Physiological Responses to Stress in Sharks

11 Physiological Responses to Stress in Sharks Gregory Skomal and Diego Bernal Contents 11.1 Introduction ........................................................................................................................460 11.2 Stress ....................................................................................................................................460 11.2.1 Natural Stressors .................................................................................................... 461 11.2.2 Anthropogenic Stressors ...................................................................................... 462 11.2.2.1 Ecotoxins .................................................................................................. 462 11.2.2.2 Capture ..................................................................................................... 462 11.3 Physiological Effects ..........................................................................................................463 11.3.1 Primary Stress Response ......................................................................................464 11.3.1.1 Catecholamines .......................................................................................464 11.3.1.2 Corticosteroids .........................................................................................464 11.3.2 Secondary Stress Response ..................................................................................465 11.3.2.1 Hyperglycemia ........................................................................................466 11.3.2.2 Acid-Base Disturbances .........................................................................466 11.3.2.3 Electrolyte Levels ....................................................................................469 11.3.2.4 Hematology and Heat Shock Proteins .................................................469 11.3.3 Tertiary Stress Response ....................................................................................... 470 11.4 Baseline ...............................................................................................................................471 11.5 Intra- and Interspecific Variation in Stress Responses .................................................472 11.5.1 Metabolic Scope .....................................................................................................472 11.5.2 Endothermy ............................................................................................................473 11.6 Compensatory Mechanisms .............................................................................................477 11.7 Recovery and Survivorship .............................................................................................. 478 11.7.1 Laboratory Studies ................................................................................................. 478 11.7.2 Field Studies ...........................................................................................................480 11.7.2.1 Conventional Tagging ............................................................................480 11.7.2.2 Acoustic Tracking....................................................................................480 11.7.2.3 Archival Tagging .....................................................................................482 11.8 Conclusions .........................................................................................................................483 Acknowledgments ......................................................................................................................484 References .....................................................................................................................................484 459 © 2010 by Taylor and Francis Group, LLC 460 Sharks and Their Relatives II: Biodiversity, Adaptive Physiology, and Conservation 11.1 Introduction Despite their long evolutionary history (~400 million years; Grogan and Lund 2004), elasmobranchs are now facing new substantial anthropogenic threats, including habitat degradation and fisheries interactions, which cause acute and chronic stress that may exceed stress levels typically imposed by natural events (e.g., seasonal habitat changes, predator avoidance). Although the physiological stress response of teleosts has been studied for decades (e.g., Adams 1990a), much of this work has centered on salmonids due to the ease of maintaining and manipulating these fishes in captivity as well as the economic interests of the aquaculture industry and the recreational fishing sector. The global expansion of both commercial and recreational fisheries coupled with the man- dated release of captured fish due to bag limits, quotas, and minimum sizes, prompted the expansion of stress studies beyond freshwater teleosts to their marine counterparts. However, studies of elasmobranch stress physiology have lagged far behind despite the rapid proliferation of directed fisheries for, and a significant increase in the incidental capture of, these species over the last two decades (NMFS 2008) and the possibility that modifications to coastal habitats, which are important to many elasmobranch species, could increase stress. In this chapter, we review our understanding of the physiological responses to stress in elasmobranchs and highlight gaps in our current knowledge. Although some elasmobranch stress responses are similar to those exhibited by teleosts, the inherent physiological differences between these two groups create marked differences. Because this chapter centers on the responses of sharks and their relatives to stress (i.e., perturbations to homeostasis), we strongly recommend that the reader be familiar with elasmobranch physiology and homeostasis (e.g., Shuttleworth 1988a; Carlson, Goldman, and Lowe 2004; Evans, Piermarini, and Choe 2004). 11.2 Stress Numerous definitions of “stress” have emerged in the literature (Adams 1990b), but few Downloaded by [University of Denver] at 11:25 28 August 2015 have diverged far from the working definition established by Brett (1958): stress is a physiological state produced by an environmental factor that extends the normal adaptive responses of the animal, or disturbs the normal functioning to such an extent that the chances for survival are significantly reduced. In essence, stress is a condition during which the homeostasis of an organism is disrupted by intrinsic or extrinsic stimuli called stressors (Wendelaar-Bonga 1997). The stressor not only threatens or disturbs homeostatic equilibrium, but may also elicit a coordinated response by the animal to compen- sate for the disruption in an attempt to overcome the threat (Wendelaar-Bonga 1997). The responses may be behavioral (e.g., swimming out of a hypoxic environment), physiological (e.g., a decrease in metabolic rate with declines in ambient oxygen levels), or a combination of both. In general, there are increased metabolic costs associated with the stress response because energy is reallocated from one or several metabolically demanding activities (e.g., growth, reproduction) to others (e.g., respiration, locomotion, tissue repair) in order to restore homeostasis. When compensatory mechanisms (or the energetic reserves necessary to deal with the costs associated with these mechanisms) are not sufficient to correct for © 2010 by Taylor and Francis Group, LLC Physiological Responses to Stress in Sharks 461 large homeostatic perturbations in either the short term (acute) or long term (chronic), the stress response is dysfunctional and reduced fitness and/or mortality is likely (Wendelaar- Bonga 1997). Although the stress response is inherently organismal, it is often referred to as an inte- grated response because it encompasses multiple organizational levels ranging from the molecular to those of populations and ecosystems (Adams 1990b; Wendelaar-Bonga 1997). Again, in the case of elasmobranchs, many of these levels have yet to be explored and researchers are just beginning to examine the basic physiological stress response in mem- bers of this group. To date, most studies on elasmobranch stress have concentrated on small benthic species that can be readily maintained in captivity [e.g., Port Jackson shark (Heterodontus portusjacksoni); Cooper and Morris 2004]. However, work on teleosts has shown that the species-specific characteristics and magnitude of the stress response are tightly linked to overall level of swimming activity (e.g., aerobic and anaerobic scope) and life history (e.g., sluggish benthic vs. active pelagic species; Wood 1991). Given the diversity of elasmobranch activity levels and life history patterns, it is unlikely that there is a “typi- cal” stress response in this group. A stressor is often classified as acute or chronic, depending on its frequency, duration, and intensity. Acute stress results from the rapid (from minutes to hours) exposure to a stressor or a series of stressors. For example, the capture and handling of a shark may comprise several stressors, including a bout of exhaustive exercise, physical trauma from hooking or netting, and exposure to air. Although acute responses tend to be immediate, the behavioral and physiological consequences may manifest themselves over a period of hours to days (e.g., Skomal 2006). In contrast, chronic stress arises from continuous or periodic exposure to one or more stressors over longer time periods (from days to weeks). For example,

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