Life history shifts in an exploited African fish following invasion by a castrating parasite

Evolutionary theory predicts that infection by a parasite that reduces
future host survival or fecundity should select for increased investment
in current reproduction. In this study we use the
cestode Ligula intestinalis and its intermediate fish
host Engraulicypris sardella in Wissman Bay,
Lake Nyasa (Tanzania) as a model system. Using
data about infection of E. sardella fish hosts
by L. intestinalis collected for a period of 10 years,
we explored whether parasite infection affects the fecundity of the fish
host E. sardella, and whether host reproductive investment has
increased at the expense of somatic growth. We found that L.
intestinalis had a strong negative effect on the fecundity of its
intermediate fish host. For the non-infected fish we observed an increase
in relative gonadal weight at maturity over the study period, while size
at maturity decreased. These findings suggest that the life history
of E. sardella has been shifting towards earlier
reproduction. Further studies are warranted to assess whether these
changes reflect plastic or evolutionary responses. We also discuss the
interaction between parasite and fishery-mediated selection as a possible
explanation for the decline of E. sardella stock in the
lake.