| Abstract | There are accumulating reports showing that the El Niño/Southern Oscillation (ENSO) has a significant effect on the population dynamics of marine fishes. However, the influence of ENSO on the physiology of fishes, as possible components of those ecological changes in fish populations, is not fully understood. This study investigated the cellular, physiological, and organismal stress responses in a wild fish population under natural thermal stress. The sea surface temperature at the subtropical ocean of Okinawa, Japan, was the highest in the last 10 years (>32°C) during the summer of 1998 with a strong ENSO. To examine the effects of those unusually warm ocean temperatures on fish, we compared tissue 70-kDa heat shock protein (HSP70) levels and growth rates between the ENSO summer of 1998 and the normal summer of 1999 in a common fish species in Okinawa, the Indo-Pacific sergeant, Abudefduf vaigiensis. We also conducted a complementary heat shock experiment in the laboratory. The field collected Indo-Pacific sergeant had significantly higher muscle HSP70 levels in 1998 than 1999. Higher muscle HSP70 and plasma cortisol levels were observed at 32°C than at 28°C in the laboratory heat shock experiment, indicating that the highest summer ocean temperature in 1998 was sufficient for the fish to up-regulate the cellular and physiological stress responses. In support of this effect, otoliths showed slower growth rates of the fish during the summer of 1998; this may reflect the significant energetic cost of these stress responses. |
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