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STOCKTON FWO: Predation Experiments on Marked and Unmarked Delta Smelt
California-Nevada Offices , June 24, 2010
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Figure 1. Circular tank including circular net and the hoist system used in predation tests using juvenile striped bass fed on cultured adult delta smelt (photo USFWS).
Figure 1. Circular tank including circular net and the hoist system used in predation tests using juvenile striped bass fed on cultured adult delta smelt (photo USFWS). - Photo Credit: n/a
Figure 2. Galen Tigan (right) and Luke Ellison (left, UC Davis Fish Conservation Lab) introducing juvenile striped bass in circular tank prior to predation tests (Photo: USFWS).
Figure 2. Galen Tigan (right) and Luke Ellison (left, UC Davis Fish Conservation Lab) introducing juvenile striped bass in circular tank prior to predation tests (Photo: USFWS). - Photo Credit: n/a
Figure 3. Luke Ellison, right (UC Davis Fish Conservation Lab) and Tim Matt, left front and Bill Beckett, left background (DFG) introduce cultured adult delta smelt in circular tank during predation test to evaluate potential differences in striped bass predation between marked and unmarked delta smelt (Photo:USFWS).
Figure 3. Luke Ellison, right (UC Davis Fish Conservation Lab) and Tim Matt, left front and Bill Beckett, left background (DFG) introduce cultured adult delta smelt in circular tank during predation test to evaluate potential differences in striped bass predation between marked and unmarked delta smelt (Photo:USFWS). - Photo Credit: n/a

by Gonzalo Castillo, Stockton FWO
Stockton Fish and Wildlife Office Biologist Gonzalo Castillo in collaboration with Joan Lindberg (UC Davis Fish Culture and Conservation lab, FCCL, Byron), Jerry Morinaka (California Department of Fish and Game, Stockton) and Brent Bridges (U.S. Bureau of Reclamation, Byron) conducted predation tests in spring 2010 to further validate mark-recapture methods for Delta smelt.

Hatchery origin Delta smelt were provided as prey to striped bass to investigate potential differences in predation between unmarked vs. fish marked with different colors. Comparisons included: 1) differences between calcein-photonically marked fish and unmarked fish and 2) differences among fish groups photonically marked with three different colors and exposed to calcein. Calcein was applied for 5 minutes (5 g/L) to 150 fish and 150 unmarked fish were used as control (mean size 7.3 cm FL). The following week, all calcein marked fish were divided in three groups of c.a. 50 fish each, and were photonically marked in the anal fin (green, white and blue marks).

Striped bass was selected as predator because this species is considered a key piscivore in the south Delta.  The striped bass used in this study were deprived of food one week prior to predation trials to enhance feeding response.  Juvenile striped bass used in predation experiments (mean size 37.5 cm FL, range 28-40 cm) were of a size range commonly present in winter to early spring in the Clifton Court Forebay where the Delta smelt mark and recapture experiments took place in 2008-09.

Predation experiments were conducted in a circular tank (2.43 m diameter x 0.90 m height). To facilitate counting of uneaten prey and to terminate predation trials, a 2.43 m diameter circular net was inserted in the tank prior to introduction of any fish to retain all delta smelt.  A hoist system was used to lift the net and to count the prey remaining after one or two hour intervals following their introduction into the tank.  The net was completely lifted out the water to terminate experiments when it was estimated that about 50% of the fish had been eaten (Figure 1).

Four striped bass were used in each of the four predation tests. Pre-tests revealed that predation increased when striped bass were introduced in the tank 24 h in advance of prey and allowed to acclimate to their surroundings. Thus, predators were introduced 24 h before prey in the tests (Figure 2). Up to 60 delta smelt were introduced in the tank in each test (10 delta smelt per photonic mark group x 3 photonic colors plus 30 unmarked delta smelt control). To initiate feeding tests, equal numbers of marked and unmarked prey were transferred into two five gallon tanks and released simultaneously from opposite sides of the tank containing predators (Figure 3). 

Results showed no evidence of predation difference between marked fish (calcein and photonically marked) and unmarked fish.  Although delta smelt with green photonic marks seemed to have experienced slightly lower predation relative to fish with blue or white marks, such differences were not statistically significant.  Earlier experiments also showed no significant differences in mortality between marked and unmarked delta smelt maintained in the lab without exposure to predators. These new experiments further support the mark-recapture assumption that there are no differences in predation induced mortality between marked and unmarked delta smelt and among the three photonic marked groups.

 

 


Contact Info: Scott Flaherty, , Scott_Flaherty@fws.gov



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