During January–May, surface manifestation of multiple, individual basin‐scale fronts accentuate the central North Pacific Subtropical Frontal Zone (STFZ) system. The most prominent of these fronts are climatologically located at 32°−34°N and at 28°−30°N latitudes [herein nominally referred to as the ‘Subtropical Front’ (STF) and the ‘South Subtropical Front’ (SSTF), respectively], although considerable interannual variability in both position and intensity is observed. This seasonally dynamic system is also the region typically targeted by the Hawaii‐based swordfish (Xiphias gladius) longline fishing fleet, where the presence, position, and strength of the convergent fronts are believed to play a key role with regard to the catch and catch rates of swordfish. Information furnished by a recent series of meridional hydrographic surveys and concurrent satellite remote sensing data elucidate structural patterns and coupling of the physics and biology associated with these fronts. This enables a re‐characterization of the spring North Pacific STFZ and offers new insight into the seasonal variability of the phytoplankton dynamics in the subtropical North Pacific. On synoptic time scales, geographical positioning of the fronts may be systematically identified through surface outcropping of diagnostic thermohaline isopleths and therefore readily discerned from both shipboard surveys and by spaceborne sensors. The STF during spring can be characterized by the surface expression of the 34.8 isohaline and the 17°C isotherm within the frontal gradient. Biologically, the STF marks the transition from low chloropigment (chlorophyll + phaeopigments), nutrient‐depleted surface waters to the south to a more productive regime to the north. To the south, the 20°C and 35.0 surface isotherm and isohaline, respectively, are characteristically embedded in the thermohaline gradients associated with the SSTF. A sharp increase in depth‐integrated chloropigment is also observed at the SSTF and is ascribed to an increase in the concentration and thickness of the subsurface chloropigment maximum (SCM) prompted by the shoaling of the nutricline with the thermocline structure into the euphotic zone.

KEY WORDS: albatross, bycatch, Hawaii, longline fishery, seabird.Capture in longline fisheries is a critical threat to most albatross and large petrel species. 1-3 Blackfooted Phoebastria nigripes and Laysan P. immutabilis albatrosses are the predominant seabird species incidentally caught in Hawaii longline fisheries. This study reports results of a trial in the Hawaii pelagic longline tuna and swordfish fisheries comparing four experimental treatments' seabird capture rates and commercial viability. Two research fishing trips were conducted between 1 April and 17 May 2003 on a Hawaii-based pelagic longline vessel, at traditional fishing grounds south of the Northwestern Hawaiian Islands, between 21°41′N and 25°08′N, 173°58′W and 167°43′W.Two of the treatments employed were setting branch lines through 9 m and 6.5 m long underwater setting chutes, which release baited hooks beneath the sea surface, in an attempt to prevent diving seabirds from reaching them. The design of the underwater setting chute, illustrated in Molloy et al., 4 is similar to that used in this present trial. When setting with the 9 m and 6.5 m chutes on the research vessel, 5.4 m and 2.9 m of the chute's shaft was underwater, respectively.A third treatment, called side-setting, entailed setting from the side of the vessel, with other gear design the same as conventional approaches when setting from the stern. The crew throwing baited hooks was 8 m forward from the port-stern corner. Baited hooks were thrown forward, close to the side of the vessel's hull, to protect baits from seabirds. A bird curtain was used, 4.9 m forward from the portstern corner, when side-setting to increase the effectiveness of this mitigation method by preventing birds from establishing a flight path along the side of the boat where baited hooks were being deployed. The hypothesis is that when side-setting, baited hooks will be set close to the side of the vessel hull where seabirds will be unable or unwilling to pursue the hooks. By the time the stern passes the hooks, the hooks will have sunk to a depth where seabirds cannot locate them or cannot dive.A fourth treatment was blue-dyed bait. Bait was completely thawed and dyed blue by soaking in a large tub with dissolved blue food coloring (Virginia Dare FD & C Blue no. 1) powder at a concentration of 4 g/L of water for 1-4 h to achieve regulatory-required darkness. The hypothesis is that dyed bait is difficult for birds to detect because it reduces the contrast between bait and sea color.Research on the efficacy of blue-dyed bait and underwater setting chutes for pelagic longline fisheries at reducing seabird bycatch has been conducted previously. 5,6-10 This present study is the first assessment of the effectiveness of side-setting at reducing seabird capture.Setting occurred only during daylight to enable observations of seabird interactions with fishing gear. Both tuna and swordfish gear used 60 g swivels attached within 1 m of the hook, a weighting design selected by the Hawaii Longline Association. Gilman et a...