Contents lists available at ScienceDirect Deep-Sea Research Part II journal homepage: www.elsevier.com/locate/dsr2 Revisiting the Regime Problem hypothesis: 25 years later Vanessa Izquierdo-Peña a , Salvador E. Lluch-Cota a, ⁎ , Martín E. Hernandez-Rivas b , Raul O. Martínez-Rincón a a CONACYT - Centro de Investigaciones Biológicas del Noroeste, S.C., Av. Instituto Politécnico Nacional 195, Playa Palo de Santa Rita Sur, La Paz, B.C.S. 23096, Mexico b Centro Interdisciplinario de Ciencias Marinas-Instituto Politécnico Nacional, Av. Instituto Politécnico Nacional s/n, Playa Palo de Santa Rita, La Paz, BCS 23096, Mexico ARTICLE INFO Keywords: Alternation Synchrony Small pelagics Sardines Anchovies Sardinella Climate ABSTRACT Small pelagic fish species have been a central research topic for more than 60 years because of their ecological role, their economic importance, and the apparent multidecadal large fluctuations in catches, synchronic fluc- tuations from remote regions, and the alternation between the abundance of sardine and anchovies within each system. Strong evidence supports the observation that low-frequency fluctuations of small pelagics occur naturally. However, the synchrony and the alternation have been questioned. In this study, the Regime Problem hypothesis is revisited by updating the fish catch time series to 2015, and by incorporating into the analyses stocks not originally considered due to the time series lengths. The specific questions addressed here are: 1) whether the synchronic fluctuations of small pelagics catches from remote regions and the species alternation between species within each system, observed for the last century in the Regime Problem literature, remains after updating the time series, and 2) If a multidecadal signal can also be detected in regions that were not considered in the original Regime hypothesis framework. Results indicate that the multidecadal synchrony between systems and alternation between species remains clear for the Kuroshio and Humboldt systems. The California small pelagics cannot be considered to be in phase with the other systems, that in Benguela alternation is not evident during at least the last two decades, and that there is not a single worldwide mode of variability teleconnecting all small pelagics. 1. Introduction Small pelagics have been a central research topic for more than 60 years because of their ecological role, their economic importance, and their involvement in one of the major, and still unsolved, scientific mysteries: the multidecadal abundance synchrony and large fluctua- tions of remote populations. Kawasaki (1983) was the first to suggest the synchronic fluctuations of sardine catch time series in three remote regions of the Pacific Ocean (Japan, California, and Humboldt). Sub- sequently, it was argued that, besides synchrony, sardines and ancho- vies from distant systems (Japan, California, Humboldt, and Benguela) were showing a pattern of alternation between periods of high and low abundance, linked to the expansion and contraction of the occupied habitat, and independently of the management strategies and local environmental characteristics of these systems. This pattern has been addressed under the concept of the Regime Problem, a term adopted by the Regime and the SCOR WG98 groups, after the contributions of the illustrious scientist John Isaacs (Lluch-Belda et al., 1989; Schwartzlose et al., 1999; Chavez et al., 2003). While small pelagics fish stocks fluctuations are mostly governed by the combined effects of internal density-dependent processes, and ex- ternal environmental-driven forces (Rouyer et al., 2012; Lindegren et al., 2013), their relative contribution through time and space, and especially the underlying mechanisms linking climate and population dynamics remain unsolved. Hypotheses range from temperature af- fecting recruitment (Takasuka et al., 2007, 2008) and spawning habitat availability (Lluch-Belda et al., 1989, 1992), to current flows and up- welling activity determining retention of larvae within productive systems (Bakun, 1998; MacCall, 2009) and the size composition of the phyto- and zooplankton consumed by the fish (van der Lingen et al., 2006; Rykaczewski and Checkley, 2008). To date, there is not a gen- erally accepted theory to explain the nature and mechanisms governing such fluctuations (MacCall, 2009), which has significant implications on the management of these fisheries (Lindegren et al., 2013). Strong evidence supports the observation that low-frequency fluc- tuations of small pelagics occur naturally, not only through the fisheries history but also before any human influence (Baumgartner et al., 1992; McClatchie et al., 2017). However, the synchrony between remote https://doi.org/10.1016/j.dsr2.2018.11.003 ⁎ Corresponding author. E-mail address: slluch@cibnor.mx (S.E. Lluch-Cota). Deep-Sea Research Part II 159 (2019) 4–10 Available online 06 November 2018 0967-0645/ © 2018 Elsevier Ltd. All rights reserved. T