Dr Julian Pepperell
The Black Marlin Phenomenon
Australia is blessed with the best big marlin fishery in the world. After nearly 40 continuous years, this fishery has taught us a lot, but there is still much to be learned.
Sometimes we tend to take things a bit too much for granted. Take the annual appearance of the magnificent black marlin off the Great Barrier Reef, for example. By any standards, this is an extraordinary phenomenon, but over the years, we have tended to become somewhat complacent about it, assuming that it will happen like clockwork again and again and again. And while there have been some slow seasons along the way, fortunately, over a period of nearly 40 years, the marlin have indeed kept appearing on cue. Such continued sustainability is an outstanding result for any fishery, so let’s have a look at the history of this fishery, and see if there are any lessons to be learned.
Soon after the end of the second world war, Japanese longline fleets began their great expansion into unfished waters. By the early 1950s, some vessels were fishing off northeast Australia, and had discovered the fertile grounds of the Coral Sea together with the seasonal aggregation of black marlin. The average annual take of black marlin by the Japanese fleet from then until 1981 (when they were excluded from the region) was about 2,000 fish, although the catch was as high as 14,000 in the mid 1960s. By that time, the recreational fishery was just beginning. This really took off in 1966 with the landmark capture of a 1,064 pound marlin in 1966 by Richard Obach, fishing from George Bransford’s ‘Sea Baby’. This single fish put Cairns on the map, fired up a fleet of charter boats, and the fishery has flourished ever since.
In the early days of the fishery, as was generally the case for game fishing world wide, most fish were boated and weighed. Annual reports of the Cairns Game Fishing Club show that in some seasons, hundreds of fish were taken, with up to 40 ‘granders’ weighed in a single year. While many today would undoubtedly frown at this practice, at the time, the numbers being taken were not regarded as large certainly only a fraction of the Japanese catch. Happily however, at the same time, tagging was taking off, with the first black marlin released off the GBR in 1968 by the legendary Bob Dyer. During those early years, marlin were tagged at a steady, if rather low rate. Nevertheless, this pioneering work, analysed by American scientist Jim Squire and Cairns GFC secretary, Daphne Nielsen, resulted in some very important findings. During this time, the Japanese longline fleet was fishing virtually within earshot of the charter fleet, and as a result, were handing in quite a few tags while others were being returned from much further afield throughout the southwestern Pacific ocean.
From the beginning, research and the charter fishery have gone hand in hand. In the early 1970s, Japanese marlin biologist Izumi Nakamura visited Cairns and was permitted to dissect several giant black marlin. He arranged for the carcasses to be carefully buried, and returned the following year to unearth the skeletons for further study. Since then, scientists from near and far have come to study these remarkable fish. Unfortunately, funding for billfish research has always been in short supply and studies have usually had to be undertaken on shoestring budgets, often in researchers’ own time. Thanks to the charter fleet though, there has always been willing cooperation from anglers, captains and crew.
In a nutshell, what has this research shown? The original tagging of thousands of black marlin along the reef has shown that these fish are capable of very long distance movements, while popup satellite tagging has confirmed and added to the overall picture. Ultrasonic tracking showed that fish rarely dive deeper than 200 metres, that they tend to stay deep during the day, but come to the surface at night. Inspection of gonads and the finding of larvae indicated that the annual arrival of fully adult black marlin in the Coral Sea adjacent to the reef is most likely an aggregation of spawning or pre-spawning fish. And examination of the fishing diaries of long term charter captains and deckhands has revealed that low catch rates usually occur during seasons in which higher than average sea surface temperatures are experienced. Interestingly, these records also suggested that the best catch rates generally occur around the quarter moons.
Although these and other studies have revealed much about black marlin biology, there are still huge gaps in our knowledge. For example, we still don’t have a good understanding of what the true size of the black marlin population is, whether a large percentage of fish come back to the reef each year, whether or not the seasonal population near the reef is static or undergoes constant emigration and immigration, where the big females go after they leave the reef, what conditions are good or bad for spawning, whether spawning takes place every year, how often individual fish spawn, and so the list goes on. By the time this piece goes to print, another black marlin season will be well under way along the Great Barrier Reef. Let’s hope the fish appear on cue, and may it always be so.