Emigration of penaeid shrimp from the once-through cooling lake of Cedar Bayou Steam Electric Generating Station, Baytown, Texas

Abstract

Migration and distribution of two decapod shrimp, Penaeus setiferus and P. aztecus, in the cooling-water system of a power plant on the Texas coast were investigated, with emphasis on the seaward emigration from the cooling lake. Samples were collected every two weeks for a year by straining water leaving the lake over a drop structure, trawling in the cooling lake, and flushed from the intake screen. Shrimp catches at the drop structure were positively correlated with the standing population in the cooling lake. Significantly more shrimp passed over the drop structure nocturnally than diurnally. Shrimp caught at the drop structure diurnally averaged either smaller or similar in size to the shrimp taken there at night. The passage of a cold front enhanced migration and initially increased and then reduced the mean size of shrimp passing the drop structure. Higher percentages of the shrimp population in the cooling lake emigrated near new moon than full moon. The moon-phase effects appeared to be due to the moon cycle itself rather than to the intensity of moon light. Significantly more shrimp of both species emigrated during the hours of ebbing than incoming tide. As the lake surface was above high tide level, an endogenous timing mechanism rather than environmental factors associated with tidal rhythms controlled this emigration pattern. A temperature drop of up to 0.23 C/hour significantly increased P. aztecus emigration. Shrimp catch decreased with increased hourly temperature change. More shrimp passed over the cooler half of the drop structure during warmer months and over the warmer half when the temperature was low. In the cooling lake during the cooler months, more shrimp appeared in the warmer waters than in the colder areas, and the converse was true during the warmer months. A laboratory thermal resistance study showed that resistance time of shrimp to thermal shock was affected by acclimation temperature, temperature increase, salinity and species. The recovery of marked shrimp from the discharge canal 16 hours after their release in early November, showed that some parts of the discharge canal were favorable for shrimp habitation. The increased temperature of the thermal effluent in the cooling system increased shrimp growth in cold months. The thermal advantage disappeared after mid-April. An estimated 11 million shrimp per year passed over the second drop structure. Nocturnal harvest is most economical since very few emigrate during daytime.