The Reproductive Biology of Siscowet and Lean Lake Trout in Southern Lake Superior

Abstract

AbstractLean and siscowet morphotypes of lake trout Salvelinus namaycush in Lake Superior are thought to be genetically separate, but the reproductive isolating mechanism is unknown. The testicular and ovarian cycles and reproductive hormone levels of these morphotypes were determined from May to October in populations east and west of the Keweenaw Peninsula in southern Lake Superior. The gonadosomatic index (GSI) increased from August to October for lean and siscowet males and females east of the Keweenaw Peninsula and for siscowets west of the Keweenaw Peninsula. Circulating estradiol‐17β (E2) levels and ovarian GSIs increased simultaneously in females of both morphotypes. However, circulating 11‐ketotestosterone (11‐KT) levels in lean and siscowet males were not significantly elevated until October even though testicular GSIs increased by August. Transcripts of the follicle stimulating hormone (FSH) beta subunit (an indirect measure of FSH activity) increased in lean and siscowet males and females during August and September, when GSIs were increasing for both morphotypes. The seasonal changes in GSIs and hormone levels indicate that both lean and siscowet individuals in southern Lake Superior populations undergo reproductive maturation at the same time in the fall; therefore, reproductive timing does not appear to genetically isolate the morphotypes in these populations. A proportion of the sampled females (lean lake trout: 54%; siscowets: 42%) exhibited no increase in GSI from August to October, strongly suggesting that in any given year some proportion of the population does not reproduce. This was also observed in males but at a lower percentage (19–20%). Fish that did not have maturing gonads from August to October also had lower E2 and 11‐KT levels than maturing fish. Fecundity measured for lean and siscowet lake trout was not statistically different and was similar to historical values.