The daytime foraging depth of swordfish in the North Pacific was investigated with data from an 8 yr tagging program, using 28 Wildlife Computer pop-up archival tags deployed on swordfish in the North Pacific. The tags transmitted data from 1°S to 44°N latitude and from 206 to 249°E longitude. Five tags were recovered, providing a full archival record that showed that when swordfish did not engage in daytime basking behavior, they remained within a narrow range of light level during both day and night, suggesting swordfish stay within a soundscattering layer (SSL) to feed during both day and night. Daytime mean depth of non-basking swordfish ranged from 32 to 760 m. Seventy-seven percent of the daytime mean depth could be explained with a generalized additive model that used 3 environmental indices: satellite-derived surface chlorophyll as a proxy for light at depth, oxygen at 400 m obtained from the World Ocean Atlas, and temperature at 400 m inferred from the tag data. This model, when used in a predictive mode, generated a basin-wide map of swordfish daytime mean depth that showed depths exceeding 600 m to the north of Hawaii, shoaling to 300 m off the coast of California. This information could improve daytime swordfish catch by longliners and potentially allow them to switch from shallow night sets that result in interactions with sea turtles. This approach in effect defines the habitat of swordfish prey, giving us insight into the vertical behavior of those mid-trophic level organisms inhabiting the SSL. Our model could be easily applied to other deep-foraging species. KEY WORDS: Xiphias gladius · Swordfish · Satellite tracking · Generalized additive model · Habitat modellingResale or republication not permitted without written consent of the publisher

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.

SUMMARYCranial endothermy evolved independently in lamnid sharks, billfishes and tunas, and is thought to minimize the effects of ambient temperature change on both vision and neural function during deep dives. The opah, Lampris guttatus, is a large epipelagic-mesopelagic predator that makes repeated dives into cool waters to forage. To determine if L. guttatus exhibits cranial endothermy, we measured cranial temperatures in live, decked fish and identified potential sources of heat and mechanisms to conserve heat. In 40 opah (95.1±7.6 cm fork length), the temperature of the tissue behind the eye was elevated by a mean (±s.e.m.) of 2.1±0.3°C and a maximum of 6.3°C above myotomal muscle temperature (T m ), used as a proxy for ambient temperature. Cranial temperature varied significantly with T m and temperature elevation was greater at lower T m . The proximal region of the paired lateral rectus extraocular muscle appears to be the primary source of heat. This muscle is the largest extraocular muscle, is adjacent to the optic nerve and brain and is separated from the brain only by a thin layer of bone. The proximal lateral rectus muscle is darker red in color and has a higher citrate synthase activity, indicating a higher capacity for aerobic heat production, than all other extraocular muscles. Furthermore, this muscle has a layer of fat insulating it from the gill cavity and is perfused by a network of arteries and veins that forms a putative counter-current heat exchanger. Taken together, these results support the hypothesis that the opah can maintain elevated cranial temperatures.

No abstract