Sagitta lyra, a biological indicator species in the subarctic waters of the eastern Pacific Ocean

In the Gulf of Alaska the chaetognath, Sagitta lyra, was most frequently found in areas where the temperature at 150 m exceeded 6 °C and the salinity exceeded 33‰. Water of this character was mainly associated with a northward intrusion of warm water past the Canadian coast. Comparison of surface gillnet catches of salmon with occurrence of S. lyra showed that when S. lyra was present in the zooplankton, the salmon catch was small and when it was absent the catches of salmon could be quite large. This change in catch was even observed (on two occasions) on successive days at a station where the physical properties of the water did not change detectably.