Seasonal abundance and vertical distribution of mesopelagic calycophoran siphonophores in Monterey Bay, CA

Abstract

The seasonal abundance and vertical distribution patterns of a group of small calycophoran siphonophores (principally Chuniphyes multidentata and Lensia conoidea) were investigated using a remotely operated vehicle (ROV) deployed in Monterey Bay, California. Abundance was assessed along 295 horizontal transects covering a depth range of 100-1000 m over a three and a half year period. The vertical distribution of the study animals changed seasonally, coupled to the onset and cessation of upwelling in the bay. While numerical abundance peaked after upwelling, there was no significant difference between seasons. The siphonophores were more broadly distributed over the depth range sampled during the upwelling or Shallow Mixed Layer (SML) period, than during the non-upwelling or Deep Mixed Layer (DML) period. There were no significant differences in abundance or distribution patterns between years except in 1993, when there were significantly more siphonophores observed during the SML period than during the DML period. This may reflect effects resulting from the 1992-1993 El Nino event. The abundance of these siphonophores was negatively correlated with that of Nanomia bijuga, a physonect siphonophore of similar size and feeding behavior found in the bay. The siphonophores studied here appear from preliminary data to migrate vertically, possibly with two separately migrating groups.