7821

• UCREFRP Catalog Number: 7821
• Author: Hamilton, S. J., et al.
• Title: Evaluation Of Toxicity To Larval Razorback Sucker Of Selenium-Laden Food Organisms From Ouray NWR On The Green River, Utah.
• USFW Year: 1996.
• USFW - Doc Type: \
• Copyright Material: NO

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NR treatments, it is clear that a stress threshold of some sort was exceeded in the SI, S3,
S4, and S5 treatments before it was exceeded in SR and NR. This observation implies a
food-derived stress caused the differences in survival because all fish were held in the same
water. Alternatively, the delayed mortality in the SR and NR treatments could have been a
result of an interaction between S 1 water and food organisms from SR and NR.

Steve Severson (U.S. Fish and Wildlife Service, Ouray Native Fish Facility, Vernal,
Utah) observed fish on June 1 in studies 1 (day 11) and 2 (day 6) when the feeding rate was
20 organisms/fish and noted their stomachs looked full. Likewise, prior to daily feedings of
live zooplankton, uneaten live zooplankton from the previous day were observed in the
exposure beakers, thus suggesting larval razorback sucker were apparently satiated.
Nevertheless, the feeding rate was doubled to 40 organisms/fish after June 1.

In the present study, larvae were fed 20 organisms/fish (125 organisms/L or 12.5
organisms/L per fish) initially and 40 organisms/fish (250 organisms/L or 25 organisms/L
per fish) through the majority of the studies 1 and 2 and all of studies 3 and 4. Papoulias
and Minckley (1990) reported in their laboratory study conducted at 180C that 30 to 60
nauplii of brine shrimp (Artemia salina) per fish per day was sufficient to maintain high
survival and no reduced growth, whereas 12 or less nauplii per fish per day resulted in
substantial mortality and reduced growth. Papoulias and Minckley (1992) reported high
larval survival (77, 90, and 67%) in ponds with invertebrate densities of 43, 24, and 12
organisms/L, respectively. Our supplemental feeding study confirmed that as little as 20
food organisms per fish per day will maintain a high survival rate. Even 24-day old larvae
fed 20 organisms per fish in the supplemental feeding study -- half the rate in Ouray study
3 -- had 97 % survival.

Likewise, Marsh and Langhorst (1988) concluded that nutritional factors such as type,
number, or size of available foods did not affect survival or growth of larval razorback
sucker in Lake Mohave reservoir, which had a low 1.55 zooplanktonlL (reported as 1,554
zooplanktonlm3) or in Arizona Bay backwater, which had 0.38 zooplanktonlL (reported as
377 zooplanktonlm3). The organism densities in the field studies by Papoulias and Minckley
(1992) and Marsh and Langhorst (1988) encompassed all possible sizes, whereas the
zooplankton used in our study were preselected to be < 0.425 mm, which would have
represented only a portion of the various sizes present in a field situation and presumably
available to larval fish.

Larval razorback sucker in the present study were fed either 125 or 250 organisms/L.
This density is 8 and 17 times, respectively, greater than the 15 organisms/L, which included
all possible sizes, reported by Cooper and Severn (1994) for Sheppard Bottom pond 3. By
increasing the density of zooplankton and preselecting small sizes in the present study, the
likelihood that larval razorback sucker would successfully consume food was increased
substantially. This approach also reduced the energy cost to larvae to search and capture
prey. In natural wetlands fish would have unlimited access spatially to food organisms, but
low food density would require larvae to forage over greater areas, which could in turn cost

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