Scientists on the NOAA ship Oscar Elton Sette are surveying surface waters of the Kona coast of the Island of Hawaii to assess billfish spawning

April 24, 2006
A plankton net, known as an Isaacs-Kidd trawl, being towed across the surface of the water to collect billfish eggs and larvae off the Kona coast

The NOAA Ship Oscar Elton Sette is engaged in an 8-day research cruise to calm, leeward waters off the Kona coast of the Island of Hawaii to assess spawning by billfish. The Kona coast, on Hawaii's western side, is renowned among rod-and-reel sportsfishers as a hotspot for billfishes. Six species of billfishes live there; however, only blue marlin, striped marlin, and shortbill spearfish are typically caught by sportsfishers. Swordfish are caught seasonally in offshore Hawaiian waters by commercial fishers while sailfish and black marlin are caught rarely.

The research cruise is under the direction of Chief Scientist Robert Humphreys of the Fisheries Biology and Stock Assessment Division, NOAA Pacific Islands Fisheries Science Center, headquartered in Honolulu. Humphreys is leading a team of scientists interested in learning more about the little-known spawning activity of billfishes. But instead of pursuing the magnificent adult billfishes like sportsfishers do, the scientists will tow a net through the surface waters to collect the minute eggs and larvae produced by spawning adults. By capturing eggs and larvae, scientists can learn more about which billfish species are spawning in the area, observe when and where the eggs and larvae occur, and determine if spawning is associated with certain measurable environmental conditions, such as sea surface temperature and salinity.

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Blue marlin, swordfish, and shortbill spearfish larvae (from top to bottom) collected in a surface Isaacs-Kidd plankton tow

Collection and identification of billfish eggs and larvae present many challenges. One of the early obstacles scientists had to overcome during previous cruises was difficulty in identifying the species of larvae caught in the net tows. And since only swordfish eggs had been previously described in the scientific literature, PIFSC biologists didn't know exactly what the eggs of the other billfish species would look like. Fortunately, biologists working in other parts of the Pacific Ocean were tackling the same problems. Collaborating scientists from the Scripps Institution of Oceanography and the Southwest Fisheries Science Center (both in La Jolla, California) developed a reliable identification technique for billfish. The method relies on the fact that billfish species differ in their DNA, so the identity of eggs and larvae can be determined by conducting a DNA assay. Until recently, the technique had only been applied in land-based laboratories. It was modified especially to be used at sea onboard the Sette.

In conducting the DNA assay at sea, a single eyeball from each larva, or an entire egg, is processed and species identification is established in about 3 hours. The DNA-based identification is rapid enough to enable scientists to return to a site where tows earlier in the day yielded verified billfish larvae and eggs and collect additional samples of these relatively rare billfish specimens.

Results of such analyses are helping PIFSC scientists understand the spawning patterns of each billfish species and the environmental conditions that promote or retard spawning activity. Such knowledge may be used to identify other locations in the Pacific where particular species of billfish are likely to spawn.