John K. Carlson

Extinction risk and conservation of the world’s sharks and rays

The rapid expansion of human activities threatens ocean-wide biodiversity. Numerous marine animal populations have declined, yet it remains unclear whether these trends are symptomatic of a chronic accumulation of global marine extinction risk. We present the first systematic analysis of threat for a globally distributed lineage of 1,041 chondrichthyan fishes—sharks, rays, and chimaeras. We estimate that one-quarter are threatened according to IUCN Red List criteria due to overfishing (targeted and incidental). Large-bodied, shallow-water species are at greatest risk and five out of the seven most threatened families are rays. Overall chondrichthyan extinction risk is substantially higher than for most other vertebrates, and only one-third of species are considered safe. Population depletion has occurred throughout the world’s ice-free waters, but is particularly prevalent in the Indo-Pacific Biodiversity Triangle and Mediterranean Sea. Improved management of fisheries and trade is urgently needed to avoid extinctions and promote population recovery. DOI: http://dx.doi.org/10.7554/eLife.00590.001

Is the collapse of shark populations in the Northwest Atlantic Ocean and Gulf of Mexico real

Abstract Increasing fishing pressure on sharks stocks over recent decades has resulted in declines of many populations and led to increasing concerns for their conservation. The extent of these declines, however, has been highly variable—the result of the level of fishing, ocean conditions, and the life history of individual species. Two recent articles have described the collapse and possible extirpation of shark populations in the northwest Atlantic Ocean and Gulf of Mexico. Herein, we examine the results of these two papers commenting on the data sets used, comparing them to other available data sets, and critically evaluating the analyses and conclusions. We argue that these conclusions have been overstated because: (1) the analyses were based on a limited number of data sets, (2) the data sets themselves are inadequate to describe the status of all shark populations in the northwest Atlantic Ocean and Gulf of Mexico reported in these studies, (3) available data sets that could produce different concl...

Metabolism, Energetic Demand, and Endothermy

CONTENTS 7.

Habitat use and movement patterns of bull sharks Carcharhinus leucas determined using pop‐up satellite archival tags

Habitat use, movement and residency of bull sharks Carcharhinus leucas were determined using satellite pop-up archival transmitting (PAT) tags throughout coastal areas in the U.S., Gulf of Mexico and waters off the south-east U.S. From 2005 to 2007, 18 fish (mean size = 164 cm fork length, L(F)) were tagged over all seasons. Fish retained tags for up to 85 days (median = 30 days). Based on geolocation data from initial tagging location to pop-off location, C. leucas generally travelled c. 5-6 km day(-1) and travelled an average of 143.6 km. Overall, mean proportions of time at depth revealed C. leucas spent the majority of their time in waters <20 m. They exhibited significant differences among depths but were not found at a particular depth regardless of diurnal period. Most fish occupied temperatures c. 32 degrees C with individuals found mostly between 26 and 33 degrees C. Geolocation data for C. leucas were generally poor and varied considerably but tracks for two individuals revealed long distance movements. One fish travelled from the south-east coast of the U.S. to coastal Texas near Galveston while another moved up the east coast of the U.S. to South Carolina. Data on C. leucas movements indicated that they are found primarily in shallower waters and tend to remain in the same location over long periods. While some individuals made large-scale movements over open ocean areas, the results emphasize the importance of the coastal zone for this species as potential essential habitat, particularly in areas of high freshwater inflow.

Age and growth of the bonnethead shark, Sphyrna tiburo, from northwest Florida, with comments on clinal variation

Age and growth rates of the bonnethead shark, Sphyrna tiburo, from northwest Florida were estimated from vertebrae collected between October 1992 and October 1995. The von Bertalanffy growth equation was fit to male and female vertebral age data. Initial growth was rapid (≈ 200 mm TL) for both sexes from age 0–1. At age 2 growth slowed for males but continued for females. Similar to many species of sharks, females grew slower than males (K = 0.28 and K = 0.69, respectively) but attained a larger maximum size (L∞=1226 and L∞=897). Maximum age was estimated in males and females to be 8+ and 12+ years, respectively. Growth of young-of-year sharks was 21 to 30 mm TL per month determined by three different methods. A comparison of age and growth estimates from populations at more southerly latitudes suggest that clinal variation in total length may be evident among bonnethead sharks in the Gulf of Mexico with females reaching larger sizes in northern areas as compared to south Florida.

Diagnosing the dangerous demography of manta rays using life history theory

Background. The directed harvest and global trade in the gill plates of mantas, and devil rays, has led to increased fishing pressure and steep population declines in some locations. The slow life history, particularly of the manta rays, is cited as a key reason why such species have little capacity to withstand directed fisheries. Here, we place their life history and demography within the context of other sharks and rays. Methods. Despite the limited availability of data, we use life history theory and comparative analysis to estimate the intrinsic risk of extinction (as indexed by the maximum intrinsic rate of population increase rmax) for a typical generic manta ray using a variant of the classic Euler–Lotka demographic model. This model requires only three traits to calculate the maximum intrinsic population growth rate rmax: von Bertalanffy growth rate, annual pup production and age at maturity. To account for the uncertainty in life history parameters, we created plausible parameter ranges and propagate these uncertainties through the model to calculate a distribution of the plausible range of rmax values. Results. The maximum population growth rate rmax of manta ray is most sensitive to the length of the reproductive cycle, and the median rmax of 0.116 year−1 95th percentile [0.089–0.139] is one of the lowest known of the 106 sharks and rays for which we have comparable demographic information. Discussion. In common with other unprotected, unmanaged, high-value large-bodied sharks and rays the combination of very low population growth rates of manta rays, combined with the high value of their gill rakers and the international nature of trade, is highly likely to lead to rapid depletion and potential local extinction unless a rapid conservation management response occurs worldwide. Furthermore, we show that it is possible to derive important insights into the demography extinction risk of data-poor species using well-established life history theory.

Relative abundance and size of coastal sharks derived from commercial shark longline catch and effort data.

In the north-west Atlantic Ocean, stock assessments conducted for some commercially harvested coastal sharks indicate declines from 64 to 80% with respect to virgin population levels. While the status of commercially important species is available, abundance trend information for other coastal shark species in the north-west Atlantic Ocean are unavailable. Using a generalized linear modelling (GLM) approach, a relative abundance index was derived from 1994 to 2009 using observer data collected in a commercial bottom longline fishery. Trends in abundance and average size were estimated for bull shark Carcharhinus leucas, spinner shark Carcharhinus brevipinna, tiger shark Galeocerdo cuvier and lemon shark Negaprion brevirostris. Increases in relative abundance for all shark species ranged from 14% for C. brevipinna, 12% for C. leucas, 6% for N. brevirostris and 3% for G. cuvier. There was no significant change in the size at capture over the time period considered for all species. While the status of shark populations should not be based exclusively on abundance trend information, but ultimately on stock assessment models, results from this study provide some cause for optimism on the status of these coastal shark species.

Age and growth of the scalloped hammerhead shark, Sphyrna lewini, in the north-west atlantic ocean and Gulf of Mexico

The scalloped hammerhead, Sphryna lewini (Griffith & Smith, 1834), is a globally exploited species of shark. In order to gain insight into the life history of this species in the USA waters, age and growth was examined from specimens (n = 307) captured from the north-west Atlantic Ocean and from the Gulf of Mexico. The von Bertalanffy growth model resulted in growth parameters of L∞ = 214.8 cm fork length (FL), k = 0.13 year–1, t0 = –1.62 year for males and L∞ = 233.1 cm FL, k = 0.09 year–1, t0 = –2.22 year for females. The oldest age estimates obtained for this population were 30.5 years for both males and females, which corresponded to FL of 234 cm and 241 cm respectively. Bowker’s test of symmetry and Index of Average Per Cent Error suggests that our ageing method represents a non-biased and precise approach to the age assessment. Marginal increments were significantly different between months (Kruskal–Wallis P = 0.017) with a distinct trend of increasing monthly increment growth beginning in January. When compared to previously published studies, our growth estimates suggest slower growth than populations in the Pacific Ocean but faster growth than previously reported in the Gulf of Mexico.

Age and growth of the blacknose shark, Carcharhinus acronotus, in the western North Atlantic Ocean with comments on regional variation in growth rates

We examined the age and growth of the blacknose shark, Carcharhinus acronotus, in the western North Atlantic Ocean by obtaining direct age estimates using vertebral centra. We verified annual deposition of growth increments with marginal increment analysis and validated it by analyzing vertebrae marked with oxytetracycline from a female blacknose shark held in captivity. Von Bertalanffy growth parameters indicated that female blacknose sharks have a lower growth constant (k), a larger theortical maximum size (L∞), and are longer lived than males. We compared these growth parameters for blacknose sharks in the western North Atlantic Ocean to growth parameters for blacknose sharks collected in the eastern Gulf of Mexico to test for differences between regions. Females in the western North Atlantic Ocean have a significantly lower L∞, lower k, and a higher theoretical longevity than females in the Gulf of Mexico. Males in the western North Atlantic Ocean have a higher L∞<>, lower k, and higher theoretical longevity than males in the Gulf of Mexico. The significant differences between these life history parameters for blacknose sharks suggest that, when possible, future management initiatives concerning blacknose sharks should consider managing the populations in the western North Atlantic and the Gulf of Mexico as separate stocks.

Age and Growth of the Blacknose Shark, Carcharhinus acronotus, in the Eastern Gulf of Mexico

Age and growth of the blacknose shark, Carcharhinus acronotus, from the eastern Gulf of Mexico was estimated by counting bands on the vertebral centra from 123 individuals. Back-calculated von Bertalanffy growth functions were constructed for populations in northwest Florida and Tampa Bay, Florida. Von Bertalanffy growth function parameters for males in northwest Florida (L, = 963.1 mm FL, K = 0.59, to = -0.754 yr) were significantly different from those in Tampa Bay (L, = 801.0 mm FL, K = 0.771, to = -0.797 yr), as were those for females (L, = 1136.5 mm FL, K = 0.352, to = -1.212 yr in northwest Florida; L, = 1241.3 mm FL, K = 0.237, to = -1.536 yr in Tampa Bay). Theoretical longevity, estimated as the age at which 95% of L, is reached, varied from age 10-16 yr for females and 4.5-9.0 yr for males, depending on geographic area. The oldest sharks aged were 4.5+ yr. Centrum edge and marginal increment analyses lent support to the hypothesis that narrow dark bands are formed during winter months. Length-frequency analysis verified back-calculated size at ages 0, 1, and 2. The growth dynamics of blacknose sharks are similar to those of other relatively small, fast-growing, short-lived species of sharks.