Wind-mixing effects on feeding success and condition of blue whiting larvae in the Porcupine Bank area

Prey environment, feeding success and condition of blue whiting Micromesistius poutassou larvae were analysed in contrasting meteorological situations west of Ireland in the spring of 2 consecutive years (1994 and 1995). While larval abundance in both years of the study was approximately equal, there were marked differences in the physical and biological environment between the 2 years. 1994 was characterised by strong, unidirectional wind stress, which probably caused the low overall abundance of prey organisms recorded in this year. Abundance of copepod eggs and nauplii was considerably higher in 1995, when wind speeds were lower and wind direction was more variable. Copepod nauplii were also significantly larger than in 1994. Larval feeding success in both years was highly variable, but with considerably higher feeding intensities in 1995 than in 1994. For the determination of larval condition, 3 different condition indices were used, Fulton’s K index (FCI) and body height to body length ratio for between-year comparison, and carbon and nitrogen content for estimation of immediate effects of wind stress on larval condition. In 1994, the FCI of blue whiting larvae was significantly lower than in 1995, and 11% of the larvae were significantly lighter at length than their conspecifics, suggesting that a part (11%) of the larval population at any one time fed less successfully at low prey density and high turbulence levels. No differences between the years were observed using the index of body height at the anus versus standard length. Analyses of relative C and N content of blue whiting larvae in relation to varying wind stress showed that while wind stress had a minor negative effect on C content, relative N content was significantly reduced with increasing wind speed, indicating that at periods of high wind-mixing more protein is catabolised in order to satisfy increasing energy demands, thus leaving less material to be assimilated for growth.