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Do Some Atlantic Bluefin Tuna Skip Spawning?

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Do Some Atlantic Bluefin Tuna Skip Spawning? SCRS/2006/088 Col. Vol. Sci. Pap. ICCAT, 60(4): 1141-1153 (2007) DO SOME ATLANTIC BLUEFIN TUNA SKIP SPAWNING? David H. Secor1 SUMMARY During the spawning season for Atlantic bluefin tuna, some adults occur outside known spawning centers, suggesting either unknown spawning regions, or fundamental errors in our current understanding of bluefin tuna reproductive schedules. Based upon recent scientific perspectives, skipped spawning (delayed maturation and non-annual spawning) is possibly prevalent in moderately long-lived marine species like bluefin tuna. In principle, skipped spawning represents a trade-off between current and future reproduction. By foregoing reproduction, an individual can incur survival and growth benefits that accrue in deferred reproduction. Across a range of species, skipped reproduction was positively correlated with longevity, but for non-sturgeon species, adults spawned at intervals at least once every two years. A range of types of skipped spawning (constant, younger, older, event skipping; and delays in first maturation) was modeled for the western Atlantic bluefin tuna population to test for their effects on the egg-production-per-recruit biological reference point (stipulated at 20% and 40%). With the exception of extreme delays in maturation, skipped spawning had relatively small effect in depressing fishing mortality (F) threshold values. This was particularly true in comparison to scenarios of a juvenile fishery (ages 4-7), which substantially depressed threshold F values. Indeed, recent F estimates for 1990-2002 western Atlantic bluefin tuna stock assessments were in excess of threshold F values when juvenile size classes were exploited. If western bluefin tuna are currently maturing at an older age than is currently assessed (i.e., 10 v. 8 years), then F thresholds for a juvenile-adult fishery decrease to very low levels (F20=0.13; F40=0.07). Because juveniles of either population are known to mix widely, recovery of western Atlantic bluefin tuna should depend upon sufficiently protective minimum size limits that are commonly applied throughout North Atlantic and Mediterranean Sea fisheries. RÉSUMÉ Au cours de la saison de frai du thon rouge de l’Atlantique, la présence de certains adultes a été constatée à l’extérieur des zones de frai connues, ce qui donne à penser qu’il existe des zones de frai inconnues ou que nos connaissances actuelles sur l’évolution de la reproduction du thon rouge sont fondamentalement erronées. Sur la base de perspectives scientifiques récentes, l’omission de la fraye (retard de la maturation et fraye non annuelle) est possiblement un phénomène courant chez les espèces marines ayant une longévité modérée comme le thon rouge. En principe, l’omission de la fraye représente une compensation entre la reproduction actuelle et future. En ne réalisant pas la reproduction, un individu peut obtenir des bénéfices en terme de survie et de croissance provenant de la reproduction différée. Dans une gamme d’espèces, l’omission de la fraye a été corrélée positivement avec la longévité, mais pour les espèces autres que l’esturgeon, les adultes frayent à des intervalles d’au moins une fois tous les deux ans. Une gamme de divers types d’omission de la fraye (constante, à un âge plus précoce, à un âge plus avancé, omise ponctuellement et retards de la première maturation) a été modélisée pour la population du thon rouge de l’Atlantique ouest afin de tester ses effets sur les points de référence biologiques de la production d’œufs par recrue (EPR) (stipulés à 20% et 40%). A l’exception de retards extrêmes de la maturation, l’omission de la fraye n’avait un effet que relativement faible sur la réduction des valeurs seuil de la mortalité par pêche (F). Cela s’est avéré particulièrement vrai par rapport aux scénarios de pêcheries de juvéniles (âges 4-7), où les valeurs seuil de F ont été considérablement réduites. En réalité, les récentes estimations de F pour les évaluations de stock de thon rouge de l’Atlantique ouest de 1990-2002 dépassaient les valeurs seuil de F lorsque l’on exploitait les classes de taille de juvéniles. Si la maturation actuelle du thon rouge de l’ouest se produit à un âge plus avancé que ce qui est actuellement évalué (c’est-à-dire 10 v. 8 ans), alors les seuils de F pour une pêcherie de 1 Chesapeake Biological Laboratory, University of Maryland Center for Environmental Science, Solomons, Maryland 20688 United States, e- mail [email protected] 1141 juvéniles-adultes diminuent jusqu’à des niveaux très faibles (F20=0,13; F40=0,07). Etant donné que l’on connaît le mélange important des juvéniles des deux populations, le rétablissement du stock de thon rouge de l’Atlantique ouest devrait dépendre de limites de tailles minimales suffisamment protectrices qui sont généralement appliquées dans les pêcheries de l’Atlantique Nord et de la Méditerranée. RESUMEN Durante la temporada de puesta del atún rojo del Atlántico, algunos adultos desovan fuera de las zonas de desove conocidas, lo que sugiere o bien zonas de desove desconocidas o errores fundamentales en nuestros conocimientos actuales sobre la temporada reproductiva del atún rojo. Basándose en recientes perspectivas científicas, la omisión de la reproducción (maduración retardada y reproducción no anual) es posiblemente dominante en especies marinas de moderadamente larga vida como el atún rojo. En principio, la omisión de la reproducción es una compensación entre la reproducción actual y futura. Al no realizar la reproducción, un ejemplar puede obtener beneficios de crecimiento y supervivencia que se incrementan con el aplazamiento de la reproducción. En una gama de especies, se ha hallado una correlación positiva entre la omisión de la reproducción y la longevidad, pero para las especies distintas a los esturiones, los adultos desovan en intervalos de al menos una vez cada dos años. Se modeló una gama de tipos de omisión de la reproducción (constante, en etapas más jóvenes, en etapas adultas, omisión ocasional, y retrasos en la primera madurez) en la población de atún rojo del Atlántico occidental para probar sus efectos en los puntos de referencia biológicos de la producción de huevos por recluta (estipulados en 20% y 40%). Con la excepción de retrasos extremos en la madurez, la omisión de la reproducción tuvo un efecto relativamente escaso en reducir los valores umbral de la mortalidad por pesca (F). Esto era especialmente cierto en comparación con los escenarios de pesquería juvenil (edades 4-7) en la que los valores umbral de F descendieron significativamente. De hecho, las estimaciones recientes de F para las evaluaciones del stock de atún rojo del Atlántico occidental de 1990- 2002 superaban los valores umbral de F cuando se explotaban las clases de talla juvenil. Si actualmente el atún rojo occidental está madurando a una edad más avanzada que la estimada actualmente (a saber, 10 v. 8 años), entonces los umbrales F para una pesquería de adultos- juveniles descenderían hasta niveles muy bajos (F20 = 0,13; F40 = 0,07). Dado que se sabe que los juveniles de cada una de las poblaciones presentan un amplio nivel de mezcla, la recuperación del atún rojo del Atlántico occidental dependería de tallas mínimas suficientemente protectoras, que se aplican comúnmente en todas las pesquerías del Atlántico norte y Mediterráneo. KEYWORDS Bluefin tuna, reproductive behavior, spawning seasons 1. Bluefin tuna: Spawning in the wrong place at the wrong time? During the spawning season (May-July), Atlantic bluefin tuna sometimes occur outside spawning centers (the Gulf of Mexico [GOM] or Mediterranean Sea) in parts of the Central Atlantic Ocean and elsewhere. Electronic tagging has provided definitive evidence of this behavior. Lutcavage et al. (1999) relocated twelve large bluefin tuna (190-263 cm SFL) over a broad region of the Central-North Atlantic during May-July. Previous conventional tagging studies and subsequent intensive archival tagging efforts supported this mode of behavior in large bluefin tuna. Based upon continuous tracking of individuals, Block et al. (2005) observed that many adults (> 200 cm CFL) persisted in North American continental shelf and central Atlantic waters during spring and summer months. This behavior prompts three hypotheses: (1) Spawning occurs in regions outside the GOM and Mediterranean (Lutcavage et al. 1999); (2) Bluefin tuna are becoming mature at larger sizes and older ages than previously observed or expected (Block et al. 2005); or (3) Bluefin tuna are not obligate annual spawners: they sometimes “skip” spawning. Several years of directed sampling in the central Atlantic have failed to find spawning condition bluefin tuna (ICCAT, 2007). In addition, ichthyoplankton collections in the central Atlantic have failed to recover larval bluefin tuna (Fromentin and Powers, 2005). Certainly, scientific sampling of unknown spawning regions in the Central Atlantic and elsewhere is a daunting task. Therefore it will be difficult to reject the hypothesis of alternate spawning areas for bluefin tuna. On the other hand, the second and third 1142 hypotheses have not received due treatment and may be the more logical inferences given the variability of reproductive schedules in marine fishes. The purpose of this essay is to provide a prospectus for skipped spawning (SS), with delayed maturation as a special “case” of skipped spawning in Atlantic bluefin tuna. This review was prompted by increased empirical evidence for SS and modeling exercises showing its consequence to population dynamics (Rideout et al. 2005; Jørgensen et al. 2006). Evidence supports the generality of SS, particularly for long-lived migratory fishes with large energetic demands for reproduction (Figure 1; Table 1). Here, SS refers narrowly to the coupled migration and spawning by bluefin tuna and other fishes, also known as non-annual spawning cycles. More broadly, SS also includes failed oogenesis due to retention and/or resorption of oocytes (Rideout et al. 2005). The focus here is on Atlantic bluefin tuna and other species that as adults sometime occur in alternate habitats during spawning periods.
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