How seasonal temperature variations may influence the structure of annual squid populations

Previous laboratory studies have established that temperature is highly influential in affecting both the growth and life span of Cephalopoda. A major unresolved question is how such sensitivity to environmental factors may determine population structure. Extending a hypothesis first advanced by Forsythe (1993, in: Recent Advances in Fisheries Biology (T. Okutani, R. K. O'Dor, & T. Kubdera, eds.), Tokyo, Tokai University Press), we formulate a simple individual-level model which incorporates a two-stage growth response for squid hatchlings exposed to continuous seasonal fluctuations of temperature. We show that in seasonally fluctuating temperatures such a growth ‘duality’ can directly affect the size distribution of squid over the year and we demonstrate how in seasonally warming waters this mechanism may lead to younger individuals surpassing the size of older individuals. By postulating that development status can be inferred directly from size, we go on to propose a circle-map model of the whole squid life cycle. Numerical investigations into the iterative dynamics support the hypothesis that the progeny from such a population can rapidly separate to a low number of tightly synchronized cohorts within a few generations.