Genetic Assessment of Sheepshead Stock Structure in the Northern Gulf of Mexico: Morphological Divergence in the Face of Gene Flow

Abstract

Strategies for conservation and management of marine species are increasingly relying on molecular genetic data for the delineation of independent fishery stocks. These data may be more reliable and less subjective than morphological or ecological variables, although their biological interpretations can be problematic. Here, we review existing morphological data synergistically with a new molecular data set for sheepsheads Archosargus probatocephalus. Two named subspecies of sheepshead exist in the northeastern Gulf of Mexico (A. probatocephalus probatocephalus and A. probatocephalus oviceps). Although these subspecies exhibit divergent morphology, it is unclear whether morphological divergence represents meaningful differentiation for management. We show that frequency distributions of each of five meristic counts are significantly different between the subspecies (P < 0.05). However, Bayesian structure analysis of microsatellite genotypes indicated that all Gulf of Mexico sheepshead populations constitute a single stock (posterior probability ≈ 0.9999). Variance partitioning of mitochondrial DNA (mtDNA) haplotypes suggests significant but limited divergence between subspecies (genetic differentiation index FST = 0.036, P < 0.005). Patterns of pairwise mtDNA genetic distance and microsatellite divergence suggest that isolation by distance, rather than subdivision among independent genetic stocks, is driving the significance of variance analyses. Overall, sheepshead molecular genetic data indicate very limited genetic subdivision between the subspecies despite considerable divergence of morphological characters. These results have multiple interpretations, each of which carries implications for the management of this species in the Gulf of Mexico.

Age and growth of sheepshead (Archosargus probatocephalus) in Tampa Bay, Florida

“…Music and Pafford 2 could validate scale annuli only in sheepshead younger than age 5, and annuli in scales of sheepshead older than age 2 have been reported to be unreadable (Schwartz, 1990; Wenner 3 ). Age has been underestimated in sheepshead and other fish species when scales were used, and age estimates from the use of scales have been lower than those derived from otolith sections (Beamish and McFarlane, 1983;Carlander, 1987;Lowerre-Barbieri et al, 1994;Dutka-Gianelli, 1999). In our discussion, the only studies considered in growth comparisons are those in which ages were estimated on the basis of validated otolith annuli.…”

“…Subspecific distinction is based partly on pigmentation (size and number of vertical body bars) and meristic counts (lateral line scales, gill rakers, and dorsal fin spines and rays), both of which overlap considerably between the 2 subspecies (Caldwell, 1965). Results of recent genetic analyses in which mtDNA of sheepshead from the Gulf of Mexico and South Atlantic indicated that a single panmictic population of sheepshead exists within the range of this species from Texas through North Carolina (Anderson et al, 2008;Seyoum et al, in press). More detailed microsatellite analysis, however, has revealed a significant genetic break at the subspecies boundary in the Florida panhandle (Apalachee Bay), providing genetic support for the validity of 2 subspecies of sheepshead within its range in the United States (Seyoum et al, in press (Munyandorero et al 1 ).…”

“…Elsewhere, age and growth studies of juvenile and adult sheepshead have been conducted in Georgia (Music and Pafford 2 ), North Carolina (Schwartz, 1990), South Carolina (Wenner 3 ), Louisiana (Beckman et al, 1991), and northwest Florida (DutkaGianelli and Murie, 2001). Validated (Music and Pafford 2 ) and unvalidated (Schwartz, 1990) ages have been determined also from scales.…”

“…Validated ages determined from otolith sections have been used to estimate von Bertalanffy growth parameters for sheepshead from Louisiana (Beckman et al, 1991), South Carolina (Wenner 3 ), and northwest Florida (Dutka-Gianelli and Murie, 2001). All 3 studies noted a high variability in size at age for sheepshead, reported von Bertalanffy growth parameters, and the predicted sizes at age varied considerably among the 3 studies (Dutka-Gianelli and Murie, 2001).…”

“…Each study relied almost exclusively on the fishery (commercial or recreational) for its samples. Beckman et al (1991) indicated that because the gear types used were more apt to catch certain sizes of fish than others and because fishermen occasionally sorted the catch before supplying the researchers with samples, the age and size structures of the sheepshead analyzed probably did not represent the overall population of sheepshead in Louisiana. Other researchers also have determined that reliance upon samples obtained only from the fishery can cause misrepresentation of the size distribution and age structure of a population (Miranda et al, 1987;Hilborn and Walters, 1992;Wilson et al, 2015).…”