Agreement of net and acoustical methods for surveying euphausiids using a net based LED strobe light system

No abstracts are to be cited without prior reference to the author.

Euphausiids are well known for their ability to avoid capture by standard oceanographic plankton nets. During a study of euphausiid/herring interactions in Franklin Basin (Gulf of Maine), MOCNESS tows were made at night and during daylight to provide ground‐truth for acoustic surveying of the euphausiids and fish. A 1‐m2 MOCNESS was equipped with an LED‐based strobe light with peak output between 490 and 520 nm that filled a region in front of the net with light several orders higher than normal. Concurrent acoustic data were collected at 43, 120, 200, and 420 kHz. To evaluate its efficacy in increasing euphausiid capture, two horizontal tows were done at a site where euphausiids (mostly Meganyctiphanes norvegica) were present. During each tow (6075 m at night; 160–190 m during daylight), four of the eight nets sampled with the strobe flashing and four sampled with the strobe off, in a random sequence. There was a significant increase in biovolume catch of zooplankton when the strobe light was activated with total displacement volume increased by a factor of 2.2 at night and by a factor of 5.5 during the day. This was due largely to a higher abundance of euphausiids between 10 and 35 mm during the night (factor of 4.5) and during the day (factor of 11.0). Euphausiids caught with the strobe light on accounted for most of the observed volume backscattering at 43 and 120 kHz, whereas those caught with the strobe light off did not. In addition, with the strobe light on the abundance of euphausiids collected by the nets matched the acoustically estimated abundances.