An early morning departure from Kleinbaai Harbour on 20 January, with the initial goal of deploying underwater video systems, took a turn as news arrived that a large, deceased fish was found floating in Jouberstdam, brought to our attention by fisherman Johan Stoop. The team on board research vessel Lwazi, consisting of skippers Hennie Odendaal and Dickie Chivell, marine biologist Amy Webber, guide Sonja Fleck and Marine Dynamics Academy intern and volunteer, Thomas Merdy and Anja Baumgart, swiftly made their way to where the fish was being held alongside the fishing vessel.
A smooth transfer of the evident swordfish was completed, towing it back to the harbour where it was transported to the International Marine Volunteer Lodge for further analysis and dissection.
Identifying the specific species of swordfish was completed before measurements and dissection, led by Amy Webber, with the assistance of Sonja Fleck in the presence of Marine Dynamics Academy interns and volunteers. The rigid pectoral fins and grooves of the lower jaw were some of the main indicators that the fish being studied was a Black marlin (Istiompax indica). Other species of marlin include the striped marlin, white marlin and blue marlin, of which one of 262cm was dissected by the DICT and MDA team in 2021. The black marlin is typically found within the tropical and sub-tropical waters of the Indo-Pacific. Occasionally during summer months, the odd individual can be found around the temperate Atlantic waters of the Cape. Black marlin are highly migratory, found from the water surface to as deep as 200m, feeding predominantly on shoaling fish. Below, the distribution of the species is highlighted, with darker colouration indicating a higher probability of black marlin presence (AquaMaps, 2019).
The individual collected was measured to be 365cm in total, with the maximum length recorded at 448cm. Upon collection, the black marlin was in good condition, however the bill of the upper jaw was broken, not allowing for the full length to be established. The black marlin has one of the shortest bills out of the marlin species, typically standing at less than 50% of the total body length. Using a species-specific length/weight curve, the weight of the fish was estimated to be 440kg as based on the lower jaw to tail fork length of 346cm, with the largest maximum weight ever recorded sitting at 707.6kg in 1953. The black marlin displays sexual dimorphism, a trait in which the females grow larger than the males due to their longer lifespan. Females are estimated to live as long as 13 years, with males living to 7 years in comparison. Their life expectancy differs based on a range of external environmental conditions; however sex is the most important factor. Male individuals have a faster maturation rate, maturing at 50-80kg whilst females mature at 200kg. The size and weight of the individual provided evidence that it was a mature female.
After all required measurements were taken, the necropsy began for internal inspection of the individual to investigate as to whether there were any clues of the death. All organs were intact, with no surface signs of predation. The stomach contents showed full digestion, with no indication as to what the last meal may have been. Within the bay, warmer waters ranging from 20-24°C through the month of December could be a possible reason as to why the black marlin was found within Atlantic waters. No clear signs of internal stress were found. The continued inspection of the internal organs confirmed the black marlin to be a mature female, with the ovaries sighted within the body cavity. Samples of the gill filaments, pectoral fin and gonads were collected and stored to aid further investigation into the biology of billfish.
To demonstrate the complexity of the black marlin biology, the eye was removed. Marlin species have impeccable eyesight, relying largely on vision to identify their prey species. A unique adaptation that increases predation efficiency is the close alignment to the to muscular structure that allows heat transfer via a counter current exchange of the large blood vessel leading to the back of the eye. Increased temperature allows the fish to view more ‘frames per second’, an advantageous feature when swimming at high speeds.
Black marlin are fished throughout the Indian ocean, particularly in China, Taiwan, Japan, Korea, Sri-Lanka and Indonesia. Recreational angling of the species is also very popular, with vulnerability being particularly high due to their long migratory routes offering limited protection through the use of Marine Protected Areas. The IUCN status of this specific species currently lies at Data Deficient, meaning not enough is known about these fish to accurately assess their population stability, stressing the value and importance of the biological insight the stranding has allowed the DICT and wider community to achieve. Sincere appreciation to Johan Stoop and his team for allowing to the DICT and Marine Dynamics Academy staff to explore this unique and rare scientific practice.
References
AquaMaps (2019, October). Computer generated distribution maps for Istiompax indica (Black marlin), with modelled year 2050 native range map based on IPCC RCP8.5 emissions scenario. Retrieved from https://www.aquamaps.org
Length/Weight Curve for Black Marlin: Lizard Island Game Fishing ClubMerrett, N. R. (1970). Gonad development in billfish (Istiophoridae) from the Indian Ocean. Journal of Zoology, 160(3), 355-370.
ORI Fish Fact Sheet – Black Marlin: https://saambr.org.za/wp-content/uploads/2023/03/ORI-Fish-Fact-Sheet-Black-Marlin-FINAL.pdf
Sun, C. L., Chang, H. Y., Liu, T. Y., Yeh, S. Z., & Chang, Y. J. (2015). Reproductive biology of the black marlin, Istiompax indica, off southwestern and eastern Taiwan. Fisheries Research, 166, 12-20.
Sun, C. L., Yeh, S. Z., Liu, C. S., Su, N. J., & Chiang, W. C. (2015). Age and growth of Black marlin (Istiompax indica) off eastern Taiwan. Fisheries Research, 166, 4-11.
