VERTICAL-DISTRIBUTION AND MIGRATION OF FISH LARVAE IN A NORTHEAST PACIFIC BAY

Fish larvae were collected at 6 depths in Auke Bay, southeastern Alaska, USA, on 4 days in May and June 1987. On 3 dates samples were collected between 08:00 and 13:00 h, on the other date samples were collected every 4 h during 24 h. Vertical distributions of light, temperature, salinity, chlorophyll and copepod nauplii were also measured. Wind speed data were available from a nearby station. Smelt (Osmeridae) larvae, the most abundant larval fish taxon, migrated to the surface at midnight, whereas walleye pollock, flathead sole, rock sole, poacher (Agonidae) and northern smooth-tongue moved deeper at dusk. During the day, most fish larvae were concentrated from 5 to 15 m, broadly overlapping the highest abundances of copepod nauplii, although poacher and northern smoothtongue were consistently deeper (15 to 25 m). Diurnally, smelt larvae were shallow (5 to 10 m), and pycnocline depth accounted for more variability in their mean depth than any other biotic or physical variable. Mean depths of the other 5 most abundant taxa were in the same rank order by depth in all 4 sets of daytime samples, and depth of isotherms below the pycnocline accounted for more variation in their mean depths than did any other variable. The diurnal depth distributions we observed apparently result from temperature preferences among marine larval fishes, with descent at dusk as the most common form of diel vertical migration.