ower Growth Resu ts in Larger Oto iths: An Exprimenta with Guppies David Reznick Department of Biology, University of California, Riverside, CA 9252 t, USA Eric Kindbeck Department sf Zoology, University sf Maryland, College Park, M D 28742, USA and Heather Bryga Department of Biology, University sf California, Riverside, CA 9252 8, USA Reznick, Do, E. Lindkck, and H. Bryga. 1989. Slower growth results in larger stoliths: an experimental test with guppies (PoecFBia reticuiata). Can. ). Fish. Aquat. Sci. 46: 108-1 12. We demonstrate that slowly growing guppies (Poeci8ia reticubata) have larger otoliths than equal-sized, rapidly growing guppies. This retationship has been suggested by previous authors, Baaa t they cornpared wild or pond- reared fish from different poopurlations, thereby confounding their observations with the different environments. Our experiment controlled for the genetic background, food quality, and the aquatic environment and assessed the influences of food availability and growth rate on relative otolith size. This difference in relative otolith dimensions could aid in comparing growth rates among populations or could improve the use of dolitk dimen- sions for estimating population age structures. In both cases, these methods could also complement data based on counting daily increments or annuli on otoliths and other structures to characterize growth rates and age structures. Les auteurs montrent que les guppys (Psecibia reticuiata) croissance lente prksentent de plus gros otolithes que les guppys 3 croissance rapide de meme taille. L'hypth&se dkne telle relation avait d4jA 616 faite par d%utres auteurs, mais ceux-ci avaient compare des poissons sauvages ou 6lev6s en &tangs de populations differentes de sorte que leurs observations se trouvaient obscurcies par les 6carts entre les environnements. Dans [a pr6ente expkrience, les pararn&tres relatifs aux ant&$dents g$n6tiquesf 3 la qualit6 des aliments et 3 Peenvironnement aquatique ont 6t6 contr8l6s et on a 6valu6 Bes effets de la dispnibilit6 de la nourriture et du taux de croissance sur la taille relative des otoiithes. Cet &art de taille relative pourrrait permettre de cornparer les taurx de croissance de populations diff6rentes ou de faciliter I'utilisation de ces param&tres pour l'estirnation be la structure des ages des populations. Dans les deux cas, ces methodes perrnettraient de completer les donnkes repssant sur le d6nom- brement des accroissernents quotidiens ou des anneauw des otolithes, oaa d'aketres structures, et servant 3 carac- @riser les taux de croissance et la structure des iges. Received October 26, 1987 Accept& August 29, 1988 (J946N ish otolith are vdued primarily as m indicator of age; however, they u e dso valuable for the infomation they contain on a fish's past history. Such infomation includes somatic growth rate (e.g. Vok et 81. f 984; Rad&e et al. 1985; Secor and Bean 1986; reviewed by C m p m a md Neilson 1985), tempramre regime (reviewed by Rad&e 1984), periods of low resource avdlability (e.g . Rice et al. 1985; Jones and Brothers 19871, or trmsitions between life history stages (e.g. Brothers and McFa1md 198 1; Victor 4982, 1983; Neilson et al. 1985a). One way for otoliths to yield i d o m d i o n on somatic growth rate is that slowly gowing fish appear to have larger m&or heavier otoliths than equal-sized, rapidly growing fish (Tern- plemen md Squires 1956; Wilson 1984; Secsr md Dem 1986). If this observation is generally true, then the relative size of otolith could provide m index of relative growth rate. Relative p w t h rates could therefore be evaluated with statisticd com- parisons m o n g body size - otolith size regression lines. Otolith size and body size data are far more easily obtained than gen- erating gowt-h curves from counts of daily or mnud rings (Seeor md Dean 1986). Because these studies were based on either c o m p ~ s o n s among different populations or on variations among wild-caught individuals, the ultimate cause of the dif- ferences in relative otolith dimensions may be &t~bu&ble to factors other than growth rate differences, such as unknown environmental effects . We have exp~mentally evaluated the relationshp between growth rate md otolith size to see if slowly growing fish have relatively luge otolifs. We manipulated growth rate by con- trolling food avdlability. It was thus possible to generate sim- ilar-sized fish of different ages free from confounding environ- rnentd influences. Matedds and Methds We pedomed a two-way experiment with high m d low levels of food availability, sampling fish after either 6 or 16 wk of controBled feeding. There were therefore four treatments: low fsd, 6 wk (Lo-6); low food, 16 wk (Lo-16); high food, 6 wk 108 Can. 9. Fish. bkquao. Ses'., VQ~. 46, 1989 Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Riverside (UCR) on 11/06/13 For personal use only.