Interaction of ocean currents and resident micronekton at a seamount in the central North Pacific

Seamounts are often characterized by unique faunas, which may include vertically migrating micronekton species. The two dominant seamount-associated micronekton at Southeast (SE) Hancock Seamount on the northern Hawaiian Ridge are the sternopthychid fish Maurolicus muelleri and the lophogastrid mysid Gnathophausia longispina. Distribution patterns for both species, based on midwater trawl and acoustic data, were compared with acoustic Doppler current profiler-measured ocean currents during four research cruises in 1987 and 1988. Populations of both species migrated above the seamount nightly. The distributions of these two species over the seamount were analyzed through acoustic surveys over the seamount flanks and summit. Differences between distributions in the first and second halves of the night were regulated by a combination of behavioral and physical processes. Ocean currents affected the localized nighttime aggregations over different regions of the summit within and between cruises. Currents over the summit had the potential to advect animals an average of 6–7 km per night during the first two cruises and about 2 km per night during the last two cruises. Net and acoustic results indicated that G. longispina and M. muelleri were displaced to the downstream side of the summit through the night, but the distances of displacement were less than would have been predicted by advection alone. Both G. longispina and M. muelleri appeared to resist advective loss from the seamount, although the short-term distributional patterns of the two species were subject to current-mediated influences. Very few specimens were taken 5 and 10 km from the summit or above 100 m depth. Behavior patterns apparently facilitated the maintenance of these populations at this isolated seamount.