HCNSO Student Theses and Dissertations

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Defense Date


Document Type

Thesis - NSU Access Only

Degree Name

M.S. Marine Biology


Oceanographic Center

First Advisor

David W. Kerstetter

Second Advisor

Charles F. Cotton

Third Advisor

Robin L. Sherman


Pteroplatytrygon violacea is the only member of the Dasyatidae family that exhibits an entirely pelagic behavior. Age, growth and reproduction characteristics were estimated for western north Atlantic populations, where it is a common bycatch species of commercial pelagic longline fisheries targeting swordfish and tuna. Ages were assigned by counting band-pair deposition on vertebral centra sections. An annual pattern of band-pair deposition was validated through marginal increment analyses. Age estimates ranged from 2.5 to 8 years for males and 3 to 10 years for females. The von Bertalanffy growth model, a modified form of the von Bertalanffy, a two-parameter form of the von Bertalanffy with a fixed length-at-birth, the Gompertz growth model and a logistic model were fitted to sex-specific observed size-at-age data. Models were fitted using maximum likelihood estimation and nonlinear least squares methods. Resulting models were evaluated based on biological rationale and Akaike’s information criteria. All growth models yielded similar estimates, however the two-parameter Von Bertalanffy growth model provided the best fit for both males (L∞ = 54.79 cm (Disc width, DW) and k = 0.44 year-1) and females (L∞ = 65.56 cm (DW) and k = 0.26 year-1). Reproduction was determined by assessing maturity and reproductive stages. Sexual dimorphism was observed where females grew significantly larger than males. Males were determined mature when DW was greater than 38cm when claspers were shown to be fully calcified and vas deferens were coiled. Females were determined to reach maturity between 40 and 50 cm DW although a lack of smaller sized females prevented a more accurate estimate of first maturity. Females were observed pupping near term embryos ranging from 14.2 to 16 cm DW in mid-July and late September indicating possibilities of two litters per year. Large oocytes where observed in an ovary in a female with a uterus containing seven eggs with a mean mass of 2.80 g indicating that P. violacea exhibit vitellogenesis proceeding simultaneously with gestation. A clear seasonality for the reproductive cycle was not apparent however it seems parturition occurs in late spring and early fall when conditions are optimal for neonate survival. Data on the age, growth and reproduction of western north Atlantic populations of P. violacea will contribute to the lack of life history characteristics of this common bycatch species.

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