using the same
methods it is repeatable later by another individual with real hope for comparable
results. Methods for determining numbers of fi sh, if they are GPS based, can similarly indicate changes in locations of aggregations.
With extremely dense aggregations comprised of many hundreds to thousands of
fi sh, such as occurs in the Nassau grouper, cubera, Lutjanus cyanopterus , dog
snapper, L. jocu , or twin-spot snapper, L. bohar, things are different (Fig. 9.11 , also
see Fig. 12.24). Whaylen et al. ( 2004 , 2006 ) attempted to determine the number of
Nassau grouper in an aggregation by several divers estimating the dimensions of the
P.L. Colin
distance covered each minute, and numbers of fi shes counted on a per minute basis.
This allowed for an average density to be calculated for each species allowing
for statistical treatment of data and better comparisons over time (Yvonne Sadovy
de Mitcheson, Asap Bukurrou, Scott Kiefer and PC unpublished data, Palau
Conservation Society 2010 ) .
Permanent survey areas can also be set up as quadrats of equal area, which are
each surveyed independently. Kadison et al. ( 2006 ) delineated an area 1.5 km
long by 100 m wide of Grammanik Bank, an elongate shelf edge reef feature in
the US Virgin Islands, into six equal segments each about 160 m in length. Divers
swam or used Diver Propulsion Vehicles 5 m off the bottom of 30–40 m depth to
survey each section of the reef in a 20 m wide transect, counting species and
numbers below them. Short narrow (30 m by 2 m) transects (Nemeth 2005 ) were
carried out by a diver swimming at a constant speed with the tape rolling out
behind and using a 1 m piece of PVC pipe marked at 5 cm intervals to assist in
estimating fi sh lengths. Shapiro et al. ( 1993 ) divided up a red hind aggregation
site into large square sections using ropes and four divers simultaneously swam
the entire grid.
If fi sh are aggregated across a broad area and not in a single tight cluster, the
“GPS density” method mentioned earlier can be used to construct maps of fi sh density
and allow repeatable surveys. A GPS receiver in a waterproof fl oat is towed by a
single or multiple observer(s) swimming tracks across the aggregation area including areas where no fi sh occur. The GPS records its position every 15 s to 1 min while
data on numbers of fi sh seen within a given swath width by the observer are recorded
for each minute (using a watch synchronized with the GPS time). Later the downloaded position data provide the distance surveyed in each minute, the swath width
provides an area (of a rectangle) and fi sh numbers observed produces a density
measurement within the rectangle. The central position measurement of the rectangle
(at 30 s point for each min) gives the geographic centre of the rectangle. The series
of geolocated density measurements, as well as the locations where no fi sh occur, is
used to construct a map of the distribution of fi sh during a given survey (Fig. 9.1 ).
This method will be described in greater detail elsewhere (Colin et al. in prep), but
the basic method produces high quality information on distribution of fi sh within a
spawning aggregation, and estimates of the total number of fi sh within an aggregation. The outer limits of an aggregation can also be delineated, if distinct, (Fig. 9.10a ).
The area occupied by an aggregation day to day can be shown as well as change
occurring over time (Fig. 9.10b ). This is, perhaps, the closest to an archival method
for documenting the status of spawning aggregations yet available. Using the same
methods it is repeatable later by another individual with real hope for comparable
results. Methods for determining numbers of fi sh, if they are GPS based, can similarly indicate changes in locations of aggregations.
With extremely dense aggregations comprised of many hundreds to thousands of
fi sh, such as occurs in the Nassau grouper, cubera, Lutjanus cyanopterus , dog
snapper, L. jocu , or twin-spot snapper, L. bohar, things are different (Fig. 9.11 , also
see Fig. 12.24). Whaylen et al. ( 2004 , 2006 ) attempted to determine the number of
Nassau grouper in an aggregation by several divers estimating the dimensions of the