Identification and Characterization of Shark Nursery Grounds along the Mississippi and Alabama Gulf Coasts
Glenn R. Parsons and Eric R. Hoffmayer
Abstract.—From October 1997 to September 2000, we conducted a survey of shark nursery grounds in the northern Gulf of Mexico extending from Bay St. Louis, Mississippi to Perdido Bay, Alabama. The objectives of the survey were to identify shark pupping/nursery grounds, determine their extent, and characterize the environmental conditions prevalent. Collections were made from March to October of each year with at least four sites sampled each month, two sites in Mississippi waters and two sites in Alabama waters. Collections were made using a gill net fished from 1500 until 2200 hours each day. A total of 100 collections were made during the study, resulting in the capture of more than 2,200 sharks. Young-of-the-year and juvenile sharks were collected from many areas in the Mississippi Sound with many sharks taken around Cat, Ship, Horn, Petit Bois, Round, and Dauphin islands. Shark populations along the Mississippi and Alabama gulf coasts are dominated by three species, the Atlantic sharpnose shark Rhizoprionodon terraenovae, the blacktip shark Carcharhinus limbatus, and the finetooth shark C. isodon. Other species captured included the bull shark C. leucas, the scalloped hammerhead Sphyrna lewini, the bonnethead S. tiburo, the spinner shark C. brevipinna, the blacknose shark C. acronotus, and the sandbar shark C. plumbeus. We used analysis of variance to compare the environmental factors present at sites where sharks were present with those at sites where sharks were not present and found significant differences in surface and bottom dissolved oxygen when Atlantic sharpnose sharks were present, surface and bottom temperature and surface dissolved oxygen when finetooth sharks were present, and surface and bottom temperature when blacktip sharks were present. We used unweighted poisson regression to examine the effect of environmental factors on catch per unit effort (CPUE) (sharks 100 m net–1 h–1) and found that surface salinity significantly altered catch of Atlantic sharpnose sharks, surface and bottom temperature and surface dissolved oxygen significantly altered finetooth shark CPUE, and both surface and bottom temperature and dissolved oxygen altered blacktip shark CPUE. To consider interspecific interactions between the three dominant species, we used the Yule coefficient of association and found that young of the year of the three most common species were significantly, positively associated. Future studies of shark abundance and distribution should consider the interactions between co-occurring species. The Mississippi Sound, associated barrier islands, and the lower reaches of the Mobile Bay are important nursery grounds for several shark species, particularly blacktip, Atlantic sharpnose, and finetooth sharks.