Northwest Fisheries Science Center

The Main Deck

Acoustic and trawl adventures in the Northeast Pacific

This portal tracks the research and sea-going activities of the Fisheries Engineering and Acoustic Technologies (FEAT) Team from NOAA¿s Northwest Fisheries Science Center.  Follow us as we use acoustics, trawling, and oceanographic sampling to learn about the Northeast Pacific Ocean.

Back deck of Bell M. Shimada
Acoustic echogram of hake
trawl catch

Day versus night

By Sandy Parker-Stetter
January 29, 2016

A few of our original winter survey questions focused on Pacific hake “aggregations” (referring to a group of fish that aren’t necessarily behaving/swimming in an organized way as they do in a “school”) and what they do at night. We wondered if they stayed at the same depths during day and night. Further, if they did move up/down, we wondered if it was just males, just females, or both. We wonder about a lot of things.

Wondering aside, we are conducting 24-hour operations so have had lots of chances to see what hake do in the darkness and in the daylight. 

We see the adult hake on the acoustic displays both day and night, but the aggregations change. On the echogram, you can see that during the day the adults are in a tight layer down deep. During the night, they are much more spread out, but still identifiable, in shallower water. This is different than during the summer when the hake are very dispersed, and difficult to identify, at night. During the dusk period, the hake move up. During the dawn period, the hake move down. 

Acoustic echogram showing adult hake deep in the water during the day (left) and much shallower at night (right).  Image credit NWFSC-FRAM-FEAT

The dawn/dusk movement takes place over several hours, but the layer stays intact. The compound echogram shows the hake shallow at night (where we fished them), their migration back down at dawn, and the hake down deep during day (where we again fished them). You can also see the layer being disrupted by a piece of equipment that was lowered down to 500 m. Our trawls suggest that both males and females are doing this day/night migration.

Compound echogram showing the hake layer (in white) at night, moving down, and then during day.  Note that when we lowered a temperature sensor in the water, the layer was disrupted and briefly disappeared!  Image credit Sandy Parker-Stetter (NWFSC)

Tagged: Winter hake survey

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