Posted by Corey Eddy
8 May 2012
University of Massachusetts Dartmouth graduate student Corey Eddy was recently onboard a purse seinefishing vessel in the eastern Pacific Ocean. He is studying sharks, specifically the physiological stress response, survival, and post-release behavior of sharks captured during purse seining operations. Corey explains the focus of his project in this video.
Hmm. Where to begin? I’m writing this from the airport in Guayaquil, Ecuador, headed back to Boston. The cruise came to an abrupt end when a crewmember tragically passed away. As I was writing a draft of this blog, there was a huge commotion outside my door, a lot of anxious voices, a lot of hurried footsteps. When the boat suddenly changed speed and direction, it became obvious something was seriously wrong. I won’t say much more than that. After speaking with the captain and ISSF, we decided to continue with the blog and to be honest with events, while remaining respectful. We all felt it was important to acknowledge what happened, as I am trying to describe life at sea in addition to the research. I didn’t know the man well, but he always greeted me with a smile, a thumbs-up, and “todo bien (everything’s good)” when I’d see him out on deck or below. We didn’t talk much, unfortunately. May he rest in peace.
There are a lot of stories that I wanted to share so in tribute to the entire crew for hosting me on this trip, I’ll share a few.
A couple weeks ago, March 14th actually, we came across a whale carcass as we were en route between FADs. There wasn’t much left and I don’t know much about whales, so I can’t tell you what species. I did hear that it was possibly a pilot whale. Check out the photo. Anyway, not to get too gory, but the last three feet were chewed to the bone by sharks.
Most of the flesh of the head was gone too. It was really interesting to see the very clear bite marks, but what was even more interesting was that the crew turned it into a FAD. Three of the guys jumped in to wrap the carcass with netting, before tying that to a man-made raft and one of our buoys. Keep in mind the fresh bite marks and the fact that it was still bleeding.
Now I’m going to jump ahead in time, back to April 25th, the day I actually began writing this entry. We’d been busy for over a week, having fished every day between the 17th and 25th, twice one day and three times another. After a slow and discouraging start, we had put a good amount of fish on the boat, almost 400 tons. Our best set was 95 tons. With that came a few sharks; 20 actually. I put out PSATs on five silkies and three scalloped hammerheads. I wasn’t expecting to tag hammers on this cruise, but it was an unforeseen and fortunate opportunity that I couldn’t pass up. After all, I’m studying the sharks regularly taken as bycatch in the purse seine industry; silkies are just the most common species, but hammers are encountered quite often in the area we were fishing. What made it an especially good opportunity was that we caught five in one set and the physical condition (although subjective) of these three ranged from excellent to fair. They were also just about the same size and all male, so we may have a good look into how length of the capture event affects their survival and behavior. In about 45 days, when the tags pop-up and transmit data, we shall see.
In that time, I’d also been able to take blood samples from two silky sharks. One was actually gilled (meaning he got his head stuck in the net, trying to get out), but survived and actually swam away with a surprising amount of energy. I didn’t tag him with a PSAT however, as he didn’t go through the entire capture process and, again, I’m trying to minimize the stress of those we do. The second blood sample came from another male, but this one went came up in the brail, several minutes after the hammers. Although he was moving his jaws, eyes, and whole body while on deck, I didn’t see him swim away after release, but they don’t always do that. That one also didn’t get a PSAT.
I suppose this is my chance to talk a little about why I’m taking blood samples. I’m studying the physiological stress response that silkies exhibit following capture. I may be repeating myself, but it has been generally assumed that bycatch survives when returned to the sea. Unfortunately, we now recognize this may not be true and the animals (sharks, turtles, tuna, whatever you’ve got), may actually succumb to stress or injury and die after days or even weeks. For that reason, I am studying their physiological response, to see what goes on inside their bodies that may ultimately kill them. Sharks typically undergo short periods of physical exercise when capturing prey or evading other predators. They are built for that and have species-specific limitations on how much activity they can manage. However, they are not designed for extreme bouts of physical exercise characteristic of capture by fishing gear. In these circumstances of prolonged duress, they may actually exercise themselves to death. Therefore, I am looking at key indicators of the stress response to see how physiological, biochemical, and molecular changes may alter the function of the cardiovascular system. If the system cannot sufficiently deliver oxygen and nutrients throughout the body, while also removing wastes, the shark’s behavior and chances of survival may ultimately be compromised. Using the relative physical condition of sharks, we can compare the physiological response of those that donate blood with the behavior and survival rate of those that are tagged. With that, we can see how the sharks react physiologically to the capture event and how this stress response may indicate the extent to which their behavior and survival may be affected. An understanding of how these sharks respond to capture in purse seines may allow resource managers and the fishing industry to modify fishing techniques to minimize the occurrence of post-release delayed mortality.
Although the journey came to a tragic end, I was able to collect some valuable data and I had a memorable time doing it. While I already mentioned seeing my first hammerheads and mako sharks, I also saw my first whale shark, a 2 meter juvenile that was alongside the boat as we passed a FAD. I saw a lot of longliners while we were at sea too, and I had hoped to talk about the
extreme amounts of bycatch frequently taken in that industry and the role it plays in shark finning. Aside from the work aspects, I also enjoyed a few great meals on the Via Simoun and several great conversations with the captain and crew. I might even have inspired the captain to buy a new boat and start catching invasive lionfish in the Atlantic before they become the ocean’s greatest man-made disaster. Well, that’s a topic of conversation for another blog…