316
P.L. Colin
9.6.8 Measuring Sizes of Fishes
The size of fi sh in an aggregation is, after the numbers present, the most useful
information to acquire. Size sampling often requires fi sh capture and this may not
perfectly refl ect the actual size distribution of fi sh in the aggregation because of size
or sex selectivity of fi shing gear. Size information can also be acquired “remotely”.
This might be done by pointing lasers, a set distance apart, at fi shes and have divers
photograph them. This will not work for all species, as Kadison et al. ( 2006 ) tried
using green lasers to estimate lengths of grouper, but found that large fi sh had an
extreme aversion to laser beams. Often observers estimate the size of fi shes visually; a technique which has value, but for which the data are innately unverifi able
and consequently must be treated as inaccurate A variety of methods has been proposed to train observers to estimate lengths of fi shes underwater (Bell et al. 1985 )
which appear to increase the ability to closely estimate lengths under the controlled
conditions of training. There is no guarantee similar results are obtained on fi shes in
the fi eld and the skills of length estimation required regular retraining. It must be
remembered that visual size estimates are not accurate. The problems of using estimated lengths in fi shery and conservation considerations are similar to the estimates
of fi sh numbers present in aggregations; the data are useful but any pretensions of
accuracy are specious.
Three dimensional (3D) imagery has great promise as a means of determining
sizes of fi sh in an aggregation (Klimley and Brown 1983 ) . Rand et al. ( 2006 ) used
3D videography to capture images of a Nassau grouper aggregation and extract size
estimates of individual fi sh, construct a size frequency curve for the fi sh and determine nearest neighbour distances. Similar methods have been applied to submersible and ROV images (Rochet et al. 2006 ) . Some earlier efforts using stereo images
have been listed elsewhere (Colin et al. 2003 ) and are not repeated here.
9.6.9 Working with Gametes and Eggs
If eggs are collected in the fi eld, or fi sh are spawned artifi cially the characters of the
eggs (shape, size, and presence of pigment or oil globules) can be determined by
microscopic examination. For fi eld collection, immediately after spawning the area
where the gametes have been released can be marked with a small squirt of fl uorescein dye from a squeeze bottle. After this delay the water near the dye marker is
swept with the hand net for a short period and the contents collected in the net put
into a plastic bag. Most planktonic reef fi sh eggs should be handled gently just after
spawning, but can be easily manipulated once hardened up after fertilization. After
an hour or so they are quite robust and can usually be safely washed in a sieve and
gently handled.
Appeldoorn et al. ( 1988 ) provide a thorough discussion of various tracers useful
in tracking the dispersal and fate of pelagic fi sh eggs. Kifl awi et al. ( 1998 ) and
P.L. Colin
9.6.8 Measuring Sizes of Fishes
The size of fi sh in an aggregation is, after the numbers present, the most useful
information to acquire. Size sampling often requires fi sh capture and this may not
perfectly refl ect the actual size distribution of fi sh in the aggregation because of size
or sex selectivity of fi shing gear. Size information can also be acquired “remotely”.
This might be done by pointing lasers, a set distance apart, at fi shes and have divers
photograph them. This will not work for all species, as Kadison et al. ( 2006 ) tried
using green lasers to estimate lengths of grouper, but found that large fi sh had an
extreme aversion to laser beams. Often observers estimate the size of fi shes visually; a technique which has value, but for which the data are innately unverifi able
and consequently must be treated as inaccurate A variety of methods has been proposed to train observers to estimate lengths of fi shes underwater (Bell et al. 1985 )
which appear to increase the ability to closely estimate lengths under the controlled
conditions of training. There is no guarantee similar results are obtained on fi shes in
the fi eld and the skills of length estimation required regular retraining. It must be
remembered that visual size estimates are not accurate. The problems of using estimated lengths in fi shery and conservation considerations are similar to the estimates
of fi sh numbers present in aggregations; the data are useful but any pretensions of
accuracy are specious.
Three dimensional (3D) imagery has great promise as a means of determining
sizes of fi sh in an aggregation (Klimley and Brown 1983 ) . Rand et al. ( 2006 ) used
3D videography to capture images of a Nassau grouper aggregation and extract size
estimates of individual fi sh, construct a size frequency curve for the fi sh and determine nearest neighbour distances. Similar methods have been applied to submersible and ROV images (Rochet et al. 2006 ) . Some earlier efforts using stereo images
have been listed elsewhere (Colin et al. 2003 ) and are not repeated here.
9.6.9 Working with Gametes and Eggs
If eggs are collected in the fi eld, or fi sh are spawned artifi cially the characters of the
eggs (shape, size, and presence of pigment or oil globules) can be determined by
microscopic examination. For fi eld collection, immediately after spawning the area
where the gametes have been released can be marked with a small squirt of fl uorescein dye from a squeeze bottle. After this delay the water near the dye marker is
swept with the hand net for a short period and the contents collected in the net put
into a plastic bag. Most planktonic reef fi sh eggs should be handled gently just after
spawning, but can be easily manipulated once hardened up after fertilization. After
an hour or so they are quite robust and can usually be safely washed in a sieve and
gently handled.
Appeldoorn et al. ( 1988 ) provide a thorough discussion of various tracers useful
in tracking the dispersal and fate of pelagic fi sh eggs. Kifl awi et al. ( 1998 ) and