Have you ever been stressed out? At some point everyone on this planet has experienced some form of stress. Furthermore, it’s probable that every living animal get’s stressed to some degree.
Figure 1: A shortfin mako shark caught by accident on a tuna fishing trip. Photo Credit: Fiona Ayerst
Stress is defined as a circumstance of disrupted homeostasis. An animal is in homeostasis when factors of its internal environment, such as pH, temperature and oxygen levels, are stable. Physical, chemical and perceived stressors can induce suspended homeostasis. Examples of physical stressors include starvation, and the handling, capture, confinement and transport of animals. Chemical stress can be prompted by contaminant and pollution exposure, and change in variables such as temperature, salinity, oxygen and acidification. Lastly, perceived stressors are considered stimuli that lead to a startled reaction, such as the presence of a predator. These stressors are not a detriment to an animal’s health, unless the strength or period of exposure overwhelms the physiological response mechanisms.
Different organisms have adapted their own specific physiological response mechanisms that allow homeostasis to be re-established, and ultimately allow the animal to overcome stress. Nevertheless, scientists have classed physiological stress responses into three major non-specific categories: primary, secondary and tertiary. Primary responses are defined as a change in hormone levels and/or neurotransmitter function. Secondary responses are classed as cellular, metabolic, osmoregulatory and immune changes. Tertiary responses are considered changes in the whole-body of the animal, such as growth, locomotion, feeding, disease resistance, and even anxiety and aggression.
Figure 2: Scientists are trying to further understand the influence of fishing on stress in sharks. It is believed that stress is species-specific, like for this salmon shark. Photo Credit: NOAA
Sharks are animals that deal with stress everyday. Changes in water temperature, salinity, pressure and light are examples of environmental variables that can stress these organisms. Furthermore, the process of escaping predators, locating and capturing prey, competition and finding mates are examples of ecological stressors. Nevertheless, nothing stresses a shark more than human induced stressors such as fishing. Sharks are fished for numerous reasons including pleasure, money, as well as for research. Fishing includes the process of catching the animal, and introducing it to a foreign environment from its natural habitat. Sharks are caught, indirectly or directly, via techniques such as single hook and line, long-lines, trawls and gill nets. In recent years, there has been a great amount of research that investigates induced stress on sharks from fishing, and based on levels of stress, the likelihood of their post-release survivorship.
When a shark is caught, indicators of stress that could be recorded are time of day, initial depth of the shark, the fight time (from hooking till it reaches the surface), the size and type of shark, location of the hook, and how long the shark is exposed to air. Blood samples can be taken from the shark to investigate the levels of plasma electrolytes (Na+, Cl-, Mg2 +, Ca2 +, and K+), metabolites (glucose and lactate), blood hematocrit, and heat shock protein (Hsp70). From these analyses, scientists can determine which indicator(s) can induce the most stress, in addition to the likelihood of mortality. Afterwards, sharks are often tagged, allowing scientists to investigate behavioral changes and post-release survivorship.
Figure 3: The giant upper lobe of the thresher shark is used to knock out prey by a whip-like action. They are brought to the surface backwards, which is very stressful because they are ram breathers. Photo Credit: Paul E. Ester
It has been found that all fishing variables described in the previous paragraph stress out sharks. However, the degree to which they stress is species-specific. For example, the thresher shark is extremely sensitive to fishing. This shark is unique because it mainly relies on its tail for predation. The upper lobe of the caudal fin in this animal is extremely long. This allows the shark to use it as a whip, which knocks out its prey. The tail becomes hooked when the shark is caught during fishing. The thresher shark is a ram breather, meaning it has to swim forward to pump oxygenated water over its gills (see the following blog article for more information: “Breathing Mechanisms in Sharks”: http://atlanticsharks.org/blog/?p=53. When the shark is pulled up to the boat, it is pulled backwards, making breathing very difficult because water cannot be pumped through the gills in this direction.
Fishing for sharks often leads to varying levels of stress that can overwhelm their physiological response mechanisms, and can prohibit recovery. Understanding the relationship between stress and stressors can teach us about the physiology of these organisms, the biological and ecological impacts that fishing has on these animals, and the importance of species-specific conservation and management. This information could help conserve shark populations.
Please remember-If you have questions or comments, please comment below, and I will respond as soon as I can.
Scott Seamone, ShARCC Intern