Age and growth of black marlin, Makaira indica, in east coast Australian waters

Abstract

Sagittae, anal and dorsal fin spines were collected from black marlin, Makaira indica, in eastern Australian waters to evaluate and interpret age information from these structures. Presumed annual marks in sections of the third anal and third dorsal spines from 69 fish ranging in size from 8.5 to 451 kg were examined. Sectioned otoliths were also examined for presumed daily microincrements using both light and scanning electron microscopy. Spine radius and sagitta weight each varied predictably with the length and weight of the fish. The time period between consecutive growth marks was checked against the growth of these hardparts recovered from a tetracycline-injected and recaptured fish at large for 6 months. The increase in the size of fin spines from the recaptured fish was consistent with calculated growth curves derived from counts of presumed annual zones. Lengths and weights at age, based on these bands, are presented for black marlin. External ridges were observed on the edge and ventral surface of the rostrum of the sagitta, but they were inconsistently clear to be reliably counted. Sagittae displayed an internal record of microincremental growth on a scale that suggested a daily periodicity. Confident and consistent enumeration of these increments was restricted to fish <40 kg (~1600 mm lower jaw–fork length). A daily cycle of increment deposition was not confirmed by a count from the recaptured fish that fell short of the 198 days at large. The discrepancy was most likely associated with the difficulties of enumerating microincrements. The ages assigned to black marlin in this study support ages suggested from analysis of length–frequency data on juvenile fish. They underline the restraints previously recognized in decoding sagittae of blue marlin and confirm the utility of fin spines in providing age information for istiophorids.