Hawaii-based Longline Fishery Catches Younger Age Groups of Striped Marlin

Reliable stock assessments require sound information on biological parameters such as the age-composition of the catch. Research at PIFSC is providing new information on the ages off striped marlin caught by Hawaii longline vessels. The study is a team effort by the principal investigator Dr. R. Keller Kopf of Charles Sturt University, Australia, and scientists in the PIFSC Fisheries Research and Monitoring Division. Support for the work has been provided by the NOAA-funded Pelagic Fisheries Research Program of the University of Hawaii Joint Institute for Marine and Atmospheric Research.

The research team determined ages of striped marlin caught by Hawaii-based longliners and sampled by observers deployed on the vessels by the NOAA Fisheries Pacific Islands Regional Office. Marlin heads and dorsal fins collected by the observers were dissected at the PIFSC Aiea Heights Research Facility to extract the bony tissues ("hard parts") used for age determination. These include sagittal otoliths from the head and 4th and 5th dorsal fin spines from dorsal fin. After they were cleaned, the hard part samples were shipped to Dr. Kopf where they were sectioned and aged based on standard methods developed during a prior age-and-growth study of striped marlin harvested in the east Australian longline fishery. One aspect of the methodology is to corroborate the 1st and 2nd annual growth bands in dorsal spine sections using ages determined by analyzing otoliths from the same fish. Since dorsal fin spine sections typically reveal several growth marks formed during the fish's first year of life, the otolith-derived ages help to determine which mark is the first true annular mark (indicating the first year of growth). The Hawaii catches have provided very small striped marlin (weights of 6-10 kg) for study and otolith ageing of these specimens has helped confirm the early rapid growth of this species.

Striped marlin exploited by the Hawaii longline fleet attain a size of ~103 cm EFL (eye-to-fork length) in their first 6 months and 135 cm EFL at an age of 1 year. Based on a range of about 85-185 cm EFL for the samples studied, preliminary results indicate that 3 or 4 year-classes of striped marlin are contributing to the catch of the Hawaii-based commercial longline fishery. In comparison, the age-and-growth study in eastern Australia by Dr. Kopf revealed ages up to 8 years for the those striped marlin and the presence of primarily large fish; only a small proportion of striped marlin in the eastern Australian catch are as small as the larger fish in the Hawaii fishery (<185 cm EFL). Our preliminary results indicate that the region in which the Hawaii-based longline fishery operates is home to predominantly smaller, younger striped marlin. Presumably larger and older striped marlin leave the Hawaii fishing grounds well before they reach maximum size. Whether these fish move into the western Pacific or to other region(s) is unknown. This collaborative age-and-growth study is planned for completion by the end of 2012.

Hard parts from the head and dorsal fin of striped marlin provide information on the fishi's age and growth rate. (A) Tiny 
               sagittal otoliths are extracted from the fish's head; (B) Spines are dissected from the fish's anterior first dorsal fin 
               and the 4th and 5th spines are used for ageing.
Hard parts from the head and dorsal fin of striped marlin provide information on the fishi's age and growth rate. (A) Tiny sagittal otoliths are extracted from the fish's head; (B) Spines are dissected from the fish's anterior first dorsal fin and the 4th and 5th spines are used for ageing.