Diel horizontal migration of the Hawaiian mesopelagic boundary community observed acoustically

Abstract

ABSTRACT:The mesopelagic boundary community off the leeward coasts of 2 Hawaiian Islands, Oahu and Hawaii, was investigated with an echosounder modified to read directly into a laptop computer. Acoustic sampling was conducted over a total distance of 12.6 km off the Waianae coast of Oahu and 46.3 km off the Kona coast of Hawaii. The density of organisms was determined using echo energy integration, and relative abundance was determined in a way analogous to catch-per-uniteffort. The vertical range of mesopelagic organisms expanded as the mesopelagic layer rose and then compressed as it descended. The vertical range of the layer off the Kona coast was larger than that off the Waianae coast, possibly because of the greater bottom depth off Kona. Near midnight, the boundary community 3 km from the shoreline was split into 2 distinct layers, one beginning approximately 25 m from the surface and one beginning approximately 90 m from the surface. The density and the relative abundance of mesopelagic organisms were consistently higher off the Waianae coast than the Kona coast. However, the density of organisms observed in both locations was high, reaching a maximum of 1800 organisms m -3 off Waianae and 700 organisms m -3 off Kona. The maximum relative abundance off Waianae neared 100%, while off Kona it never exceeded 70%. In both locations, organisms were found within 1 km of shore, in waters much shallower than their assumed daytime habitat. The temporal patterns of relative abundance and density of organisms in waters closest to the shores of each island resembled a bell curve, with a peak in relative abundance and density around midnight. In waters further from shore, the temporal patterns in relative abundance and density had a bimodal distribution, with peaks around both 21:00 and 03:00 h. These patterns in relative abundance and density are significantly affected by the distance of the sampling location from the shoreline, but not by the depth of the sampling site. The data suggest that the organisms of the mesopelagic boundary community undergo a diel horizontal migration that is reciprocal, 1.8 km in 2 h toward shore and then the same distance in approximately the same amount of time away from shore, in addition to their well-established vertical migration. The temporal patterns of the horizontal component of the migration are predictable and are conserved between days, phases of the moon, seasons, and islands.

Using active acoustics to compare lunar effects on predator–prey behavior in two marine mammal species

“…This feature was not determined to be spatially correlated in previous studies in New Zealand, so no space effects were taken into account (Benoit-Bird et al 2004). However, in Hawaii, distance from the shoreline is a critical parameter in determining scattering layer depth (Benoit-Bird et al 2001), so a minimum depth was calculated for each distance from the shoreline. For both locations, the time relative to sunset at which the scattering layer passed 150 m on its upward migration and the time relative to sunrise that 150 m was passed on its downward migration were measured, taking distance from shore into account in only the Hawaii data.…”

“…The scattering layer off New Zealand remained at least 30 m from the surface in all samples. In Hawaii, horizontal migration of micronekton meant that vertical movement was constrained by the seafloor as the layer approached the island (Benoit-Bird et al 2001, BenoitBird & Au 2006. This horizontal migration also resulted in differential depth movement, with layers closer to the shoreline also found closer to the surface than those that made less extensive horizontal migrations.…”

“…Off both Hawaii and Kaikoura, the behavior of the mesopelagic animals combined with the local topography brings these prey layers unusually close to the shoreline (Robertson et al 1978, Cipriano 1992, Benoit-Bird et al 2004, Benoit-Bird & Au 2006. In both habitats, the mesopelagic micronekton layers undergo extensive diel migrations, moving from daytime depths of up to 600-1000 m, where they are inaccessible to dolphins, to within meters to a few tens of meters of the surface at night, where they are exploited by the dolphins (Cipriano 1992, Benoit-Bird et al 2001, Benoit-Bird & Au 2003b. Both species of dolphins follow diel movements in their prey, resulting in substantially shallower dives near the middle of the night than near crepuscular periods (Benoit-Bird & Au 2003a, Benoit-Bird et al 2004 One of the most significant factors affecting the behavior of diel migrators is light, both solar and lunar (Blaxter 1974).…”

Occurrence of White Sharks (Carcharodon carcharias) in Hawaiian Waters

“…Spinner dolphins closely follow the horizontal and vertical movements of these boundary community prey organisms [28]. The shallowest and most nearshore position of the boundary community, and associated spinner dolphins, is typically at midnight [29]. Both the Weng and Jorgensen papers provided figures detailing the transition from deep to shallow behavior occurring during crepuscular periods.…”

Importance of Thin Plankton Layers in Hawaiian Food Web Interactions: Research Spanning from Physical Circulation to Spinner Dolphins

“…A bottom-mounted ADCP was deployed at the 25 m site to measure full water column current patterns in the absence of the scattering layer. Previous attempts to utilize an acoustic Doppler current profiler (ADCP) during the layer's migration (night time hours) have had limited success because of high scattering from the rapidly moving mesopelagic micronekton (Benoit-Bird et al, 2001). Consequently, in the past only currents between mesopelagic patches or above the mesopelagic layer could be measured.…”