Dispersal of adult Black Marlin (Istiompax indica) from a Great Barrier Reef spawning aggregation

Adult and juvenile black marlin (Istiompax indica) were obtained between 1987 and 1991 from the recreational fishery operating along the Queensland coast, eastern Australia. Thirty males between 8.5 kg and 83.5 kg and 26 females (11.6-64 kg) were sourced from the nearshore fishing grounds off Cape Moreton, Dunk Island and Cape Bowling Green. Twenty five large females (295-525 kg) and 3 males (59-158 kg) were secured from the Lizard Island Game Fish Club's annual October tournaments off the Ribbon reefs.Date of capture, weight (kg), lower jaw-fork length (LJFL cm) and gonad weights (gms) were recorded for each specimen. Gonad tissue was routinely taken from the mid-section of a gonad, randomly left or right, and placed directly into 10% neutral buffered formalin solution. Histological sections were prepared and a software driven computer dissecting microscope was utilized for processing the histological sections to recover imagery, oocyte counts and measurements. Larval billfish sampling took place over several days in October and November 1988 and 1989 in the vicinity of the recreational fishing fleet supplying adult specimens for the reproductive component of this study. Sampling took place along the extent of embayments at First Corner and Second Corner on No. 10 Ribbon Reef and in a southerly facing embayment on Yonge Reef. The contents of the cod end of the net from each tow were preserved in 70% ethanol. Fish larvae were separated and retained from each sample for later identification of the billfish larvae. Measurement software and calibrated imagery were used to provide standard lengths for each larva to 0.1 mm following rehydration. Six larvae identified as black marlin based on visual characters were submitted to DNA analysis (400 bases of the mitochondrial DNA control region compared to known black marlin sequences). Larval black marlin were aged based on published age-length relationships and growth models for larval blue marlin. The smallest istiophorid larva recorded in this study was 1.7 mm and this value was used as size at hatching. An instantaneous growth rate of 0.0976 enabled calculated age estimates to be assigned to each larva. Between 2002 and 2009, a total of 67 Popup satellite archival transmitting (PSAT) tags were deployed on black marlin captured via rod-and-reel from recreational fishing boats fishing the GBR between Cairns and the Ribbon Reefs. Tags were deployed between September and December on fish assessed to be in good health after capture. Sea surface temperature (SST) records from the PSAT tags were used to determine the mean temperature experienced by black marlin within the study area. Daily SST values were obtained for each tagged fish remaining in the study area by accessing the PSAT transmitted temperature/depth data.To compliment the PSAT tag results, data on black marlin from the east coast of Australia, tagged using conventional tags were extracted from the tagging database owned and maintained by Industry and Investment NSW.
 An integrated study of black marlin (Istiompax indica) reproductive biology, larval sampling and adult black marlin tagging was undertaken to provide evidence of spawning and to document the geographic range of the black marlin stock that frequents the GBR.

Organization: Australian Ocean Data Network