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906 HAINES AND MODDE ratory studies were age-O Colorado squawfish pro- duced at Dexter (New Mexico) National Fish Hatchery. During the study, the fish were held at temperatures ranging from 190C to 210C and fed BioKiowa diet. Prior to marking, fish were anesthetized with MS-222 (tricane methanesulfonate). A dental in- oculator (Syrijet Mark II) was used to inject red tattoo ink intracutaneously in the caudal peduncle as described by Laufle et al. (1990). After marking, the fish were held in an aquarium for 10-15 min to confirm dye penetration. The elastic polymer (provided by Northwest Marine Technology) was injected subcutaneously on the dorsal surface left of the dorsal fin with a I-mL insulin syringe and 28-gauge needle. The polymer was implanted as a liquid and subsequently solidified into an elastic solid. For the third treatment, the fish's left pelvic fin was clipped. Mark longevity and survival.-The study began 4 December 1992 when fish averaged 49.8 mm TL (SD = 7.04 mm). Fish with each of the three marks were held in flow-through troughs (2.0 m X 0.5 m X 0.1 m) for 142 d. Three replicate lots con- sisting of 120 fish (40 fish of each mark) were tested. Deaths were recorded daily. Fish were ex- amined for marks on 22 December 1992 and 12 February, 30 March and 22 April 1993. On 22 April 1993, all fish having recognizable elastic polymer marks were placed in a cage in an outside pond and held an additional 104 d. On 4 August 1993 the number of fish having a recognizable mark were counted. One-way analysis of variance was used to assess differences in mean length, mortality, and mark retention among marking treatments. Percentages were arcsine-transformed to more closely approx- imate normality. Tukey's honestly significant dif- ference method was used to compare differences among means (Wilkinson 1990). Vulnerability of marked fish to predation.- Marked and unmarked (control) age-O Colorado squawfish were exposed to predation by large- mouth bass Micropterus salmoides. Marking pro- cedures were the same as described in the previous section. Two recovery times (IS and 120 min) were tested; recovery time was defined as the time elapsed between marking and exposure to preda- tion. The experiment was conducted in six circular tanks (1.8 m in diameter and 0.9 m deep) and consisted of two trials. Each tank held two large- mouth bass (205-255 mm TL) that had been placed in the tanks 2 or 3 weeks prior to the experiment and fed fathead minnows Pimephales promelas. Food was withheld for at least 3 d prior to begin- ning each trial. For each trial, 144 Colorado squawfish were di- vided into six lots of 24 fish (6 fish with each of the three marks and 6 controls); each lot was ran- domly assigned to a tank. Three tanks received fish that had recovered IS min and three received fish that had recovered 120 min. After 3 min, sur- viving Colorado squawfish were counted. Data were transformed to instantaneous mortality rates per min for each largemouth bass and analyzed by means of a two-way analysis of variance (4 marks X 2 recovery times) with three replications, in which the variance assigned to date of trial and tank were blocked (Wilkinson 1990). Field-marking evaluation.-Based on laboratory experiment results, the syringe-injected elastic polymer was selected to determine the feasibility of using mark-recapture techniques to study movement patterns of young Colorado squawfish and estimate abundance and winter survival. Field studies conducted in the autumn of 1992 and spring of 1993 were designed to answer four ques- tions: (I) Can the mark be easily applied and rec- ognized in the field? (2) How much postmarking mortality occurs? (3) How much immigration and emigration occur? and (4) What biases and pre- cision can we expect from an optimally designed mark-recapture study? The study area was a 16.1-km reach of the Green River, beginning 3 km below Jensen, Utah. For each population estimate, three sampling passes were made. With one exception, sampling passes consisted of seining (4.5 X 1.9 m, 4.0-mm mesh) all backwaters over 30 m2 within the study reach; the first pass in autumn consisted of seining only large backwaters (70-800 m2, N = 6). Each seine haul was approximately 8 m in length (range, 4- 10 m). Age-O Colorado squawfish received marks unique to each backwater. No fish were marked on the third pass. Captured Colorado squawfish were placed in a 20-L bucket filled with river water containing 0.5% added salt, anesthetized within 20 min with MS-222 (0.1 g/L), and marked with red elastic polymer. Fish recovered in a 20-L bucket of river water with 0.5% salt for 20-30 min and then were placed in a floating net-pen until the seining and marking of all specimens were completed; elapsed holding time ranged from 0.5 to 3.0 h.No salt was added to the holding buckets the first 2 d of sam- pling. All marked fish were released into the back- water of origin. On five occasions, postmarking ~