Larval distribution of offshore spawning flatfish in the Gulf of Alaska: potential transport pathways and enhanced onshore transport during ENSO events

Offshore and deepwater spawning flatfish species in the Gulf of Alaska, such as arrow-tooth flounder Atheresthes stomias and Pacific halibut Hippoglossus stenolepis, have juvenile nurseries that are inshore, in bays or at the mouths of bays. Larvae must emigrate from their spawning areas along the continental slope and outer shelf towards shore, in a direction across the prevailing Alaskan Stream. Using a 20 yr time series of data from ichthyoplankton surveys in the Gulf of Alaska, we examine patterns of variability in larval halibut and flounder distributions that may reflect processes resulting in successful recruitment to nursery areas. Several patterns can be observed in these data. Eggs and the smallest-sized larvae are distributed deep in the water column along the outer shelf and slope. Larger larvae tend to be located in the upper water column and farther inshore over the continental shelf. Larger larvae are also associated with deep-sea valleys and troughs that penetrate the shelf. Thus, these topographic features may serve as transport pathways to juvenile nursery grounds. ENSO (El Nino-Southern Oscillation) conditions and warm-year anomalies are linked to recruitment strength of Pacific halibut. Variability in larval transport as related to ENSO and other conditions that enhance onshore advection may play an important role in the recruitment of flatfishes to their nursery grounds. The results of this analysis indicate that larvae of both species are more abundant in coastal areas during El Nino events, and that a higher proportion of larvae are transported inshore during El Nino years.