Our objective was to determine the maximum effective rearing density for Atlantic salmon (Sahno salar) at 17.5°C, a temperature typically used in hatcheries to accelerate growth. We reared 5.8-g (mean weight) parr for 80 d to final rearing densities of 14-55 kg/m 2 (rearing unit bottom area) or 80-310 kg/m 3 (rearing unit volume). Weight gain was slightly depressed in fish at a final rearing density of 26 kg/m 2 (146 kg/m 3 ), but mortality, food conversion, and gain in length were not affected. At densities greater than 26 kg/m 2 , fish growth was slower and food conversion was higher, but mortality was unaffected. Atlantic salmon may be reared at 17.5°C to densities of at least 14 kg/m 2 (80 kg/m 3 ) without adverse effects on growth and health, but final rearing densities should not exceed 26 kg/m 2 .

American shad Alosa sapidissima from the Columbia River on the Pacific coast and the Delaware River on the Atlantic coast were reared from 3 June to 24 October 1986 in two adjacent hypalon-lined ponds. Although fish from the Columbia River were introduced into ponds 29 d after those from the Delaware River, they grew significantly faster and attained a greater final weight. Fish from the Columbia River also had lower mortalities at all test salinities and temperatures than fish from the Delaware River. Electrophoresis revealed allelic differences between the two stocks at one locus (creatine kinase). We conclude that the two stocks of American shad are sufficiently different so that managers should not introduce them to different river systems without careful consideration.

A new technology for treating waters contaminated with acid mine drainage involves the dissolution of limestone particles using carbon dioxide at pressures above ambient. Because of the fish health risks associated with episodes of high carbon dioxide levels in treated waters, we subjected three species of fish, brook trout (Salvelinus fontinalis), slimy sculpin (Cottus cognatus), and blacknose dace (Rhinichthys atratulus), to 24 h exposures of elevated dissolved carbon dioxide (CO2) at three levels, ranging from 1.0 (low) to 6.3 (high)%, under laboratory conditions. We measured blood physiological variables as well as behavior, including feeding responses, before, during, and after exposure. Physiological responses differed by species, but all species had elevated hematocrits after 1 h of exposure. Brook trout hematocritis were higher at medium and high levels of CO2 than in a control group (0.0% CO2) after 24 h of exposure. Slimy sculpin hematocrits were higher in medium- and high-level exposure groups than in controls after 1 h, but not after 24 h, of exposure. Blacknose dace hematocrits were higher in all three exposure groups than in controls after 1 h but only in medium-level exposure groups after 24 h. Brook trout plasma glucose was significantly higher in medium- and high-level exposure groups after 1 h, and in the high-level group after 24 h, than in controls. Slimy sculpin plasma glucose was not significantly different in elevated CO2 exposure groups from that of controls throughout exposure. Branchial ventilation was significantly greater in all species at elevated CO2 during exposure, indicating stress; however, no difference was observed between treatment and control groups of blacknose dace after 24 h, indicating acclimation. Pectoral fin beats and cough rates were not consistently related to CO2 exposure throughout the study. Brook trout had the longest lasting reaction to stress at lower levels of CO2 among the three species tested. Many of the 11 observed behavioral variables, related to swimming, feeding, social, and illness factors, were affected by elevations of dissolved CO2. Two to seven behavioral variables (18-64% of those measured) were affected by treatment level of dissolved CO2 with a trend by species for the number of variables affected: brook trout > blacknose dace > slimy sculpin. However, behavioral sensitivity to treatment level was greatest in blacknose dace. Recovery to pre-treatment activity rates for most behavior patterns (including feeding) was observed 24 h after cessation of exposure in all three species. Recovery was independent of treatment level, was most rapid in blacknose dace, and was slowest in brook trout. Overall, slimy sculpin was least affected behaviorally by elevated CO2. Although all three species showed stress response and changes in behavior at moderate levels of CO2 (> or = 2%), brook trout and blacknose dace showed evidence of ability to avoid harmful CO2 levels by swimming out of affected waters, whereas the slimy sculpin showed minimal behavioral change...

A new technology for treating waters contaminated with acid mine drainage involves the dissolution of limestone particles using carbon dioxide at pressures above ambient. Because of the fish health risks associated with episodes of high carbon dioxide levels in treated waters, we subjected three species of fish, brook trout (Salvelinus fontinalis), slimy sculpin (Cottus cognatus), and blacknose dace (Rhinichthys atratulus), to 24 h exposures of elevated dissolved carbon dioxide (CO2) at three levels, ranging from 1.0 (low) to 6.3 (high)%, under laboratory conditions. We measured blood physiological variables as well as behavior, including feeding responses, before, during, and after exposure. Physiological responses differed by species, but all species had elevated hematocrits after 1 h of exposure. Brook trout hematocritis were higher at medium and high levels of CO2 than in a control group (0.0% CO2) after 24 h of exposure. Slimy sculpin hematocrits were higher in medium- and high-level exposure groups than in controls after 1 h, but not after 24 h, of exposure. Blacknose dace hematocrits were higher in all three exposure groups than in controls after 1 h but only in medium-level exposure groups after 24 h. Brook trout plasma glucose was significantly higher in medium- and high-level exposure groups after 1 h, and in the high-level group after 24 h, than in controls. Slimy sculpin plasma glucose was not significantly different in elevated CO2 exposure groups from that of controls throughout exposure. Branchial ventilation was significantly greater in all species at elevated CO2 during exposure, indicating stress; however, no difference was observed between treatment and control groups of blacknose dace after 24 h, indicating acclimation. Pectoral fin beats and cough rates were not consistently related to CO2 exposure throughout the study. Brook trout had the longest lasting reaction to stress at lower levels of CO2 among the three species tested. Many of the 11 observed behavioral variables, related to swimming, feeding, social, and illness factors, were affected by elevations of dissolved CO2. Two to seven behavioral variables (18-64% of those measured) were affected by treatment level of dissolved CO2 with a trend by species for the number of variables affected: brook trout > blacknose dace > slimy sculpin. However, behavioral sensitivity to treatment level was greatest in blacknose dace. Recovery to pre-treatment activity rates for most behavior patterns (including feeding) was observed 24 h after cessation of exposure in all three species. Recovery was independent of treatment level, was most rapid in blacknose dace, and was slowest in brook trout. Overall, slimy sculpin was least affected behaviorally by elevated CO2. Although all three species showed stress response and changes in behavior at moderate levels of CO2 (> or = 2%), brook trout and blacknose dace showed evidence of ability to avoid harmful CO2 levels by swimming out of affected waters, whereas the slimy sculpin showed minimal behavioral change...

Although considerable information exists on habitat use by stream salmonids, only a small portion has quantitatively examined diurnal and nocturnal habitat variation. We examined diel variation in habitat use by age-0 and age-1þ brook trout (Salvelinus fontinalis) during summer and autumn in a headwater stream in northern Pennsylvania. Habitat variables measured included cover, depth, substrate, and velocity. The most pronounced diel variation occurred in the use of cover during both seasons. Both age-0 brook trout and age-1þ trout were associated with less cover at night. Age-0 brook trout occupied swifter water during the day than at night during both seasons, but the difference was not significant. Increased cover, depth, and substrate size governed the habitat of age-1þ brook trout. Our findings support the need for a better understanding of diel differences in habitat use of stream salmonids when considering habitat enhancement and protection.