Validated age and growth estimates for Carcharhinus obscurus in the northwestern Atlantic Ocean, with pre- and post management growth comparisons

Abstract

Age and growth estimates for the dusky shark, Carcharhinus obscurus, were derived from vertebral centra collected in the northwestern Atlantic Ocean. Sample collection spanned the years prior to and following the implementation of management measures . Growth was compared pre-and post-population depletion and pre-and post-management to investigate the possibility of density-mediated shifts in age and growth parameters over time. There was no evidence of difference between periods for either sex. Additionally, bomb radiocarbon dating was used to determine the periodicity of band pair formation. Results support the traditional interpretation of annual band pairs up to approximately 11 years of age. After this time, vertebral counts considerably underestimate true age. Maximum validated ages were estimated to be between 38 and 42 years of age (an increase of 15 to 19 years over the band count estimates), confirming longevity to at least 42 years of age. Growth curves estimated using only validated data were compared to those generated using band pair counts. Logistic growth parameters derived from validated vertebral length-at-age data were L ∞ = 261.5 cm FL, L o =85.5 cm, t o =4.89 year and g= 0.15 year −1 for the sexes combined. Revised estimates of age at maturity were 17.4 years for males and 17.6 years for females.

Evidence for systemic age underestimation in shark and ray ageing studies

“…Although predominantly detected in two orders, the Lamniformes and Carcharhiniformes, these were also by far the most intensively studied (see Appendix S2). Age underestimation does not seem to be restricted to particular life history types and has been reported in some relatively small and short‐lived species such as blue‐spotted mask rays ( Neotrygon kuhlii , Dasyatidae) (Pierce & Bennett, ) as well as large and long‐lived species such as dusky sharks ( Carcharhinus obscurus , Carcharhinidae) (Natanson et al., ).…”

“…(); m McPhie and Campana (); n Natanson (); o Natanson et al. (); p Natanson et al. (); q Passerotti et al.…”

“… it results from growth zones becoming vanishingly small and unresolvable on the margin of ageing structures as individuals get larger and older, and as growth decreases (Chin et al., ; Francis et al., ; Hamady et al., ), or it results from a temporary or permanent cessation of growth and growth zone formation (Natanson et al., ) potentially occurring at any life stage, including in response to a reallocation of resources to reproduction (Harry et al., ; Natanson, ) or from “feast and famine”‐feeding behaviour (Andrews & Kerr, ). …”

“…As such, a positively biased (larger) L ∞ associated with this apparent absence of older individuals would lead to a negatively biased (lower) k , somewhat analogous to the bias generated by length‐selective fishing gears such as gillnets (Thorson & Simpfendorfer, ; Walker, Taylor, Hudson, & Cottier, ). This phenomenon was seen in northwest Atlantic dusky sharks; using validated data points corrected for age underestimation led to an increase in the estimated growth coefficient of 6%–17% and a decrease in L ∞ of 8%–9% (Natanson et al., ). Growth coefficients, k , estimated from vertebral ageing of school sharks, where ages are known to be substantially underestimated, were 24%–26% lower than estimated from two long‐term tagging studies (Moulton et al., ).…”

Skate-ing on thin ice: molecular ecology of longnose skates in the Southeast Pacific Ocean

“…A variety of mineralised structures may be used for age and growth estimation in chondrichthyans, i.e., vertebrae (Francis et al, 2001;Gedamke et al, 2005;Ainsley et al, 2014;Natanson et al, 2014), dermal denticles (Moura et al, 2004;Campana et al, 2006;Serra-Pereira et al, 2008), and caudal thorns (Henderson et al, 2004;Francis & Ó Maolagáin, 2005;Arkhipkin et al, 2008;Francis & Gallagher, 2009). A popular ageing technique in elasmobranch is the sagittal-sectioning of vertebral centra from the thoracic region (Cailliet et al, 2006).…”