Monsoonal wet season influences the migration tendency of a catadromous fish (barramundi Lates calcarifer)

Many animals exhibit partial migration, which occurs when populations
contain coexisting contingents of migratory and resident individuals. This
individual-level variation in migration behaviour may drive differences in
growth, age at maturity, and survival. Therefore, partial migration is
widely considered to play a key role in shaping population demography.
Otolith chemistry and microstructural analysis were used to identify the
environmental and individual-specific factors that influence migratory
behaviour in the facultatively catadromous barramundi (Lates calcarifer)
at two distinct life history stages: firstly, as juveniles migrating
upstream into fresh water; and secondly, as adults or sub-adults returning
to the estuarine/marine spawning habitat. Monsoonal climate played an
important role in determining the migration propensity of juveniles:
individuals born in the driest year examined (weak monsoon) were more than
twice as likely to undergo migration to freshwater than those born in the
wettest (strong monsoon) year. In contrast, the ontogenetic timing of
return migrations to the estuary by adults and sub-adults was highly
variable and not strongly associated with the environmental parameters
examined. We propose that scarce resources within saline natal habitats
during lower rainfall years may provide an ecological incentive for
juveniles to migrate upstream, whereas more abundant resources in higher
rainfall years may promote resident life histories within estuaries. We
conclude that inter-annual climatic variation, here evidenced by monsoonal
strength, likely plays an important role in driving the persistence of
diversified life-histories within wild barramundi populations.