Trophic studies of mesopelagic lanternfishes (Myctophidae) in the Perth Canyon

The diet of mesopelagic lanternfishes (Myctophidae) was investigated in the Perth Canyon off the Western Australian coast. Previous studies have investigated the abundance and diversity of larval fishes in the waters of the canyon but most components of the pelagic food web and their relationships have not yet been studied. Taking advantage of their diel migration pattern, larval Diaphus sp. (n = 80) and adult Myctophum asperum (n = 41) and Myctophum phengodes (n = 7) were captured at night using a large 1 mm mesh size surface net. Concurrent netting of zooplankton was also undertaken to assess the availability of prey using 150 µm, 500 µm and 1 mm mesh nets. The gut contents of the myctophids were identified and revealed almost no nighttime feeding among larval Diaphus sp. confirming the relationship between ontogenetic stage, eye development and an inability to feed at night. In contrast, the morphometrics of adult M. asperum and M. phengodes and the size and number of prey in the diet revealed a positive relationship between fish size and prey size, but no significant increase in the number of prey as fish size increased. Calanoid copepods were consumed by 83% of M. asperum and 100% of M. phengodes specimens and, respectively, they constituted 39% and 62% of the number of prey items in their guts. Euphausiid adults were consumed by 48% of M. asperum and 100% of M. phengodes specimens and, respectively, constituted 15% and 25% of the number of prey items in their guts. Selectivity was assessed using Ivlev’s and Chesson’s indices and, contrary to their abundance in the diet, calanoids were not typically positively selected with both Myctophum species showing a preference for euphausiids. This investigation represents the first study of the role of myctophids in the pelagic food web in the Perth Canyon and Southeast Indian Ocean and as such is an initial baseline to fill this knowledge gap