Is the increased abundance of southern fish species in the southern North Sea during recent years a signal of a long-term trend, or can it be explained by random variability? Time-series data on the abundance of 12 southern fish species were derived from the 1st quarter International Bottom Trawling Surveys 1970-1994. Two periods of increased abundance were identified, one in the mid-1970s and one around 1990. Both periods coincided with positive anomalies of winter temperature and salinity. These anomalies in turn are correlated with the southerly wind component over. The Netherlands, suggesting increased inflow of Atlantic Water through the Strait of Dover. The conclusion is that the increased abundance does not reflect a long-term trend, but is the effect of temporary increased in southerly winds over the southern North Sea, resulting in increased transport of southern fish species into the North Sea, and favourable temperature conditions during winter.