The Featured Research page is designed to highlight current research in the Large Pelagics Research Laboratory. In this installment, we highlight the work of two graduate students working on the reproductive biology of bluefin tuna, Gilad Heinisch and Jessie Knapp. Also,we've included some background to tuna reproduction

Reproductive Mysteries of the Atlantic Bluefin Tuna

North Atlantic bluefin tuna ( Thunnus thynnus ; ABFT) is a highly migratory species inhabiting the North Atlantic Ocean and adjacent seas. Findings of the Large Pelagics Research Center reveal some of the mysteries associated with migration patterns of this magnificent species, as well as raises questions about the scientific community's understanding of its reproductive behavior.

ABFT fisheries are managed as two separate stocks: East and West. T rans-Atlantic migrations and a high percentage of mixing have been documented, and natal homing is assumed to be the dominant pattern of the species' reproductive behavior. ABFT of the eastern stock spawn during spring-summer in the Mediterranean Sea, whereas ABFT of the western stock spawn during the spring in the Gulf of Mexico and the Florida straits.

ABFT is a gonochoristc teleost (bony fish) and after sexual differentiation to males and females is completed, individuals retain their sex throughout their life time without reversing it (sex reversal is a common phenomenon among bony fishes).

Mature ABFT are believed to spawn every year. A single female's spawning season will last 3-4 weeks and it is agreed that a 200 lb. female will spawn between 50- 100 million eggs per season. These fish are daily spawners, i.e. each female release a batch of eggs every day at approximately 28 hr. intervals. Accordingly, during spawning season the female's ovary will contain oocytes at different developmental stages. Below are female ovaries and male testis with associated fat (photos by Gilad Heinisch).


Figure 1. Examples of male testes with fat body (left) and female ovaries (right).

The prevailing view among the scientific community is that there is a great discrepancy between the two stocks ' maturity schedules , although key issues have yet to be addressed. In the Mediterranean Sea ABFT of the eastern stock reach sexual maturity at the age of 3-5 years old. I t is believed that ABFT of the western stock achieve sexual maturity between 8-12 years of age . L eading bluefin tuna scientists (e.g. Ray Baglin, Frank Mather, Pete Wilson) and fisherman (Frank Cyganowski, Kip Farrington) believed that ABFT also spawn in other locations in the West Atlantic, based on catches of smaller bluefin tuna with ripe gonads near the Gulf Stream edge, the Azores, and in the Caribbean Sea.

Recently, our lab published a study based on Jen Goldstein's thesis work (Goldstein et al., 2007). The authors had examined 195 gonads of commercially landed bluefin tuna from the Gulf of Maine and found great variation in gonadal status among the alleged post spawning fish. This heterogeneity could not be the outcome of a relative homogenous reproductive behavior. The authors have postulated that natural variation, alternative spawning grounds and/or skipped spawning events are scenarios that could explain their observations. Because our lab also tracks the migration and behavior of fish tagged in the same area with PSAT tags, we are also able to comment on the dispersal patterns of fish entering and leaving the region. There was no evidence that the smaller fish had come from the Mediterranean that season.

Furthermore, reports from pop-up satellite tag over the last decade have repeatedly documented ABFT swimming outside known spawning grounds during the spawning season. Lutcavage et al. (1999) reported that twelve pop up satellite tags were successfully released from ABFT during May – July 1998 in the mid-Atlantic region bounded by Bermuda and Azores . The total number of successfully released tags in that study was seventeen (out of 20 tagged individuals). In other words, 70% of the giant ABFT tagged were not present in the known spawning grounds during the spawning season. Similar observations were reported by Galuardi et al. (2008).

The discrepancy of the age at first maturity between the western and eastern stocks and the postulated spawning grounds in the West Atlantic are key issues in bluefin tuna biology. Our lab is focused on resolving these outstanding questions, with 2 PhD projects, and current collaboration with Mediterranean researchers. The “Tag A Tiny” project, launched in 2007, was specifically designed to track migration pattern of young ABFT tagged in the Gulf of Maine .

This work will undoubtedly shed new light on bluefin migration , sexual maturity and reproduction patterns.