8190

• UCREFRP Catalog Number: 8190
• Author: Papoulias, D. and W. L. Minckley
• Title: Effects of Food Availability on Survival and Growth of Larval Razorback Suckers in Ponds
• USFW Year: 1992
• USFW - Doc Type: Transactions of the American Fisheries Society
• Copyright Material: YES

FEEDING BY LARVAL RAZORBACK SUCKERS
fects due to location. Three ponds did not con-
form with the design because of uneven distri-
bution among localities, so one was arbitrarily
assigned to each group. Each group received dif-
ferent fertilization treatments-341 kg commer-
cial alfalfa pellets and 57 kg P04 per hectare ("high"
fertilization), 170 and 28 kg/hectare ("medium"),
and no application ("low")-before they were filled
with pumped groundwater between 8 and I 1 Feb-
ruary 1985. Half these amounts were applied to
fertilized ponds a week later, then every 2 weeks
through 3 April.
Adult female razorback suckers from Lake Mo-
have received three injections of chorionic gonad-
otropin at 220 IU/kg body weight at 24-h intervals
for 3 d (Hamman 1985) beginning 11 February.
Ova could be expelled with slight pressure 24 h
after the third injection, and were stripped and
fertilized by milt from naturally ripe males from
Lake Mohave. Fertilized eggs were incubated at
21°C in Heath trays'. Hatching began 18 Febru-
ary, and larvae swam actively by 23 February. On
25 February, 9.4-10.7-mm (TL) larvae (mean ±
SE, 9.96 ± 0.04 mm, N = 50) were stocked in
our experimental ponds at a rate of 250,000/hect-
are. Fish received no supplemental feeding.
We divided sampling periods into "weeks" of
7 d duration. Three of four replicate ponds in each
treatment were randomly selected for sampling
each week. Limnological sampling began the week
after ponds were filled (week 0) and continued
through week 7. At the end of week 0, larvae were
stocked. Fish sampling began week l and contin-
ued through week 7. The experiment was con-
cluded at week 8.
We collected single 2-L samples for chlorophyll
analyses (from 1000 to 1400 hours) from 0.5-m
depths at pond centers. Samples were filtered
through 0.8-µm-pore filters, extracted in 90% ac-
etone, and analyzed spectrophotometrically at 665
nm. Quantitative estimates of chlorophyll a were
not attempted; results were used as an index of
relative concentrations of phytoplankton.
We measured surface water temperatures and
sampled invertebrates and fish between 1000 and
1400 hours and from 2000 to 2400 hours. Total
numbers and volumes of invertebrates per cubic
meter were calculated by summing the numbers
or volumes computed for each of three sampling
areas in a pond. We separated ponds into "shal-
lows" (the area from water's edge to level bottom),
"bottom" (defined by height of a device that sam-
pled between 8.0 and 30.5 cm above bottom), and
"open water" (from surface to 30.5 cm above the
341
substrate in pond centers). Relative volumes of
sampling regions were similar among all ponds;
open water constituted 65% of the volume, bot-
tom 20%, and shallows 15%. Our plankton tows
were made with a standard plankton net (l 9.0-cm
diameter, 80-µm mesh) towed at constant speed
the length of each pond in open-water and bottom
regions, and a standard distance in the shallows.
The net was mounted on a sled (Pennak 1978) for
sampling the bottom and shallows. Samples were
preserved in 5% buffered formalin.
Invertebrate samples were agitated and 1.0-mL
aliquots were pipetted to a Sedgewick-Rafter cell.
Field or strip counts were made (APHA et al. 1980)
and data were expressed as numbers per liter. Or-
ganisms were identified to major groups (families
or higher categories) for analysis. We measured
lengths and widths of individual invertebrates by
ocular micrometer on a compound microscope and
assigned geometric shapes to reconstruct volumes
(Edmondson and Winberg 1971). Biovolume of
invertebrates was expressed as cubic millimeters
.of invertebrates per cubic meter of water. Values
for numbers and biovolume were weighted by
habitat volume to derive overall means for ponds.
Widths of invertebrates were also used for esti-
mating size-selective feeding by larval razorback
suckers (Hunter 1981).
During daytime sampling we incidentally cap-
tured larvae in plankton tows and actively at-
tempted to capture larvae by towing a plankton
net behind a boat. At night we sampled using a
spotlight and dip net to attract and collect the
phototactic larvae (Marsh and Langhorst 1988).
The few larvae that were incidentally captured in
our night plankton samples were pooled with those
caught with the spotlight. During weeks I and 2,
both day and night samples were analyzed to ob-
tain data on initial feeding (week 1), and provide
adequate sample sizes (week 2). Night collecting
was perfected by week 3, and the relatively inef-
fective daytime sampling was suspended. Larvae
were preserved in 5% buffered formalin.
We measured upper lip length (ULL), or tip of
upper lip to junction of upper and lower lips, as
an index of mouth size (mouth size = ULL x 2';
Shirota 1970). Total larval length was measured
with an ocular micrometer on a dissecting micro-
scope. Preserved specimens were blotted dry and
weighed to the nearest 0.01 mg. Guts were excised
and food items were removed, identified, and
counted. As with the invertebrate samples, food
items were measured (width and length), and as-
signed a geometric shape to reconstruct volume.