ENVIRONMENTALLY-INDUCED SPATIOTEMPORAL VARIATIONS IN THE FECUNDITY OFBROWN TROUT SALMO-TRUTTA L - TRADE-OFFS BETWEEN EGG SIZE AND NUMBER

1. Resident brown trout Salmo trutta in the Esva River basin (north Sp ain) live in a patchy environment with tracts of riparian forest or me adow along stream banks. This study assessed whether the reproductive traits of brown trout from four contrasting sites reflected site-speci fic factors. 2. Length at maturity (10.5-11 cm of 1 + individuals) was the same in the four sites examined but slowest growers in slow-growi ng sub-populations delayed maturity for 1 year relative to fast-growin g fish. The analysis of monthly variations in egg size and number sugg est that two 'decisions' in two consecutive years are required to comp lete spawning. The first concerns the number of eggs, determined when trout are still 0 +, and the second concerns egg size. 3. At three sit es, egg size and number did not differ significantly between years but highly significant interannual variations were apparent at another si te. Fish length was the major determinant of egg size and number at al l sites but for any given length, brown trout at sites where the fish exhibited higher growth rates spawned more, but smaller, eggs than tho se at slow-growing sites. This spatial pattern was identical to the te mporal pattern exhibited by trout at another site. The combination of temporal (year-to-year) and spatial (between rivers) variations in egg size and number showed a significant negative correlation, supporting the operation of a trade-off between these two traits. 4. The trade-o ff between egg size and number seems to be determined by site-specific factors, with slow-growing trout at sites which are fully covered by canopy spawning fewer, but larger, eggs than fast-growers in unshaded sites.