315
9 Studying and Monitoring Aggregating Species
as “number of fi sh counted” for each area and sampling time. Data were reported to
suggest “trends”, rather than total numbers of fi shes in aggregations; these defi ciencies made it impossible for subsequent workers to repeat their fi eld measurements
except in the case of the fully surveyed site.
9.6.7 Hydroacoustic Surveys
One way to survey fi sh numbers without having to enter the water is by using
hydroacoustic surveys. Perhaps the best attempt to conduct a hydroacoustic survey
of a reef fi sh spawning aggregation is that of Taylor et al. ( 2006 ) where relatively
careful underwater surveys were combined with the acoustic data. They defi ned the
extent of and numbers present (roughly 2,000 fi sh) in a Cayman Islands Nassau
grouper aggregation, stressing the importance of “ground truthing” (i.e. combining
this approach with diver surveys of numbers present to ensure that the hydroacoustic
method refl ects numbers of fi sh species of interest present). They found that hydroacoustic surveys tended to overestimate the abundance of fi sh, but appears to provide
a good measure of the spatial extent of the aggregation (Chap. 12 ).
Ehrhardt and Deleveaux ( 2007 ) attempted similar work for Nassau grouper in
the Bahamas without ground-truthing. Their estimate of the area covered by the
aggregation off High Cay, Andros at 555,000 m
2 is much larger than any known
Nassau grouper aggregation, while their estimate of fi sh numbers was much higher
than any other that had been surveyed in recent years in the country, or indeed
anywhere currently in the range of the species. They estimated a total of 10,500 fi sh
of an average weight of 2.01–2.13 kg. This reported weight is immediately suspect
as these values are below even the lower limit for aggregating Nassau grouper. Colin
( 1992 ) found Nassau grouper from aggregations at Long Island, Bahamas to weigh
between 2.7 and 9.7 kg (Colin 1992 ) , with Nassau grouper from other aggregations
having similar weights. These disparities call into question the accuracy of their
results, emphasizing the need for ground-truthing, and could be highly misleading
if used directly for management, for example in stock assessment. Similarly
Johnston et al. ( 2006 ) did broad scale acoustic surveys for red hind aggregations
covering hundreds of km of the Puerto Rico insular shelf, but lacked adequate
ground-truth validation. Although extensive schools of fi sh were recorded, it is
unknown whether the data do represent spawning aggregations (or even red hind) or
other fi shes hovering above the bottom.
Hydroacoustic surveys remain a developing technology, with good potential for
assessing spawning aggregations of reef fi shes if applied rigorously. Stringent protocols for ground-truthing and assessing accuracy are essential. The unknown number
of species which could produce signal returns similar to target species, as well as the
attraction of predators to sites (Heyman et al. 2005 ; Nemeth et al. 2007 ) means
hydroacoustic surveys without careful ground truth will not provide reliable data
and will tend to overestimate fi sh numbers.
9 Studying and Monitoring Aggregating Species
as “number of fi sh counted” for each area and sampling time. Data were reported to
suggest “trends”, rather than total numbers of fi shes in aggregations; these defi ciencies made it impossible for subsequent workers to repeat their fi eld measurements
except in the case of the fully surveyed site.
9.6.7 Hydroacoustic Surveys
One way to survey fi sh numbers without having to enter the water is by using
hydroacoustic surveys. Perhaps the best attempt to conduct a hydroacoustic survey
of a reef fi sh spawning aggregation is that of Taylor et al. ( 2006 ) where relatively
careful underwater surveys were combined with the acoustic data. They defi ned the
extent of and numbers present (roughly 2,000 fi sh) in a Cayman Islands Nassau
grouper aggregation, stressing the importance of “ground truthing” (i.e. combining
this approach with diver surveys of numbers present to ensure that the hydroacoustic
method refl ects numbers of fi sh species of interest present). They found that hydroacoustic surveys tended to overestimate the abundance of fi sh, but appears to provide
a good measure of the spatial extent of the aggregation (Chap. 12 ).
Ehrhardt and Deleveaux ( 2007 ) attempted similar work for Nassau grouper in
the Bahamas without ground-truthing. Their estimate of the area covered by the
aggregation off High Cay, Andros at 555,000 m
2 is much larger than any known
Nassau grouper aggregation, while their estimate of fi sh numbers was much higher
than any other that had been surveyed in recent years in the country, or indeed
anywhere currently in the range of the species. They estimated a total of 10,500 fi sh
of an average weight of 2.01–2.13 kg. This reported weight is immediately suspect
as these values are below even the lower limit for aggregating Nassau grouper. Colin
( 1992 ) found Nassau grouper from aggregations at Long Island, Bahamas to weigh
between 2.7 and 9.7 kg (Colin 1992 ) , with Nassau grouper from other aggregations
having similar weights. These disparities call into question the accuracy of their
results, emphasizing the need for ground-truthing, and could be highly misleading
if used directly for management, for example in stock assessment. Similarly
Johnston et al. ( 2006 ) did broad scale acoustic surveys for red hind aggregations
covering hundreds of km of the Puerto Rico insular shelf, but lacked adequate
ground-truth validation. Although extensive schools of fi sh were recorded, it is
unknown whether the data do represent spawning aggregations (or even red hind) or
other fi shes hovering above the bottom.
Hydroacoustic surveys remain a developing technology, with good potential for
assessing spawning aggregations of reef fi shes if applied rigorously. Stringent protocols for ground-truthing and assessing accuracy are essential. The unknown number
of species which could produce signal returns similar to target species, as well as the
attraction of predators to sites (Heyman et al. 2005 ; Nemeth et al. 2007 ) means
hydroacoustic surveys without careful ground truth will not provide reliable data
and will tend to overestimate fi sh numbers.