Oceanographic Influences on the Diet of 3 Surface-Migrating Myctophids in the Eastern Tropical Pacific Ocean

We examined the stomach contents of 3 vertically migrating myctophid fish species from the eastern tropical Pacific (ETP) Ocean and used a classification tree to examine the influence of spatial, biological, and oceanographic predictor variables on diet. Myctophum nitidulum (n = 299), Symbolophorus reversus (n = 199), and Gonichthys tenuiculus, (n = 82) were collected with dip nets from surface waters, and prey taxa were quantified from bongo net tows from August through November 2006. A classification tree produced splits with longitude and sea surface salinity (SSS), thereby separating 3 geographically and oceanographically distinct regions of the ETP (offshore, nearshore, and intermediate), where diet was similar among the 3 species. Myctophids consumed, primarily, ostracods offshore (76.4% mean percentage by number [MNi]), euphausiids nearshore (45.0%), and copepods (66.6%) in the intermediate region. The offshore region was characterized by a greater abundance of ostracods in the zooplankton community (17.5% by number) and within a deep mixed- layer depth (MLD) (mean 52.6 m, max 93.0 m). SSS was low in the nearshore region (< 32.9 psu) and the MLD was shallow. The intermediate region represented a transition zone between the oceanographic condition of the offshore and nearshore regions. Our results indicate that these 3 myctophid species share a similar regional diet that is strongly influenced by longitude, ostracod availability, SSS, and MLD.