In this study, the authors examined late atretic follicles in the ovaries of Atlantic bluefin tuna, Thunnus thynnus (Linnaeus 1758), at different times of the year using transmitted light and epifluorescence microscopy. Atresia (degeneration and resorption of developing ovarian follicles) is a natural process involved in fecundity downregulation in teleosts and is substantially enhanced in stressful conditions. Early (α and β) atretic stages of yolked oocytes have a relatively short duration in seasonally reproducing species, whereas later (γ and δ) atretic follicles (LAF) persist for longer time in the ovary, serving as a sign of previous vitellogenic activity. LAF can thus be used as reliable markers of maturity during non-reproductive periods. Lipofuscin granules accumulate in the cytoplasm of LAF cells as a result of lysosomal digestion of oocyte components. Taking advantage of the well-known autofluorescent properties of lipofuscins, LAF may be identified in unstained histological sections under fluorescence microscopy using appropriate excitation and emission wavelengths. The authors explore in this study the applicability of fluorescence microscopy to provide a fast and effective method to assess late atresia in fishes. This method may be particularly useful to determine sexual maturity in individuals sampled long after the spawning season, where LAF are difficult to detect in standard histological sections. Furthermore, LAF autofluorescence provides a rapid way to quantify late atresia in fishes using image analysis.
Two distinct contingents of Atlantic bluefin tuna (ABFT), migrants and residents, occur in the Strait of Gibraltar (SoG). Long-term residents are commercially exploited all year round using hook and line gears, whereas migrants are caught by traps in springtime as they pass through the SoG towards Mediterranean spawning grounds. While reproductive features of migrants have been widely studied, the life history of residents remains poorly investigated. The relative contribution of this subpopulation to ABFT eastern stock productivity is therefore unknown. Reproductive traits of resident ABFT were monitored throughout the year, and were compared with those of migrants crossing the SoG in spring. To assess maturation timing, gonads were classified into maturation stages based on histological features. Our results indicate biometric and reproductive differences between SoG residents and migrants. ABFT caught by hook and line gears were smaller on average than those collected from traps. The females sampled from the hook and line fishery in May and early June showed, on average, significantly lower GSI values than those caught by trap. In contrast, the GSI values of males sampled in May were not significantly different between gears. Histological analyses showed a more advanced reproductive condition in trap-caught migrants. Delayed maturation schedules of residents in comparison to migrants would result in less egg production and poorer larval survival rates. The present findings encourage further investigation of reproductive schedules in unstudied subpopulations for a better understanding of ABFT dynamics.
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