Abstract
We investigated the environmental drivers of larval abundance of anchovy Engraulis encrasicolus and sardine Sardinops sagax in Algoa Bay, Eastern Cape (South Africa). This study comprised a pre-drought post-drought time period, comparing the responses of the fish larvae to different factors before and after the drought. The current study presents, for the first time, which environmental variables are affecting the anchovy and sardine larvae populations in the region. Easterly wind speed and zooplankton density were the only environmental variables that presented a significant change between the pre- and post-drought periods, increasing after the drought. Generalized additive models (GAMs) were used in order to explore the effects that environmental factors might have in the abundance of anchovy and sardine larvae in Algoa Bay. Specifically, the GAM that best explained the deviance of the anchovy larvae dynamics included the covariates rainfall, easterly wind speed, Chl a concentration, sardine larvae abundance and the interactions SST*Chla and sard*SST. The GAM best explaining sardine larvae abundance included only the easterly wind speed as a covariate. This model showed that there was a positive relationship between the higher values of wind speed and sardine larvae abundance.
Highlights
Fluctuations in the small pelagic fish populations have a major impact on the ecosystem and on fisheries (Cury et al 2000)
The response of small pelagic fish species to the environmental variability is manifested as inter-annual oscillations in recruitment success (Cury and Roy 1989; Cushing 1996; Bakun 2006); environmental regime shifts have resulted in abundance and distributional changes between sardine and anchovy in most upwelling areas worldwide (Roy et al 2007; Coetzee et al 2008)
New insights have demonstrated that recruitment variability can result from numerous abiotic and biotic processes, including fishing, maternal condition and other anthropogenic factors, operating on many different spatio-temporal scales, and represents an integrated process acting throughout early life stages of fishes (Houde 2008)
Summary
Fluctuations in the small pelagic fish populations have a major impact on the ecosystem and on fisheries (Cury et al 2000). The response of small pelagic fish species to the environmental variability is manifested as inter-annual oscillations in recruitment success (Cury and Roy 1989; Cushing 1996; Bakun 2006); environmental regime shifts have resulted in abundance and distributional changes between sardine and anchovy in most upwelling areas worldwide (Roy et al 2007; Coetzee et al 2008). The combination of physical factors and food availability has become widely accepted as the main environmental determinant of larval fish survival (Leggett and Deblois 1994; Cushing 1996; Costalago et al 2011), and thereby fish population dynamics (Beaugrand and Reid 2003). With the increasing threat of global climate change, there is an urge to understand which environmental factors influence the early stages of key species like sardine and anchovy and thereby further comprehend how these factors alter population growth
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