AES 2012: Deepwater Chondrichthyans Symposium

Selected tweets from the deepwater chondrichthyans symposium at the 2012 meeting of the American Elasmobranch Society

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  1. The American Elasmobranch Society is a non-profit professional organization of shark, ray, skate, and chimaera scientists. Each year, AES holds an annual conference in a different North American city where members meet and present their research. The 2012 meeting took place from August 8-14 in Vancouver. Here is a full schedule of all the conference talks (including those by other scientific societies which participated in the meeting), but not all talks were live-tweeted.
  2. Habitat associations of deepwater chondrichthyan life history traits. Cassie Rigby.
    Abstract: Life history traits are important indicators of the productivity of a species, and its ability to tolerate fishing pressure. Using a variety of life history traits (male and female traits of maximum size, size at maturity, age at maturity, longevity, growth rate and size at birth) from a wide range of chondrichthyans we demonstrate that there are life history differences between shelf, oceanic and deepwater habitats. This included deepwater species having lower growth rates, later age at maturity and higher longevity than both shelf and oceanic species. This analytical review used a larger suite of species than previously assessed and also undertook an examination of deepwater species life history traits associated with depth and geographic range. We examined the patterns in life history traits by depth (upper, mid and deep slope), region (North and South Atlantic, North and South Pacific) and latitudinal zone (Polar, Temperate and Tropical). North Atlantic female deepwater chondrichthyans were found to have significantly higher growth rates and lower longevity than those in the South Atlantic and size at birth was smallest in species inhabiting the deeper slope habitats, Polar to Temperate zones and the North Atlantic region. There was a trend for both male and female chondrichthyans of lower growth rate, later age at maturity and higher longevity with increasing depth. However this trend was not significant when phylogenetic relatedness and size were taken into account which suggests that differences in species composition and size among habitats can drive these trends and stresses the importance of accounting for these to clearly determine the influence of habitat on life history traits. These associations of life history traits with habitat, both among deep, shelf and oceanic species and within deepwater species will be discussed within the context of ecological theories and conservation strategies.
  3. Bathymetric limits of chondrichthyans in the deep sea. Jack Musick. 
    Abstract: Chondrichthyans are largely absent in abyssal (>300m) habitats in in most regions of the world ocean are uncommon below 2000m. The deeper-living chondrichthyans include certain rajids, squaliforms and holocephalans. Several hypotheses have been erected to explain the absence of chondrichthyans from the abyss. These are mostly based on energetics: Deep-sea food webs are impoverished due to their distance from primary production, and chondrichthyans, occupying the highest trophic levels, cannot be supported due to entropy among trophic levels. We examine this hypothesis by comparing trophic levels, calculated from dietary data, of deep-sea chondrichthyans with those of deep-sea teleosts. Chondrichthyans were mostly above trophic level 4, whereas all the teleosts examined were below that level. The potential prey field for both chondrichthyans and teleosts declines in biomass and diversity with depth, but teleosts appear to have more flexibility in their feeding mechanisms and food habits.
  4. The diversity, distribution and demographic population structure of deep water elasmobranchs in the northeast Exuma Sound, the Bahamas. Edd Brooks (presented by Dean Grubbs). 
    Abstract: There is a fundamental lack of basic taxonomic, biological and ecological information pertaining to the majority of deep water species, in particular elasmobranchs, largely due to the logistical challenges of sustained ecological investigation in this remote and hostile ecosystem. The Exuma Sound is a deep water inlet of the Atlantic Ocean ranging in depth from 500 – 2000 m and characterized by steep walls along its margin. The sound is in close proximity to land (<3 km), facilitating the sustained investigation of its deep water elasmobranch fauna over extended periods of time. A total of 69 deep water longline surveys were conducted from September to December in both 2010 and 2011 (depth: 472.6 – 1024.1 m; seabed temperature: 15.6 – 5.9 °C), resulting in the capture of 144 sharks of at least eight different species. This does not include a potentially new species of Centrophorus currently undergoing morphological and genetic assessment. Elasmobranch species richness declined significantly with increasing distances from the edge of the Exuma Sound (ρ = -0.295, p = 0.014), increasing depth (ρ = -0.242, p = 0.045), and increasing seabed water temperatures (ρ = 0.288, p = 0.016). Distance from the edge of the Exuma Sound was a significant predictor for the presence or absence of Squalus cubensis, Mustelus canis insularis, Centrophorus spp., Hexanchus nakamurai and Centroscymnus owstoni. Furthermore, depth and temperature were significant predictors of the presence or absence of S. cubensis, M. canis insularis and C. owstoni. Depth was also a significant predictor of presence or absence of H. nakamurai. There were no predictable trends in the abundance of Hexanchus griseus, Galeus springeri or Pseudotriakis microdon. Significantly skewed sex and/or maturity ratios were identified for a number of species suggesting that this is a common life history trait in deep water elasmobranchs. The results of this study indicate that the use of deep water longline surveys in the Exuma Sound, and similar oceanographic areas, is an efficient and cost effective method for the sustained investigation of deep water elasmobranchs, and is vital for the effective management of this especially vulnerable group of species.
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