Sunday, May 20, 2012

Hawaiian Reef Sharks: possible competition for food causing massive decline

For reef sharks, commercial shark fishing isn't the only thing that threatens their survival.  In reef communities near populated islands, an additional threat comes from the taking of fish by local fishermen - fish that often constitute a major portion of the sharks' diets.  When local and/or commercial fishermen compete for the same food source as reef shark species, it can be a crippling blow to the shark population.

A recent study by Hawaii's Joint Institute for Marine and Atmospheric Research showed a drastic reduction in reef shark populations around populated islands in Hawaii as opposed to more uninhabited islands or pristine reefs.   

"We found 90-97 percent decline in reef shark abundance: white tip, grey, galapagos and nurse sharks," said Marc Nadon, a researcher with the Institute.   

The researchers have not been able to determine a more specific cause but look to accidental bycatch (sharks are now more protected, at least from legal commercial shark fishing, due to recent legislation) and overall fishing pressure as contributing factors.

"70 percent of reef shark diet is reef fish, so if you remove the food source it would be logical that reef shark would follow the same trend and decline," said Nadon. 

While the researchers will be doing more studies this fall, their research's concern with competition for food has support based on what has been observed in other island nations.  Both Samoa and the Marianas have seen major declines in reef shark populations around populated islands compared to other unspoiled reefs.

5/21/12 - As a follow-up, I received some information from Dr. Chris Lowe, director of the Shark Lab at the California State University at Long Beach.  He has some concerns regarding the accuracy of the researcher's claims based on the reliability of the study's methods.  We'll have to wait and see what future studies produce.  Here are Chris' comments:

"Don't buy the 97% decline. There are some methodology problems here. I don't dispute that there are more sharks in the Northwestern Hawaiian Islands, but not by 97%. These numbers are based on towboard diver surveys and anyone who has dived in a remote location knows that a diver is a novel things - sharks come flocking to you. However, in places like the main Hawaiian Isl. sharks tend to avoid divers (spearfishermen) - who shoot at them when they approach their fish. We've set 100s of short longlines all around the most populated island (Oahu) to catch sharks to tag and have caught a lot of sharks even though we rarely saw them diving in those areas. This suggests there are behavioral responses of sharks to divers in these areas that make it likely to overestimate sharks in remote locations, while underestimating them in populated locations. In addition, diver surveys are limited to 100', but we know that reef sharks are deeper around the Main Hawaiian Isl. and this is probably because there is more food at those depths. This depth shift could be due to human depletion of reef fishes, but it doesn't mean there are 97% fewer sharks, they are just outside their survey area."


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