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The Spiny Issue of Ageing Spiny Dogfish: Historical Dogma vs. New Methods  

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Figure 1. (A) Spine and (B) vertebrae thin section from the same animal. White dots denote annuli.

The dogma of using the dorsal fin spine to age spiny dogfish (Squalus suckleyi) has been in existence for over 30 years. With these well established methods, the species has a rather long history of published literature on age and growth. However, a problem with this method is that the dorsal fin spine, which protrudes from the body, is sometimes broken and often worn, thus creating lost or difficult-to-read annuli. Recent research on an Atlantic congener (Squalus acanthias) found that a technique using histological staining of vertebrae thin sections made it possible to count annuli, thus eliminating the sources of uncertainty associated with worn spines. However, this vertebral method has yet to be tested in the much longer-lived North Pacific spiny dogfish.

The North Pacific Research Board funded a study to examine both ageing methods and to determine the best for use for spiny dogfish. This is a collaborative effort between the Center's Auke Bay Laboratories, Age and Growth Lab, and the University of Alaska Fairbanks, with inter-agency participation from the Northwest Fisheries Science Center and Washington Department of Fish and Wildlife. Our study examines both age structures (Fig. 1) and compares inter- and intra-reader as well as inter-lab variability in reading annuli to determine which method produces the most precise ages for the North Pacific spiny dogfish. Results suggest a substantial decrease in intra- and inter-reader variability when the vertebrae method is compared to the dorsal fin spine method. Preliminary analyses also show that there are multiple sources of measurement error when using the spine method, sources that do not exist with the vertebrae method, and that inter-reader variance increases substantially more with increasing sample age with the spine method than with the vertebrae method. Future work will include completion of sectioning, staining and reading of vertebrae and spines (for a total of almost 400 samples of each), readings by at least one more reader, marginal increment analysis, and an inter-lab exchange.

By Cindy Tribuzio  


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