Sheepshead Genetics

The sheepshead, Archosargus probatocephalus, is a marine fish that lives primarily in estuarine habitats from Nova Scotia, Canada, to Rio de Janeiro, Brazil. Sheepshead is the 10th most important recreational harvest making it a valuable fishery throughout the southeastern United States. In the Gulf of Mexico and the Atlantic combined recreational and commercial landings along Florida’s coast was the highest of all states until 2013.

Sheepshead have long been separated into three subspecies based on meristic traits, particularly counts of pigmented bars on the sides of the body. The ranges of two of the subspecies that occur in North America meet at Apalachee Bay, Florida. Members of the subspecies that occurs to the east of the Apalachee Bay boundary generally exhibit six bars on both sides of the body and are classified as A. probatocephalus probatocephalus. In contrast, most fish west of the boundary (to Campeche Bank, Mexico) exhibit five bars on both sides of the body and are classified as A. p. oviceps. The third subspecies A. probatocephalus aries occur from Belize to Bahia de Sepetiba, Brazil. No specimens from this subspecies were available for study.

The morphological differences of the sheepshead subspecies was expected to be associated with genetic differences, but initial genetic analysis did not show any genetic divergence between the described subspecies. For this reason, the sheepshead was assumed to be a single randomly interbreeding population that was found in both the Gulf of Mexico and the Atlantic Ocean.

Recently using sheepshead-specific microsatellite markers developed at FWRI, the genetic population structure of this fish was found to be composed of three population clusters demarcated in the western Gulf of Mexico, the eastern Gulf of Mexico and the Atlantic Ocean (see Figure). The two Gulf of Mexico clusters were separated by a genetic break located at Apalachee Bay, close to the boundary between the two morphologically defined subspecies. The second genetic break separated the eastern Gulf and Atlantic clusters and coincided with a discontinuity of estuarine habitat between Miami and Palm Beach that restricts gene flow between populations. Although this area had not been recognized as a genetic boundary for sheepshead populations in the past, it had been widely recognized as a boundary for many other near-shore fish species. The population structure discovered here cannot be equated to evolutionary divergence and does not serve to validate the morphological classifications of the sheepshead subspecies. That work would require the use of extensive single nucleotide polymorphism (SNP) analysis between the subspecies.

Because the population trend is stable, Sheepshead is listed as a species of “least concern” in the IUCN Red List of Threatened Species. At present, this fish is considered a perfect substitute species as others species become more limited through regulations. This situation is a potential concern since it could lead to a great stress on sheepshead fishery, eventually destabilizing the stable population trend.

In Florida, the management of this fish is not based on the definition of the subspecies as a baseline designation of stocks, but between Gulf and Atlantic populations. If changes were to be initiated in response to a changed status of this fish, one aspect of the fishery that will require attention would be the replacement of the present management policy on a par with the newly discovered fragmentation of the sheepshead into three clusters. This is of considerable interest to the state of Florida which is the only state where all three clusters exist and where most landings are made. This can be easily accomplished by adopting northwest Florida as a separate management unit within the Florida Gulf, which at present has not acquired such a status due to lack of adequate data.