Warmer waters mean fewer fish

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In a probable sign of things to come, scientists have revealed that the unseasonably warm temperatures caused by El Niño events exert profound effects on reef fish populations in the South Pacific.

The study, conducted by Alain Lo-Yat and coauthors and to be published in a forthcoming issue of the journal Global Change Biology, was conducted on the atoll of Rangiroa in French Polynesia. 

Over a period of four years (January 1996–March 2000), the authors studied the recruitment of larval fishes to the reef, comparing their data with remote sensing data of sea surface temperature (SST) anomalies, surface current flow and chlorophyll-a (Chl-a) concentrations.

This period of study included an intense El Niño event (April 1997–May 1998) between two periods of La Niña conditions (January–March 1996 and August 1998–March 2000).

Many coral reef fishes are broadcast spawners, and will scatter their eggs far and wide.  The larvae usually develop in open waters, before making their way back to the reef.  The authors examined the recruitment of Rangiroa Atoll by collecting larval fishes using crest nets, which are fine-meshed nets that resemble rectangular windsocks.

Based on their catches, the authors recorded a substantial decline (up to 51%) of larval recruitment during the El Niño event, when SSTs increased by up to 3.5°C.  At the same time, they noted that Chl-a concentrations were concurrently at their lowest (implying that the primary productivity of the reef was greatly depressed). 

The authors hypothesise that the combination of increasing SST and declining productivity may have caused more fish larvae to die from starvation (because the larvae are poikilothermic, increasing temperatures would increase their metabolic rate and require them to eat more to survive), resulting in lower rates of larval supply to reefs. 

At the same time, it is also possible that the increased SSTs depressed the breeding of the adult fish; this lowered spawning output would further compound the effect.

The authors conclude that extreme climatic events may, intermittently, have devastating effects on fish populations, with complete cessation of the replenishment of populations. 

Co-author Stephen Simpson warned: "Coral reef fisheries provide food and livelihoods for hundreds of millions of people throughout the world and underpin a multi-billion dollar tourism industry. Our study shows that warmer waters may leave fish stocks on reefs in serious trouble, which will have far-reaching consequences for the people around the globe who are dependent upon them."

For more information, see the paper: Lo-Yat, A, SD Simpson, M Meekan, D Lecchini, E Martinez and R Galzin (2011) Extreme climatic events reduce ocean productivity and larval supply in a tropical reef ecosystem. Global Change Biology DOI: 10.1111/j.1365-2486.2010.02355.x