Abundance, horizontal and vertical distribution of epipelagic ctenophores and scyphomedusae in the northern Bering Sea in summer 2017 and 2018 : Quantification by underwater video imaging analysis

We examined the abundance and horizontal and vertical distributions of epipelagic ctenophores and scyphomedusae in the northern Bering Sea using an underwater video camera during July of 2017 and 2018. The effects of environmental and biological parameters on the distribution of these species were evaluated by generalized additive modelling (GAM). In 2017, the dominant ctenophore, Bolinopsis infundibulum, was mainly distributed in the north and west of St. Lawrence Island (SLI), and their vertical distribution varied with the region but not by the time of day. We found that B. infundibulum was distributed in the upper pycnocline north of SLI, but below the pycnocline west of SLI. Biological interactions with other gelatinous zooplankton may explain these regional differences in vertical distribution; GAM analysis revealed a negative interaction between B. infundibulum and the large scyphomedusa, Chrysaora melanaster, which occurred in the upper layer in the west of SLI. B. infundibulum may avoid that layer to reduce feeding competition. For the ctenophore, Beroe sp., vertical and horizontal distributions were similar to those of B. infundibulum, and GAM analysis also revealed a positive interaction for both species. As B. infundibulum is an important prey of Beroe sp., a prey-predator interaction may result from their similar horizontal and vertical distributions. Standing stocks of epipelagic ctenophores and scyphomedusae in 2018 were low compared to those in 2017, by a factor of 1/20 (C. melanaster) and1/90 (Beroe sp.). This might be due to annual differences in water mass in this region, in that the thermal conditions characterized by a high abundance of the dominant B. infundibulum in 2017 (8 degrees C) were absent in 2018. As this drastic decrease in standing stock in 2018 was apparent for both ctenophores and scyphomedusae, food availability was hypothesized to be poor that year

Horizontal and vertical distribution of the appendicularian community and population structure in the Bering and Chukchi Seas during the summer of 2007

The horizontal and vertical distributions of the appendicularian community and the population structure of dominant species in the southeastern and northern Bering Sea shelf and the Chukchi Sea were studied during the summer of 2007. The feeding impact was also calculated and the results were compared with those found in 1983-1996. Oikopleura vanhoeffeni was the dominant species in this region and stage I specimens with small tail lengths ( : TL 14 mm) dominated in 1983-1996 and, considering the generation length of O. vanhoeffeni (ca. one year), the main spawning of O. vanhoeffeni would not have occurred before summer from 1983-1996. In 2007, O. vanhoeffeni are considered to have spawned earlier because newly recruited small individuals are more abundant. Recently, the timing of the sea ice retreat is becoming earlier in this region, and the fastest ice-free timing was reported in 2007. Earlier sea ice retreat may induce an altered timing of primary production and other biological phenology. Thus, the early spawning of O. vanhoeffeni in 2007 may result in the dominance of smaller specimens observed in that year