Tenacious Sharks and their Hangers-On

Russ Vetter from Leg II, SHARK CRUISE. Today is the day we plan to deliver the open-ocean (pelagic) stingrays in our holding tanks to a researcher at the Wrigley Marine Science Center on Catalina Island. To accommodate this extra activity, we start the day earlier. This means getting up at 4:00 a.m. to begin the CTD cast and setting the longline at 5:30 a.m. The grumblings of the early morning will hopefully give way to the rewards of getting to stretch our legs on Catalina Island this evening.

The morning set proves to be full of a nice variety of types and sizes of animals. We have business non-stop down on the tagging ramp. The highlight of the morning is a very large male blue shark in excellent condition. This fellow has plenty of energy and has no interest in getting into the shark cradle. He tries to dive deep and there is no way I can control him. When he makes a dive there is nothing to do but let him pull against the ship. If the leader breaks, so be it, there is nothing I can do. After several dives, with myself as resistance, he tires enough to head to the surface and we use a dip in the waves to lift him up and into the cradle. It’s not pretty, and he is so large his tail hangs off the back of the cradle, but he is eventually restrained and we go to work. He receives both a satellite position only tag (SPOT) tag on his dorsal fin and a pop-off archival tag (PAT) on his side. Off he goes. This should be a fun animal to track.

Animals come fast and furious on the second set. The animals tend to be larger makos that require a fair amount of muscle to get them ready for the tagging procedure. Regrettably, we have two animals that are tail hooked and not ventilating. I don’t think any of us have seen this before with mako sharks, although this is very common with thresher sharks. Thresher sharks use their immense tails to stun their prey prior to feeding and often get tail hooked when hitting the baits on our longline. When a shark is unable to move forward through the water the gills and gill slits collapse since they are not designed to have water flowing in the reverse direction. When animals are landed in this condition we place a water perfusion hose in the mouth and keep the animal submerged in the cradle. Sometimes they revive sometimes not. Regrettably, these animals do not.

The stomachs of the two makos prove extremely interesting. All cruise long we have seen that most makos have scars on their skin from rows of suckers presumably from squid tentacles (see previous blog). Lately the Humboldt squid has been observed as a year-round resident of the Cailfornia Current. This squid, which can weigh more than 70 pounds, is a permanent resident of warmer waters off Mexico. Speculation has been rampant that this might be a harbinger of global warming or an indication that the top predators of the Current are at such low levels that “outsiders” can now thrive in the Current. Whatever the truth, the presence of large numbers of huge squid is a new fact of life and we have taken then on hook and line for scientist John Field who is studying this phenomenon. The stomachs of the two large makos prove to be filled with Humboldt squid.

In the open ocean, where hiding places are rare, each shark becomes an ecosystem. When sharks are encountered, they often have smaller fish that use them for protection and as a source of food. The remora is a small, highly evolved fish that uses sharks as a free ride and free food source. Remoras have a modified dorsal fin that is composed of a series of plates that serve as a suction cup on the top of their head. They use the suction cup to attach to the shark and briefly pop on and off when the shark feeds. Their attachment to the shark, both literal and figurative, is tenacious and they often stay with the shark even when the cradle lifts the shark out of the water! -- ABOARD THE DAVID STARR JORDAN, Off Catalina Island, California.