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The sixgill sharks of the genus Hexanchus (Hexanchiformes, Hexanchidae) are large, rarely encountered deep-sea sharks, thought to comprise just two species: the bluntnose sixgill Hexanchus griseus (Bonaterre, 1788) and the bigeye sixgill Hexanchus nakamurai (Teng, 1962). Their distribution is putatively worldwide in tropical and temperate waters, but many verified records for these species are lacking, and misidentification is common. Taxonomic uncertainty has long surrounded H. nakamurai in particular, with debate as to whether individuals from the Atlantic constitute a separate species. Using 1,310 base pairs of two mitochondrial genes, COI and ND2, we confirm that bigeye sixgill sharks from the Atlantic Ocean (Belize, Gulf of Mexico, and Bahamas) diverge from those in the Pacific and Indian Oceans (Japan, La Reunion, and Madagascar) with 7.037% sequence divergence. This difference is similar to the genetic distance between both Atlantic and Indo-Pacific bigeye sixgill sharks and the bluntnose sixgill shark (7.965% and 8.200%, respectively), and between the entire genus Hexanchus and its sister genus Heptranchias (8.308%). Such variation far exceeds previous measures of species-level genetic divergence in elasmobranchs, even among slowly-evolving deep-water taxa. Given the high degree of morphological similarity within Hexanchus, and the fact that cryptic diversity is common even among frequently observed shark species, we conclude that these results support the resurrection of the name Hexanchus vitulus Springer and Waller, 1969 for bigeye sixgill sharks in the northwest Atlantic Ocean. We propose the common name "Atlantic sixgill shark" for H. vitulus, and provide new locality records from Belize, as well as comments on its overall distribution. [ABSTRACT FROM AUTHOR]

Sea turtle bycatch in longline fishing gear is an ongoing threat to the recovery of sea turtle populations. While considerable research has focused on pelagic longline fisheries, very little attention has been paid to captures of sea turtles in bottom longlines. Estimates of sea turtle takes in the shark bottom longline fishery have raised concern that this fishery may be impacting loggerhead sea turtle, Caretta caretta (Linnaeus, 1758), populations. However, there are no current management actions designed to reduce the bycatch of sea turtles in fisheries that target sharks using bottom longline gear because no studies have identified which factors, if any, influence the capture of a loggerhead sea turtle. We used generalized linear models to determine which factors influence the probability of loggerhead sea turtle captures in the shark bottom longline fishery, and which factors are related to at-vessel mortality. While a variety of fishing techniques were considered as factors, no particular fishing method was found to predict a capture of a sea turtle. The most significant factor in predicting the capture of a loggerhead sea turtle in shark bottom longline gear was the area fished. Soak time was found to predict at-vessel hooking mortality with the median time for a mortality to occur was 14-15 hrs. While no definitive fishing factor was identified in the capture or mortality of a loggerhead sea turtle, research is needed using controlled methods for further examining the factors affecting captures of sea turtles in this and other bottom longline fisheries. [ABSTRACT FROM AUTHOR]

This study generates and updates mathematical conversions among body length and weight measurements of sharks commonly encountered in the western North Atlantic Ocean. At the initiation of individual research programs, standardized measurements are determined to meet program objectives, yet these measurements often vary among programs and may differ within programs over time. Since length is of vital importance to understanding the basic biology of a species (e.g., growth, length at maturity) and to enforce management regulations based on size, it is necessary to have length-length and length-weight conversions to be able to standardize measurements for individual species. We compiled length and weight data on sharks from nine research programs operating in the western North Atlantic Ocean from Canada through the Gulf of Mexico to obtain length-length and length-weight conversions for 27 species and 3 genera consisting of 6 species. Length-length and length-weight conversions are presented for all species using over the body fork length as the independent variable. This study updates and expands previous conversions with new information. [ABSTRACT FROM AUTHOR]