Using a multimodel approach to determine the age and growth of an endangered small shark species in the southern Gulf of Mexico

The bonnethead shark (Sphyrna tiburo), recently upgraded to endangered by the IUCN red list, is frequent in artisanal fisheries’ catches of the southern Gulf of Mexico (GOM). The age and growth parameters were determined from 162 vertebral centra collected from bonnethead sharks (28.2–112.1 cm total length; TL) between 2010 and 2019 in the southern GOM. Two readers determined age estimates using two rounds of independent band pair counts (opaque and translucent). The average percent error (APE) and coefficient of variation (CV) were estimated to assess count precision, and a Bowker test of symmetry was performed to determine potential bias between readers’ counts. The edge and marginal increment analyses were conducted to validate the band pair deposition periodicity, and a multimodel approach was used to select the best growth model. The reading precision was good because APE and CV values were 7.9% and 5.6%, respectively, and the Bowker’s symmetry test indicated no systematic differences between readers. The validation analyses did not provide evidence to determine the band pair deposition periodicity; thus, it was assumed that one band pair is deposited yearly. The oldest male and female were five (95.4 cm TL) and eight (112.1 cm TL) years. The Gompertz growth function provided the best statistical fit and best describes the growth of bonnethead sharks. The parameters were L∞=113 cm, k2=0.49/year, L0=39 cm for females, and L∞=83.6 cm, k2=0.79/year, L0=40.2 cm for males. These are the first age and growth parameters estimated for the bonnethead shark in the southern GOM, which is the second most landed shark and needs a population assessment for proper management.