Sperm characteristics in perch Perca fluviatilis L

Sperm production is a crucial factor for the potential reproductive success of males in many species (Nakatsuru & Kramer, 1982). Especially under strong sperm competition, higher sperm numbers in the ejaculates are likely to increase a male’s chance of fertilization (Parker et al., 1996). But sperm production is limited by physiological constraints, e.g. by testis size, which often depends on the body size of a male (Schärer & Robertson, 1999). In many fish species, large males produce a multiple of the sperm number of small males (Petersen & Warner, 1998). Moreover, sperm allocation tactics can vary according to the size and status of a male or its amount of available sperm (Kazakov, 1981; Gross, 1996; Taborsky, 1998). Allometries in sperm production and gonad characteristics between small and large males have been described in several species, e.g. small males having relatively larger testes or higher sperm densities in the seminal fluid or differences in hormone production (Gage et al., 1995; Leach & Montgomerie, 2000; Uglem et al., 2002). Sperm flagellum length, motility or longevity can also depend on the size and status of a male (Gomendio & Roldan, 1991; Gage et al., 1995; Stockley et al., 1997; Taborsky, 1998; Scaggiante et al., 1999; Mazzoldi et al., 2000). Small males might at least partially compensate their physical inferiority in sperm competition. Males of the perch Perca fluviatilis L. start developing their testes in August which reach maximum size in October. Testes remain at a similar level (1–2% of body mass) throughout the following months until the beginning of the spawning season (Le Cren, 1951). Testes begin to ripen early in April and show