Archival tags were used to study the seasonal movements, migration patterns and vertical distribution of juvenile North Pacific albacore (Thunnus alalunga). Between 2001 and 2006, archival tags were deployed in North Pacific albacore in two regions of the Northeast Pacific: (i) off Northern Baja California, Mexico and Southern California, and (ii) off Washington and Oregon. Twenty archival tagged fish were recovered with times at liberty ranging from 63 to 697 days. Tagged albacore exhibited five distinct, seasonal migratory patterns. Depth and temperature data also showed a broad range of vertical behaviors. In certain regions such as off Baja California, Mexico, juvenile albacore make frequent dives to depths exceeding 200 m during the day and remain in the surface mixed layer at night, whereas off Oregon and Washington they remain near the surface both day and night. Water temperatures encountered ranged from 3.3 to 22.7°C. Peritoneal temperatures were significantly higher by an average of approximately 4°C, as expected in these warm-bodied fish. This study provides a comprehensive examination of horizontal and vertical movements of juvenile albacore in the Northeast Pacific. The results reveal diverse behavior that varies regionally and seasonally as albacore move among different habitats throughout the entire North Pacific.
This study compares the critical oxygen saturation (O 2crit ) levels of the shiner perch Cymatogaster aggregata obtained using two different methods wherein hypoxia is induced either by the fish's respiration (closed respirometry) or by degassing oxygen with nitrogen (intermittent-flow respirometry). Fish exhibited loss of equilibrium at a higher O 2 saturation in the closed respirometry method when compared with the intermittent-flow method. Utilization of closed respirometry yielded O 2crit measurements that were almost twice as high as those obtained with intermittent-flow respirometry. The lower hypoxia tolerance in closed respirometry is consistent with additional stress, caused by a build-up of ammonia and carbon dioxide and a faster rate in dissolved oxygen decline. The results indicate that these two methods of determining hypoxia tolerance in aquatic organisms are not comparable, and that much care should be given to method choice.
Fronts-i.e., the boundaries between water masses-are ubiquitous in the world oceans and have been shown to significantly influence pelagic ecosystems with enhanced local productivity and increased abundances of forage fish and top predators. Here we use data from archival tags to document how four juvenile albacore tunas foraged at and exploited a thermal front. Of the 3098 observed trips, the albacore mainly swam across the front between the warm side above the thermocline and the cold side below the thermocline with an average of 78 6 20.4 cross-frontal trips per fish per day. The warm frontal surface waters provided a thermal resource, allowing the tuna to maintain higher body temperatures and thus forage more efficiently in the food-rich waters of the cold side of the front. Foraging success of the tunas decreased as the cross-front thermal gradient weakened. This first look into small-scale use of fronts by a top predator demonstrates that ephemeral, submesoscale oceanic features can play a significant role in pelagic ecology.Oceanic fronts, strong horizontal gradients in temperature, salinity and/or density, are biogeochemical hotspots caused by the physics associated with the abutment of water masses with different properties (L evy et al. 2012;Woodson and Litvin 2014). The increased remote and in situ sampling of our oceans in recent decades has demonstrated that fronts are ubiquitous features, occurring across a spectrum of temporal and spatial scales (L evy et al. 2012). Although the term "front" covers a wide array of physical features, most higher trophic level, ecological work has focused on mesoscale fronts (spanning 10-100 km) that are both persistent (lasting from weeks to months) and recurring (seasonally prominent This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Scientific Significance StatementThe boundaries between water masses, known as "fronts," are oceanic hotspots-known to harbor increased abundances of bacteria, plankton, forage fish, and pelagic predators. Fronts are complex structures that vary in both time and space. The influence of these dynamics on open ocean ecosystems is still an active area of research, and it is not clear how large predators, such as tunas, exploit these features. We used a novel approach to show that albacore tuna use fronts as both a temperature and a food resource, but that their foraging success deteriorates as the horizontal scale of the front increases in size.
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