Shrimp and redfish studies, Bryan Mound brine disposal site off Freeport, Texas, 1979-1981. Volume V(B): Brine avoidance/attraction bioassays on redfish.
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A circular tank with a water depth of 77 cm and diameter of 5.49 m was used to monitor the behavior of the redfish or red drum (Sciaenops ocellata) in response to various temperature and salinity gradients. Photoelectric gates separated eight peripheral compartments from the open center of the tank and monitored fish entering and leaving the compartments. The experimentation was based on free choice, non-conditioning design. The 100 fish used had a standard length of 42.75 +- 1.44 cm (x +- SE) and generated an average of 83.12 +- 24.66 compartment entries per hour of monitoring. Redfish were determined to be most active in our system during the first six hours of the dark phase, though a minimum adjustment period of three hours was necessary for this species to begin normal exploratory behavior. A comparison of various brines made from salt dome salt and artificial sea salts (Instant Ocean) indicated that redfish did not readily discriminate among these solutions as determined by the activity measures employed. In addition, brines made with Brazos River water as a diluent were not distinguished from brines made from tap water as a diluent. Redfish neither avoided nor were attracted to the 2 ppt brine gradients, as compared to control compartments receiving no brine. In separate experiments, attraction/avoidance for brines at each of three different gradient strengths (0.5, 1.0 and 2.0 ppt) were tested on fish acclimated at three different ambient temperatures (15, 20, 25 degrees C). In addition, in the same protocol, brine heated 2 degrees C above background was compared to unheated brine using separate compartments. Redfish at colder acclimations of 15 and 20 degrees C appeared attracted to heated brine only if the saline gradient in the test compartments was low (0.5 ppt above background). At an acclimation temperature of 25 degrees C during fall, the unheated brine appeared attractive compared to both the heated brine and the appropriate controls. A separate experiment conducted at 25 degrees C acclimation during spring indicated a similar trend, but the results were not statistically significant. We hypothesize that redfish may use saline gradients as part of their cueing system for movements from estuaries to the open gulf, as well as for localized movements as required for behavioral thermoregulation. Direct observation also indicated that the redfish responded to slight saline gradients under certain conditions. However, the diffuser's maximum near- field concentration of approximately 5 ppt above ambient reported by Randal (1981) is not of sufficient magnitude to harm any life stage of this euryhaline species. In addition, even if redfish are attracted to the brine gradients, it is not likely that they would remain long in the area without further reinforcement of the behavior (e.g. cool water or food resources). None of the present monitoring programs involving sampling near the diffuser are using methods which will result in capture of large redfish which would enable confirmation or rejection of the hypothesis that redfish may aggregate around the brine diffuser under certain conditions.