311
9 Studying and Monitoring Aggregating Species
variation of fi sh numbers and timing, as well as their distribution within and relative to aggregation sites, so protocols need to be modifi ed accordingly to best characterize spawning aggregations and accommodate the different ways in which
the fi sh aggregate and move within them or the aggregation area shifts over time.
Underwater surveys typically involve transect work, using one or several transects,
but this can be challenging because of prevailing conditions or because fi sh
assemble in three dimensions. Ideally, studies should strive to design for long-term
duration and in any case the methods should be described clearly enough to allow
for repeatability.
The single line transect generally provides only a single total count, and if the
length and swath width are known, a single density measurement (Fig. 9.5a ). If
the survey transect crosses the entire aggregation, then areas of higher (which
may be a core area, Chap. 2 ) and lower density (or with no fi sh at all) are going to
be lumped together in the single density determination. An approach of measuring density of individuals within subunits of an overall aggregation site seems to
hold hope for more realistic estimates of overall numbers. If a single long permanent transect, set up using survey tape and compass, is subdivided into equal
length subunits, each of which is marked separately by fl oats or other means. Each
subunit is counted separately (but successively). This allows several measurements of density at known locations, providing increased resolution of the location and limits of the aggregation and fi sh within it and an average value to be
calculated (Fig. 9.5b ). The approach obviates the need to constantly update a single overall count of the transect (which can run to hundreds of fi sh). If the transect
is permanent, with its location suitably quantifi ed, and the same increments are
used for each survey, the surveys obtained are repeatable at another time by
another observer. Ideally, transect counts should be replicated, and possible interdiver differences examined.
Lacking a permanent transect with subdivisions, something as simple as making
fi sh counts at time intervals (1 to a few minutes) while maintaining a constant
approximate swim rate over the bottom can provide valuable relative information on
fi sh density and distribution within the aggregation. It is possible to use a method of
measuring distance “on the fl y” (Nemeth 2005 ) where time units (counts made on
some time interval) can be transformed into distance units by towing a waterproof
GPS logging positions at time intervals during the transect swims. Either method
allows determination of density of fi sh within portions of the aggregation area, an
improvement over the single transect total count.
A single permanent transect runs the risk of initially being located to represent
the overall aggregation, however, slight shifts in location of aggregated fi sh can
render a permanent transect unrepresentative. Ideally multiple permanent transects
can to be set up so that if the location of the aggregation shifts, the data from individual transects will provide evidence of this and, if necessary, new transects can be
added to include additional areas with aggregated fi sh. A SCRFA/Palau Conservation
Society partnership found a single transect aggregation monitoring project to be
unrepresentative and it was replaced by fi ve permanent parallel transects which
fully cover the present known aggregation of three species of groupers (Fig. 9.9 ).
Each transect was surveyed by an observer towing a logging GPS, which recorded
9 Studying and Monitoring Aggregating Species
variation of fi sh numbers and timing, as well as their distribution within and relative to aggregation sites, so protocols need to be modifi ed accordingly to best characterize spawning aggregations and accommodate the different ways in which
the fi sh aggregate and move within them or the aggregation area shifts over time.
Underwater surveys typically involve transect work, using one or several transects,
but this can be challenging because of prevailing conditions or because fi sh
assemble in three dimensions. Ideally, studies should strive to design for long-term
duration and in any case the methods should be described clearly enough to allow
for repeatability.
The single line transect generally provides only a single total count, and if the
length and swath width are known, a single density measurement (Fig. 9.5a ). If
the survey transect crosses the entire aggregation, then areas of higher (which
may be a core area, Chap. 2 ) and lower density (or with no fi sh at all) are going to
be lumped together in the single density determination. An approach of measuring density of individuals within subunits of an overall aggregation site seems to
hold hope for more realistic estimates of overall numbers. If a single long permanent transect, set up using survey tape and compass, is subdivided into equal
length subunits, each of which is marked separately by fl oats or other means. Each
subunit is counted separately (but successively). This allows several measurements of density at known locations, providing increased resolution of the location and limits of the aggregation and fi sh within it and an average value to be
calculated (Fig. 9.5b ). The approach obviates the need to constantly update a single overall count of the transect (which can run to hundreds of fi sh). If the transect
is permanent, with its location suitably quantifi ed, and the same increments are
used for each survey, the surveys obtained are repeatable at another time by
another observer. Ideally, transect counts should be replicated, and possible interdiver differences examined.
Lacking a permanent transect with subdivisions, something as simple as making
fi sh counts at time intervals (1 to a few minutes) while maintaining a constant
approximate swim rate over the bottom can provide valuable relative information on
fi sh density and distribution within the aggregation. It is possible to use a method of
measuring distance “on the fl y” (Nemeth 2005 ) where time units (counts made on
some time interval) can be transformed into distance units by towing a waterproof
GPS logging positions at time intervals during the transect swims. Either method
allows determination of density of fi sh within portions of the aggregation area, an
improvement over the single transect total count.
A single permanent transect runs the risk of initially being located to represent
the overall aggregation, however, slight shifts in location of aggregated fi sh can
render a permanent transect unrepresentative. Ideally multiple permanent transects
can to be set up so that if the location of the aggregation shifts, the data from individual transects will provide evidence of this and, if necessary, new transects can be
added to include additional areas with aggregated fi sh. A SCRFA/Palau Conservation
Society partnership found a single transect aggregation monitoring project to be
unrepresentative and it was replaced by fi ve permanent parallel transects which
fully cover the present known aggregation of three species of groupers (Fig. 9.9 ).
Each transect was surveyed by an observer towing a logging GPS, which recorded