Lisa J. Natanson

USE OF ISOTOPIC ANALYSIS OF VERTEBRAE IN RECONSTRUCTING ONTOGENETIC FEEDING ECOLOGY IN WHITE SHARKS

We conducted stable 13C and 15N analysis on white shark vertebrae and demonstrated that incremental analysis of isotopes along the radius of a vertebral centrum produces a chronological record of dietary information, allowing for reconstruction of an individuals trophic history. Isotopic data showed significant enrichments in 15N with increasing sampling distance from the centrum center, indicating a correlation between body size and trophic level. Additionally, isotopic values verified two distinct ontogenetic trophic shifts in the white shark: one following parturition, marking a dietary switch from yolk to fish; and one at a total length of >341 cm, representing a known diet shift from fish to marine mammals. Retrospective trophic-level reconstruction using vertebral tissue will have broad applications in future studies on the ecology of threatened, endangered, or extinct species to determine life-long feeding patterns, which would be impossible through other methods.

Rapid and simultaneous identification of body parts from the morphologically similar sharks Carcharhinus obscurus and Carcharhinus plumbeus (Carcharhinidae) using multiplex PCR.

Abstract: Many commercially exploited carcharhinid sharks are difficult to identify to species owing to extensive morphological similarities. This problem is severely exacerbated when it comes to identifying detached shark fins, and the finless and headless shark carasses typically sold in markets. To assist in the acquisition of urgently needed conservation and management data on shark catch and trade, we have developed a highly streamlined approach based on multiplex polymerase chain reaction (PCR) that uses species-specific primers derived from nuclear ribosomal ITS2 sequences to achieve rapid species identification of shark body parts. Here we demonstrate the utility of this approach for identifying fins and flesh from two globally distributed, morphologically very similar carcharhinid sharks (Carcharhinus obscurus and Carcharhinus plumbeus) intensively targeted in fisheries worldwide, and often confused for each other even as whole animals. The assay is conducted in a 4-primer multiplex format that is structured to simultaneously achieve the following efficiency and cost-reduction objectives: it requires only a single-tube amplification reaction for species diagnosis, it incorporates an internal positive control to allow detection of false-negative results, and it is novel in that it allows species identification even when DNAs from two species are combined in the same tube during the PCR reaction. The latter innovation reduces the required effort for screening a set of unknown samples by 50%. The streamlined approach illustrated here should be amenable for use in a shark conservation and management context where large numbers of samples typically need to be screened; the approach shown may also provide a model for a rapid diagnostic method applicable to species identification in general.

Vertebral growth zone deposition in pacific Angel sharks

Vertebrae and body size measurements were taken from 334 Pacific angel sharks collected from commercial gill netters off Santa Barbara, California, from Sept. 1979-Nov. 1983. Radiographs of vertebral centra from 247 specimens were studied to delineate calcified bands for age determination. The vertebrae of the smallest newborn sharks (260 mm TL) had 6 or 7 bands while those of the largest (1140 mm TL) had 42 bands. Bands were not deposited annually but were related to somatic growth. This hypothesis is supported by the number of bands in embryos and newborn Pacific angel sharks, growth of girth, and vertebral centrum dimensions, laboratory grow-out studies of tetracycline-injected sharks, and evidence from six tetracycline-injected tag returns.

Reproduction and Development of the Pacific Angel Shark, Squatina californica, off Santa Barbara, California

Specimens of Squatina californica were collected from commercial gill-net boats off Santa Barbara, California, from Jan.-Nov. 1983 for studies of reproduction. Sexual maturity in both males and females began between 900 and 1000 mm TL. Fecundity estimates from egg and embryo numbers ranged between 1 and 11. The number of young did not increase with increasing size of the female after maturity. Reproduction in the angel shark appears to follow an annual cycle. Embryo size from samples of pregnant females taken in eight months increased gradually for the 12 months between Aug. and July. Pupping occurred in spring and each female had an average of six pups. Growth rates ranged from 45 mm TL per month in very young embryos to 10 mm TL per month in those approaching the size at birth and the overall mean growth rate was 23 mm TL per month. Individual developmental morphology is described for five embryos ranging in size from 35.4-175 mm TL.

Effect of temperature on band deposition in the little skate, Raja erinacea

Laboratory experiments were conducted to determine the effects of temperature on vertebral band deposition in the little skate, Raja erinacea. Skates, injected with the antibiotic tetracycline as a biological marker, were maintained in aquaria for one year. Band deposition patterns from skates kept under fluctuating temperature conditions were compared to deposition patterns of skates kept under constant conditions; all other factors were the same. No effects of temperature on band deposition were apparent, although differences in timing of body growth were noted. Annual band deposition was confirmed.

Vertebral Bomb Radiocarbon Suggests Extreme Longevity in White Sharks

Conservation and management efforts for white sharks (Carcharodon carcharias) remain hampered by a lack of basic demographic information including age and growth rates. Sharks are typically aged by counting growth bands sequentially deposited in their vertebrae, but the assumption of annual deposition of these band pairs requires testing. We compared radiocarbon (Δ14C) values in vertebrae from four female and four male white sharks from the northwestern Atlantic Ocean (NWA) with reference chronologies documenting the marine uptake of 14C produced by atmospheric testing of thermonuclear devices to generate the first radiocarbon age estimates for adult white sharks. Age estimates were up to 40 years old for the largest female (fork length [FL]: 526 cm) and 73 years old for the largest male (FL: 493 cm). Our results dramatically extend the maximum age and longevity of white sharks compared to earlier studies, hint at possible sexual dimorphism in growth rates, and raise concerns that white shark populations are considerably more sensitive to human-induced mortality than previously thought.

Population Dynamics of the Porbeagle in the Northwest Atlantic Ocean

Abstract A virgin population of porbeagles Lamna nasus in the northwest Atlantic Ocean supported annual catches of up to 9,000 metric tons (mt) in the early 1960s before the fishery collapsed in 1967. Low and apparently sustainable catches of about 350 mt in the 1970s and 1980s allowed the stock to partially rebuild before a new fishery arose in the early 1990s. The response of the population to this renewed fishing pressure has been unclear until now. However, a new population dynamics analysis suggests that population abundance has once again declined. On the basis of more than 140,000 length measurements, an extensive catch rate index, a confirmed growth model, and a catch-at-age matrix, it appears that at least 90% of the sexually mature population has been lost as fishing mortality has increased. Independent measures of fishing mortality (F) based on Petersen analysis of tag-recaptures, Paloheimo Zs, and a population model all suggest that fishing mortality was about 0.20 in 2000. Biological referenc...

Seasonal Distribution and Historic Trends in Abundance of White Sharks, Carcharodon carcharias, in the Western North Atlantic Ocean

Despite recent advances in field research on white sharks (Carcharodon carcharias) in several regions around the world, opportunistic capture and sighting records remain the primary source of information on this species in the northwest Atlantic Ocean (NWA). Previous studies using limited datasets have suggested a precipitous decline in the abundance of white sharks from this region, but considerable uncertainty in these studies warrants additional investigation. This study builds upon previously published data combined with recent unpublished records and presents a synthesis of 649 confirmed white shark records from the NWA compiled over a 210-year period (1800-2010), resulting in the largest white shark dataset yet compiled from this region. These comprehensive records were used to update our understanding of their seasonal distribution, relative abundance trends, habitat use, and fisheries interactions. All life stages were present in continental shelf waters year-round, but median latitude of white shark occurrence varied seasonally. White sharks primarily occurred between Massachusetts and New Jersey during summer and off Florida during winter, with broad distribution along the coast during spring and fall. The majority of fishing gear interactions occurred with rod and reel, longline, and gillnet gears. Historic abundance trends from multiple sources support a significant decline in white shark abundance in the 1970s and 1980s, but there have been apparent increases in abundance since the 1990s when a variety of conservation measures were implemented. Though the white sharks inherent vulnerability to exploitation warrants continued protections, our results suggest a more optimistic outlook for the recovery of this iconic predator in the Atlantic.

Maximum age and missing time in the vertebrae of sand tiger shark (Carcharias taurus): validated lifespan from bomb radiocarbon dating in the western North Atlantic and southwestern Indian Oceans

Bomb radiocarbon analysis of vertebral growth bands was used to validate lifespan for sand tiger sharks (Carcharias taurus) from the western North Atlantic (WNA) and southwestern Indian Oceans (SIO). Visual counts of vertebral growth bands were used to assign age and estimate year of formation (YOF) for sampled growth bands in eight sharks from the WNA and two sharks from the SIO. Carbon-14 results were plotted relative to YOF for comparison with regional Δ14C reference chronologies to assess the accuracy of age estimates. Results from the WNA validated vertebral age estimates up to 12 years, but indicated that ages of large adult sharks were underestimated by 11–12 years. Age was also underestimated for adult sharks from the SIO by 14–18 years. Validated lifespan for C. taurus individuals in the present study reached at least 40 years for females and 34 years for males. Findings indicated that the current age-reading methodology is not suitable for estimating the age of C. taurus beyond ~12 years. Future work should investigate whether vertebrae of C. taurus record age throughout ontogeny, or cease to be a reliable indicator at some point in time.

Age and growth of the white shark, Carcharodon carcharias, in the western North Atlantic Ocean

Age and growth estimates for the white shark (Carcharodon carcharias) in the western North Atlantic Ocean (WNA) were derived from band pair counts on the vertebral centra of 81 specimens collected between 1963 and 2010. We used two previously published criteria to interpret band pairs and assessed the validity of each method using Δ14C levels from a recent bomb radiocarbon validation study and existing Δ14C reference chronologies in the WNA. Although both criteria produced age estimates consistent, to varying degrees, with different reference chronologies, only one was considered valid when life history information was used to select the appropriate reference chronology and minimum/maximum ages based on bomb carbon values were taken into consideration. These age estimates, validated up to 44 years, were used to develop a growth curve for the species, which was best described using the Schnute general model (sexes combined). These results indicate that white sharks grow more slowly and live longer than previously thought.