Occurrence of Two Species of Young Threadfin, Polydactylus opercularis 
and P. approximans , in the Offshore Waters of the 
Eastern Tropical Pacific Ocean 
W. L. Klawb and F. G. Alverson 1 
Two species of threadfin, 2 ( Polydactylus , Poly- 
nemidae ) , occur along the Pacific coast of the 
Americas. P. opercularis ranges from the upper 
portion of the Gulf of California to northern 
Peru; P. approximans is found from southern 
California to northern Peru (Hildebrand, 1946; 
Berdegue, 1956). The two are known collec- 
tively as "bobo” by U. S. West Coast tuna 
fishermen who capture them for bait from Santa 
Maria Bay, Mexico, to the Gulf of Guayaquil, 
Ecuador (Alverson and Shimada, 1957). Adults 
of both species commonly occur in inshore wa- 
ters, where they are utilized for food by the 
indigenous human population. The juveniles, 
however, have often been found at a considerable 
distance offshore, where they are, on occasion, 
found in quantity. 
Information concerning the offshore occur- 
rence of young Polydactylus was obtained from 
two sources: (1) logbooks kept by the masters 
of tuna clippers, in which a record of naviga- 
tional and fishing activities, including the pro- 
curement of bait, is kept; and (2) collections 
made by scientists from the Scripps Institution 
of Oceanography, Bureau of Commercial Fish- 
eries, Inter-American Tropical Tuna Commis- 
sion, and other institutions. The bobo taken by 
the tuna fishermen were captured with either a 
lampara net or a crowder, a small net used to 
concentrate bait in the wells (Godsil, 1938; 
Alverson and Shimada, 1957). Listed in Table 
1, by date of capture, are the locations and 
amounts of bobo taken for use as bait. The 
locations of capture have been plotted in Figure 
1. Practically all the collections taken by scien- 
' Inter-American Tropical Tuna Commission, La 
Jolla, California. Manuscript received December 20, 
1962. 
2 The following names are used for threadfin in 
some of the Latin American countries: raton, aleta de 
hebra in Mexico, amarillo in Ecuador, and barbudo in 
Peru. 
tists were made with the aid of a night light, an 
electric light suspended outboard of a drifting 
vessel so that the organisms attracted may be 
dipped out with a fine-mesh net. Listed in Table 
2, for each threadfin collection made by scien- 
tific personnel, Is the date and location of cap- 
ture, number, size-range, and species. The loca- 
tions of capture have been plotted in Figure 2. 
In our collections the young of both species 
were not taken beyond 100 miles offshore, north 
of the Gulf of Tehuantepec, and 250 miles off- 
shore south of the Gulf, the only exception 
being the occurrence of P approximans in the 
vicinity of the Galapagos Islands, some 600 
miles offshore (Figure 2). These limits are 
probably fairly accurate descriptions of the off- 
shore distribution as numerous collections to 
the westward (Klawe, 1963) did not yield any 
young. 
P, approximans has been recorded from 27: 
38 M and 114:50 W (off Turtle Bay, Baja 
California) in the north to 5:52 S and 81:28 W 
(off Point Aguja, Peru) in the south. P. opercu- 
laris has been recorded from 23:55 N and 108: 
00 W (Gulf of California) in the north to 
5:52 S and 81:28 W in the south. The north- 
south offshore distribution of the young, of both 
species, is similar to the north-south inshore 
distribution of the adults. 
The specimens of P, opercularis collected by 
night lighting and other methods, excepting 
the bait fishery, had a size-range of 7-87 mm, 
the vast majority being less than 50 mm in 
fork-length (Table 2). Three samples of P. 
opercularis which were given to the Commission 
by tuna fishermen had size-ranges of 46-67 mm, 
58-72 mm, and 63-89 mm, respectively (Table 
1 ) . The length-frequency distribution of two 
of these samples has been plotted in Figure 3. 
Specimens of P. approximans collected by meth- 
ods other than the bait fishery ranged in size 
166 
