Age and Growth in Sharks
Why age sharks?
Sharks are long-lived animals that grow very slowly and do not produce many young. In many parts of the world, sharks are fished commercially, thus, in order to ensure proper management of the stocks, age and growth data must be obtained. With this data, we can determine the longevity of the species as well as maximum age, age at maturity, growth rate, and differences in growth between males and females.
Most bony fishes, or teleosts, are aged by the use of banding patterns on bony otoliths or scales. In cartilaginous elasmobranchs, the otoliths, or staticonia, are similar in size to sand grains and the dermal denticles do not grow (Applegate 1967). Some elasmobranch species have hard spines that contain rings suitable for ageing (Ketchen 1975). Most species of elasmobranchs, however, do not have spines, so vertebrae have been used as the ageing structure (Haskel 1949, Ishiyama 1951, Stevens 1975, Cailliet et al. 1983a, b). Several methods have been developed to enhance the concentric pairs of opaque and translucent vertebral bands including the staining of whole vertebrae using silver nitrate (Stevens 1975) or alizarin red S (LaMarca 1966); histological sectioning and subsequent staining with hemotoxylin or acid fuschin (Ridewood 1921, Natanson 1984, Casey et al 1985); x-radiography (Cailliet et al 1983a, Wintner 2000); oil clearing (Cailliet et al 1983a); and electron microprobe analysis (Cailliet and Radke 1987). The band pairs are then counted by multiple readers and related to the age of the individual.
Our Standard Methods
We obtain our samples from sportfishing tournaments, research cruises, commercial and recreational fishermen and cooperating biologists. A 6-10" piece of backbone is removed from the shark just over the gills. The sample is kept frozen until processing. Data such as species, sex, location caught, date caught, and a measurement over the body fork length (FL) are recorded when taking vertebrae (Backbone Removal Instructions)
Individual vertebrae from the sample are cut sagitally using a gem saw with two diamond blades separated by a 0.6 mm spacer resulting in a bow-tie shaped section. The resulting sections are kept in tissue capsules and stored in 70% ETOH.
Each section is photographed with a black and white video camera attached to a stereomicroscope, using reflected light. Magnification depends on the size of the section. Band pairs (consisting of one opaque and one translucent band) are counted and measured from the images using Imaging software.
Vertebrae contain concentric pairs of opaque and translucent bands. Band pairs are counted like rings on a tree and then an age assigned to the shark based on the count. Thus, if the vertebrae has 10 band pairs, it is assumed to be 10 years old. Recent studies, however, have shown that this assumption is not always correct. Researchers must therefore study each species and size class to determine how often the band pairs are deposited because the deposition rate may change over time. Determining the actual rate that the bands are deposited is called "validation".
The majority of past studies on elasmobranch age determination have been based on the presumption that the vertebral band pairs are deposited annually (Aasen 1963, Taylor and Holden 1964, Stevens 1975). These studies based their assumption on a few actual validated studies performed on Rajid species (Ishiyama 1951, Holden and Vince 1973, Holden 1974).
Most validated elasmobranch ageing studies have shown that one band pair is deposited annually throughout an individual's lifetime (Ishiyama 1951, Holden and Vince 1973, Holden 1974, Gruber and Stout 1983, Smith 1984, Natanson et al. 2002). Studies on the periodicity of band formation in the Pacific angel shark, Squatina californica, have indicated that, in this species, the bands are deposited to strengthen the vertebral column and show no time scale relationship (Natanson 1984). Additionally, some studies suggest the possibility of two band pairs forming per year (Parker and Stott 1965, Pratt and Casey 1983, Branstetter and Musick 1994). It has also been suggested that bands form annually for the first years of life then change to no periodicity (Casey and Natanson 1992). Natanson (1993) showed that bands may cease to form annually when an individual is reproductively active. Preliminary studies on thresher sharks from the West Coast have indicated annual band pair formation (Cailliet et al. 1983b, Smith and Aseltine-Neilsen 2001). The results of these studies demonstrate the problems associated with assuming annual band periodicity and underscore the need to validate band periodicity in all ageing studies.
Combined with tagging, tetracycline (OTC) injection is a commonly used technique for validating band periodicity. Injected tetracycline circulating in the bloodstream is absorbed and deposited at sites of active growth. The tetracycline mark will glow under ultraviolet light, while growth before and after the mark does not. All growth subsequent to the mark can be attributed to deposition over the time period the shark was at liberty. Growth and deposition rates can then be derived. This technique has been used successfully in the lemon shark, leopard shark, Pacific angel shark, Atlantic sharpnose shark, sandbar shark, blacktip shark, and porbeagle shark.
In conjunction with our tagging studies, we are able to inject sharks with OTC on research vessels and on board cooperating commercial vessels. Additionally some cooperating biologists and commercial fishermen inject sharks for us. The sharks are measured, tagged, and injected with a 25 mg/kg dose of OTC; sharks are then released. At recapture, many cooperating fishermen turn in the vertebral sample as well as the basic recapture information to aid our ageing and validation studies.
Recently bomb radiocarbon dating has emerged as one of the best techniques for age validation for long-lived species (Campana 2001). This technique uses the discrete radiocarbon pulse in the environment caused by the detonation of nuclear bombs in the 1950s and 1960s as a time indicator (Kalish 1995, Campana 2001). Radiocarbon levels incorporated into the growth bands are measured and related to a reference chronology to determine the absolute age of a fish and can also be used to confirm annual age in a species (Kalish 1995, Campana 1997, 2001). This technique requires the availability of specimens that were alive between 1960 and 1970 (Kalish 1995) and more specifically, 1958-1965 for the North Atlantic (Campana 1997). Bomb-radiocarbon techniques have been used to infer age in bivalves, mammals (Turekian et al. 1982, Bada et al. 1990, Peck and Brey 1996), and more recently has been successfully used to confirm annulus age estimates from otoliths of bony fish (Kalish 1993, Campana 1997). Currently, the porbeagle is the only shark species on which this method has been successfully completed. Results indicated that band interpretation for this species is correct for at least 26 years of age (Campana et al. 2002). In addition, preliminary analysis of a shortfin mako vertebrae indicated that the currently accepted method of band counting for this species is incorrect (Campana et al. 2002). The original reported growth rate is twice as high as that indicated by the new data. This discrepancy highlights the need for further research on age validation for all shark species.
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