DETECTION AND ABUNDANCE ESTIMATION OF EUPHAUSIIDS USING AN OPTICAL PLANKTON COUNTER

An optical plankton counter (OPC) mounted on a towed Batfish vehicle w as used to detect and estimate the abundance of euphausiids in the Eme rald and La Have Basins of the Scotian Shelf, northwest Atlantic Ocean . Euphausiids (Meganyctiphanes norvegica and Thysanoessa inermis) were identified from their characteristic diameter distributions measured by the OPC. The diameter distributions containing euphausiid peaks wer e compared to several depth strata known to contain these animals. The depth strata containing euphausiids were established from acoustical tows made on the same cruise and the identification of animals was acc omplished by sampling these acoustical layers with the BIONESS net sam pler. The diel migration patterns of the euphausiids were also identif ied from these diameter distributions. Euphausiid reaction to artifici al lights mounted on Batfish and BIONESS showed increases in abundance estimates by a factor of 2x and 50x respectively. Experiments with ar tificial lights mounted on a remotely operated vehicle and camera plat forms are also described. An artificial light mounted on a towed Batfi sh resulted in a distinct increase in euphausiid abundance estimates a s measured by the OPC. Abundance estimates from the OPC are also compa red to results from acoustical tows and showed agreement to within a f actor of 2 and better.