A Novel Technique for Batoids: A Histological Approach to Age the Smooth Skate, Malacoraja senta, from the Western Gulf of Maine
Introduction and background
The vulnerability of elasmobranchs to exploitation by commercial fisheries is well documented. In the Gulf of Maine, assessment studies by the Northeast Fisheries Science Center (1999) identified three species of batoids (skates and rays) as being overfished. They are the barndoor, Dipturus laevis, thorny, Amblyraja radiata, and smooth, Malacoraja senta, skates. The lack of biological information for these skates led us (Tsang and Sulikowski) to conduct a comprehensive study funded by the Northeast Consortium (May 2001) to collect the necessary life history data for the thorny and smooth skates. Our goal was to collect information that could be used to develop an effective fisheries monitoring, management and conservation plan for these particular species. In retrospect, this was a timely study because the situation of the skates has worsened. Since 2003, commercial landings of all three species have been prohibited.
Of the existing methods available to accurately age elasmobranchs, the enumeration of growth zones in vertebral centra has proven quite efficacious in some of our studies. To date, we have successfully used this method to age the winter skate (Sulikowski et al., Fishery Bulletin 101:405-413, 2003; Sulikowski et al., in press, Environmental Biology of Fishes) and the thorny skate (Sulikowski et al., in press, Fishery Bulletin). However, these two skate species were the exception rather than the rule. For the vast majority of batoids, there have been unforeseen difficulties in using the enumeration of growth zones in vertebral centra as well as a number of other techniques that have been successfully employed to determine the age of other elasmobranchs. As a result, this has led to incomplete or absent information for many batoid species. Unfortunately, in our life history study of smooth skates in the Gulf of Maine, we have encountered similar difficulties in our attempts to ascertain their age.
We propose to use an alternative and novel method that has been used successfully to age a few species of sharks, but has yet to be attempted in skates or rays. This will allow us to complete our Northeast Consortium project and provide the valuable information needed for proper management of smooth skates. In doing so, we will also establish a working relationship with Dr. Lisa Natanson, a leading shark biologist at the National Marine Fisheries Service (NMFS) Apex Predators Program located in Narragansett, Rhode Island. Dr. Natanson not only has over 20 years of experience and technical expertise using alternative methods to age sharks, but she also has a laboratory equipped to conduct these procedures. We aim to apply for funds from NH Sea Grant and other extramural agencies so that we can fully characterize this alternative method, as well as to develop others. Our ultimate goal is to create a repertoire of standard techniques designed for use in batoid species that are as difficult to perform age assessments in as the smooth skates; and we plan to present this data at the 2006 American Elasmobranch Society meeting, which has a symposium specifically dedicated to batoid research. This venue will be a perfect opportunity to present our findings to a national audience of our peers.
The method of choice that provides the best opportunity for success employs the use of histological techniques. Vertebrae from smooth skates are cleaned of connective tissue and trimmed to size. Then, two vertebrae from each specimen will be decalcified, placed in a tissue cassette, and stored in 70% buffered formalin until processed by the laboratory of Dr. Natanson. There, the samples will be dehydrated, embedded in paraffin, sectioned with a sledge microtome (80-l00um thickness; this is the critical step), and stained with hematoxylin and eosin. The prepared slides of vertebrae will then be viewed using a binocular dissecting microscope and the age determined by counting the stained growth zones. Approximately 400 skates (200 male and 200 female) will be used for this study, which equates to the processing of 800 vertebrae.