80 
Abstract .—Gonad weights and re- 
sults of histological analyses from 85 
swordfish, Xiphias gladius, were used 
to develop a validated method for clas- 
sification of the reproductive activity of 
female swordfish based on gonad indi- 
ces (GI’s). The validated method pro- 
vides a significant improvement over 
previously published (unvalidated) 
methods. The method was shown to be 
independent of the length of individual 
fish, important when length is used as 
a criterion for selection of individuals 
from which summary statistics based 
on GI are being developed. Female 
swordfish were found to be in a repro- 
ductively active condition when GI = 
lnfgonad weight in gm)/ln(eye-fork- 
length in cm) > 1.375. Classification 
methods for species with comparable 
reproductive habits and characteristics 
may be alike, and it is speculated that 
results for other billfishes would be 
similar to those described for swordfish. 
Manuscript accepted 12 August 1996. 
Fishery Bulletin 95:80-84(1997). 
Use of gonad indices to estimate the 
status of reproductive activity of 
female swordfish, Xiphias gladius : 
a validated classification method 
Michael G. Hinton 
Inter-American Tropical Tuna Commission 
8604 La Jolla Shores Drive, La Jolla, California 92037-1508 
Ronald G. Taylor 
Michael D. Murphy 
Florida Marine Research Institute, Florida Department of Environmental Protection 
100 Eighth Avenue SE, St. Petersburg, Florida 33701-5095 
We describe a validated classifica- 
tion method that uses gonad indi- 
ces (GI’s) to determine accurately 
the reproductive condition of female 
swordfish, Xiphias gladius. This 
study uses previously unpublished 
data as well as histological analy- 
ses detailed in Taylor and Murphy’s 
(1992) study of the reproductive bi- 
ology of swordfish captured in the 
Straits of Florida. It is standard 
practice to use GI’s to identify re- 
gions and times of active spawning 
in studies of the distribution and 
structure of stocks of many species 
of fish, including swordfish (e.g. 
Kume and Joseph, 1969; Shingu et 
al., 1974; Miyabe and Bayliff, 1987; 
Sosa-Nishizaki, 1990; Nakano and 
Bayliff, 1992; Arocha and Lee, 1993; 
Arocha et ah, 1994; Gouveia and 
Mejuto, 1994; Arocha and Lee, 1995; 
Hinton and Deriso, in press). Data 
on the reproductive activity of 
swordfish are costly and difficult to 
obtain but essential to studies such 
as those noted; full use should be 
made of all available information. 
Our classification method over- 
comes problems of published meth- 
ods (e.g. Miyabe and Bayliff, 1987), 
which have the potential to reduce 
the information database of the re- 
searcher by over 50%. To our knowl- 
edge, it is the first method appli- 
cable to female swordfish to have 
been validated with data obtained 
from histological analyses, which 
provide a verifiable measure of the 
reproductive status of individual 
swordfish. 
The standard practice (Gouveia 
and Mejuto, 1994) in studies using 
values of GI for female swordfish 
has been to estimate GI = 10 4 x GW/ 
EFL 3 , where GW = gonad weight in 
grams, and EFL = length from the 
posterior edge of the orbit to the fork 
of the tail in centimeters (we note 
that without loss of generality, other 
length measurements, such as 
lower-jaw fork length (LJFL), have 
been used) (Kume and Joseph, 
1969). The latter assumed that “[fe- 
males] with gonad indices equal to 
or greater than 3 are about to 
spawn.” Miyabe and Bayliff (1987) 
modified their method by assuming 
that “only females with gonad indi- 
ces of 7.0 or greater were [about to 
spawn].” Arocha and Lee (1995) 
modified the method of Kume and 
Joseph ( 1969) when they noted that 
females with GI’s greater than 4.0 
were in prespawning condition. In 
certain applications of these meth- 
ods, e.g. comparison of average GI’s 
for different regions or time periods, 
