(Madagascar) Testing the “depth refuge hypothesis” for Octopus cyanea

The Vezo in Southwest Madagascar have established the first community managed marine protected area in the Western Indian Ocean Region. Various management techniques have been initiated over the past 6 years, including seasonal, rotating closures of O. cyanea fishing areas. The closed areas constitute a small portion of the total fishing area, and a very small portion of the total O. cyanea habitat. Closures are timed to coincide with what are thought to be important growth seasons (female fecundity, juvenile growth spurt).

The working hypothesis has been that the season of the closure is a crucial factor to: (i) boosting the O. cyanea harvest by allowing the outgrowth of newly-recruited individuals; and (ii) aiding the overall sustainability of the O. cyanea population by allowing females resident in the closed areas to breed. An alternate hypothesis is that the season is unimportant to one or both.

The “depth refuge hypothesis” posits that while the O. cyanea closure regime allows for the rapid growth of newly settled individuals in the closed areas, the population present in the shallow/inter-tidal closed areas is incidental to the larger O. cyanea population’s sustainability because the closed areas represent not only a small portion of total O. cyanea habitat but are routinely fished between closures leaving very few large, reproductive females.

If true, this hypothesis would imply a very different approach to reserve design (to maximize outgrowth), and highlight the importance of protecting the currently un- or under-fished sub-tidal populations. We should both test the hypothesis and develop conservation strategies with the potential in mind.

Fieldwork in Madagascar required.

i. Sub-tidal surveys -- SCUBA surveys for O. cyanea density (and size/gonadal maturity if  possible) at various depths including in inter-tidal closed and open areas. The student would undertake depth surveys to determine population density at various sites across the Velondriake MPA. He/she would have to apply/develop appropriate survey techniques (likely using traplines). The student should have a background in biology, dive certification, and experience doing scientific dives. She/he would be located in Andavadoaka.

No fieldwork in Madagascar required.

ii. Size structure of inter-tidal vs. sub-tidal populations from landings data (mask & snorkel vs. gleaning). With the newly clean data set, it should be straight-forward to test whether the size structure of the sub-tidal population differs from the inter-tidal. The student should then interpret their findings. Given the dataset, could be done anywhere.