Age Restriction: Predicting Patterns for Short-Fin Mako Shark
Understanding how sharks age is important for fisheries management, but since sharks and rays don’t have true bones, the traditional methods used to age fish don’t apply. John is investigating the chemistry of shortfin mako vertebrae samples to attempt to validate age patterns in all ocean basins. This, he hopes, will support fisheries management and adequate conservation of an Endangered and CITES-listed shark.
John Mohan
I am a fish ecologist with broad interests in the life history and population dynamics of fish. I am fascinated by biominerals, the calcified structures that grow incrementally and provide clues to the age of fish and the environments they experience. In my current role as an assistant professor, I am both an educator and a researcher and I mentor undergraduate and graduate students. Mentorship is extremely important to me, as my own mentors shaped the scientist I am today. When I am not in the classroom or lab, you can find me skateboarding, surfing, disc golfing, hiking...
Conservation implications of biomineralization patterns in shark vertebrae cartilage
This project will investigate vertebrae of shortfin mako sharks, a species that is known to shift biomineralization periodicity after sexual maturity. We aim to understand if the shift in biomineralization rates is consistent among sharks from different ocean basins and occurs at a specific life-stage, to improve age estimates.
We currently lack a thorough understanding of how mineralization patterns in shark vertebrae relate to age. Particularly for the shortfin mako, where the North Pacific populations exhibit shifts in the periodicity of band pair formation, it is unknown if shifting biomineralization occurs in other populations. The consequences of inaccurate age estimates for population models are severe. Given the threatened status of shortfin mako populations in the Atlantic, it is important to investigate the consistency of vertebrae biomineralization patterns, and explore the implications for conservation. Vertebral chemistry is a promising tool to help reduce bias and subjectivity in shark aging studies, and will be used in this project to explore potential shifts in biomineralization periodicity.
Accurate estimates of age are essential for effective management and conservation of exploited shark populations. Age estimates are used in population assessment models to determine longevity, growth and population productivity. Ultimately, population models predict what levels of harvest are sustainable and model predictions are used to set catch quotas. Determining age in elasmobranchs has traditionally been achieved by visually counting mineralized band pairs that form in vertebrae centra. Counting vertebrae band pairs to determine shark age, requires that the periodicity of band pair formation is known.
Validation studies using chemical mark and recapture with shortfin mako sharks in the Pacific, has revealed biannual vertebrae deposition in juvenile life stages [1], but a shift to annual deposition in adult stages after sexual maturity [2]. Knowledge gaps exists in our understanding of the mechanisms of biomineralization of shark vertebrae, and more data is need to determine if biannual to annual deposition shifts occur in all ocean basins, and how that affects ocean-specific population models. My previous research discovered that manganese patterns reflect band pair deposition [3]. Chemical histories may help elucidate the physiology behind band pair formation, and employing a comparative approach using samples from the Pacific, Atlantic and Indian Ocean basins, will increase understanding of mechanisms driving vertebrae biomineralization by exploring the consistency of patterns in all ocean basins with a single species.
1. Wells et al. 2013 Age validation of juvenile Shortfin Mako (Isurus oxyrinchus) tagged and marked with oxytetracycline off southern California. Fish. Bull. 111, 147–160
2. Kinney et al. 2016 Oxytetracycline age validation of an adult shortfin mako shark Isurus oxyrinchus after 6 years at liberty. J. Fish Biol. 89, 1828–1833
3. Mohan et al. 2018 Elements of time and place: manganese and barium in shark vertebrae reflect age and upwelling histories. Proc. R. Soc. B 285: 20181760.
- Obtain vertebrae samples of ten juvenile and ten adult mako (5 male and 5 female) from the Atlantic, Pacific, Gulf of Mexico and Indian Oceans
- Analyze the sectioned vertebrae corpus calcareum to visually determine age and relate elemental chemistry to band pair deposition
- Compare results from each ocean basin to determine if biannual to annual band pair deposition is consistent in both males and females of this species across its range
RESULTS:
There are gaps in our understanding of the physiological basis of mineralized vertebral band pairs that are used to determine age in sharks, a critical component of successful management. To resolve these knowledge gaps, we studied manganese patterns in the shortfin mako (Isurus oxyrinchus), a highly migratory shark that exhibits shifting band pair deposition in certain populations. We analyzed sectioned vertebrae from 50 adult and juvenile shortfin mako collected from the Atlantic, Pacific, and Indian Ocean basins by comparing band pair counts to two dimensional elemental profiles of complete life histories. We found that the number of manganese concentration peaks was significantly associated with the number of band pairs counted, indicating that manganese patterns may be used to objectively estimate ages of shortfin mako when combined with additional approaches. We also applied elemental mapping techniques that might resolve narrow band pairs in the oldest individuals, to further refine age estimates.
Conservation achievements:
The primary conservation achievement for this study was the confirmation of consistent patterns of manganese variation that relates to band pair deposition in shortfin mako sampled from across the world. This data supports that Mn profiles can be used to reduce subjectivity of visually identifying band pairs, as increases, and decrease in manganese concentration suggests a consistent physiological process is driving band pair formation in shortfin mako sharks sampled from all ocean basins.