Hopkins Marine Station Student Paper

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Title: Reproductive Biology of Bluefin tuna and other Scombrids
Student Author(s): Schwager, Tad W.
Faculty Advisor(s): Block, Barbara
Pages: 39
Location: Final Papers Biology 175H
Date: June 1999
Abstract: Atlantic bluefin tuna (Thunnus thynnus) are currently managed as two separate stocks, one breeding in the Mediterranean Sea and the other in the Gulf of Mexico. The fidelity of the two stocks to their respective breeding ground is currently in question. In the Gulf of Mexico, bluefin arrive in a sexually inactive state and in some time period ranging from weeks to months, develop into spawners. This western stock is reported to mature at about 190-205 cm in length and about 8-10 years of age. Currently, bluefin tuna are being tagged with pop-up satellite and archival tags, to discern their migratory movements and philopatry to a spawning ground. My objective was to examine bluefin in the areas they are currently being tagged, to discern the state of sexual maturity of fish in the different regions. Specifically, I was interested in identifying 1) what is the minimum size of first sexual activity and 2) if spawners were restricted only to the Gulf of Mexico. Knowing which fish are mature will help to interpret the movement and behaviors revealed by the tag data. Since scombrid fishes are closely related, other species provided a valuable comparison. To histologically identify the stages of wild caught bluefin tuna I compared the tissues to yellowfin tuna (Thunnus albacares) and Eastern Pacific bonito (Sarda chiliensis) held in captivity at the Tuna Research and Conservation Center. Based on a classification system from previous studies, I assigned each sample a stage of maturity. In addition, for captive fish, I collected data on gonad size at mortality, which provided an index of gonad development for corroboration with histological examinations. I found that female yellowfin tuna do not become sexually active in water temperatures below 20 degrees C, but male yellowfin do, with no apparent variation in size to first sexual maturity from wild caught yellowfin tuna. Female yellowfin at 25 degrees C showed slightly more advanced oocytes than females at 20 degrees C. Both sexes of bonito become mature in captivity and spawn in 20 degrees C water. Together, histological observations of the captive populations reveal all the stages of gonadal development needed to classify scombrid gonad samples. Based on comparative histological analyses, I concluded that all bluefin gonadal tissues (n=111) from New England and North Carolina were inactive and therefore not spawning. All fish from the Gulf of Mexico and the Bahamas (n=5) demonstrated asynchronous ovaries with many oocytes yolked and in maturation stages just prior to spawning. The lack of a fully hydrated oocyte, indicative of a currently spawning fish is probably due to the small sample size and time of capture. The Bahama fish represent the first histological observations of sexually active bluefin tuna outside the Gulf of Mexico. Understanding the reproductive biology and the size at first spawning is a crucial step for developing future management strategies.