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ICES Journal of Marine Science, 58: 1299–1317. 2001
doi:10.1006/jmsc.2001.1119, available online at http://www.idealibrary.com on

Long-term fluctuations in the eastern Atlantic and Mediterranean
bluefin tuna population

Christelle Ravier and Jean-Marc Fromentin

Ravier, C., and Fromentin, J.-M. 2001. Long-term fluctuations in the eastern Atlantic
and Mediterranean bluefin tuna population. – ICES Journal of Marine Science, 58:
1299–1317.
Long-term time-series of bluefin tuna catches from ancestral Mediterranean and
Atlantic trap fisheries are presented and analysed. The trap is a passive gear, little
modified for centuries, that catches bluefin tuna Thunnus thynnus during their annual
spawning migration. These features, together with preliminary analyses, lead us to
suggest that long-term fluctuations in trap catches could reflect those in true abun-
dance if they vary in the same manner all around the western Mediterranean and
adjacent Atlantic. To test this hypothesis, we investigated 54 time-series more than 20
years long (the longest ones spanning four centuries) of trap catches along the western
Mediterranean coasts of Italy, Sicily, Sardinia, Tunisia, Spain, and Morocco, and the
adjacent Atlantic coasts of Spain, Portugal, and Morocco. Trends and cycles were
identified using Eigen Vector Filtering and spectral analysis, and the synchrony
between short- and long-term fluctuations in trap catches was studied with the
modified correlogram of Koenig and Knops. The magnitude of fluctuations in trap
catches is large, periods of great abundance being up to seven times bigger than those
when abundance was low. More interesting was the occurrence of 100-year-long
periodic fluctuations as well as 20-year cycles. These medium- to long-term fluctua-
tions, representing more than 50% of the total variability in the time-series, were
synchronous all around the western Mediterranean and adjacent North Atlantic. In
contrast, short-term variability was synchronous at a local scale only. It is argued that
long-term fluctuations in trap catches could be considered as a proxy for those of true
abundance, and a synthetic time-series has been computed to depict them. Biological
and ecological processes that could cause such long-term fluctuations are also
discussed.
2001 International Council for the Exploration of the Sea
Keywords: periodic fluctuations, Thunnus thynnus, time-series, trap fishery, trend.
Received 20 February 2001; accepted 10 July 2001; published electronically 2 October
2001.
C. Ravier and J-M. Fromentin: IFREMER, De´partement Ressources Halieutiques,
BP 171, 34203 Se`te Cedex, France; tel: +33-499-573232; fax: +33-499-573295.
Correspondence to Jean-Marc Fromentin; e-mail: Jean.Marc.Fromentin@ifremer.fr

Introduction fluctuations in fish populations to variations in environ-
mental conditions (Cushing and Dickson, 1976;
Fluctuations in fish stock sizes are often ascribed to Cushing, 1982; Southward et al., 1988; Dickson and
human exploitation, probably because most studies Brander, 1993; Fromentin et al., 1998) or to changes in
focus on highly exploited populations (such as the North biotic processes (May, 1974; Myers and Cadigan, 1993;
Atlantic cod Gadus morhua stocks) and over relatively Fortier and Villeneuve, 1996; Fromentin et al., 2001) is
short time periods (<50 years, e.g. Garrod and not negligible.
Schumacher, 1994; Hutchings, 1996; Myers et al., 1996;
Cook et al., 1997). However, at the beginning of this In this paper, we consider several centuries of catches
century, Hjort (1914, 1926) suggested that stochastic of bluefin tuna Thunnus thynnus (Linne´ 1758) by trap
processes during the recruitment period could also fisheries, an ancestral Mediterranean fishing technique
induce natural fluctuations in stock size. A scarcity of (Doumenge, 1998). These long-term time-series could
long time-series has probably limited research on help in describing bluefin tuna population dynamics if
this issue, but the number of studies linking long-term fluctuations in catches can be considered a good proxy
of true abundance. Eastern Atlantic bluefin tuna are

1054–3139/01/061299+19 $35.00/0 2001 International Council for the Exploration of the Sea