Laboratory observations on the vertical swimming behavior of Japanese common squid Todarodes pacificus paralarvae as they ascend into warm surface waters

This study examined the effect of warm temperature on the survival of paralarvae of Japanese common squid Todarodes pacificus and on their swimming behavior as they ascended to the surface. Observations were conducted on paralarvae in Petri dishes and in 85-cm-tall, cylindrical tanks that had a warmer upper layer and cooler lower layer separated by a small thermocline. Paralarvae were obtained through artificial fertilization and reared in Petri dishes at six experimental temperatures between 20.9 and 30.4 °C. Paralarvae reared at lower temperatures survived longer than those reared at warmer temperatures, and survival decreased at temperatures above 24 °C. When the mean temperatures in the upper layer of the tanks were 24.4–26.0 °C, the paralarvae ascended through the thermocline to the surface, but when the mean temperatures in the upper layer were 29.7–29.8 °C, paralarvae stopped ascending at the thermocline. These results show that paralarvae have a temperature preference but ascend to the surface in the unfavorable temperature range. The results suggest that increasing surface temperatures at spawning grounds will negatively affect both the survival and behavior of T. pacificus paralarvae.