Aspects of the Biology of the Red Drum, Sciaenops ocellatus, in Mississippi

Abstract

Several hundred specimens of the red drum from Mississippi were critically assessed. Regression equations for standard-length (SL) versus total-length differed between males and females and between small and large members of the same sex. A single regression line represented the weight-SL relationship for males with females. For condition coefficients to be helpful, fish had to be grouped at least by sex, season, and length or stage of maturity. By 12 months of age, most fish were about 30 to 32 cm SL and their distribution ranged throughout Mississippi Sound rather than being restricted to inshore bayou and marsh habitats like younger individuals. Juvenile fish tended to have a high hepatosomatic index (HSI) in winter, and adults had a low one following spawning. The gonosomatic index (GSI) was typically lower than the HSI except for gravid fish in spring and summer. The number of oocytes available for spawning appeared to be much higher than cited in previous reports. The highest calculated fecundity, occurring in a 758 mm SL fish, was 62 million using a volumetric displacement method and 95 million using a gravimetric one. Extrapolated estimates for larger fish were much higher. Resting and yolk-vesicle stage oocytes typically had an irregular shape, an eccentrically located nucleus, and an abundance of highly basophilic substance in the ooplasm. Throughout a seasonal histological examination, both gonads contained, often in an abundance, Periodic-acid-Schiff-positive granular leukocytes. Spawning took place in late September and October one year, but apparently has occurred over longer periods when conditions of temperature and photoperiod were appropriate. No tagged, 1-year-old fish were returned from farther than 33 km from the point of release. Adults, however, apparently migrated extensively, especially from October through April. Those adults tagged several km south of the barrier islands included individuals caught 778 km away in Texas after 746 days and 316 km away in Florida after 399 days. Another fish apparently migrated at least 120 km in inshore waters in 6 days or fewer. An estimated 25 million kg or more of red drum occurred at one time between the Mississippi River and Mobile Point, Alabama. The red drum served as a host for a variety of parasites, some capable of having an adverse effect on natural stocks, on cultured stocks, and on seafood consumers, and most species known to infect the drum are listed in a table. The red drum has succumbed to some microbial agents, low dissolved oxygen concentration, rapidly dropping temperatures, and other detrimental conditions, some unidentified or unexplained. Probably, considerable mortality can be attributed periodically to environmental or environmentally-influenced conditions and to parasitic infections.