The last day of AES talks wrapped up today. It’s always a little melancholy to see this conference end, but one must get back to real life sometime. Here are the highlights from Day 4.
This morning’s talks were all about predator/prey ecology, so of course I put down roots in that room. Good thing my talk was also part of this session.
Tiger sharks in Shark Bay, Australia were a focus of the first two talks, but only as an indirect influence on what was being looked at. This is because Shark Bay is an exceptionally good area to use as a model for all sorts of fun predation effects. It also allows researchers to model these concepts using very charismatic animals like the aforementioned sharks, sea turtles, dugongs, and bottlenose dolphins. Derek Burkholder used exclusion cages to show that grazing by dugongs and sea turtles can actually restructure the entire community of seagrasses in the area. The cages keep the big grazers out, which allows for denser seagrass inside the cage but also allows the smaller grazing fish to get in. This results in a single species of seagrass, which can apparently tolerate the grazing pretty well, dominating the other two that share the area. Mike Heithaus, who has been pumping out great papers from these studies for years (including on that disproved Flipper), showed that there are two potential paths through which tiger sharks can influence the seagrass community: by scaring off large grazers like turtles and by scaring off the predators of small grazers like dolphins. It turns out that both are true to an extent. Sharks frighten large grazers into the edges of the seagrass beds, where they can quickly turn into deep water and get away. This leads to a more uniformly grazed but still divers grass community. In the interior, where dolphins fear to tread, fish grazing leads to that same seagrass species becoming dominant. This changes seasonally: when sharks are absent in the winter, dolphins can eat the grazing fish and seagrass diversity increases again.
Marcus Drymon brought tiger sharks into the fold again by discussing how they seem to take advantage of bird migrations off of Dauphin Island. In the fall, when birds are most abundant, they can occur in over half of all tiger shark stomachs. These aren’t seagulls either; they’re the same sparrows and woodpeckers that are common in backyards across the U.S. By munching on migratory birds, could tiger sharks be bringing trophic energy from land into the marine food web?
The next feeding talk was, well, me. I talked about the occurrence of striped bass in the diet of spiny dogfish off of North Carolina, and how these two species may be engaging in predatory or competitive behavior. For their part, the stripers don’t seem particularly afraid of the dogfish and the sharks don’t seem to be negatively affecting bass populations. However, the two species might be switching between their two dominant prey species depending on what the other is eating. I got some great feedback and you can check the no-doubt epic post that David “WhySharksMatter” Shiffman is planning to make from his conference tweeting for a recap from someone other than me.
Mark Royer (who has a nice field blog about his current work in Hawaii) used diet analysis to determine where leopard sharks at La Jolla, California were foraging. By matching prey species up to habitat, he was able to find that they hunted squid in La Jolla Canyon, spiny lobster in seagrass beds, and several species of fish from the sand flats. This diet did not overlap much at all with the diet of leopard sharks in another area off of California, so there may be some localized diet specialization going on.
Jenny Bigman described a tri-national program between the U.S., Canada, and Mexico to encourage people to report sightings of basking sharks. This is working in conjunction with a tagging project that tracked at least one basker as far as Hawaii. These sharks are being sighted more often, but is it a result of more sharks, more people on the water, or more awareness of the reporting program?
The afternoon saw symposia about deep sea sharks and shark conservation. Jim Gelshleichter kind of bridged the gap by looking or signs of oil exposure in deep-sea sharks in the Gulf of Mexico. It turns out that about half the signs are there, which suggests that the deep sea species are metabolizing oil in a different way than even the shallow water sharks. Good to know as researchers continue to study the effects of Deepwater Horizon.
Over on the conservation side were two interesting talks about shark bycatch in commercial fisheries. Kelsey James tried to figure out what influences a fisherman’s decision to discard a bycaught shark, and found that the species was actually the most important factor, followed by region. The most likely bycaught sharks to be kept were makos, smooth dogfish, and hammherheads in nearshore areas, with generally smaller species and dogfish more likely to be discarded. Kieran Smith looked more at ways to directly reduces shark bycatch, and found that the combination of lead or zinc and wire leaders actually created an electrical field that lead to a nearly 80% reduction in shark catch on a longline. All this at a fraction of the cost of the rare earth metals currently being tested by several researchers to accomplish this exact goal. The next step is to take these rigs offshore and test them on some pelagic species.
My final highlight goes back to the deep sea sharks, where Steve Campana tagged and tracked Greenland sharks from two different locations using pop-up satellite tags. Despite the slow, lethargic nature of these sharks, there wasn’t a single tagged shark that didn’t migrate over 1,000 km (over 621 miles) from the capture site. Sharks tagged in the Arctic made for Greenland across the Davis Strait, while those tagged off of Nova Scotia headed past the Gulf Stream to an area over the Atlantic abyssal plain. The Atlantic sharks traveled deeper and in warmer water than those in the Arctic, but both sets of sharks swam well above the bottom in the water column. Yet another species that seems to do whatever the hell it wants.
And that’s that. The end of AES feels like the end of summer camp every year, but that just means I keep coming back. This has been a great conference and I hope to continue hanging out and working with my fellow elasmophiles well into the future. Tomorrow I’ll be checking out the Vancouver Aquarium as the last official part of AES, then it’s just exploring Vancouver until I fly home. Thanks for reading.