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Fishery Bulletin 96(2), 1 998 
Table 2 
Pearson product-moment correlations between properties of the nighttime 
(2000-0400) deep-scattering layer and the thermocline depth observed in 1992 
and 1993. S v = volume scattering strength (dB). Thermocline depth = depth of 
maximum 10-m temperature gradient. ***=P<0.001, **=P<0.01. PODS92 = 
Population of Delphinus Stocks 1992 cruise; PODS93 = Population of Delphi- 
nus Stocks 1993 cruise. 
Correlations with thermocline depth 
Mean S y 
Depth of maximum S v above thermocline 
PODS92 
38 kHz 
+0.86*** 
-0.15 
150 kHz 
+0.84*** 
-0.23 
PODS93 
38 kHz 
+0.25 
-0.01 
150 kHz 
+0.43** 
+0.15 
tering occur (MacLennan and Sim- 
monds, 1992). Target strength is a 
nonlinear function of sound fre- 
quency and the cross-sectional area 
of the target (Demer and Martin, 
1995) and TS per unit of area is typi- 
cally expressed as a function of the 
dimensionless number {ka), where k 
is the acoustic wavenumber {k=2 nfk) 
and a is a characteristic dimension 
of the target (Chu et al., 1992). At low 
sound frequencies (ka< 1, “Rayleigh” 
scattering), TS drops off rapidly. 
Thus, scattering of relatively low fre- 
quency sound by small targets is 
weak. At high frequencies (ka> 1, 
“geometric” scattering), early models 
predicted TS to be approximately con- 
stant (Love, 1977). Most other models of geometric 
scattering are highly nonlinear, with deep nulls in 
individual TS versus ( ka ) curves. However, Chu et 
al. (1992) suggested that averaging over pings and 
individuals in field studies of volume backscattering 
should “smear out” these nonlinearities. 
The wavelengths of 38-and 150-kHz sound corre- 
spond to target dimensions of approximately 4 and 1 
cm, respectively. This is the effective size of macro- 
zooplankton and micronekton organisms, such as 
euphausiids, siphonophores, small fish, and squid. 
Targets <4 cm, such as copepods, will scatter sound 
strongly at 150 kHz. Scattering of 38-kHz sound will 
be relatively weak Rayleigh scattering and may be 
undetectable over system noise. These smaller tar- 
gets may have weakened the correlation between 
backscattering at the two frequencies (Fig. 3). 
We were unable to make net tows to identify acous- 
tic targets, owing to time constraints on the cetacean 
survey cruises. Therefore, we can make only an edu- 
cated guess about the composition of the observed 
deep-scattering layers. Roe et al. ( 1984), in a uniquely 
comprehensive study of a community of vertically 
migrating organisms in the temperate northeast At- 
lantic, considered fish, decapod crustaceans, mysids, 
euphausiids, amphipods, copepods, ostracods, 
siphonophores, medusae, ctenophores, and chaetog- 
naths. Ignoring small or weak scatterers, we must 
consider fish, crustaceans (decapods, mysids, and 
euphausiids), and siphonophores, as well as cepha- 
lopods, as possible components of the observed DSL’s. 
Average biomass densities of mesopelagic fish (pri- 
marily myctophids) and cephalopods are equal to, or 
slightly greater than, those of crustaceans in the east- 
ern tropical Pacific (Blackburn, 1968; Blackburn et 
al., 1970) and subtropical Pacific (Maynard et al., 
1975). Target strength depends on the composition, 
Table 3 
Correlations between dolphin sighting rates (schools and 
individuals per 100 nautical miles of effort, log-transformed) 
and mean 150-kHz volume scattering strength (S , dB) above 
the thermocline at night (2000-0400), and 0 to 400 m night 
and day (0800-1600). *** = P<0.001, ** = P<0.01, * = P<0.05. 
PODS92 = Population of Delphinus Stocks 1992 cruise; 
PODS93 = Population of Delphinus Stocks 1993 cruise. 
Dolphin sighting rates 
Schools 
Individuals 
PODS92 
Night S v above thermocline 
+0.35* 
+0.38** 
Night S v 0-400m 
+0.05 
+0.02 
Day S v 0-400m 
-0.01 
-0.02 
PODS93 
Night S y above thermocline 
+0.27 
+0.15 
Night S v 0-400m 
+0.36** 
+0.20 
Day S v 0-400m 
+0.38** 
+0.18 
shape, and orientation of the target, as well as on 
relative size. At 38-120 kHz, small fish with gas-filled 
swim bladders (e.g. clupeoids) and squid have target 
strengths 5-10 dB greater than those for crustaceans 
of the same size (Tables 6.3 and 6.4 in MacLennan 
and Simmonds, 1992; Marchal et al., 1993). Siphono- 
phores have a gas bladder that scatters sound and 
have been observed migrating with myctophids in a 
DSL off Baja California (Barham, 1966). However, 
myctophids, flying fish, and squid were much more 
abundant than siphonophores at night dipnet sta- 
tions on our cruises. 2 Therefore, mesopelagic fish and 
2 Pitman, R. 1996. Southwest Fisheries Science Center, Natl. 
Mar. Fish. Serv., NOAA, Box 271, La Jolla, CA92038. Personal 
commun. 
