Female‐biased mortality rate and sexual size dimorphism of migratory masu salmon, Oncorhynchus masou

Abstract

– We examined whether mortality and growth during the migration phase (the lacustrine phase after smolting) differ between sexes in migratory (lake‐run) masu salmon (Oncorhynchus masou) in a lacustrine population, northern Hokkaido, Japan. The sex ratio of 1+ smolts was more skewed to females than that of the returning adults in three annual cohorts examined, indicating that females have a higher mortality rate during the migration phase. Although there was no difference in smolt length between sexes, female adults were larger in body length than males of the same age: female‐biased sexual size dimorphism (SSD) occurs. We suppose that breeding selection favouring a larger body size for females than for males in this population probably requires that females undertake more intensive foraging behaviour with its greater inherent risks but faster growth rates during the migration phase. It therefore seems to be likely that the selection is the ultimate cause of the female‐biased mortality rate and SSD.

Fluctuating sex ratios, but no sex-biased dispersal, in a promiscuous fish

“…Huse 1998), can favour either male or female dispersal, or tend to cancel each other. For example, the fact that gene flow was the same for both sexes despite a strong preponderance of females amongst smolts suggests that female Atlantic salmon incurred higher migratory mortalities than males, as reported for other salmonids (Holtby and Healey 1990;Spidle et al 1998;Tamate and Maekawa 2004). Indeed, the proportion of females among anadromous breeders was typically lower than it would have been expected from the sex ratio of emigrating smolts, again suggesting that females incurred a higher marine mortality in most years.…”

“…Although little is known about inter-sexual competition in salmonids, sex-specific differences in migratory behaviour have been found at all stages of development (emerging alevins: Nagata and Irvine 1997; emigrating smolts: Tamate and Maekawa 2004;returning adults: Morbey 2000), suggesting that more than one life stage may be sensitive to cues about sex-specific competition. For some populations, early sexual maturation in freshwater has perhaps the greatest potential effect on natal dispersal in anadromous Atlantic salmon, favouring either male or female-mediated gene flow.…”

Sex-biased dispersal and growth in sablefish (Anoplopoma fimbria) in the northeastern Pacific Ocean

“…Male-biased movement has been reported in some fishes, e.g., great white shark, Carcharodon carcharias (Pardini et al 2001), and brook trout, Salvelinus fontinalis (Hutchings and Gerber 2002). By contrast, female-biased movement has been reported in partially migratory salmonids, in which migrants are larger than residents, i.e., migrants move to productive habitats to feed and attain larger body size (Jonsson and Jonsson 1993;Tamate and Maekawa 2004;Koizumi et al 2006). In addition, the movements of female hammerhead shark, Sphyrna lewini, to a pelagic habitat resulted in increased feeding success and rapid growth compared with males (Kimley 1987).…”

Influences of habitat fragmentation by damming on the genetic structure of masu salmon populations in Hokkaido, Japan

“…In the matings, anadromous males compete with each other for females, whereas resident males rely on sneak matings (Yamamoto and Edo 2002). All anadromous individuals are semelparous and range from about 30-70 cm in fork length, while some resident individuals are iteroparous and range from about 7-20 cm in fork length (Kato, 1991;Tamate and Maekawa 2004;S. Kitanishi unpublished data).…”

“…In Shumarinai Lake, masu Fig. 1 Sample locations of masu salmon within the Uryu River system salmon were originally landlocked (Osanai 1962) and also exhibit both life histories (Tamate and Maekawa 2004). A few and infrequent artificial releases of hatchery-reared fish had taken place in Shumarinai Lake.…”