Gregory B. Skomal

Key Questions in Marine Megafauna Movement Ecology

It is a golden age for animal movement studies and so an opportune time to assess priorities for future work. We assembled 40 experts to identify key questions in this field, focussing on marine megafauna, which include a broad range of birds, mammals, reptiles, and fish. Research on these taxa has both underpinned many of the recent technical developments and led to fundamental discoveries in the field. We show that the questions have broad applicability to other taxa, including terrestrial animals, flying insects, and swimming invertebrates, and, as such, this exercise provides a useful roadmap for targeted deployments and data syntheses that should advance the field of movement ecology.

Differential sensitivity to capture stress assessed by blood acid-base status in five carcharhinid sharks

Stress from fishing capture can incite potentially lethal physiological changes in fishes. Blood acid–base status has routinely been utilized to gauge the magnitude of the stress response, which is dependent on the nature of the capture event and metabolic capacity of the species in question. The mortality induced by demersal longline capture has been shown to vary among taxonomically similar carcharhinid elasmobranchs. In this study, we aimed to: (1) quantify and compare blood acid–base disturbances associated with longline capture in five carcharhinid species; (2) examine the extent to which these disturbances correspond with reported at-vessel mortality rates; and (3) investigate how interspecific differences in the physiological stress response could relate to life history, ecology, and phylogeny. Results showed that blood acid–base disturbances from longline-capture varied between species, with relative degrees of disturbance by species proportional to previously reported at-vessel mortality rates. In addition, the degree in which metabolic and respiratory acidoses influenced relative depressions in blood pH also differed by species. The differences in blood acid–base status point to discrepancies in the aerobic and anaerobic capacities among these taxonomically similar species, and are important when considering the effects of, and possible means to mitigate deleterious consequences from, longline fishing capture.

The physiological response to anthropogenic stressors in marine elasmobranch fishes: a review with a focus on the secondary response.

Elasmobranchs (sharks, rays, and skates) are currently facing substantial anthropogenic threats, which expose them to acute and chronic stressors that may exceed in severity and/or duration those typically imposed by natural events. To date, the number of directed studies on the response of elasmobranch fishes to acute and chronic stress are greatly exceeded by those related to teleosts. Of the limited number of studies conducted to date, most have centered on sharks; batoids are poorly represented. Like teleosts, sharks exhibit primary and secondary responses to stress that are manifested in their blood biochemistry. The former is characterized by immediate and profound increases in circulating catecholamines and corticosteroids, which are thought to mobilize energy reserves and maintain oxygen supply and osmotic balance. Mediated by these primary responses, the secondary effects of stress in elasmobranchs include hyperglycemia, acidemia resulting from metabolic and respiratory acidoses, and profound disturbances to ionic, osmotic, and fluid volume homeostasis. The nature and magnitude of these secondary effects are species-specific and may be tightly linked to metabolic scope and thermal physiology as well as the type and duration of the stressor. In fishes, acute and chronic stressors can incite a tertiary response, which involves physiological changes at the organismal level, thereby impacting growth rates, reproductive outputs or investments, and disease resistance. Virtually no studies to date have been conducted on the tertiary stress response in elasmobranchs. Given the diversity of elasmobranchs, additional studies that characterize the nature, magnitude, and consequences of physiological stress over a broad spectrum of stressors are essential for the development of conservation measures. Additional studies on the primary, secondary, and tertiary stress response in elasmobranchs are warranted, with particular emphasis on expanding the range of species and stressors examined. Future studies should move beyond simply studying the effects of known stressors and focus on the underlying physiological mechanisms. Such studies should include the coupling of stress indicators with quantifiable aspects of the stressor, which will allow researchers to test hypotheses on survivorship and, ultimately, derive models that effectively link physiology to mortality. Studies of this nature are essential for decision-making that will result in the effective management and conservation of these species.

The physiological response of the Caribbean reef shark (Carcharhinus perezi) to longline capture

Longline fishing is the most common elasmobranch capture method around the world, yet the physiological consequences of this technique are poorly understood. To quantify the sub-lethal effects of longline capture in the commonly exploited Caribbean reef shark (Carcharhinus perezi), 37 individuals were captured using standard, mid-water longlines. Hook timers provided hooking duration to the nearest minute. Once sharks were landed, blood samples were taken and used to measure a suite of physiological parameters. Control data were obtained by sampling an additional three unrestrained Caribbean reef sharks underwater at an established shark feeding site. The greatest level of physiological disruption occurred after 120-180min of hooking, whereas sharks exposed to minimal and maximal hook durations exhibited the least disturbed blood chemistry. Significant relationships were established between hooking duration and blood pH, pCO(2), lactate, glucose, plasma calcium and plasma potassium. Longline capture appears more benign than other methods assessed to date, causing a shift in the stress response from acute at the onset of capture to a sub-acute regime as the capture event progresses, apparently facilitating a degree of physiological recovery. Continued investigation into the physiological response of elasmobranchs to longline capture is vital for the effective management of such fisheries.

Hematological indicators of stress in longline-captured sharks ☆

For many shark species, little information exists about the stress response to capture and release in commercial longline fisheries. Recent studies have used hematological profiling to assess the secondary stress response, but little is known about how, and to what degree, these indicators vary interspecifically. Moreover, there is little understanding of the extent to which the level of relative swimming activity (e.g., sluggish vs. active) or the general ecological classification (e.g., coastal vs. pelagic) correlates to the magnitude of the exercise-induced (capture-related) stress response. This study compared plasma electrolytes (Na(+), Cl(-), Mg(2+), Ca(2+), and K(+)), metabolites (glucose and lactate), blood hematocrit, and heat shock protein (Hsp70) levels between 11 species of longline-captured sharks (n=164). Statistical comparison of hematological parameters revealed species-specific differences in response to longline capture, as well as differences by ecological classification. Taken together, the blood properties of longline-captured sharks appear to be useful indicators of interspecific variation in the secondary stress response to capture, and may prove useful in the future for predicting survivorship of longline-captured sharks where new technologies (i.e., pop-up satellite tags) can verify post-release mortality.

Convergence of marine megafauna movement patterns in coastal and open oceans

Significance Understanding the key drivers of animal movement is crucial to assist in mitigating adverse impacts of anthropogenic activities on marine megafauna. We found that movement patterns of marine megafauna are mostly independent of their evolutionary histories, differing significantly from patterns for terrestrial animals. We detected a remarkable convergence in the distribution of speed and turning angles across organisms ranging from whales to turtles (epitome for the slowest animals on land but not at sea). Marine megafauna show a prevalence of movement patterns dominated by search behavior in coastal habitats compared with more directed, ballistic movement patterns when the animals move across the open ocean. The habitats through which they move will therefore need to be considered for effective conservation. The extent of increasing anthropogenic impacts on large marine vertebrates partly depends on the animals’ movement patterns. Effective conservation requires identification of the key drivers of movement including intrinsic properties and extrinsic constraints associated with the dynamic nature of the environments the animals inhabit. However, the relative importance of intrinsic versus extrinsic factors remains elusive. We analyze a global dataset of ∼2.8 million locations from >2,600 tracked individuals across 50 marine vertebrates evolutionarily separated by millions of years and using different locomotion modes (fly, swim, walk/paddle). Strikingly, movement patterns show a remarkable convergence, being strongly conserved across species and independent of body length and mass, despite these traits ranging over 10 orders of magnitude among the species studied. This represents a fundamental difference between marine and terrestrial vertebrates not previously identified, likely linked to the reduced costs of locomotion in water. Movement patterns were primarily explained by the interaction between species-specific traits and the habitat(s) they move through, resulting in complex movement patterns when moving close to coasts compared with more predictable patterns when moving in open oceans. This distinct difference may be associated with greater complexity within coastal microhabitats, highlighting a critical role of preferred habitat in shaping marine vertebrate global movements. Efforts to develop understanding of the characteristics of vertebrate movement should consider the habitat(s) through which they move to identify how movement patterns will alter with forecasted severe ocean changes, such as reduced Arctic sea ice cover, sea level rise, and declining oxygen content.

The physiological stress response, postrelease behavior, and mortality of blacktip sharks (Carcharhinus limbatus) caught on circle and J-hooks in the Florida recreational fishery

The views and opinions expressed or implied in this article are those of the author (or authors) and do not necessarily reflect the position of the National Marine Fisheries Service, NOAA. Abstract—The blacktip shark (Carcharhinus limbatus) is the most commonly captured species in the Florida recreational shark fishery. We aimed to quantify the postrelease mortality of blacktip sharks and to determine whether hook type (circle or J) had any impact on survival. We measured capture variables (e.g., fight time, animal condition, etc.), blood gas analytes, and finescale behavior obtained by using acceleration data loggers for blacktip sharks (n=31) caught on rod and reel by recreational fishermen. Mortalities (n=3; 9.7%) all occurred within 2 h after release. Surviving sharks were monitored for 7 to 72 h (mean: 30 h [standard deviation (SD) 22]) and behaviorally recovered from capture within 11 h (SD 2.6), although larger individuals recovered faster. Hook type did not affect where a shark was hooked, animal condition, the likelihood of hook removal, or recovery time. We found relatively low levels of mortality for blacktip sharks caught in the recreational fishery that were kept in the water and had not sustained serious injuries.