Perspectives on elasmobranch life‐history studies: a focus on age validation and relevance to fishery management

Abstract

Life-history (age, growth, age validation, reproduction and demography) studies of elasmobranchs date back to the middle of the last century with major early contributions made by British fishery scientists. As predicted by Holden in the early 1970s, many sharks and rays can be vulnerable to fishery mortality because they grow slowly, mature late in life, reproduce infrequently, have relatively low fecundities and can have relatively long life spans. As has now been found, however, not all species exhibit these traits. Also, ageing structures (neural arches and caudal thorns), other than vertebrae and spines, have since been evaluated. Various methods for validating age and growth estimates have been developed and tested on numerous species of elasmobranchs. These include tag-recapture analyses, oxytetracycline injections, centrum or spine edge and marginal increment analyses, and bomb radiocarbon dating of calcified structures. Application of these techniques has sometimes not only validated relatively slow growth and long life span estimates, but also has produced other results. A brief historical perspective on the applications and limitations of these techniques for elasmobranchs is provided, along with a discussion of selected species for which these techniques worked well, did not work at all or have produced variable and conflicting results. Because many fishery management techniques utilize age or stage-specific information, often through demographic analyses, accurate information on the life histories of fished populations, especially age validation, is extremely important for the fishery management of these cartilaginous fishes.

A hierarchical Bayesian approach for handling missing classification data

“…As a result, classification data almost always include a category for counts of unclassified individuals. Handling these unknowns has been demonstrably problematic in surveys of aquatic (Cailliet, 2015;Sequeira, Thums, Brooks, & Meekan, 2016;Tsai, Liu, Punt, & Sun, 2015), terrestrial (Boulanger, Gunn, Adamczewski, & Croft, 2011;White, Freddy, Gill, & Ellenberger, 2001), and aerial (Cunningham, Powell, Vrtiska, Stephens, & Walker, 2016;Nadal, Ponz, & Margalida, 2016) species. Classification uncertainty has multiple causes, including physical and behavioral ambiguities, observer skill level, and sampling effort (time).…”

Evidence for systemic age underestimation in shark and ray ageing studies

“…While the number of age validation studies has increased in recent years, the studies reviewed here cover fewer than 5% of described species (Figures 1 and 2). As noted by Cailliet (2015) and emphasized by Figure 2, even fewer of these species have been validated convincingly. Bomb carbon dating appears to be the most effective tool currently available for age validation of sharks and rays; it is the most likely to give a true indication of longevity and detect whether age has been underestimated.…”

“…In practice, it has meant more regular stock assessments on target species, and risk assessments on non‐target components of the catch including by‐catch and threatened species. With greater numbers and diversity of sharks and rays being aged to support these assessments, corresponding studies have successfully validated part or all of the lifespan in a growing number of species (Cailliet, ).…”

“…The many reviews on shark and ray ageing have repeatedly emphasized the importance of validation to practitioners (Cailliet, 1990(Cailliet, , 2015Cailliet & Goldman, 2004;Cailliet, Smith, Mollet, & Goldman, 2006;Cailliet et al, 1986;Goldman, Cailliet, Andrews, & Natanson, 2012). Accordingly, validation has become an integral and expected part of most studies, particularly when using the most common method of counting growth zones on thin sections of vertebrae or spines.…”

“…With greater numbers and diversity of sharks and rays being aged to support these assessments, corresponding studies have successfully validated part or all of the lifespan in a growing number of species (Cailliet, 2015).…”