Ontogeny, season and trade-offs: Vertical distribution of the mesopelagic fishMaurolicus muelleri

To investigate the validity of static optimization models, the vertical distributions of two age groups of Maurolicus muelleri is compared to the optimal annual ontogenetic growth rate: mortality risk trade-offs as determined from generation-time based life-history equations. Calculations indicate that juvenile feeding rate was near the maximum for efficient conversion to growth, and at times constrained by digestion rate. Feeding rate of adults seems not to have been high enough to sustain body mass. Juveniles seem to follow the static ontogenetic trade-off between growth and survival, while adults in winter emphasize feeding far less than predicted from static optimization. Static trade-offs are thus inadequate in predicting their distributions, and models accounting for time- and state-dependencies are required. The difference between the two age groups with respect to following the annual trade-off, is explained by their different feeding-to-fitness functions: the relation between adult feeding and fecundity is continuous, while the relationship between juvenile feeding and generation time is discrete, with a considerable fitness cost for extension of the juvenile period.