Are sea trout mainly female fish?

Yes, that is normally the case.  However, male sea trout are almost as common as females in rivers where poorer environmental conditions in fresh water appear to cause most of the young fish to go to sea.  The broad picture is that females can produce both bigger and greater numbers of eggs by migrating to better feeding environments, in the case of sea trout thus enhancing the competitive value of the anadromous trait. Conversely, body size probably is less important for males, as even small males can fertilise all of the eggs of a large female.  Large, apparently physically dominant, male sea trout of similar size to the females may accompany the females, but smaller male sea trout and especially male brown trout, which are usually present nearby, often take part in the spawning acts.  This complex relationship between the sexes at spawning time is similar to the well-known participation of ripe male parr in the reproduction of Atlantic salmon. Cross-breeding of sea trout with brown trout probably helps to maintain a broad diversity of resident and migratory tactics among their progeny, leading to optimal utilisation of suitable freshwater and marine habitats.   However, even pure matings of male and female sea trout have been shown to produce juveniles that differentiate into different forms of freshwater-resident brown trout and sea trout.