All respirometers were wrapped with standard fiberglass insulation to 

 maintain constant temperatures. Plywood enclosures were placed around 

 each chamber during experiments to isolate fish from laboratory activity. 

 Each enclosure contained a 15-watt cool white fluorescent lamp, 42 centi- 

 meters above the chamber to provide constant illumination during ejcperi- 

 ments. Fish were observed through a viewing slit in each enclosure. 

 Surfaces immediately below each chamber were blackened. Changes in dis- 

 solved oxygen concentrations were monitored by Yellow Springs Instrument 

 Co., Inc. Model 54RC oxygen meters. Oxygen electrode leads were passed 

 through neoprene stoppers that sealed the fill pipes into which the elec- 

 trodes were inserted. 



b. Fish . Fish were collected by otter trawl from the Patuxent River 

 estuary. Actual collection sites ranged from the Lower Marlboro area to 

 the vicinity of Drum Point, Maryland, depending upon species and time of 

 year. Fish were kept on the collecting vessel in 80-liter plastic trash 

 cans. A constant flow of ambient river water was maintained through the 

 cans until the vessel returned to the laboratory. 



The laboratory holding facilities consisted of 208-liter polyethylene 

 tanks immersed in controlled-temperature water baths. Water in the tanks 

 continually passed through an inline protein-skimmer filtration system. 

 Patuxent River water, which passed through a 5-micrometer mesh nylon filter, 

 was usually used to supply the tanks. During the summer months of 1972 a 

 commercial marine salt mix dissolved in laboratory well water was used to 

 supply the holding tanks. Salinity of water used in the laboratory was 

 about 5 parts per thousand. Holding tank and experimental temperatures 

 were adjusted to approximate seasonal changes. 



The fish were placed in the laboratory holding tanks, where care was 

 taken to avoid overcrowding. Unhealthy or dead fish were removed imme- 

 diately. Supplemental aeration was provided when large numbers of fish 

 were held. The fish were under continuous fluorescent illumination. They 

 were not fed following capture because active digestion increases standard 

 and routine oxygen consumption (Beamish, 1964; Glass, 1968). Fish were 

 held a minimum of 3 to 5 days before oxygen consumption rates were deter- 

 mined. 



c. Measurement of Oxygen Consumption . Respirometers were filled with 

 water from the holding tanks (Fry, 1971) during experiments. As soon as 

 the water in the apparatus could completely cover the fish, each fish was 

 transferred in a bucket of water from the holding tank to the respirometer . 

 When the respirometers were full, water was circulated at 0.28 to 0.39 foot 

 per second (ft/s) to force out entrained air which was replaced simultane- 

 ously by holding tank water. In addition, flow rate was increased by 0.18 

 ft/s at 4- or 5-minute intervals to drive out trapped air. Maximum flow 

 attained during this procedure was 2.5 to 4 times the minimum experimental 

 rate, depending on species. Flow was reduced to the minimum experimental 

 exposure rate and all access points were closed. The plywood enclosure 

 was placed around the chamber. 



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