Student Connection

Just in from the Ship

May 31, 2000  We are now into our second day of work after arriving here off the Kona coast of the Island of Hawaii at sunrise on May 28th. We are currently conducting daylight surface net tows some 2-6 nautical miles offshore of the Kona coastline which is on the lee (calm) side of this island. The Kona coast is well known in sportfishing circles for it's marlin and other billfish that are caught just offshore.

We are here in Kona to collect specimens of swordfish larvae as part of a greater research study on swordfish biology that is currently underway at our National
Marine Fisheries Service, Honolulu Laboratory. Swordfish larvae collected during this trip will be used in genetics studies of the swordfish population around the Hawaiian Archipelago. We would like to determine whether the larvae born in Hawaiian waters are genetically similar to the adult swordfish captured by Hawaii-based longline fishermen in the fishery north
of Hawaii. If swordfish in one area have little tendency to breed with swordfish from another, over time genetic differences (as exhibited within their DNA) develop that may serve to distinguish one breeding group from another separate breeding group. Knowledge of these differences and whether the adult swordfish in a fishery consist of one or several different groups can be important for managing these fisheries.

In the North Pacific, fishing for adult swordfish occurs
typically in the colder northern waters while spawning occurs only in the warm surface waters of the sub-tropics and tropics. As young, swordfish develop and grow in these warm surface waters and after some 2 years of age begin to move into the cooler, northern waters where larger, energy rich squid and other prey appears to be more abundant.

Other information that these larval swordfish specimens can yield involves an analysis of their otoliths (earbones). Based on electron and light microscopy, counts of the presumed daily growth rings embedded within these otoliths can provide information on date spawned, age (in days), and growth rate of these larvae.

The capture of swordfish larvae is difficult since few specimens if any are typically captured during a single surface tow. During previous cruises, catches of at least one swordfish larvae have been made when nets were towed through a surface slick, a trail of smooth water across the sea surface that stands out against the background of rippled water. These surface slicks are
typically seen off the Kona coast and sometimes contain a line of floating debris in which numerous young fishes can be found. Some of our highest catches of swordfish
larvae have been achieved by towing through these debris slicks.

The type of net tow gear that we use to capture swordfish larvae is called an Issacs-Kidd midwater trawl or IKMT. This type of trawl was meant
to collect organisms deep beneath the surface but for our purposes, we have found that the IKMT makes an excellent surface towing device. The IKMT is towed along the starboard (right) side of our ship for one hour at a time before we retrieve the net and collect the sample from the container at the back of the net called the "cod end." Once the cod end is removed, the sample is poured into a plastic tray and sorted. Swordfish larvae are measured and then placed individually in plastic vials and stored frozen. We have also tested another surface net called the 1-meter ring net. This net has a smaller mouth opening and smaller net mesh and should be able to capture swordfish larvae of a size smaller than the IKMT can collect.

By mid-afternoon on May 29th, we have captured a total of 20 swordfish larvae. These larvae range in total length (from tip of bill to tail) from 3/4 of an inch to 3 1/2 inches long. Most of the swordfish larvae caught have been around an inch in total length. We rarely catch swordfish larger than 3 inches as these fish can probably avoid the oncoming net by diving below or to the side of the on-coming net. We tow the net at about 3.5 knots speed which is about equivalent to a slightly slower than average walking speed. We do not tow the net faster since increasing the speed would probably start to mutilate the larvae from the addition water flow
pressure. Our last day of tows for swordfish larvae will be on Tuesday, May 30th and afterwards we will depart the Kona coast to return to our home port at Snug Harbor, Honolulu.

Last modified May 24,  2000