Hawaii FEP - Western Pacific Fishery Council

the cooler water tuna (e.g., bluefin, albacore) is more predictable and defined, with taggingstudies documenting regular, well-defined seasonal movement patterns relating to specificfeeding and spawning grounds. The oceanic migrations of billfish are poorly understood, but theresults of limited tagging work conclude that most billfish species are capable of transoceanicmovement, and some seasonal regularity has been noted (NMFS 2001).In the ocean, light and temperature diminish rapidly with increasing depth, especially in theregion of the thermocline. Many pelagic fish make vertical migrations through the water column.They tend to inhabit surface waters at night and deeper waters during the day, but several speciesmake extensive vertical migrations between surface and deeper waters throughout the day.Certain species, such as swordfish and bigeye tuna, are more vulnerable to fishing when they areconcentrated near the surface at night. Bigeye tuna may visit the surface during the night, butgenerally, longline catches of this fish are highest when hooks are set in deeper, cooler watersjust above the thermocline (275–550 m or 150-300 fm). Surface concentrations of juvenilealbacore are largely concentrated where the warm mixed layer of the ocean is shallow (above 90m or 50 fm), but adults are caught mostly in deeper water (90–275 m or 50–150 fm). Swordfishare usually caught near the ocean surface but are known to venture into deeper waters. Swordfishdemonstrate an affinity for thermal oceanic frontal systems that may act to aggregate their preyand enhance migration by providing an energetic gain through moving the fish along withfavorable currents (Olson et al. 1994).The Hawaii Archipelago’s position in the Pacific Ocean lies within the clockwise rotating NorthPacific Subtropical Gyre, extending from the northern portion of the North Equatorial Currentinto the region south of the Subtropical High, where the water moves eastward in the NorthPacific Current. At the pass between the MHI and the NWHI, there is often a westward flowfrom the region of Kauai along the lee side of the lower NWHI. This flow, the North HawaiianRidge Current (NHRC), is extremely variable and can also be absent at times. The analysis of10 years of shipboard acoustic Doppler current-profiler data collected by the NOAA researchvessel Townsend Cromwell shows the mean flow through the ridge between Oahu and Nihoa,and extending to a depth of 200 meters. (J. Firing, personal communication 2005).Embedded in the mean east-to-west flow are an abundance of mesoscale eddies created from amixture of wind, current, and seafloor interactions. These eddies can rotate either clockwise orcounterclockwise and have important biological impacts. For example, eddies create verticalfluxes, with regions of divergence (upwelling), where the thermocline shoals and deep nutrientsare pumped into surface waters enhancing phytoplankton production, and also regions ofconvergence (downwelling) where the thermocline deepens. Sea–surface temperatures aroundthe Hawaii Archipelago experience seasonal variability, but generally vary between 18°–28° C(64°–82° F) with the colder waters occurring more often in the NWHI.Significant sources of interannual physical and biological variations around Hawaii are El Niñoand La Niña events. During an El Niño, the normal easterly trade winds weaken, resulting in aweakening of the westward equatorial surface current and a deepening of the thermocline in thecentral and eastern equatorial Pacific.Water in the central and eastern equatorial Pacificbecomes warmer and more vertically stratified, with a substantial drop in surface chlorophyll.69