Daniel W. Fuller

Environmental context explains Lévy and Brownian movement patterns of marine predators.

An optimal search theory, the so-called Lévy-flight foraging hypothesis, predicts that predators should adopt search strategies known as Lévy flights where prey is sparse and distributed unpredictably, but that Brownian movement is sufficiently efficient for locating abundant prey. Empirical studies have generated controversy because the accuracy of statistical methods that have been used to identify Lévy behaviour has recently been questioned. Consequently, whether foragers exhibit Lévy flights in the wild remains unclear. Crucially, moreover, it has not been tested whether observed movement patterns across natural landscapes having different expected resource distributions conform to the theory’s central predictions. Here we use maximum-likelihood methods to test for Lévy patterns in relation to environmental gradients in the largest animal movement data set assembled for this purpose. Strong support was found for Lévy search patterns across 14 species of open-ocean predatory fish (sharks, tuna, billfish and ocean sunfish), with some individuals switching between Lévy and Brownian movement as they traversed different habitat types. We tested the spatial occurrence of these two principal patterns and found Lévy behaviour to be associated with less productive waters (sparser prey) and Brownian movements to be associated with productive shelf or convergence-front habitats (abundant prey). These results are consistent with the Lévy-flight foraging hypothesis, supporting the contention that organism search strategies naturally evolved in such a way that they exploit optimal Lévy patterns.

Vertical Movements and Habitat Utilization of Skipjack (Katsuwonus pelamis), Yellowfin (Thunnus albacares), and Bigeye (Thunnus obesus) Tunas in the Equatorial Eastern Pacific Ocean, Ascertained Through Archival Tag Data

Skipjack, yellowfin, and bigeye tunas were caught and released with implanted archival tags in the equatorial eastern Pacific Ocean between 2004 and 2006. The depth and temperature data from five recovered archival tags for each species were analyzed and compared among species while at liberty within the same geographically defined area. Evaluations of the timed depth records resulted in discrimination of distinct vertical movement patterns for each species, including association with floating objects, daytime foraging below the thermocline in depths of the deep scattering layer, surface orientation, and deep diving in excess of 500 m. During the daytime, skipjack and yellowfin tunas occasionally exhibited repetitive bounce-dive foraging behavior well below the thermocline to depths of the deep scattering layer, between 225 and 400 m. Bigeye tuna were foraging during the daytime at similar depths of about 225–475 m, except when undertaking upward forays. The deepest dives recorded for skipjack, yellowfin, and bigeye tunas were 596, 1022, and 1695 m, respectively. The vertical habitat utilization distributions indicate that skipjack, yellowfin, and bigeye tunas, when not associated with floating objects, spent 99, 96, and 92%, respectively, of their time above the thermocline during the night, but spent 37, 44, and 57%, respectively, of their time below the thermocline during the day. The apparent species-specific physiological abilities and tolerances to environmental characteristics of their vertical habitat, including dissolved oxygen and temperature, are evaluated. Results of this study on the comparative vertical movements, behavior, and habitat utilization for these species, should also be useful in evaluations of species-specific vulnerability to purse-seine and longline fisheries in this region.

Comparative Performance of Current-generation Geolocating Archival Tags

The performances of the current-generation Lotek Wireless LTD2310 and the Wildlife Computers Mk9 geolocating archival tags were compared. The depth, temperature, and light level sensors of 15 LTD2310 and 15 Mk9 archival tags were evaluated through hydrocasts of these, along with a calibrated Sea-Bird SBE39 temperature and depth probe, to nearly 500 m. Three experiments were conducted; each included five archival tags of each type simultaneously deployed in hydrocasts, along with the SBE39 probe. In all three experiments, the average differences between depth sensors on the Mk9 archival tags and the SBE39 were significantly greater than those between the LTD2310 archival tags and the probe depths for the hydrocast stops at about 500 m, 300 m, and 200 m. The standard errors about the average depth values for those hydrocast stops in Experiments 1 and 2, were greater for the LTD2310 tags, but for Experiment 3 the standard errors were greater for the Mk9 tags. The average differences between the LTD2310 and Mk9 archival tag temperatures measured by their stalk sensors and the SBE39 probe temperatures were similar in all three experiments over a temperature range of from about 9° to 27° C. The standard errors about the average temperature values were similar in all three experiments. The temperatures recorded by the Mk9 archival tag body temperature sensors lagged significantly, while those of the LTD2310 sensors were close to the temperatures recorded by the SBE39 probe during descents and ascents. The standard errors about the average tag body temperature values in all three experiments are greater for the Mk9 tags. Following the stabilization of light sensors at maximum depths (about 500 m) and darkness, during the three hydrocast ascents the 15 LTD2310 and 15 Mk9 archival tag light sensors indicated an average sensitivity to light at 440 m and 380 m, respectively. Two separate experiments conducted with archival tags implanted in the peritoneal cavity of tunas provided estimates of the accuracy and precision of geolocation based on ambient light level data. The computed distances between the average estimated geolocations, from three

Assessing niche width of endothermic fish from genes to ecosystem

Significance In large pelagic fish, as in birds and mammals, significant questions remain concerning the selective pressures that drove the evolution of endothermy. We examined cold tolerance and niche breadth on molecular, organism, and food web levels in three closely related tuna species. We show that energetic benefits of increased cold tolerance in fishes are context dependent: advantageous when quality prey is abundant, but not when such prey is scarce. We broaden our findings to the global level using published literature to show that endothermic bluefin species in all the world’s oceans specialize, to some degree, on high energy prey. Our multifaceted approach demonstrates the concept that the advantage of a “beneficial” adaptation is dependent on environmental and anthropogenic influences. Endothermy in vertebrates has been postulated to confer physiological and ecological advantages. In endothermic fish, niche expansion into cooler waters is correlated with specific physiological traits and is hypothesized to lead to greater foraging success and increased fitness. Using the seasonal co-occurrence of three tuna species in the eastern Pacific Ocean as a model system, we used cardiac gene expression data (as a proxy for thermal tolerance to low temperatures), archival tag data, and diet analyses to examine the vertical niche expansion hypothesis for endothermy in situ. Yellowfin, albacore, and Pacific bluefin tuna (PBFT) in the California Current system used more surface, mesopelagic, and deep waters, respectively. Expression of cardiac genes for calcium cycling increased in PBFT and coincided with broader vertical and thermal niche utilization. However, the PBFT diet was less diverse and focused on energy-rich forage fishes but did not show the greatest energy gains. Ecosystem-based management strategies for tunas should thus consider species-specific differences in physiology and foraging specialization.