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Lindegren, M, Checkley DM.  2013.  Temperature dependence of Pacific sardine (Sardinops sagax) recruitment in the California Current Ecosystem revisited and revised. Canadian Journal of Fisheries and Aquatic Sciences. 70:245-252.   10.1139/cjfas-2012-0211   AbstractWebsite

Small pelagic fish typically show highly variable population dynamics due, in large part, to climate variability. Despite this sensitivity to climate, few stocks of pelagic species are managed with consideration of the environment. The Pacific sardine (Sardinops sagax) represents a notable exception, for which sea surface temperature (SST) from the Scripps Institution of Oceanography (SIO) pier has been used, until recently, to adjust exploitation pressure under warm (favorable) and cold (unfavorable) climate conditions. Recently, the previously established temperature-recruitment relationship was reassessed using different methods, resulting in abandonment of the temperature-sensitive harvest control rule in 2012. In this study, we revisit the previous temperature-recruitment relationship using the original methodology and an updated data set from 1981 to 2010. In contrast to the recent reassessment, we find temperature explains significant variability in recruitment and recruitment success. We also show that mean annual SST averaged over the present California Cooperative Oceanic Fisheries Investigations area is a better predictor of recruitment variability than SST at the SIO pier. We propose that sustainable management of the Pacific sardine should consider climate variability and that the basis for this be periodically updated and revised to inform management with the best available science.

Mullin, MM, Checkley DM, Thimgan MP.  2003.  Temporal and spatial variation in the sizes of California current macrozooplankton: analysis by optical plankton counter. Progress in Oceanography. 57:299-316.   10.1016/s0079-6611(03)00103-4   AbstractWebsite

Macrozooplankton in the southern California sector of the California Current had been reported to decrease from 1951 to 1998. We analyzed some of the same preserved samples of macrozooplankton taken in non-El Nino years 1955, 1956, 1966, 1981, 1984, 1991, 1995, and 1996, and also 1999, with the optical plankton counter (OPC) to determine whether all size categories changed similarly over time. The results from 1955 to 1996 could be interpreted either as a linearly decreasing trend (total biovolume decreased by 45%) or as a regime shift (decrease of 38% from pre- to post-1975 regimes). The largest zooplankters (>2.7 mm equivalent circular diameter, ecd) were relatively more important, in terms of biovolume, at night than by day, and offshore than onshore. Their biovolume decreased by the greatest relative amount, and biovolume of the smallest zooplankters (0.75-0.80 mm ecd) decreased hardly at all. The decrease in relative importance of large zooplankters was most evident in winter and spring, and was qualitatively similar by day and at night and in nearshore and offshore regions. Total biovolume increased in 1999 to the pre-1975 level, consistent with a possible shift to a new regime. Our results are consistent with (i) a change in biovolume of the large zooplankton over the sampled period; and (ii) a regime shift in the mid-1970s and, possibly, the late 1990s. The laboratory OPC is a useful instrument for the rapid and cost-effective analysis of preserved samples of zooplankton. (C) 2003 Elsevier Ltd. All rights reserved.

Jahncke, J, Checkley DM, Hunt GL.  2004.  Trends in carbon flux to seabirds in the Peruvian upwelling system: effects of wind and fisheries on population regulation. Fisheries Oceanography. 13:208-223.   10.1111/j.1365-2419.2004.00283.x   AbstractWebsite

We hypothesized that change in the annual population size of guano-producing seabirds (cormorant, Phalacrocorax bougainvillii; booby, Sula variegata; pelican, Pelecanus thagus) is a response to changes in primary and secondary production of the Peruvian upwelling system. We tested this hypothesis by modeling nitrate input through upwelling to the upper layers of the ocean off Peru between 6degrees and 14degreesS using data on wind stress and sea surface temperature. The model predicted the amount of carbon fixed by primary production each year from 1925 to 2000, which was then apportioned to the Peruvian anchovy (Engraulis ringens) biomass and ultimately to the seabird population and the anchovy fishery, the largest single-species fishery on Earth. The model predicted a marked increase in primary production as a consequence of increasing wind stress. It overestimated the anchovy biomass after the collapse of the fishery in 1972, but closely predicted the growth of seabird populations from 1925 to the mid-1960s, and their decline thereafter, explaining about 94% of the variation in seabird numbers from 1925 to 2000. The model indicates the seabirds consumed 14.4% of the available anchovies and, thus, that seabirds consumed 2.3% of the new production, before the development of the anchovy fishery, and only 2.2% of the available anchovies and 0.3% of the new production after the development of the fishery. The model results clarify the roles that environmental and anthropogenic factors may have had in regulating the guano-producing seabird populations. It indicates that the growth of seabird populations from 1925 to 1955 was likely a response to increased productivity of the Peruvian upwelling system and that the subsequent drastic decline in seabird abundance was likely due to competition for food with the fishery, which caught similar to85% of the anchovies, which otherwise would have been available for the seabirds. This model also shows that an increase in oceanic primary production promotes reproductive success and population growth in higher trophic level organisms.