Marlin Structure & Function

What do we really know about marlins? How big do they get? How fast do they swim? Do they see colours? Do they change sex as they grow? These questions are amongst the common ones asked by anglers and onlookers at tournament "weigh-ins" after they have asked what it is. Since the world leading policy by Australian fisheries to exclude commercial marlin fishing from prime recreational waters since 1987 the east coast has seen the development of what could be argued the worlds best recreational bill fishery. But have we learned much about marlin since then? It is the aim of these articles is to answer some of the questions posed above and to provide a review of the state of our knowledge. First however, a bit of background about marlin in the Pacific Ocean is in order.

The tropical and subtropical waters of the Pacific Ocean are home to three of the largest and fastest blue water predators, the black, blue and striped marlins. Blue marlin have been weighed in at over 800 kg (1800 lbs) and have been estimated in excess of 900 kg from carcasses from commercial longline fishing vessels. Their size, speed and athletic ability make marlin spectacular sporting fishes, sought after by recreational fishermen and women who often target marlins and their billfish relatives in special billfish tournaments. Marlin are also highly regarded at the table and support significant commercial billfish fisheries.

The total Pacific Ocean catch of marlins is estimated to have been around 30,000 tonnes in 1985, about 250,000 fish. Of this, about 90% is taken by longline fishing vessels which primarily target tuna species. Billfish comprise about 18% of the total tuna longline catch and represent an important by catch for these vessels. The recreational and subsistence billfish catches are believed to be small but not insignificant compared with the longline by catch. The rapid growth and subsequent decline of the recreational and commercial swordfish fisheries off the east coast of Florida between 1977 and 1985 has helped to focus attention on billfish species and great progress has recently been made toward billfish management. The International Commission for the Conservation of Atlantic Tunas (ICCAT) has been able to negotiate agreements on management strategies for the Atlantic Ocean and action management plans such as the Atlantic Billfish Management Plan (Approved by US Secretary of Commerce in 1988) which seeks to "Maintain the highest availability of billfishes for the recreational fishery...". The Governments of Australia since 1979, Mexico since 1983, and New Zealand since 1987 have attempted to enhance their recreational billfish fisheries by exclusion of longline vessels from parts of their 200 mile Exclusive Economic Zones (EEZ).

These actions have highlighted many features of recreational billfish fisheries including their environmental sensitivity, high financial return per fish captured and significant contributions to the successful cooperative tag and release programmes.

The need to bring together all available information on billfishes from time to time has been recognised and acted upon by several organisations. The National Marine Fisheries Service (NMFS) sponsored the first International Billfish Symposium in 1972 and co-sponsored with the National Coalition for Marine Conservation (NCMC) and a number of other bodies the second International Billfish Symposium in 1988. Justifiably these meetings were mostly concerned with the problems surrounding effective management and considered at length subjects such as stock identification and assessment, age, growth and early life history development and reproduction. However, as a result there is almost nothing about the structure or function contained within the proceedings of these symposia.