Blue marlin(Makaira nigricans) and and white marlin (Tetrapturus albidus) are found throughout the tropical and temperate Atlantic Ocean. Although there are no large-scale directed fisheries for either species, they are a popular gamefish and are targeted in small-scale artisanal fisheries. However, the main source of fishing mortality for these species is the longline fisheries for tuna and swordfish, where they are caught as by-catch. Stock assessments of blue and white marlin carried out in 2000 and 2002 by ICCAT (the Internation Commission for the Conservation of Atlantic Tuna) indicated that both species are significantly overfished and continue to suffer overfishing; however, both assessments have a high degree of uncertainty in their estimates of current stock status. Effective management cannot be implemented until this uncertainty is reduced. One of the major sources of uncertainty is the lack of basic biological data, including life-history parameters such as size-at-age and growth rates. The age and growth models we develop for this project will provide this information.
Sampling is being carried out by our research collaborators in several countries throughout the Caribbean and the Atlantic, including Venezuela, Brazil, Martinique, the US and the Gulf of Guinea. Samples are coming from both artisanal and commercial fisheries. Observers at port or on-board fishing vessels collect the anal fin from all landed marlin, along with information on the size and sex of the animal.
Many fish can be aged by counting rings that form in certain hard bony structures in their bodies, like counting rings in a tree. The most commonly used hard part is the otolith or "ear stone", a calcified structure in the ear that helps fish sense and maintain their orientation while swimming. However, the otoliths of billfish are very small and fragile, making them difficult to extract and read. Instead, our study is using thin sections cut from one of the bony spines in the anal fin.
Even though the fin spines of marlin are easier to collect and read than otoliths, there is one disadvantage to using them. As the fish gets larger, the vascular tissue in the center of the spine spreads outward, obscuring the earliest rings. Therefore, we must find a way to correct for this vascularization, to make sure we are assigning the fish the correct age. One way that this has been done for other species is to measure the size of the earliest rings in young fish, whose fin spines have not become so heavily vascularized, and then comparing the size of these rings to the size of the vascularized area in older fish to estimate how many rings have been lost.
Another challenge of ageing billfish--in fact, a challenge of ageing all fish, regardless of technique--is validating the ages obtained. That is, we assume that each ring represents one year, but in order to validate the model, we must show that this is the case. The most reliable technique for doing this is chemical mark-recapture: a fish is caught, tagged, and injected with a chemical such as oxytetracycline which leaves a visible mark in the bony parts of the fish. The fish is then released. When the fish is recaptured later, the number of rings formed after the chemical mark is compared to the number of years the fish was at large. If these numbers are the same, then the rings are formed once a year. Unfortunately, the recapture rates of tagged marlin are so low that this technique is impractical.
Instead, we are using the relative marginal increment analysis technique to validate our ages. In this technique, the distance from the edge of the last ring to the edge of the spine is measured for each sample. This is the marginal increment. The samples are seperated by sex, age, and month, and the average marginal increment is calculated for each month. If the rings are being formed once a year, there should be a month where the marginal increment is close to zero (that is, the new ring has just formed). If we see this pattern, then we can say with some confidence that each ring we count represents a year of age.
This work is part of the Ph.D. dissertation of
Katie Drew
University of Miami
Marine Biology and Fisheries
Rosenstiel School of Marine and Atmospheric Science
4600 Rickenbacker Causeway
Miami, Florida, 33149
Phone: (305) 421-4924
Fax: (305) 361-4457
E-mail: kdrew@rsmas.miami.edu