Trading with coral reef resilience

Difficult trade-off between parrotfish trade and protecting crucial ecological functions

Reef fish for sale at a fish market in Zanzibar: a new study argues for a better balance between fishing and making sure reef fish can maintain crucial ecological function of grazing algae on coral reefs. Photo: N. Kautsky/Azote

A recent study conducted by centre researchers Matilda Thyresson, Magnus Nyström and Beatrice Crona dives deeper into the drivers behind exploitation of the parrotfish, an important herbivorous coral reef fish.

Published in the latest issue of Coastal Management, their analysis of ten locations in Zanzibar and one on the Tanzanian mainland, shows that parrotfish is both an important source of food for reef dependent people and a provider of a crucial ecological function of grazing algae on coral reefs.

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"Overfishing of parrotfish and other herbivores can have significant impacts on whole reef ecosystems because their removal may allow algae to outcompete corals. This in turn can have staggering implications for future fisheries. However, there has been little research done specifically on the drivers behind the exploitation of parrotfish," says Matilda Thyresson.
Size matters
Thyresson and her colleagues decided to investigate the trade of parrotfish with a special emphasis on the linkages between ecological function, market price, and socio-economic drivers behind their exploitation.
Their analysis of fisheries data in combination with interviews with fishers, hotel staff, traders and auctioneers resulted in three overall findings.
First, parrotfish are an important part of the fish trade in Zanzibar which occurs at all scales, from local consumers to international tourists. Second, size is important as larger fish generate much higher market price. Third, size also determines to which market the parrotfish is sold.
The study also revealed that all sizes of parrotfish are exposed to exploitation, leaving no size-refuge for escaping harvest.
No place to hide
This is bad news for coral reefs because herbivorous fish are not only important for suppressing algae, but also for removing them once they have become abundant.
If you remove such essential functions you risk altering ecosystem processes to the point of no return.

"This shows the importance of coupling ecological knowledge with a better understanding of the socio-economic drivers behind the exploitation of parrotfish, from global market drivers to local livelihoods," Matilda Thyresson continues.

Governance up the commodity chain
The authors believe there is a mismatch between the current minimum legal catchable size of parrotfish (10 cm) in Zanzibar and their ecosystem functions that underpin both reef resilience and associated fisheries.
"At 10 centimetres, many species of parrotfish are still juvenile. We do not advocate a ban on parrotfish fisheries, but to ensure future healthy reefs and sustainable parrotfish fisheries, adjustments in size regulations should be complemented with management of for instance certain areas, seasons or gear," says co-author Magnus Nyström.

As concluded earlier it is also crucial for governance to move up the commodity chain and aim regulatory measures at traders, rather than solely at the fishers.
All in all, the study adds several essential socio-economic aspects to the list of "operational indicators of resilience" needed to predict vulnerability before abrupt phase shifts occur (see previous study on the cornerstone of resilience).

Source: Thyresson, M. Nyström, M. and B. Crona. 2011. Trading with resilience: Parrotfish trade and the exploitation of key-ecosystem processes in coral reefs. Coastal management. 39:4:396-411.



Thyresson, M. Nyström, M. and B. Crona. 2011. Trading with resilience: Parrotfish trade and the exploitation of key-ecosystem processes in coral reefs. Coastal management. 39:4:396-411.

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Matilda Thyresson is a PhD student whose field of study is coral reefs with focus on the relationship between social and ecological systems.

Magnus Nyström's research is focused on the effects from human interventions, such as overfishing (including trade) and pollution on ecosystem functions and processes - and how this impacts on resilience in ecological and social-ecological systems.
Beatrice Crona is an Assistant Professor at the centre with a PhD in Marine Ecotoxicology /Natural Resource Management.


Stockholm Resilience Centre is a collaboration between Stockholm University and the Beijer Institute of Ecological Economics at the Royal Swedish Academy of Sciences

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