9438 - Laserfiche WebLink

Home Browse Search

404 COPEIA, 2000, NO.2 water temperatures below unmodified hypolim- nial-release dams in the basin are near 10 C year-round, whereas spring-summer tempera- tures in river reaches unaffected by dams ap- proach 20 C (U.S. Geological Survey records). Our 14 C experimental temperature simulated mixing of tributary and mainstem tailwater flows or eddy return channels and other low- velocity "backwater" habitats in tailwater reach- es that may warm above main channel temper- atures. Because early life stage fishes may drift (Ber- ry, 1988; Robinson et aI., 1998), be transported via flood (R. Valdez and Ryel, R. J. unpubI.), or actively disperse from spawning streams into tailwater reaches, we examined temperature ef- fects on three age groups (7 days, 14 days, and 41 days posthatch) of each species to evaluate effects of differential timing of entrance to cold tailwaters from warm tributaries. Eighteen 38-liter aquaria were individually supplied with recirculating hatchery water in- side the temperature-controlled lab under nat- ural light conditions supplemented with illumi- nation from wide spectrum fluorescent lights during daytime. Two one-quarter-hp chillers equipped with mechanical, biological, and chemical filters were connected to two sets of six aquaria maintained at 10 ::!:: 0.5 C and 14 ::!:: 0.5 C, respectively. A third set of six aquaria was supplied with ambient 20 ::!:: 0.5 C water but was otherwise treated identically. Temperatures in each aquarium were monitored daily with maxi min thermometers. Growth experiments were initiated with 6-8- day (hence age 7-day) , 13-15-day (hence age 14- day), and 39-43-day (hence age 41-day) post- hatch groups of each species. Swim-up (7-day) larvae were acclimated to 20 C for a minimum of 24 h prior to initiation of experiments, whereas older age groups were reared at this temperature. Between 50 and 100 fish from each experimental group were transferred to aquaria maintained at experimental (10 C, 14 C) and control (20 C) temperatures. Fish (n = 5-23 unless fewer fish remained on last sampling date) were sampled from each aquarium by fine-meshed dip net and weighed and measured [total length (TL)]. Because of a limited availability of G. CYPha, an attempt was made to return individuals to experimental aquaria alive following collection of data. Gila CYPha were anesthetized in 125 mg/L MS-222, placed on blotter paper for 5 sec, weighed and measured, and returned to respective aquaria. Age 7-day and age 14-day G. CYPha groups were weighed to the nearest 0.1 mg and measured to the nearest 0.1 mm; fish in the age 41-day G. CYPha group were weigbed to the nearest 0.01 g. Samples of P. lucius, C. latipinnis, and X. tex- anus were sacrificed and preserved in 10% for- malin prior to collection of length and weight data (no corrections for shrinkage). Fish were blotted 5 sec and weighed (nearest 0.1 mg) and measured (nearest 0.1 mm). Development.-We microscopically examined all preserved specimens of C. latipinnis and X. tex- anus to determine transformation from larval to juvenile stages. For X. texanus and C. latipinnis, D. E. Snyder and R. Muth (unpubI.) reported that total lengths at loss of the preanal finfold coincided with lengths at transformation, and rarely were finfolds lost without concomitant ac- quisition of the adult complement of fin rays. We therefore used presence/absence ofthe pre- anal finfold as the single determinant of life stage. For G. CYPha and P. lucius, we considered total lengths at loss of finfolds (D. E. Snyder, unpubl.) as representative of size at transfor- mation. We assessed time to transformation for these species by comparing experimental repli- cate mean lengths to D. E. Snyder's (unpubI.) reported lengths at loss of finfolds. Temperature shock.-Thermal-shock experiments, to simulate conditions experienced by fishes en- tering cold tailwaters from warm tributaries, were conducted on young G. CYPha (5-7-dayand 11-13-day posthatch), X. texanus (7-9-day, 11- 13-day, and 42-43-day posthatch), and C. latipin- nis (7-8-day, 14-15-day, and 42-43-day post- hatch). Culture and rearing were as described for growth experiments. Fishes were acclimated to 20 C water over 24 h. One to three replicates of 20-100 fish each were then transferred sep- arately to aquaria at the following temperatures: 10 ::!:: 0.5 C, 12 ::!:: 0.5 C, and 14 ::'::: 0.5 C. Rep- licate controls at 20 ::!:: 0.5 C for all groups were handled in like manner. Fish behavior was ob- served intensively for several minutes following initiation of each treatment and thereafter for I-min periods at various intervals over 1-5 h. Analyses.- The Levene test determined that var- iances of length and weight data (and transfor- mations) within species-age-temperature groups were not homogeneous. We employed the non- parametric Kruskal-Wallis (K-W) test to com- pare mean ranks of untransformed lengths and weights across experimental temperatures for each species and age group at each sampling date. Significance levels were adjusted for mul- tiple tests with the Bonferroni sequentially re- jective multiple test procedure of Holm (1979). "'"