Chapter 8: Fin Rays and Spines
Jesse R. Fischer and Jeff D. Koch
Fin spines and rays have long been used to estimate the age of several groups of marine and freshwater fishes (e.g., sturgeons, Kler 1916; suckers, Beamish and Harvey 1969; sharks and rays, Holden and Meadows 1962; billfishes, Hill et al. 1989; catfishes, Sneed 1951). Their preferential use for estimating age and growth for some taxonomic groups is related to morphological characteristics (e.g., lack of scales, poorly developed otoliths) that prohibit use of other calcified structures. Moreover, nonlethal removal makes fin rays and spines more applicable for estimating the age of long-lived taxa, species of elevated conservation concern (Jelks et al. 2008), and species of high economic value (Metcalf and Swearer 2005; Kopf et al. 2010). Since fin ray and spine removal may not appreciably affect swimming ability, growth, or survival (Collins and Smith 1996; Stevenson and Day 1987; Michaletz 2005; Zymonas and McMahon 2006; Nguyen et al. 2016), the ability to recapture individually marked fish makes nonlethal validation of age estimation procedures possible (Chapter 3). However, the accuracy and precision of ages estimated from fin rays and spines have not been evaluated for many species.
Nearly all fish possess fin rays or spines. Ray-finned fishes (class Actinopterygii) make up more than 95% of all described species of fish (Helfman et al. 1997) and, as the name suggests, have fins consisting of skin supported by calcareous spines, soft rays, or both. Although chondrichthyans may not have fin rays, some species of sharks and rays have dorsal fin spines and are often aptly named (e.g., Spiny Dogfish). Even the extinct spiny sharks (order Acanthodii) appeared to have prominent spine-like leading edges on most fins from fossilized specimens. Finally, although not used in age and growth studies, hagfish and lampreys have fin rays consisting of rods of cartridge.