(n,s5) 
Z 
Z 
(23,22 8) 
132) 
MEAN PLANKTON VOLUME (cc71000m*) 
*2 DAS SAASSY("7, 20.6) 
| Sa SI 
CRUISE DATES 
Figure 11.—Comparison of mean zooplankton volumes for stations 
sampled north of lat. 27°15'N and stations south of that latitude for 12 
eastern Gulf of Mexico cruises, 1972-74. Numbers of stations and 
standard errors of the mean are given in parentheses above bar graphs. 
northern sector (Table 3). There was no apparent seasonal 
pattern that explained the occurrence of the highest mean 
egg abundance in the southern sector for six cruises (Fig. 
12). The mean abundance of fish larvae was highest in the 
northern sector during only 2 of the 12 cruises (Table 3; 
Fig. 12). 
Spawning by fishes seems to be more intensive in the 
northern sector, but larval abundance usually is not highest 
there. Higher mortality rates in the north, transport out of 
that area toward the south, or transport of larvae into the 
southern sector from outside the sampling area could 
account for the observed distributions. Mean zooplankton 
volumes usually were highest in the northern sector but 
differences between sectors were not often great, pooled 
means differing by only 7 cc/1,000 m* between the two 
areas (Table 3). 
The seasonality of abundance that was observed pre- 
_ viously, when the whole shelf area was considered, also was 
| apparent in each of the sectors (Figs. 11, 12). Zooplankton 
_ volumes were lowest in winter and highest in summer for 
both the northern and southern sectors, although the degree 
_ of increase during summer was greatest in the southern 
_ sector. Abundance of ichthyoplankton also reflected the 
| seasonal pattern of relative scarcity in winter and increased 
abundance in summer in each of the sectors. 
Distribution Relative to the 50-m Depth Contour 
The 50-m isobath divides the shelf study area into 
approximate halves (Fig. 1). Zooplankton volumes and 
ichthyoplankton abundance and concentrations were ex- 
amined in relation to this depth contour to determine if they 
differed between nearshore and offshore areas (Table 4; 
Figs. 13, 14). The nature of water masses, rather than depth 
is a factor determining zooplankton and ichthyoplankton 
abundance, but the 50-m depth is a convenient, common 
contour for comparing cruises. Gulf Loop Current water 
may occasionally intrude onto the west Florida continental 
15 
(25,252.68) 
(23,1606) 
(n,s5) 
(24, 232 4) 
(25, (27.2) 
(19,146 0) 
~ 
€ 
° =~ 
= s 
2 = 
3 9009 $ Rate a 
eS ae 
o = a . i = 
a S = 2 = 3 North 
aero A lis | Ie 23 r 
Fe = (Ast it =e 37 é 
pele [BA 
. eee 
Zwortis |Y A it Z 
> a |G A Z 
z Z 4 Z Zi 
ry Z Z 4 South 
2 A | 4 
FEB. may JUN, SEPT, NOV, MAY JUN> JUL 
72 72 72 72 73 73 
(27, 63.4) 
4 
g 
Y 
Z 
Py: 
2 = 
“9 
a 
may 
4 
REEMA SASS (25, 100.0) 
RSS (26 94.2) 
KSSH (2! 6 45) 
MEAN NUMBER OF FISH LARVAE UNDER 10m? 
SSS (26, 69.8) 
Qe Uh 
KISS 
RQny (15, 801) 
= 
< 
a 
© 
N 
UM, 
. Ne May JUN 
i“) 73 7 
Figure 12.— Mean fish egg and fish larvae abundance. Comparisons are 
made for stations sampled north of lat. 27°15 N and stations south of that 
latitude for 12 eastern Gulf of Mexico cruises, 1972-74. Numbers of 
stations and standard errors of the mean are given in parentheses above 
bar graphs. 
2 RSS (9. 1831) 
‘o 
rv 
= 
z 
‘| 
aE 
SEPT. 
72 
Ne 
UL, | AUG. 
73 
i 
F 
7 
~ 
2 
) 
shelf where depths range from 50 to 200 m (Maul 1974) and 
the character of zooplankton and ichthyoplankton could be 
influenced by such intrusions (Austin 1974). We did not 
determine if differences in plankton species composition 
occurred during this study. 
Mean volumes of zooplankton were greatest at stations 
less than 50 m deep on 8 of the 10 cruises where stations 
in both depth zones were sampled. The two exceptions 
occurred in February 1972 and June-July 1973 when the 
mean volumes of zooplankton were slightly higher over the 
outer shelf. Pooled mean zooplankton volumes for the 10 
cruises were calculated for each of the two depth zones 
(Table 4). Mean volume at depths shallower than 50 m 
exceeded that at deeper stations by a factor of 1.78. For the 
10 cruises in which both depth zones were sampled, mean 
zooplankton volumes at stations shallower than 50 m 
exceeded 150 cc/1,000 m? seven times, but mean plankton 
volume exceeded that amount on only one cruise at stations 
deeper than 50 m. 
Mean fish egg abundance usually was higher at stations 
less than 50 m deep but mean larval abundance was always 
higher at the deeper stations for the 10 cruises. One reason 
for this observation is that fish larvae range in age from 1 
day to more than 3 wk. Fish eggs, by contrast, usually hatch 
within 2 days of spawning in the eastern Gulf, precluding the 
possibility of numbers accumulating over a period in any 
area. Therefore, abundance of eggs would tend to be highest 
where the eggs were spawned in greatest numbers. Other 
reasons that larval abundance may be relatively low at 
shallow stations are that a higher proportion of shallow- 
water species are demersal and furthermore their larvae 
become demersal at a younger age than larvae of demersal, 
offshore species. 
Seasonal fluctuations of zooplankton and ichthyoplank- 
ton abundance were most apparent at stations less than 50 m 
deep. Plankton volumes usually increased in summer at 
stations located over deeper water (Fig. 13) but the relative 
increase in zooplankton was less than at shallower stations. 
