326 Abstract—Examination of otolith Southern flounder (Paralichthys morphometric variation has been shown to provide improved descrip- lethostigma) stock structure inferred tions of stock structure for several marine fish species. We examined from otolith shape analysis spatial variation in otolith shape of southern flounder (Paralichthys Stephen R. Midway (contact author)1 lethostigma) to understand popula- 2 tion structure at the following geo- Steven X. Cadrin graphic levels: ocean basin (Atlantic Frederick S. Scharf1 and Gulf of Mexico); regional coastal waters (Texas, Florida) and (Georgia, Email address for contact author: [email protected] South Carolina); and local coastal waters (North Carolina). To reduce 1 Department of Biology and Marine Biology variability, we considered only age- University of North Carolina Wilmington 1 female fish. From digitized otolith 601 South College Road images, we extracted descriptions for Wilmington, North Carolina 28403 common shape indices and elliptical Present address for contact author: Coastal Carolina University Fourier coefficients and found strong Department of Biology evidence for differences at the ocean P.O. Box 261954 basin scale, but only weak evidence Conway, South Carolina 29528 for structure at either within-basin 2 (i.e., among states) or within-state School for Marine Science and Technology (local) spatial scales. Our finding of University of Massachusetts Dartmouth inferred stock structure differences 200 Mill Road, Suite 325 between the ocean basins aligns with Fairhaven, Massachusetts 02719 the geographic break in the distri- bution of this species—the absence of this species from the southern portion of Florida—as well as with recent genetic findings. Currently, Although many marine populations agement of any species (Stephenson, state-level management of southern flounder in both areas does not ac- once were considered to be panmic- 1999; Ying et al., 2011) because ig- count for any basin-wide population tic on the basis of large geographi- noring population structure can lead mixing and, therefore, by default, as- cal ranges and larval dispersal over to negative outcomes, such as loss of sumes a separate unit stock for each long distances, results from im- genetic diversity and reduction in the state, although our findings indicate proved stock identification methods yield-generating potential of a stock that mixing could be extensive. Ad- (e.g., mtDNA, parasite community, (Pawson and Jennings, 1996; Bailey, ditional sources of information (e.g., and shape analyses) are calling into 1997; Booke, 1999). Within fisheries genetics, life history traits) collected question some initial assumptions science, stock definitions vary but at appropriate spatial scales should of population homogeneity (Cadrin are focused largely on consistency be examined to confirm suspected levels of mixing and to determine et al., 2005). For example, although of unique traits—the characteristics suitable management strategies for only very small amounts of gene flow that distinguish a stock should re- the conservation of southern floun- may be required to homogenize con- main constant through time and be der stocks throughout their ranges. siderable genetic variation without unique to that stock (Ihssen et al., selection (Palumbi, 2003), evidence 1981; Booke, 1999) for both conser- is accumulating from multiple spe- vation and harvest purposes (Cadrin, cies to support the existence of fine- 2000). scale geographic structure in several Methods of phenotypic stock iden- adaptive traits (Conover et al., 2006). tification have expanded greatly Regardless of their genetic simi- from abundance and meristic ap- larities or differences, fish stocks proaches to now include the use of possessing variable traits that can both natural and artificial tags, ex- Manuscript submitted 7 August 2013. affect their responses to harvest still amination of life history traits, pop- Manuscript accepted 9 September 2014. must be delineated clearly to achieve ulation genetics, and morphometric Fish. Bull. 112:326–338 (2014). management objectives related to outlines (reviewed in Cadrin et al., doi:10.7755/FB.112.4.9 yield maximization and biomass con- 2005). Recently, the study of closed- servation (Ricker, 1958; Begg et al., form structures, such as otoliths and The views and opinions expressed or 1999; Conover et al., 2006). Spatial scales, has increased with the advent implied in this article are those of the author (or authors) and do not necessarily structure of fish stocks and the al- of computers that are able to rapidly reflect the position of the National location of fishing effort, therefore, analyze large amounts of data. In Marine Fisheries Service, NOAA. should be considerations in the man- addition, otoliths are collected rou- Midway et al.: Stock structure of Paralichthys lethostigma inferred from otolith shape analysis 327 tinely for age determination purposes, making large landings took place in North Carolina. Over the same sample sizes available for shape analyses. Consequent- period, recreational landings were about 50% lower in ly, numerous examples of successful discrimination of magnitude and were more evenly distributed among fish stocks based on otolith morphometrics and shape states between the Gulf of Mexico and South Atlan- descriptors exist. tic basins (NMFS4). However, recreational harvest may Campana and Casselman (1993) were among the be a primary factor that is contributing to population first to use otolith shape as an indicator of stock varia- declines in the Gulf of Mexico (Froeschke et al., 2011), tion. They conducted an exhaustive study of all 3 types and these declines have prompted a new stock en- of otolith pairs in which they found evidence of struc- hancement program in Texas aimed at supplementing turing among spawning groups of Atlantic cod (Ga- natural reproduction. dus morhua) in the Northwest Atlantic, in addition to Because no directed fishery exists for southern differences in otolith shape among age groups, sexes, flounder in offshore habitats and exchange of individu- and year classes. Begg and Brown (2000) used otolith als among states is not well understood, state manage- shapes to challenge successfully the assumption of a ment agencies assume unit stocks on the basis of state single stock of haddock (Melanogrammus aeglefinus) boundaries. However, population structure that does at Georges Bank, and DeVries et al. (2002) clarified not coincide with state boundaries has been shown previous tag and genetic data when they used oto- with other flounders in the Northwest Atlantic that liths to successfully distinguish stocks of king mack- share geographic ranges of a similar size and life his- erel (Scomberomorus cavalla) from the Gulf of Mexico tory characteristics with the southern flounder. These and the Atlantic that were sampled during their win- flounders include the southern flounder congener sum- ter mixing off southern Florida. More recently, otolith mer flounder (P. dentatus [Burke et al., 2000]), winter shape analysis has been done at varying spatial scales flounder (Pseudopleuronectes americanus [DeCelles and for dolphinfish (Coryphaena hippurus [Duarte-Neto et Cadrin, 2011]), and yellowtail flounder (Limanda fer- al., 2008]), North Atlantic saury (Scomberesox saurus ruginea [Cadrin, 2010]). saurus [Agüera and Brophy, 2011]), and anglerfish Interestingly, each of these studies reported stock (Lophius piscatorius [Cañas et al., 2012]) to help clari- structuring at varying scales. Summer flounder were fy questions about geographic population structure. found to have structure related to the biogeographic The southern flounder (Paralichthys lethostigma) oc- boundary of Cape Hatteras in North Carolina, whereas curs in the Northwest Atlantic and Gulf of Mexico from evidence indicated that winter flounder and yellow- North Carolina to Texas; however, this species does not tail flounder had population structures of much finer occur around the southern tip of the Florida peninsula scales, including the existence of up to 3 stocks within (Gilbert1). Southern flounder in the South Atlantic and New England waters. Yet despite evidence for within- Gulf of Mexico basins are considered separate genetic basin genetic homogeneity (Anderson et al., 2012), stocks (Anderson et al., 2012). Management for the flounder residency within specific estuaries for the range of this species occurs generally at the individual first few years of life may create regional phenotypic state level, despite a high likelihood of within-basin differences that reflect local adaptation. For example, mixing during offshore spawning migrations of adults estimates of the von Bertalanffy growth coefficient (K) and the possibility of year-round offshore residents for female southern flounder vary considerably among 2 3 (Watterson and Alexander ; Taylor et al. ). states. For fish in Texas, Stunz et al. (2000) estimated Southern flounder support important commercial K at 0.75, and Fischer and Thompson (2004) estimated and recreational fisheries throughout their range, K at 0.51 for Louisiana fish. Within the South Atlantic with females contributing most to the landings be- basin, estimates of K have been lower: 0.23 for fish in cause growth is greater in females than in males. In South Carolina (Wenner et al.5) and 0.28 for fish in 1990–2010, more than 30,000 metric tons were landed North Carolina (Takade-Heumacher and Batsavage6). commercially, and the vast majority (~98%) of these 4 NMFS (National