RECONSTRUCTION OF THE HISTORY OF PACIFIC SARDINE AND NORTHERN ANCHOVY POPULATIONS OVER THE PAST TWO MILLENNIA FROM SEDIMENTS OF THE SANTA

We present a composite time series of Pacific sardine and northern anchovy fish-scale-deposition rates developed from the anaerobic varved sediments of the Santa Barbara Basin off southern California. These series were constructed by integrating and adding to the data sets developed by Soutar and Isaacs and cover the period from A.D. 270 through 1970. We offer these series as a current best estimate of the sardine and anchovy scale-deposition rates and point out the sources and ranges of uncertainty remaining in the data. This includes providing a detailed analysis to determine the strength of the signal compared to the noise in scale-deposition rates. We also recalibrate the scale-deposition data using available population estimates; we use this recalibration to hitidcast the variability in sardine and anchovy stocks through aimost 2000 years. Spectral analysis of the scale-deposition series shows that sardines and anchovies both tend to vary over a period of approximately 60 years. I n addition, the anchovies fluctuate at a period of 100 years. The anchovy and sardine series show a moderate correlation over long time scales of several centuries or more; the correlation of shorter-period components in the time series is virtually nil. The scale-deposition record shows nine major recoveries and subsequent collapses of the sardine population over 1700 years. The average time for a recovery ofthe sardine is 30 years. A recovery is defined as an increase from less than one to over four million metric tons of biomass. We find that the current recovery is not unlike those of the past in its rate or magnitude. RESUMEN Presentamos series de tieinpo de las tasas de depositacion de escainas de la sardiiia del Pacific0 y de la anchoveta nortefia obtenidas de 10s sedimeiitos laminados anaerobicos de la cuenca de Santa Barbara, a1 Sur de California. Estas series fueron construidas a1 intcgrar y completar 10s datos de Soutar 'Manuscript completed while a visiting scientist at the Marine Life Research Group, Scripps Institution of Oceanography, University of California, San Diego. 24 e Isaacs e incluyeii de 270 a 1970, 1l.C. Ofrecemos Cstas series coni0 la mejor estimacioii a la feclia de las tasas de depositaci6n de escamas de sardiiias y anchovetas y subrayaiiios las filentes 1rangos de iiicertiduiiibre quedando en 10s datos. Se iiicluye L I I ~ aiiilisis detallado para distinguir entrc la fuerza de la sefial respecto a1 ruido en las tasas de depositaci6n de escamas. TambiCn ofrecemos una recalibracioii de 10s datos de depositaci611 utilizando estiinacioiies disponibles de la abundancia de la poblacioii. Utilizarnos esta recalibracioii para estiinar la variabilidad de 10s stocks de sardinas y anchovetas durante casi 2000 afios. El cilculo del espectro de las series de depositacion muestran que tanto las sardinas como las anchovetas tienden a variar con un periodo de aproximadamente 60 afios, mientras que por otro lado las anchovetas fluct~an con pcriodo de 100 arios. L is series de anchovetas y sardiiias muestran una correlacion mediaiia en escalas temporales de siglos o mayores, mieiitras que la correlacion en periodos n i i s cortos es virtualmcnte nula. El registro de depositaci6n de escamas inuestra que durante 1700 afios la poblacion de sardinas tuvo nueve recuperaciones principales seguidas de nueve colapsos. El ticmpo proiiiedio de recuperacion de la sardina es dc 30 afios; se define rccuperacion como un incremento en la biomasa de menos de uno a m i s de cuatro inilloiies de toneladas. Encoiitrainos que el proceso de recuperacidii actual es similar a 10s del pasado en SLI tasa y inagnitud. INTRODUCTION Traditional efforts to deterniiiie the fundamental time scales, and the sources, of variability in population sizes of Pacific sardine and other small pelagic fishes of tlie California Current are hindered because these stocks expand and contract over periods of several decades or longer. After roughly 60 years of annual biomass estimates for tlie Pacific sardine, fisheries scientists are able to describe only one major oscillation in its abundance. This oscillation appears to be nearing completion as the current sardine recovery gets well under way. BAUMGARTNER ET AL.: HISTORY OF PACIFIC SARDINE AND NORTHERN ANCHOVY POPULATIONS CalCOFl Rep., Vol. 33,1992 Fortunately, the annually layered (varved) sediments in the Santa Barbara Basin off southern California provide a natural historical record of pelagic fish populations (Soutar 1967). Not only does the Santa Barbara Basin underlie a portion of the spawning grounds of the Pacific sardine and northern anchovy, but also anaerobic conditions below sill depth preserve fish scales within a much more detailed chronological framework than is normal for marine sediments. Because they prevent the homogenization of sediments that results from mixing by benthic animals, anaerobic conditions produce a yearly memory of processes in the pelagic ecosystem. Soutar and Isaacs (1969) developed time series of fish-scale counts for small pelagic species including the Pacific sardine and northern anchovy; these series were based on the analysis of a piston core from the Santa Barbara Basin and extend back over nearly two millennia. After developing these long time series, Soutar and Isaacs turned their attention to constructing shorter series covering the 160 years from 1810 through 1970. These series were based on a much expanded and therefore more reliable data base of four box cores (Soutar and Isaacs 1974). Development of these high-quality time series of scaledeposition data into the twentieth century also made it possible to compare and integrate the paleoecological record with direct estimates of population biomasses. These data constituted the first continuous time series of fossil fish and offered a picture of variability over periods significantly greater than a century. Sequences of fossil scales have since been constructed from the slope sediments off Peru (DeVries and Pearcy 1982) and for the Benguela margin off West Africa (Shackleton 1986). But the lack of well-developed, continuously varved records in these places has so far prevented the reconstruction of time series to equal the quality of those from the Santa Barbara Basin. An overriding lesson from the Santa Barbara records is that in the past both sardines and anchovies experienced large natural fluctuations which were clearly unrelated to fishing, and that abrupt natural declines, similar to the collapse of the sardines during the 1940s, are not uncommon. An obvious shortcoming in the study by Soutar and Isaacs (1969) is that the very small area sampled by a single piston corer (7.6-cm diameter) is used to represent scale deposition over the entire basin. Another constraint on their results derives from the uncertainty in the underlying chronology due to imperfect preservation of the annually deposited layers over the entire length of the record. One of the goals of this paper is to provide improved composite time series of sardine and anchovy scale deposition covering most of the past two millennia. We developed these new time series by integrating data from a second piston core with the original data presented by Soutar and Isaacs (1969), and by substituting the box core data for the upper portion of the two piston cores. The new sardine and anchovy series provide significantly more reliable estimates of the scaledeposition rates (SDR’s) and are now presented as a continuous record from approximately A. D. 300 through 1970. The evolution of the improved SDR series is documented in the following section, along with a description of the efforts to achieve a more accurate chronological base for the series. A second goal of this paper is to examine the quality and inherent limitations of the composite sardine and anchovy time series constructed by averaging information from the two piston cores. We have made a detailed analysis of the strength of the SDR signals relative to the noise that tends to obscure the information in which we are interested. This analysis is presented in the section “Signal-to-Noise Ratios in Scale-Deposition Rates, ” which documents the range of uncertainty in the SDR values and evaluates our current effort to provide reliable series for hindcasting the changes in fish populations. How the SDR series can be used to describe past variability in the sizes of sardine and anchovy stocks is illustrated in the section “Hindcasting Population Sizes from Scale-Deposition Rates. ” Our final goal is to describe the principal time scales over which the variability occurs in the SDR series and to apply this information to a historical analysis of the inferred recoveries and collapses. This goal is met in the sections “Time Scales of Variability in the SDR Series” and “Recoveries and Collapses of the Pacific Sardine. ” We also compare past recoveries with the current one. RECONSTRUCTING THE T IME SERIES OF SCALE-DEPOSITION RATES The essential foundation for reconstructing any time series from the sediment record is the chronology assigned to a sediment column. Detailed radiometric dating of the modern laminated sediments in the Santa Barbara Basin with 210Pb and 228Th/232Th has demonstrated that the light-dark lamina pairs are annually deposited varves (Koide et al. 1972; Soutar and Crill 1977; Bruland et al. 1981). Varved sediments began to form in the Santa Barbara Basin as early as 8000 years ago and have provided a much more detzi!pd chronological framework for reconstructing paleoecologic and climatic histories than is