Sex steroid hormones and behavior reveal seasonal reproduction in a resident fin whale population.

Fin whales in the Gulf of California constitute a resident population genetically isolated from the rest of the North Pacific Ocean. Its small population size and the scarce information available about its dynamics in a semi-enclosed sea underline the importance of conducting studies about its reproduction. Given the monsoonal regime that dominates the oceanographic habitat of this region, we hypothesized seasonality in the population's reproductive activity. To test this, we validated and assayed testosterone and progesterone from blubber biopsies of free-ranging individuals. Lactating females exhibited low progesterone concentrations, whereas a group of females of unknown reproductive stage, but with extremely high progesterone concentrations, showed strong evidence of separation and were considered to be likely ovulating or pregnant. A seasonal model of testosterone concentrations showed a high peak during the late summer. This trend was supported by the first documentation of courtship events and by the recording of a female with high progesterone concentration during summer and re-sighted with a calf 1 year later. Therefore, the breeding in this resident population would be seasonal, as it is in migratory baleen whales, but occurring during the summer/autumn, which is the least productive season in the Gulf of California. Our study represents an important input to assist in future management policies of this protected population.