What’s the Main Purpose?
Even though istiophorid billfishes (which include the marlins, spearfishes, and sailfish) are popular with sportfish anglers all over the world, there is still a lot to learn about the biology of these species. For example, the number and geographic extent of billfish populations in the Atlantic, Pacific, and Indian oceans is unknown or incompletely known for multiple billfish species– this is what my research is focused on. Specifically, I am working to find out just how many populations of striped marlin (Kajikia audax) there are in the Indo-Pacific. I’m equally interested in answering this question for white marlin (K. albida) in the Atlantic. Understanding the population structure of striped marlin and white marlin in their respective ocean basins will help scientists and managers identify appropriate strategies for species management and assessment, and promote long-term genetic diversity necessary for species persistence. This information will also provide an understanding of how populations of these species interact with each other, and which angler groups interact with which population(s). We may even get an idea of how much individual source populations contribute to mixed population assemblages.
How Does it Work?
This research project is genetics-based and will mainly consist of comparing groups of white marlin and striped marlin from different geographic locations to see if there are genetic differences. If a significant level of genetic differences exists between two groups of striped marlin, for example, this is evidence that two different populations may be present. To do this work, I need samples of striped marlin and white marlin from throughout the distributional ranges of these species….which is essentially the whole world. This is where a network of anglers, agencies, and scientists is essential. For the past several months I have been working to identify people in diverse geographic locations that have access to striped marlin and white marlin. The success of this project depends on the ability to collect samples of these species through this network. The sampling process is quick and easy: I just need a small piece of tissue from a hooked fish. The tissue sample can be small (about the size of your fingernail) and is placed in a tube of preservative to keep the DNA fresh for me to work with back in the lab at VIMS. For a released fish, this sample is typically taken from a fin prior to release (any fin, whichever is easiest to get to!). If a fish is being landed, the sample can come from anywhere on the fish (typically muscle or fin). The date, geographic location of where the fish was caught, and sample number are recorded….and that’s it! In some cases (especially if the fish is being landed), it is possible to obtain additional information, such as length, weight, sex, and paired gonad weight…..this information is especially helpful for comparing the genetics of different demographic groups, such as males vs. females, or juveniles vs. adults.
How Can I Help?
Sign up to sample! I need about 50 samples of striped marlin or white marlin from each of multiple geographic locations throughout the Atlantic, Pacific, and Indian oceans. Sampling will occur from 2015-2017….so I can always use more help! If this project sounds like something you could contribute to, just get in contact with me via email at email@example.com and I’ll be glad to give you a sampling kit to get started.
More About White Marlin
White marlin (Kajikia albida) are found in the Atlantic Ocean, from about 45°N to 45°S, and mostly inhabit surface waters with temperatures of 24-29°C. An average size for white marlin is about 150 cm lower jaw fork length (LJFL) and 22 kg in weight (about 50 pounds), although this varies by geographic location and between sexes. Based on the longest time-at-large reported for a tagged white marlin, this species is capable of living to at least 15 years of age. Satellite tagging data indicate that white marlin take frequent dives to 100-200 m depth, presumably for feeding. White marlin are typically associated with ocean fronts, submarine canyons, and other oceanic features where prey species aggregate.
Several popular sport fisheries for white marlin exist Atlantic-wide, from the Azores to Angola to the Dominican Republic. Over the past few years, the Mid-Atlantic United States has developed a reputation as one of the best places in the Atlantic for catching white marlin. Recreational fisheries for white marlin and other istiophorid billfishes are primarily catch and release. White marlin are also caught as bycatch in pelagic longline fisheries targeting tunas and swordfish, and are targeted by small-scale artisanal fisheries in some regions.
In the Atlantic Ocean, white marlin are managed by the member countries of the International Commission for the Conservation of Atlantic Tunas (ICCAT). ICCAT previously recognized two populations of white marlin in the north and south Atlantic separated by 5°N, but switched to a single Atlantic-wide population model in 2000. The appropriateness of the single-stock model is uncertain, as suggested by evidence from satellite and conventional tagging of white marlin. It is important that we figure out the best management model for white marlin, however, because this species is currently thought to be overfished in some parts of its range.
Random Cool Facts
- White marlin look nearly identical to the roundscale spearfish (Tetrapturus georgii; also known as the hatchet marlin), the latter of which was just confirmed as a distinct species in 2006. White marlin and roundscale spearfish are still easily confused with one another, and need to be examined closely to tell them apart.
- Satellite tagging of white marlin shows that this species can travel over 70 miles a day, and is capable of trans-oceanic movements. One of the longest distances travelled by a tagged and reported white marlin was over 6,500 km (nearly 4,000 miles).
- Like other billfishes, white marlin have heater organs associated with their eyes and brain. The purpose of these organs is to keep these structures warm at depth, which helps explain how these species can feed at depths in excess of 200m.
More About Striped Marlin
Striped marlin are found in the Indian and Pacific oceans, generally between 45°N and 45°S, and in waters 20-25°C. In the Pacific Ocean, the distribution of striped marlin is similar to a sideways horseshoe, with continuous distributions north and south of the equator and along the eastern Pacific. An average striped marlin might be 155-185 cm eye fork length, though this is highly variable among geographic regions and also between sexes. Striped marlin are typically found in surface waters above the thermocline, but satellite tagging data show dives to 40-100 m depth presumably for feeding. Counting growth rings on dorsal fin spines and time-at-large in tagged fish indicate that striped marlin can live to at least 10-12 years of age.
Similar to white marlin, striped marlin are prized by sportfish anglers throughout their range and competitive sport fisheries are found in locations including Australia, New Zealand, Ecuador, and Kenya. Striped marlin are also caught as bycatch in pelagic longline fisheries targeting tunas and swordfish, but in some of these fisheries striped marlin are opportunistically targeted or are valuable incidental catch. Small-scale artisanal fisheries also exist for this species in some locations.
Multiple populations of striped marlin are recognized in the Pacific Ocean, however, the exact geographic extent of and boundaries among populations are relatively unclear. In the eastern Pacific Ocean, striped marlin are managed by the Inter-American Tropical Tuna Commission (IATTC). The Western and Central Pacific Fisheries Commission (WCPFC) manages striped marlin in the western and central Pacific. The most recent stock assessments of striped marlin in the Pacific Ocean indicate that the western and central North Pacific stock is overfished and experiencing overfishing, while the southwestern stock is likely to be approaching an overfished state. The eastern Pacific stock appears to be healthy, and is not thought to be overfished or experiencing overfishing. In the Indian Ocean, striped marlin are managed as a single ocean-wide population by the Indian Ocean Tuna Commission (IOTC). According to a 2014 assessment of population status, striped marlin are thought to be overfished and experiencing overfishing in this region. The relationship between Indian Ocean and Pacific Ocean striped marlin is currently unknown, but this is something I hope to understand better with my research.
Random Cool Facts
- Like the other billfishes, striped marlin grow extremely fast and can reach up to 45% of maximum size within the first year of life. In the southwest Pacific Ocean, striped marlin grow especially rapidly and are capable of reaching 75% of maximum size in the first year.
- Information from tagging and genetic studies have identified multiple populations of striped marlin in the Pacific Ocean, the most dynamic of which has a range that seasonally spans most of the North Pacific from Taiwan to southern California, USA.
- One of the longest distances travelled by a tagged and reported striped marlin was over 6,700 km (nearly 4,200 miles).