8146 - Laserfiche WebLink

Home Browse Search

NOTES sunlight in early larvae of Colorado squawfish maintained under simulated natural conditions. Methods Fertilized Colorado squawfish eggs (1 d old) were obtained from Dexter National Fish Hatchery, New Mexico, in May 1990. Eggs were incubated in aflow-through system that received well water at 18°C. Embryos were examined twice daily, and otoliths (sagittae and lapilli) were first observed 3 d after fertilization, about 2 d before hatching. Four groups of 3-d-old embryos and four groups of newly hatched larvae (175 specimens/group) were acclimated to distilled water at 18°C. Eggs and larvae were then placed in I -L glass beakers containing a tris-buffered solution (pH 6.8) of 350 mg tetracycline hydrochloride (TC)/L of distilled water for 24 h (eggs) or 12 h (larvae) (Muth and Nesler 1989). Following TC exposure, eggs and larvae were reacclimated to well water and placed in 3.8-L rearing jars. Containers were covered during and following marking to shield specimens from overhead fluorescent lights. Samples of six to eight larvae per group were fixed and preserved in 95% ethanol immediately after hatching (em- bryos) or marking and at daily intervals thereafter. Tests to determine the effect of sunlight on tet- racycline marks were started at swim-up, about 5 d after hatching. Two of the four groups marked as embryos and two of the four groups marked as larvae were left in rearing jars and held in the laboratory. The other two groups from each treat- mentwere placed in white plastic tubs (34 cm long x 29 cm wide x 14 cm deep) with 8-10 cm of aerated well water over a 2-cm-deep layer of washed, light-colored sand. Tubs were suspended in an outdoor trough of circulating water to mod- erate temperature. Water in the trough was cooled to about 18-20°C each morning; water in the plas- tic tubs never exceeded 25°C. Approximately 25- 339'0 of the surface area of each tub was shaded to simulate natural cover from direct sunlight. Movements of larvae between shaded and un- shaded areas were documented. Tubs were ex- posed to direct sunlight for 6-9 h/d over a 7-d period. Five larvae per group were selected for mark examination from samples preserved immediate- ly after hatching or marking, at swim-up (just be- fore start of outdoor tests), and at the end of days 1, 3, and 7 after swim-up. sagittae and lapilli were extracted, mounted whole in glycerin on glass slides, and examined with incident UV light under a compound microscope for presence and quality 667 of fluorescent marks. Mark quality for each otolith was ranked as absent, faint, lucid, or bright with respective values of 0, 1, 2, or 3 (Muth et al. 1988; Muth and Nesler 1989). Values were averaged within each sample of five fish, and the mean was used as a subjective comparison of mark quality among samples. Results and Discussion Exposure to sunlight over a 7-d period had no detectable effect on quality or retention of TC marks in otoliths of Colorado squawfish larvae. Marks in all otoliths examined from exposed and unexposed larvae, whether marked as embryos or larvae, were distinct and lucid to bright with mean mark quality values of 2.6-3.0. Survival of em- bryos or larvae during and immediately after marking was nearly !00%. Thereafter, mortality in all groups held indoors or outdoors was less than 20%. Certain aspects of the study were intended to simulate natural conditions. Observations of hatchery fish (Hamman 1981) suggest that in the wild Colorado squawfish larvae remain hidden in the substrate (shielded from sunlight) prior to swim-up. This period was simulated by the inter- val between marking and test exposure to direct sunlight. During this interval, calcification of the otoliths may have been sufficient to cover and protect TC marks. Had specimens been exposed to direct sunlight immediately after marking, marks might have been affected. However, Muth et al. (1988) determined that presence and quality of fluorescent marks in otoliths of larvae of fat- head minnows Pimephales promelas were unaf- fected by exposure immediately after TC marking to artificial white light in the 340-650-nm range (<_ 1,000 lx, 12 h/d). Partial shading of the outdoor tubs to simulate natural cover allowed larvae to periodically avoid exposure to direct sunlight. For the first 2-3 d after swim-up, larvae tended to remain in or near the shade but made frequent, near-surface forays into unshaded areas. Even these periodic exposures to sunlight should have been sufficient to produce a detectable negative effect on mark quality or re- tention. The time spent in unshaded areas in- creased as larvae developed; 5 to 6 d after swim- up, larvae favored unshaded areas. By day 6 after swim-up, larvae had noticeable melanophore pig- mentation on the dorsal surface of the head and nape. In the wild, early larvae of Colorado squaw- fish are typically found in turbid water, which in itself reduces their exposure to sunlight.