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74
<br />Fish Hatchery (NFH), New Mexico. Glass jars
<br />(3.91) containing larvae were arranged in rows in
<br />indoor raceways; with position effects minimized
<br />by aconstant-temperature (18° C) water bath and
<br />uniform lighting. Continuous water flow at
<br />---100 ml min-', which maintained jars at full ca-
<br />pacity,was provided from 2.54 cm pipes perforated
<br />above each container. Jars were tilted to facilitate
<br />outflow and screened to prevent loss of fish and
<br />food.
<br />Adult (Lake Mohave) female suckers received
<br />three intramuscular injections of 220IU chorionic
<br />gonadotropin kg'' body weight at 24 h intervals
<br />(Hamman 1985) beginning 11 February 1985.
<br />About 24 h after the third injection, eggs were
<br />stripped and fertilized by milt from naturally-rip-
<br />ened, Lake Mohave males. Fertilized eggs were
<br />incubated at 21° C in Heath® trays (Inslee 1982).
<br />Hatching began on 18 February, and larvae first
<br />swam actively on 23 February. On 25 February, 9.6
<br />to 10.7 mm TL (Mean = 9.96 ± 0.04 (SE) mm,
<br />n = 50) larvae were stocked at 10 jar '; studies
<br />lasted 50 days, until 16 April.
<br />We examined starvation in Experiment 1 and the
<br />pattern of razorback sucker larvae mortality over
<br />time in the total absence of food; six replications
<br />(jars) were performed. The time to irreversible
<br />starvation was measured in Experiment 2. Larvae
<br />were considered irreversibly starved when, after
<br />being without food for a predetermined period of
<br />time, death could not be prevented by offering
<br />food. The point of irreversible starvation was de-
<br />fined as the number of days to a cumulative larval
<br />mortality > 50%. This point was determined by
<br />delaying introduction of food to the jars containing
<br />the larvae. Seven treatments with 3 replicates (jars)
<br />were established such that individual jars of larvae
<br />were without food for 7, 11, 15, 19, 23, 27 and 31 d
<br />after hatching, then offered newly-hatched brine
<br />shrimp (Artemia Salina) nauplii at 3001-' for the
<br />remaining 50, 46, 42, 38, 34, 30, and 26 d, respec-
<br />tively, of the experiment. In Experiment 3, we
<br />studied the effect of ration size on mortality and
<br />growth of razorback sucker larvae. Six treatments,
<br />each replicated in 3 jars, consisted of individual jars
<br />of larvae fed brine shrimp nauplii at concentrations
<br />of 5, 10, 50, 100, 500, and 10001'` .
<br />For experiments 2 and 3, the individual jars of 10
<br />larvae were offered the predetermined numbers of
<br />live Artemia nauplii three times d'' at 8 h intervals.
<br />Nauplii were from San Francisco Brand' eggs in-
<br />cubated in aerated 3.91 jars at 27-28° C for 36 h, by
<br />which time most had hatched. Jars were agitated
<br />and allowed to stand for 1/2 h. During this time
<br />nauplii accumulated near the bottom from where
<br />they were siphoned, nearly free of egg cases, to a
<br />beaker. Densities in concentrated samples were
<br />estimated by five counts of 1.0 m1 aliquots in a
<br />Sedgewick-Rafter cell. Appropriate amounts were
<br />prepared by dilution for selected levels of feeding.
<br />Levels of food are quoted as the nominal feeding
<br />rate, e.g., a container receiving a nomina150 nau-
<br />plii 1'' feeding'' (150 nauplii 1'' d'') is quoted as 50
<br />nauplii 1''.
<br />Individual containers (including those not re-
<br />ceiving food) were inspected for dead larvae and
<br />siphoned clean of accumulated materials prior to
<br />each feeding. Dead larvae were preserved in 5%
<br />buffered formalin. Not all dead larvae were reco-
<br />vered; some became lodged and undetected be-
<br />neath screens, some were inadvertently killed by
<br />siphoning, and others may have decomposed. Such
<br />larvae were considered as mortalities. To estimate
<br />minimum prey density necessary for 50% survival,
<br />unaccounted mortalities were distributed over
<br />dates when known mortalities occurred according
<br />to the following formula (O'Connell & Raymond
<br />1970):
<br />
<br />where m„ =total dead on day n; d„ =dead observ-
<br />ed on day n; P =initial population; S =survivors;
<br />and D =dead observed on all days. All mortalities
<br />were recorded by date, and if possible, measured.
<br />Surviving larvae were preserved in 5% buffered
<br />formalin and measured at the conclusion of the
<br />experiment.
<br />Data for Experiments 2 and 3 were statistically
<br />evaluated using the Kruskal-Wallis test (p < 0.05).
<br />Means in text are followed by ± one standard er-
<br />rOT.
<br />Results
<br />Experi~
<br />Median
<br />suckerl
<br />ing. Th~
<br />30 d pos
<br />0.2 mm
<br />Experin
<br />No mon
<br />food on
<br />receiving
<br />hatch ha
<br />26.7%.
<br />among 7
<br />rates occ
<br />for 27 (8f
<br />Table 1. E:
<br />(top) and u
<br />after 50 d a
<br />razorback s
<br />hatching. D
<br />fetences (1
<br />means.
<br />Age at initi;
<br />(d after hart
<br />7
<br />11
<br />15
<br />19
<br />23
<br />27
<br />31
<br />7 ,
<br />11 ,
<br />15 ,
<br />19
<br />23 1
<br />27
<br />31
<br />
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