As shark biologists, we often need to get up close and personal with sharks in order to attach tags and collect necessary samples to answer critical questions about shark migrations, physiology, fisheries interactions, reproductive biology, etc. Over the years, scientists have developed various methods of capture and handling, which often involve removing sharks from the water and placing them in a cradle, sling, or even on a boat deck. These methods are used by shark biologists all over the world, and as scientists, we want to ensure the utmost safety of both the scientists and the sharks.
It is also common for scientists to use hoses to irrigate the gills of sharks while they are out of the water (helping them breath) and place a dark and wet cloth, such as a towel, over their eyes to keep them calm so that scientists can work quickly. To verify that our subjects are being handled properly and returned to the water in a minimally stressed state, we sample blood from each shark!
Yes, you’ve heard us talk about the importance of blood sampling a few times! However, through continued efforts in the lab, we have a little more information to share with you now! These data are not only applicable to the work we do with OCEARCH, but also for any shark scientist removing sharks from the water for research!
So, lets get started! With the blood samples we collected from white sharks Genie, Betsy, and Katharine in Cape Cod, and Lydia in Jacksonville, we can begin to understand the health of these sharks not just after they are captured, but also while they are on the cradle!
There are several parameters within a blood sample that indicate how much stress the shark is experiencing. Just as your doctor can use a blood sample to check your health, we can do exactly the same thing with sharks. For example, lactate, sometimes called lactic acid, is a commonly used indicator of physiological stress. Lactate is produced by muscles during periods of intense exercise. So, for example, when your muscles burn after running fast, this is caused by the build-up of lactic acid. Fish are no different. When a fish is hooked, it will use its swimming muscles to get away (think fight-or-flight!) and lactic acid is produced! By measuring the level of lactate in the blood, we can get a sense of the shark’s health.
Another parameter to investigate is potassium, which I have found through my PhD work to be correlated with shark mortality. There appears to actually be a potassium threshold level, above which mortality rates are much higher. It has been shown that high levels of potassium (above the possible threshold of 7mmol/L) cause disruptions in heart function (specifically, it induces bradycardia, or slowing of the heart beat), thereby bringing about mortality.
So, what levels of lactate and potassium are we measuring in sharks while on the cradle? To answer this question, we have blood-sampled white sharks at various times during the 15-or-so minutes that they are on the M/V OCEARCH cradle. This allows us to not only assess stress levels associated with the capture event, but also investigate how stress may be exacerbated while on the cradle.
This brings me to the two figures below, which show levels of lactate (Figure 1) and potassium (Figure 2) in each white shark at various sampling times (indicated on horizontal axis) while on the cradle. The color-coded lines indicate any changes that may have occurred between each blood sampling.
So, what do these figures tell us? Well, considering that lactate values in a free-swimming, uncaptured shark is thought to be around 0.5 mmol/L, we see the white sharks are reacting to being captured with some degree of physiological stress, which we would expect. However, the magnitude of the stress response is much lower than that measured in several other species caught on a different type of scientific sampling gear ( Read: Blood Samples Provide Stress Comparison Across Species), and the porbeagle shark is shown for comparison. In addition, time on the cradle does not appear to exacerbate lactate levels in the white shark’s blood.
It’s also exciting that we are seeing white shark blood potassium values near baseline or “unstressed” values obtained from other studies. This, again, verifies low stress levels in white sharks throughout the tagging process. Similar to lactate, potassium values do not appear to increase during the tagging event. Therefore, both figures indicate to us that the tagging event, or removing the sharks from the water, is not exacerbating the stress response in these sharks.
Also, thanks to Dr. Nick Whitney’s accelerometer data and the continuous tracks from these sharks, we know that the white sharks are quickly recovering from their capture events and resuming their migrations.
So, overall, we see little evidence that the tagging event, bringing the sharks out of the water, exacerbates stress, which is exactly what any scientist who removes sharks from the water to tag them wants to know! We have performed similar data assessments with other species of sharks during the Galapagos and Chile expeditions with OCEARCH, and the results are similar! This is good news not only for the OCEARCH science team, but also for shark research in general, which often involves removing sharks from the water for short periods of time to perform necessary tag deployments and biological sampling.
Dr. Heather Marshall, Postdoctoral Research Fellow
Dr. Greg Skomal, Senior Scientist and Program Manager
Massachusetts Division of Marine Fisheries