Age and growth of the Australian sharpnose shark,Rhizoprionodon taylori,from north Queensland,Australia

Synopsis Age and growth were studied inRhizoprionodon taylori using specimens caught in Cleveland Bay, North Queensland, Australia. Von Bertalanffy growth parameters were estimated using three different techniques: vertebral ageing, back calculation and length frequency. Vertebrae from 138 specimens were sectioned and narrow circuli counted to estimate age. Marginal increment analysis verified that circuli were produced annually in late summer, probably as a result of stress during the mating season. The oldest female was 7 and male 6 years old. Von Bertalanffy growth parameters estimated from vertebral ageing data for males were t_O = 0.410 yr, K = 1.337, L = 652.2 mm, and for females t_O = 0.455 yr, K = 1.013 and L = 732.5 mm. Growth parameters determined by length frequency and back calculation techniques concurred with those from vertebral ageing. Growth of the 0+ age class was very rapid, averaging 140% of the size at birth in the first year. Males and females matured after only one year, the lowest age at maturity reported in the family Carcharhinidae. Annual growth increments decreased rapidly after maturity, and little growth occurred after three years.     

Ontogenetic changes in the diet of the sandbar shark, <Emphasis Type="Italic">Carcharhinus plumbeus</Emphasis>, in lower Chesapeake Bay and Virginia (USA) coastal waters

This study describes the diet of the sandbar shark, Carcharhinus plumbeus, highlighting differences in diet within various regions of the Virginia (USA) nursery area, as well as ontogenetic changes in diet. Stomach samples were obtained in 2001 and 2002 from 232 sharks caught by gillnets or longlines. Historical data from the Virginia Institute of Marine Science (VIMS) Shark Ecology Program were also analyzed. Ontogenetic changes in diet were evident, with crustacean prey decreasing in frequency with increasing shark size, and elasmobranch prey importance increasing with increasing shark size. Whereas previous research in Chincoteague Bay, VA showed the blue crab, Callinectes sapidus, was the dominant crustacean in sandbar shark diet, the mantis shrimp, Squilla empusa, dominated the crustacean portion of the diet in this study. Differences in diet of sharks were observed among locations within the study area. Small juveniles (≤80 cm precaudal length) in the lower Chesapeake Bay ate more fishes, whereas Eastern Shore juveniles ate more crustaceans. Crustacean prey items varied among locations along the Eastern Shore, with more portunid crabs consumed in waters near Wachapreague and more mantis shrimp consumed near Sand Shoal Inlet. Our study showed that Carcharhinus plumbeus is a generalist predator and is thus unlikely to strongly impact the population of any particular prey species, and in turn is not likely to be strongly affected by fluctuations in abundance of a single prey species.     

Olfactory morphology of carcharhinid and sphyrnid sharks: Does the cephalofoil confer a sensory advantage?

Many hypotheses have been advanced to explain the adaptive significance of the sphyrnid cephalofoil, including potential advantages of spacing the olfactory organs at the distal tips of the broad surface. We employed comparative morphology to test whether the sphyrnid cephalofoil provides better stereo-olfaction, increases olfactory acuity, and samples a greater volume of the medium compared to the situation in carcharhiniform sharks. The broadly spaced nares provide sphyrnid species with a significantly greater separation between the olfactory rosettes, which could lead to an enhanced ability to resolve odor gradients. In addition, most sphyrnid species possess prenarial grooves that greatly increase the volume of water sampled by the nares and thus increase the probability of odorant encounter. However, despite a much greater head width, and a significantly greater number of olfactory lamellae, scalloped hammerhead sharks do not possess a greater amount of olfactory epithelial surface area than the carcharhiniform sandbar sharks. Therefore, sphyrnid sharks might not possess any greater olfactory acuity than carcharhinids. Despite this, there are clear olfactory advantages to the cephalofoil head morphology that could have led to its evolution, persistence, and diversification. persistence, and diversification.     

Catch composition and aspects of the biology of sharks caught by Thai commercial fisheries in the Andaman Sea

Catch composition, landing patterns and biological aspects of sharks caught by commercial fishing fleet operating in the Andaman Sea were recorded from landing sites in Ranong province of Thailand over a period of 1 year. Of the 64 species previously reported in the existing Thailand checklist, only 17 species were recorded in this study. Shark landings from the Andaman Sea appear now to be dominated largely by bamboo sharks Chiloscyllium spp. (Hemiscylliidae), which contribute c. 65% of the total number of sharks recorded. The carcharhinid sharks comprised c. 30·5% to the total catch, while the remaining c. 4·5% of landings comprised sharks from the families Squalidae, Stegostomatidae, Sphyrnidae and Triakidae. The catch composition is remarkably different from the previous landing survey in 2004, in that the current study found noticeable declines in landings of slow‐growing, late‐ maturing and low‐fecundity species (especially sphyrnid and carcharhinid species). The absences of many species and changes in life‐stage composition suggest that the populations of these groups may be close to collapse. The results from this study emphasize the urgency for additional research and monitoring efforts and also the need for management incentives in order to manage shark fisheries effectively in the Andaman Sea.     

Catch composition and reproductive biology of whaler sharks (Carcharhiniformes: Carcharhinidae) caught by fisheries in Indonesia

The first detailed information on the biological aspects of members of the economically important shark family Carcharhinidae in Indonesian waters is given. Carcharhinids dominated the landings of sharks at sites visited, comprising almost 60% of the total biomass of all sharks recorded, over an intensive 5 year sampling period. A total of 26 species of carcharhinids, representing half of the species in the Carcharhinidae, were recorded. The most abundant species, in terms of numbers recorded, were Scoliodon laticaudus and Carcharhinus falciformis , while the most abundant in terms of biomass were Prionace glauca , C. falciformis and Carcharhinus obscurus . Biological data are presented for 22 of the 26 species. The total length at which males attain maturity ( L ₅₀) was determined for 10 species. The biological aspects of a number of species, most notably Carcharhinus sorrah and Loxodon macrorhinus , were found to differ from those recorded in the literature for other regions.     

Assessing the overlap between the diet of a coastal shark and the surrounding prey communities in a sub-tropical embayment

An elasmobranch survey of sub-tropical Hervey Bay, Australia, captured the slit-eye shark Loxodon macrorhinus at only one of three sites sampled. The dietary composition of this small shark species was compared to the prey communities within Hervey Bay to test whether prey availability was driving this observation. Dietary analysis of prey groups revealed that teleosts dominated the diet, per cent index of relative importance, % I(RI) (79·5%) and per cent geometric index of importance, % G(II) (52·7%), with shrimp-like invertebrates and cephalopods identified as the most important invertebrate prey groups. Baited remote underwater video (BRUV) used to sample prey communities at each site, demonstrated a highly diverse and significantly different community composition among the sites. There was no significant overlap between the diet of L. macrorhinus and any of the prey communities detected by BRUVs according to one-way analysis of similarities and the simplified Morisita index. Habitat electivity analysis revealed affinity of L. macrorhinus for the site with the highest water clarity (Secchi disc depth), opposing that of three other shark species. Overall, the results suggest that the distribution of L. macrorhinus is not driven by prey availability but other factors such as water clarity, predator avoidance or a reduction in interspecies competition.     

Parthenogenesis in a large‐bodied requiem shark,the blacktip Carcharhinus limbatus

Genetic evidence is provided for parthenogenesis in a large‐bodied shark, the blacktip Carcharhinus limbatus, from the speciose and commercially important family Carcharhinidae, the first verified case of asexual development in this lineage and only the second for any chondrichthyan. The parthenogenetic embryo exhibited elevated homozygosity relative to its mother, indicating that automictic parthenogenesis is the most likely mechanism. Although this finding shows that parthenogenesis is more common and widespread in sharks than previously realized and supports the early existence of parthenogenetic abilities in vertebrates, the adaptive significance of automixis in these ancient fishes remains unclear.     

Reproductive biology,multiple paternity and polyandry of the bull shark Carcharhinus leucas

To improve understanding of bull shark Carcharhinus leucas reproductive biology, we analysed reproductive traits from 118 bull sharks caught along Reunion Island coasts (Western Indian Ocean), including 16 gravid females. Specific microsatellite loci were used to investigate the frequency of multiple paternity. Males and females reached maturity at c. 234 cm and 257 cm total length (L_T), respectively, and litter sizes ranged from 5 to 14 embryos. Analysis of the 16 litters collected in various months of the year indicated that parturition occurs between October and December, with a size at birth c. 60–80 cm L_T and that the gestation period is probably c. 12 months. Assuming a 1 year resting period and a period of sperm storage (4–5 months) between mating (in June–September) and fertilisation, the reproductive cycle of bull sharks at Reunion Island would be biennial. At least 56.25% of the litters investigated were polyandrous, sired by 2–5 males. Several males that each sired several litters conceived during the same or distinct mating seasons were detected, suggesting both a seasonal aggregation of sharks to mate and some male fidelity to mating site. Altogether, these findings provide valuable information for both shark risk management and conservation of the species in the Western Indian Ocean.     

Reproductive biology of Carcharhinus acronotus in the coastal waters of South Carolina

The reproductive biology of blacknose sharks Carcharhinus acronotus in the western North Atlantic Ocean was studied by examining specimens collected in the coastal waters of South Carolina. Males begin the maturation process between 875 and 910 mm fork length ( L _F), as indicated by the presence of functional claspers and siphon sacs. The presence of vitellogenic oocytes and developing oviducal glands and uteri indicated that females begin to mature at c . 870 mm L _F. Length at which 50% of the population reached maturity was 896 and 964 mm L _F, equivalent to 4·3 and 4·5 years, for males and females, respectively. Gonado‐somatic indices suggested that spermatogenesis and vitellogenesis began after December. Mating took place during the end of May and the beginning of June. Fertilization occurred during late June and early July, suggesting that female blacknose sharks were capable of sperm storage. Based on the timing of fertilization and occurrence of females carrying near‐term pups in late May and early June, the gestation period for blacknose sharks was c . 11 months. Female blacknose sharks reproduced biennially based on the absence of vitellogenic oocytes in near‐term females and there being no indication of vitellogenesis in postpartum females. Male blacknose sharks were capable of reproducing annually as indicated by turgid genital ducts, which were observed in all mature males collected during late May and early June.     

Reproductive cycle of the blacknose shark Carcharhinus acronotus in the Gulf of Mexico

The reproductive periodicity of blacknose sharks Carcharhinus acronotus in the northern Gulf of Mexico was determined by examining reproductive tissues from specimens collected monthly from 2002 to 2005. Monthly changes in shell gland mass, right ovary mass and ovarian follicle diameter were assessed for 74 mature females. Temporal changes in testes mass, testes width and proportion of mature spermatocysts were examined for 64 mature males. Trends in female reproductive tissues suggested an annual peak in reproductive activity during June and July, while trends in male variables suggested an annual reproductive peak during May and June. Although male and female reproductive activity peaked in different months, a strong synchronicity existed between the proportion of mature spermatocysts and the diameter of the largest ovarian follicle. Based on these results, the mating season of blacknose sharks lasts from mid-May to July in the Gulf of Mexico. Maximum embryo sizes were observed in May, which suggested that partition occurs during late May or early June. Results indicate that blacknose sharks have a clearly defined annual cycle in the Gulf of Mexico. This conclusion is further supported by the complete absence of gravid females without vitellogenic ovarian follicles among all mature females examined.