An oceanographic characterization of swordfish (Xiphias gladius) longline fishing grounds in the springtime subtropical North Pacific

Abstract

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.

Life history of the neon flying squid: effect of the oceanographic regime in the North Pacific Ocean

“…This contour coincides approximately with the 20°C SST front (Polovina et al 2000), which is moderately productive (i.e. less productive than the TZCF, but more productive than the STD; Polovina et al 2000, Seki et al 2002. Thus, the productivity of the STFZ increases after late autumn (November).…”

Mesoscale eddies influence distribution and aggregation patterns of micronekton in the Mozambique Channel

“…Therefore, schooling behaviour of prey organisms plays a major role in the feeding success of marine fish predators that roam the open sea, often in shoals (Ménard & Marchal 2003 and eddy-eddy interactions likely enhance biological production in otherwise poor waters, sustaining stocks of potential prey for micronektonic species through bottom-up processes. The micronekton, in turn, can be sought out by upper trophic level predators such as tuna and tuna-like species (Seki et al 2002, Domokos et al 2007), turtles (Polovina et al 2004, Lambardi et al 2008 or seabirds (Nel et al 2001, Weimerskirch et al 2004, Hyrenbach et al 2006. Indeed, several tropical seabird species rely to a large extent on the association with subsurface predators such as tuna, which concentrate prey organisms at the surface while hunting, thus allowing birds to snatch their prey at or above the sea surface (Le Corre & Jaquemet 2005).…”

“…Several studies have demonstrated that eddies markedly enhanced geochemical transformations and primary production in the subtropical Pacific Ocean (Seki et al 2001, BenitezNelson et al 2007, in the Atlantic Ocean (McGillicuddy et al 1998, Garçon et al 2001, in the Bering Sea (Mizobata et al 2002) and in the Indian Ocean (Quartly & Srokosz 2004, Waite et al 2007). Mesoscale eddies also affect the upper-trophic-level organisms by influencing the horizontal and vertical distribution of mesozooplankton and fish larvae (Bakun 2006, Muhling et al 2007, affecting movement patterns of turtles (Polovina et al 2004, Lambardi et al 2008, attracting tuna (Young et al 2001) or swordfish (Seki et al 2002), and influencing the foraging strategy of seabirds (Nel et al 2001, Weimerskirch et al 2004, Hyrenbach et al 2006.…”

Performance of pop-up satellite archival tags

“…PSAT retention time was also in versely related to vertical distribution of chl a. Epipelagic species, which had the poorest retention times, spend significantly more time in the photic zone (~150 to 200 m), where the vertical distribution of chl a is at its highest concentration (Marshall 1966, Furuya 1990, Longhurst 1998, Seki et al 2002, Pérez et al 2006. Therefore, unless epi pe lagic species migrate into deeper (> 200 m) and cooler waters for ex tended periods, or away from areas with high chl a concentration, the op portunity for fouling organisms to accumulate on PSATs, as opposed to those devices being carried by deeper-diving species, is likely greater.…”