Staaf et at: Distribution of ommastrephid paralarvae in the eastern tropical Pacific 
79 
in the Gulf of California only the jumbo squid has been 
reported to spawn (Gilly et ah, 2006; Staaf et ah, 2008; 
Camarillo-Coop et ah, 2011), and, to our knowledge, 
no other adult ommastrephid has been described from 
this region, although adults of purpleback squid have 
been reported from the area near the mouth of this 
gulf (Olson and Galvan-Magana, 2002). In the south- 
ern hemisphere, the Peru Current System has yielded 
only jumbo squid paralarvae (Sakai et ah, 2008). In the 
large intervening equatorial region, paralarvae of both 
purpleback squid and jumbo squid are present (Oku- 
tani, 1974; Ueynagi and Nonaka, 1993). 
The data that form the basis of this knowledge were 
collected through a variety of methods. Samples from 
both the Pacific and Gulf coasts of the Baja California 
Peninsula and from the Peru Current were collected 
primarily during subsurface oblique tows with bongo 
nets (Ramos-Castillejos et ah, 2010; Camarillo-Coop et 
ah, 2011; Sakai et ah, 2008). By contrast, the central 
region of the eastern tropical Pacific (ETP) has been 
sampled extensively during surface tows with neuston 
nets, yielding higher densities of paralarvae (Ueynagi 
and Nonaka, 1993; Vecchione, 1999). In the ETP, densi- 
ties can be extremely high, as in the case of more than 
10,000 very small paralarvae of the SD complex from 
a single surface tow conducted during the 1986-87 El 
Nino (Vecchione, 1999). By contrast, the greatest num- 
ber of SD-complex paralarvae reported from the Baja 
California Peninsula is 20, collected with a bongo net 
(Camarillo-Coop et ah, 2011). 
Surface tows effectively sample only the top 10-20 
cm of the water column, but subsurface oblique tows 
typically sample from the surface to depths of about 
200 m. Because oblique tows sample a broader, deeper 
range of habitats than surface tows, discrepancies in 
paralarval abundance and size between the 2 types 
of tows may reflect different vertical habitat prefer- 
ences at different stages of development. For example, 
if recently hatched paralarvae exhibit a preference for 
surface waters, surface tows would be far more effec- 
tive at capturing these animals because oblique tows 
spend very little time at the surface (10-20 cm). And 
if paralarvae begin to occupy greater depths as they 
grow, while their numbers decrease because of natu- 
ral mortality, oblique tows would be likely to capture 
fewer, larger individuals than would surface tows, as 
has been seen for the ommastrephid Todarodes pacifi- 
cus (Yamamoto et ah, 2002; 2007). 
Although high surface abundances can be represen- 
tatively sampled by surface tows, any narrow subsur- 
face band of high abundance, as might occur at a pyc- 
nocline, would be undersampled by oblique tows. How- 
ever, a strong association of paralarvae with a subsur- 
face feature in preference to the surface could still be 
detected by a greater likelihood of capture in oblique 
rather than in surface tows, as has been found for 
the northern shortfin squid ( Illex illecebrosus ), which 
shows a relationship with the subsurface interface be- 
tween slope water and the Gulf Stream in the Atlantic 
(Vecchione, 1979; Vecchione et ah, 2001). 
Diel vertical migrations, typical of adult ommas- 
trephids, also could drive different abundances in sur- 
face and oblique tows. This result was found in loliginid 
paralarvae (Zeidberg and Hamner, 2002), but the situa- 
tion is less clear for ommastrephids (Piatkowski et ah, 
1993; Young and Hirota, 1990). The few surface tows 
during which paralarvae of northern shortfin squid 
were collected in the Middle Atlantic Bight were con- 
ducted at night (Vecchione, 1979) — a finding that could 
indicate a nighttime migration to the surface, but the 
numbers are too small to strongly support this idea. No 
significant differences in paralarval abundance of pur- 
pleback squid have been found between daytime and 
nighttime tows in Hawaii (oblique and horizontal tows 
from the surface to a depth of 200 m; [Harman and 
Young, 1985]) or Japan (horizontal tows from the sur- 
face to a depth of 200 m; [Saito and Kubodera, 1993]). 
On cruises conducted by NOAA in the ETP, ecosys- 
tem data (including plankton samples) from a large 
geographic area have been collected regularly and ar- 
chived for many years. In this study, we present the 
first analysis of planktonic squid from this data set, 
focusing on the ommastrephids jumbo squid and pur- 
pleback squid. Our aims are 1) to compare surface and 
oblique tows conducted at the same location and time 
to determine differences in paralarval distribution and 
abundance due to sampling method, 2) to address ques- 
tions of species-specific depth preference and vertical 
migration, 3) to uncover relationships between paralar- 
val abundance and oceanographic features, and 4) to 
use molecular techniques on a subset of samples to de- 
termine whether the 2 species have distinct spawning 
areas or habitat preferences within their range overlap. 
Paralarval distribution is also contrasted with adult 
distribution data, collected during the 2006 cruise, to 
confirm that the study region is within the adult range 
of both species and to enhance our understanding of 
the ETP as a feeding and spawning area. 
Materials and methods 
Study area and data collection 
The ETP, where the ranges of jumbo squid and purple- 
back squid overlap, is defined by 3 large surface cur- 
rents and 2 water masses (Fiedler and Talley, 2006; 
Fig. 1A). The westward-flowing North and South Equa- 
torial Currents derive from the temperate California 
and Peru Currents, respectively. The Equatorial Coun- 
tercurrent flows eastward from the western Pacific to 
the coast of Central America. These currents define 
2 water masses: Tropical Surface Water and Equato- 
rial Surface Water, the latter cooler and fresher than 
the former. Two smaller-scale oceanographic features 
are prominent: 1) a distinct thermocline ridge at the 
interface between the North Equatorial Current and 
