293
9 Studying and Monitoring Aggregating Species
representative of the overall aggregation, i.e. males may be caught more often by
hook and line than females, spear fi shermen may specifi cally target males, or
fi shing only occurs at certain stages of aggregation formation. It may be useful to
pay for sampling (measuring, weighing) as you may be disturbing people trying
to make a living. Purchasing part of the catch allows you to keep the samples for
other work without impacting the income of fi shers. Often samples must be processed quickly for sale so researchers need to be able to work fast. If fi sh are
being cleaned and fi lleted, the carcasses may normally be discarded. These
should be retained and the fi sher paid for them. If intact (fi llets removed only) the
length can be determined, the body cavity contents (including gonads) may still
be present, tissue samples taken and otoliths removed from the head. The downside of fi shery sampling is the prospect that fi shing is wiping out the aggregation
under study.
GC – Gut Contents
Whether fi shes are feeding while engaged in aggregation is poorly known, so
examining gut contents is useful for answering this question as well as for broader
ecosystem implications. Some species seem to leave aggregation sites for part of the
day and return at the time of spawning (Chaps. 2 , 12.11 ). They may be feeding
elsewhere. The guidelines of Randall ( 1963 ) on sampling gut contents still hold.
Spearing or netting are the best means to obtain unbiased gut contents, but fi sh taken
in traps or by hook and line should be excluded from any normal gut content analysis.
Nonetheless, it is useful to know if fi sh in an aggregation will take a baited hook, or
go into a baited trap, as this may indicate how easily they can be captured on the site
using different fi shing methods, providing perspective on how easy it is to overfi sh
an aggregation.
GD – GPS Density Survey
This new technique produces a series of geolocated density measurements of fi sh
in an aggregation (or any other fi sh that can be visually surveyed) by running a
series of transects across an aggregation while towing a position logging GPS
unit. The positioned density data can be used to produce a bubble plot or contour
plot of fi sh density; estimated numbers of fi sh present in the aggregation can be
obtained (Fig. 9.1 ). This method is discussed further below.
GN – Gonad Study
Examination of gonads is important because their size, especially relative to body
size, and condition can be used to determine reproductive readiness and time of
spawning. The Gonadosomatic Index (GSI), the weight of gonads relative to that of
the fi sh (typically expressed as GSI = gonad weight/body weight * 100), can cheaply
and readily indicate seasonality (See Chap. 3 , Fig. 3.4, Chap. 12, Fig. 12.62), size
of sexual maturation (from gonads taken from adults during the spawning season),
and even lunar periodicity. The presence of hydrated eggs signals imminent spawning
within about 24 h. Gonad samples can often be taken from fi sher catches throughout
the year or by market sampling. If no fi sher catches are available, investigators may
9 Studying and Monitoring Aggregating Species
representative of the overall aggregation, i.e. males may be caught more often by
hook and line than females, spear fi shermen may specifi cally target males, or
fi shing only occurs at certain stages of aggregation formation. It may be useful to
pay for sampling (measuring, weighing) as you may be disturbing people trying
to make a living. Purchasing part of the catch allows you to keep the samples for
other work without impacting the income of fi shers. Often samples must be processed quickly for sale so researchers need to be able to work fast. If fi sh are
being cleaned and fi lleted, the carcasses may normally be discarded. These
should be retained and the fi sher paid for them. If intact (fi llets removed only) the
length can be determined, the body cavity contents (including gonads) may still
be present, tissue samples taken and otoliths removed from the head. The downside of fi shery sampling is the prospect that fi shing is wiping out the aggregation
under study.
GC – Gut Contents
Whether fi shes are feeding while engaged in aggregation is poorly known, so
examining gut contents is useful for answering this question as well as for broader
ecosystem implications. Some species seem to leave aggregation sites for part of the
day and return at the time of spawning (Chaps. 2 , 12.11 ). They may be feeding
elsewhere. The guidelines of Randall ( 1963 ) on sampling gut contents still hold.
Spearing or netting are the best means to obtain unbiased gut contents, but fi sh taken
in traps or by hook and line should be excluded from any normal gut content analysis.
Nonetheless, it is useful to know if fi sh in an aggregation will take a baited hook, or
go into a baited trap, as this may indicate how easily they can be captured on the site
using different fi shing methods, providing perspective on how easy it is to overfi sh
an aggregation.
GD – GPS Density Survey
This new technique produces a series of geolocated density measurements of fi sh
in an aggregation (or any other fi sh that can be visually surveyed) by running a
series of transects across an aggregation while towing a position logging GPS
unit. The positioned density data can be used to produce a bubble plot or contour
plot of fi sh density; estimated numbers of fi sh present in the aggregation can be
obtained (Fig. 9.1 ). This method is discussed further below.
GN – Gonad Study
Examination of gonads is important because their size, especially relative to body
size, and condition can be used to determine reproductive readiness and time of
spawning. The Gonadosomatic Index (GSI), the weight of gonads relative to that of
the fi sh (typically expressed as GSI = gonad weight/body weight * 100), can cheaply
and readily indicate seasonality (See Chap. 3 , Fig. 3.4, Chap. 12, Fig. 12.62), size
of sexual maturation (from gonads taken from adults during the spawning season),
and even lunar periodicity. The presence of hydrated eggs signals imminent spawning
within about 24 h. Gonad samples can often be taken from fi sher catches throughout
the year or by market sampling. If no fi sher catches are available, investigators may