From data to parameters: a preliminary DEB model for loggerhead turtles (Caretta caretta)

We develop a physiologically structured population model for Pacific bluefin tuna (Thunnus orientalis, PBT) by combining approaches that operate at two levels of biological organization. At the level of an individual, we rely on a Dynamic Energy Budget (DEB) representation of physiological energetics to describe metabolic processes regulating food intake, growth, maturation, and reproduction of PBT. Consequently, the modeled fish state (i.e. reserve energy, structural length, and the maturation/ reproduction investment) depends on the environmental conditions (i.e. food availability and temperature), allowing us to break free from the constraints of statistical (i.e. static) length-at-age and fecundity-at-age relationships often used as the underlying assumptions of population dynamics. At the population level, a matrix formulation based on the DEB model offers a relatively straightforward way to determine how fecundity and survival rates regulate population growth. The former rate is read off the fecundity-at-age relationship generated at the individual level, whereas the latter rate follows from natural and fishing mortalities found in the latest PBT stock assessment. We apply the model to estimate the uncertainty in the growth rate of the wild PBT population caused by the unclear feeding ecology of this species during spawning migrations. In particular, we show that if PBT experience reduced food availability while migrating, the effect on the growth rate of an individual fish may be negligible, whereas the fecundity rate may suffer a 40% decline. Such a decline has a negative effect on the population growth rate, which – for the purpose of robust fishery management - has to be compensated for by reducing fishing effort. We find that a reduction across all age groups of about 17 % relative to the present-day (2007-2009) harvesting is needed. A somewhat counterintuitive consequence is that the total catch in terms of biomass may actually increase upon implementing the reduction due to, among other factors, the dominance of larger fish in the catch