Oceanographic investigation of the American Samoa albacore (Thunnus alalunga) habitat and longline fishing grounds

Abstract

The American Samoa fishing ground is a dynamic region with strong mesoscale eddy activity and temporal variability on scales of <1 week. Seasonal and interannual variability in eddy activity, induced by baroclinic instability that is fueled by horizontal shear between the eastward-flowing South Equatorial Counter Current (SECC) and the westward-flowing South Equatorial Current (SEC), seems to play an important role in the performance of the longline fishery for albacore. Mesoscale eddy variability in the American Samoa Exclusive Economic Zone (EEZ) peaks from March to April, when the kinetic energy of the SECC is at its strongest. Longline albacore catch tends to be highest at the eddy edges, while albacore catch per effort (CPUE) shows intra-annual variability with high CPUE that lags the periods of peak eddy activity by about 2 months. When CPUE is highest, the values are distributed toward the northern half of the EEZ, the region affected most by the SECC. Further indication of the possible importance of the SECC for longline performance is the significant drop in eddy variability in 2004 when compared with that observed in 2003 -resulting from a weak SECCwhich was accompanied by a substantial drop in albacore CPUE rates and a lack of northward intensification of CPUE. From an ecosystem perspective, evidence to support higher micronekton biomass in the upper 200 m at eddy boundaries is inconclusive. Albacore's vertical distribution seems to be governed by the presence of prey. Albacore spend most of their time between 150 and 250 m, away from the deep daytime and shallow nighttime sonic scattering layers, at depths coinciding with those of small local maxima in micronekton biomass whose backscattering properties are consistent with those of albacore's preferred prey. Settling depths of longline sets during periods of decreased eddy activity correspond to those most occupied by albacore, possibly contributing to the lower CPUE by reducing catchability through rendering bait less attractive to albacore in the presence of prey.

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).…”

Top marine predators track Lagrangian coherent structures

“…In fact, all investigations on the relationship between eddies and top-predator communities, using satellite imagery observations, have shown strong ties between them (7,8). Upper predators particularly used the boundary between 2 eddies (9)(10)(11)(12). The key point is that interactions between eddies generate strong dynamic interfaces (13) and make them a complex and energetic physical environment.…”

Consequences of the life history traits of pelagic larvae on interisland connectivity during a changing climate

“…Spawning dates for each model year were identified as 6 d after the first full moon to occur later than 12 October (Mundy & Green 1996, Craig 2009. It is recognized that some spawning can occur across several days or even following successive full moons in October and November.…”