OIKOS 100: 483–492, 2003 Microgeographical variation in brown trout reproductive traits: possible effects of biotic interactions Esben Moland Olsen and Leif Asbjørn Vøllestad Olsen, E. M. and Vøllestad, L. A. 2003. Microgeographical variation in brown trout reproductive traits: possible effects of biotic interactions. – Oikos 100: 483 – 492. This study documents substantial variation in reproductive traits among populations of stream-dwelling brown trout (Salmo trutta L.) at a very small geographic scale. Within two streams, we found a parallel pattern of variation, where females living above major waterfalls produced fewer and larger eggs than conspecifics from below the waterfalls. Four additional streams were represented with either a below-waterfall site (n =2) or an above-waterfall site (n =2). When these streams were included in the analyses, there was no consistent difference in reproductive traits between females from above- and below-waterfall sites. There was no significant difference in total reproductive investment among sites within streams, but considerable variation among streams. Female first-year growth rates was estimated from scales, and differed significantly among populations. Within streams, females from below water- falls experienced higher first-year growth rates as compared to females from above the waterfalls. Within seven out of eight populations, egg size increased significantly with increasing female body length. Within three populations, we found evidence for a trade-off between offspring size and offspring number, as a negative association between fecundity and egg size independently of adult body size. Within three populations egg size decreased significantly with increasing maternal first-year growth, independently of adult body size. We suggest that the within-stream differ- ences in offspring size/number strategies are influenced by population density and growth effects. Earlier, we have shown that population densities are consistently lower below the waterfalls in these streams. The Alpine bullhead (Cottus poecilopus ) is found only below the waterfalls and could influence brown trout demography. E. M. Olsen and L. A. Vøllestad, Di. of Zoology, Dept of Biology, Uni. of Oslo, P.O. Box 1050 Blindern, NO-0316 Oslo, Norway (e.m.olsen@bio.uio.no). For a maturing female, limited resources may force offspring size to be traded off against offspring number (Sva ¨rdson 1949, Elgar 1990, Sinervo and Licht 1991). Depending on the shape of the offspring size – off- spring fitness function, maternal fitness could be maxi- mized by producing many small offspring or few large ones (Smith and Fretwell 1974, Lloyd 1987). Large offspring often have a higher survival probability than smaller offspring (reviewed by Roff 1992), and theory predicts that optimal offspring size increases with de- creasing resource abundance or quality (Sibly and Calow 1983, Parker and Begon 1986, Winemiller and Rose 1993, Hutchings 1997). This prediction rests on the assumption that larger offspring are less affected by scarcity of food than smaller offspring. As a specific case, models predict that increasing the intensity of sibling competition select for the production of smaller clutches with larger eggs (Parker and Begon 1986). Strong non-sib competition will select even stronger for larger eggs. An increase in selective predation on smaller offspring should give the same result. If off- spring fitness does not depend on offspring size (above a minimum physiological size), then fecundity should be maximised (Hutchings 1997). Salmonid fishes are excellent study subjects for test- ing predictions from evolutionary theory. These fishes often form reproductively isolated populations across a diversity of environments and exhibit high levels of Accepted 6 September 2002 Copyright © OIKOS 2003 ISSN 0030-1299 OIKOS 100:3 (2003) 483