By Charlie Gough, Marine Research Coordinator, Madagascar
Fishing affects all levels of biological organisation, from the individual to populations, affecting their demographic and genetic characteristics as well as communities and ecosystems. Among the many effects of fishing, those referring to the marine ecosystem and its functioning, trophic structure and energy flow are often thought to be some of the most important.
Trophic levels within global catches have declined significantly by 0.5 trophic levels in 50 years, a phenomenon referred to as “fishing down marine food webs”. But how does this play out on the reef and what we are seeing here in the reef fish populations of Maintirano?
It has been demonstrated that on reefs, such as Kingman reef in the Line Islands, south of Hawaii, and Palmyra atoll of the central Pacific that have not been subjected to fishing pressures, predatory fish species dominate the ecosystem accounting for up to 85% of fish biomass, a vastly different picture to that observed during marine surveys in Madagascar or the wider Western Indian Ocean where carnivorous fish species only account for less than 20%. The lower abundance and biomass of these species recorded in this area is most likely due to existence of fishers and the high level of fishing pressure that is exerted on these reefs, not from the monstrous industrial fisheries that have undermined fisheries elsewhere in the developed world but through the sheer number of traditional fishermen that now survive on these shores.
The reefs here in western Madagascar are following in the footsteps of many others that have been exploited by man. The human preference (in the majority) is for large bodied piscivorous fish species such as Grouper, Emperor and Snapper ultimately reducing their numbers on the reefs and subsequently having repercussions on the food web of the ecosystem.
As populations of large predators decrease, their prey species increase, as this happens a domino effect of predator and prey species continues with increasing and decreasing populations along the food chain, ultimately affecting all species and undermining the biological structure and functioning of the reef.
Fishermen are simply another link in this food chain, but as their prey species becomes scarce and harder to catch they adapt; changing their gear types, finding more efficient ways to catch the fish or even fishing in new locations and starting to prey on other species that are perhaps less marketable but more abundant, so despite a lower market value the sheer volume in which they can be caught makes up for the loss. Eventually these species will meet the same fate and the fishers will have to adapt again.
What we will see on the reefs of Maintirano is how fisherman populations that have migrated vast distances to escape the reduced catches in their home villages are having the same effect on fish biomass in the Barren Islands. In particular we will be interested to see how trophic levels of fish observed on the reefs here compare to pristine un-fished reefs in other places. These reefs may still only be slightly affected but they need protection, not only for the survival of the reef but also to support the Vezo communities that exist here.
My interest in all things marine stems from rock pooling as a child and rapidly tearing through nature books to see what treasures I had found in my little net as the rain hammered down on our typical family holidays to Devon. Now I have graduated from a little net on the beach to diving in the Mozambique Channel on some of the world’s remotest coral reefs, documenting the health and diversity of both the corals and the inquisitive fish that exist beneath the surface.
Having worked with Blue Ventures since 2007 I have had a lot of luck and many great opportunities to explore the coastal waters of Madagascar and have lead a number of research expeditions up and down the western coastline working along the way with local fishermen as well as national and international NGOs. My interests lie at the interface between man and the sea and how each can affect the other.
I am excited to be part of this expedition to the Barren islands, and am interested to see the impacts that people here have had on the diversity, abundance and ecological functioning of the coral reefs and their fish and invertebrate populations, in the short time that they have been exploiting them. It will also be interesting to see how the fishers themselves perceive the impacts of fishing on the resources on which they depend.This research cruise to the Barren islands will be the first time that anyone has systematically explored and documented the status of the reefs of this region, and I’m looking forward to keeping you all updated with the characters, creatures and stories great and small that we encounter on our route!
You can find out more about my work with Blue Ventures here