Roundscale Spearfish Head Morphology

The morphometric characteristics that separate white marlin Kajikia albidus from roundscale spearfish Tetrapturus georgii can be difficult for non-experts to distinguish, particularly when the animals are in the water. Using the measurements of the lower jaw tip to the longest branchiostegal ray (LJBR) and the opercle (LJOP), a discriminant function is presented that will help field observers separate the two species. The function DR-W = LJBR (3.31861) + LJOP (-2.84856) – 6.37198 classified specimens as roundscale spearfish when DR-W was > 0 and as white marlin when DR-W < 0 with a 95% correct classification rate overall. Since only the head of a specimen needs to be closely observed, the method has particular utility for tournament anglers during the hook removal process.

Data were collected as part of the POP

Beerkircher and Serafy 2011. Using head measurements to distinguish white marlin Kajikia albida from roundscale spearfish Tetrapturus georgii in the western North Atlantic. Bull. Mar. Sci. 87(1):147-153

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Unknown. Likely only Representative of the white marlin and RSSP captured at these times and places by PLL vessels.

Nothing is known about the degree of agreement between individual measurements in the POPOL data set and the true measurements, as the observer collecting the data is the sole source of the information and it cannot be cross checked with alternate sources.

See above. The accuracy of analytical results to the “true” values cannot be determined as the true values cannot be calculated without 100 % observer coverage and multiple observers on each vessel. Neither situation has ever occurred during collection of POPOL data.

No limits. The species size range captured on board pelagic longline vessels can be easily measured by observers with standard hand measuring devices.

There have been no quantitative studies to detect potential bias in POPOL.

There are no known comparable data sets .

Only dead specimens could be measured; observations of live white marlin or roundscale spearfish did not include the complete detailed measurements.

A hand centimeter measuring tool, collecting data to the nearest centimeter, can be expected to be very precise.

No information available.

No information available.

No information available.

See above, specimen needed to be dead and brought on board by the crew in order for the observer to take the complete data needed.