Water column structure influences long-distance latitudinal migration patterns and habitat use of bumphead sunfish Mola alexandrini in the Pacific Ocean

Abstract

Satellite-tracking of adult bumphead sunfish, Mola alexandrini, revealed long-distance latitudinal migration patterns covering thousands of kilometers. Horizontal and vertical movements of four bumphead sunfish off Taiwan were recorded with pop-up satellite archival tags in 2019–2020. Two individuals moved northward and traveled to Okinawa Island and Kyushu, Japan and two moved southwards; crossing the equator, to Papua New Guinea and New Caledonia. During daytime, bumphead sunfish descended below the thermocline and ascended to mixed layer depths (MLD) during nighttime. The N–S migrants, however, demonstrated different habitat utilization patterns. Instead of using prevailing currents, the northward movements of sunfish cohorts exhibited extensive use of mesoscale eddies. Fish in anticyclonic eddies usually occupied deeper habitats whereas those in cyclonic eddies used near-surface habitats. On northward excursions, fish spent most of their time in regions with high dissolved oxygen concentrations. Southward movement patterns were associated with major currents and thermal stratification of the water column. In highly stratified regions, fish stayed below the thermocline and frequently ascended to MLD during daytime either to warm muscles or repay oxygen debts. These results for bumphead sunfish present important insights into different habitat use patterns and the ability to undergo long-distance migrations over varying spatial-temporal scales and features.

Spatially varying effects of the California Undercurrent on Pacific hake distribution

“…This may be particularly important in the southern portion of the study area where the undercurrent is the strongest and age-2 hake tend to be most prevalent. Although the degree to which ocean currents may assist pelagic fish migration is poorly understood, other marine species such as turtles, whales, and bumphead sunfish (Mola alexandrini) have been shown to have ocean current-assisted migration (Gaspar et al 2006;Shillinger et al 2008;Tyson Moore et al 2020;Chang et al 2021). Supporting this migration assistance hypothesis is our finding that the center of gravity for age-2 hake had the strongest correlation with mean annual northward velocity (Table 3), suggesting the youngest and smallest hake tend to be distributed further north in years with a stronger northward flowing undercurrent.…”

Rapid physiological colouration change is a challenge - but not a hindrance - to successful photo identification of giant sunfish (Mola alexandrini, Molidae)

“…However, another individual tagged with a satellite tag first remained local for c. one month, then moved 2,700 km westward in c. two weeks (Thys et al, 2017). Similarly, some individuals tagged in the Nusa Penida MPA have been tracked moving away hundreds of km, both within and outside the sunfish season (Thys et al, 2016;Nyegaard, 2018), and most recently, four M. alexandrini tagged in Taiwan moved between 552-6,952 km; two travelled north (one reached Japan after 178 days), while two travelled south and crossed the equator, with one reaching New Caledonia after 240 days (Chang et al, 2021).…”