On the Biology of Calanus finmarchicus. III. Vertical Distribution and Diurnal Migration in the Clyde Sea-Area

Abstract

1. Previous work on vertical distribution and diurnal migration is described.2. The diurnal migration of Calanus was studied on two occasions in Loch Fyne (January and July, 1932).3. Vertical hauls were taken every three hours with a closing net dividing the total depth into six sections.4. The results for each copepodite stage of Calanus are discussed, and January and July conditions are compared where possible.5. Ova and nauplii were observed to be most abundant in the top 30 metres.6. Copepodite Stages I, II, and III were most abundant above 30 metres and only the third copepodites showed any tendency to descend as the light increased.7. Stage IV showed a migration towards the surface at night, but were generally distributed during the next day.8. It is suggested that the results for this stage are probably confused owing to its transitional nature between the young stages living at the surface and Stage V Calanus living in deep water.9. Stage V was found always in deep water, slight diurnal changes being ascribed to the presence of such Calanus as were about to moult into adults.10. Females showed definite diurnal migration in both January and July.11. Males showed a general distribution and migrated in small numbers towards the surface at night and away from it during daylight.12. Stage V copepodites and females both lived nearer the surface in January than in July, correlated with seasonal changes in the intensity of the sunlight.13. The presence of swarms of Calanus at the surface under conditions of bright sunlight is discussed.