Sharks are often thought of in terms of superlatives, and perhaps no species has racked up as many “mosts” as the Great White Shark, Carcharodon carcharias. This is the star of “Jaws” after all, and probably the species most people who aren’t devoted to being familiar with fish visualize when they hear the word “shark.” Thanks to new research by Marra and friends (2019), we’re becoming familiar with the White Shark on the most basic level of all: the genetic level.
Not every species has had its entire genome decoded, and the White Shark is one of only a handful shark species to get this level of attention. The Whale Shark and the Elephant Shark (actually a species of chimera) have also had their genetic codes mapped, providing a couple of fairly closely-related species for comparison. By comparing the full genome of the White Shark with these other shark species and other vertebrates, the authors were able to identify specific mutations that have stood the tests of time and natural selection. Many of these genes are associated with the very traits that have made sharks such incredible survivors for going on 450 million years.
Marine environments are typically considered more open than those on land when it comes to animal movement. On land, the range of a species can be limited by geographic features like mountain ranges, canyons, rivers, and anything else that might get in the way. In the ocean, however, actively swimming animals like, say, large sharks have few physical barriers and may instead be restricted by their own environmental preferences. This is why in unusually warm summers you might see tropical fishes in southern New England. Because of this, one of the anticipated consequences of warming ocean temperatures is shifting distributions of mobile and highly migratory species. Basically, changes in temperature are likely to allow marine animals to move into places they haven’t before, and if those temperature changes become consistent, these species might make regular visits or even just start staying there.
This kind of change is already happening and has been documented across a variety of marine species. Now, findings from a new paper in Scientific Reports by me and co-authors from the North Carolina Division of Marine Fisheries, Simon Fraser University, and East Carolina University show an apex predator may be joining the northward shift.
Juvenile Bull Shark captured in Pamlico Sound, North Carolina. Photo from the Smithsonian Environmental Research Center.
I’ve defaced this colonial-era image of Native American fishing methods from Roanoke Island, NC to show that sharks have been observed in North Carolina’s estuaries for quite some time. Original art: “The Manner of Their Fishing” by John White.
It’s a bit of a cliché to reference the movie Jaws when talking about sharks, but I’m going to do it anyway. There’s a pivotal scene where the giant White Shark is spotted moving into a salt pond, where it proceeds to terrorize the children of protagonist Chief Martin Brody. While no sharks are in the business of regularly eating humans, at least part of this scene is realistic: sharks do make use of inshore, estuarine environments like lagoons, bays, and the lower portions of rivers. Despite the fact that sharks are generally thought to stay out in the ocean, many species are not only comfortable entering estuaries, they actually depend on them. Some species make extensive use of estuaries as shelter from predators and/or a place to grab a bite themselves.
So it should come as no surprise that North Carolina’s Pamlico Sound, part of the Albemarle-Pamlico estuary (the second-largest in the continental U.S. after Chesapeake Bay) offers a lot of potential real estate for sharks. Unlike Chesapeake Bay, Pamlico Sound is a lagoon, which means it’s wide, shallow, mostly water, and connected to the ocean via fairly small inlets between barrier islands. It straddles the transition zone marked by Cape Hatteras between temperate and subtropical marine ecosystems, and the amount of seagrass growth there is second only to Florida in sheer area. This estuary is already well-known for its importance as habitat for such varied (as well as tasty and/or fun to catch) species as Blue Crab, Penaeid shrimps, Flounder, and Red Drum. However, the sharks of Pamlico Sound have mostly been known by scattered reports and sightings from fishermen. That is, until my co-authors and I were able to look through a nice data set to get an idea of which sharks are present in the sound and where in the estuary they might like to be. The results are now published in PLOS One, and here’s a quick summary of how we got them.
I’d like to take a moment rant about a particular pet peeve of mine, which involves the seemingly-dull subject of species common names. As you may have learned in biology class, all identified and described species are assigned a Latin scientific name, which is intended to be a universal identifier of that species regardless of where it’s coming up in conversation. However, scientific names are not typically very familiar to non-scientists, so common names remain the most, well, common way to refer to a species.
On January 1, 2016, the Southern Fried Science central server began uploading blog posts apparently circa 2041. Due to a related corruption of the contemporary database, we are, at this time, unable to remove these Field Notes from the Future or prevent the uploading of additional posts. Please enjoy this glimpse into the ocean future while we attempt to rectify the situation.
I’ve been posting very sporadically due to spending the past month or so compiling all the data from the Marine Species Distribution Survey’s Cape Lookout leg. This was an exciting part of the survey for me because it brought me back to the waters I worked in while earning my PhD, so it a lot of ways it was like coming home. I happily took the lead on the apex predator portion of the survey so that’s mostly what I’ll be recapping first, but future posts will have more details on the trap, core, and genetic surveys.
Of course a lot has changed since then. For one thing, the ocean was two meters shallower, though parts of Beaufort and Morehead City used to flood at high tide even back in 2015. The biggest change may be the collapse and migration of many of North Carolina’s barrier islands, especially after Hurricane Monty rolled through ten years ago. In my mind I still picture Cape Lookout, now an island sitting by itself southeast of the Down East Banks, as part of a chain of barrier islands that once outlined all the North Carolina sounds. Core and Shackleford Banks are still on the map, but as shallow subsurface shoals that have a nasty habit of grounding whatever daring (or foolish) freighters still land cargo in Morehead City. They do draw in a lot of fish though, and still act as a sort of sill that allows Back and Core Sounds to function pretty much as shallow lagoons. If rumors of coral growth on some of the banks are true, it’s possible that the shoals could become fixed in place again.
It’s generally thought that baleen whales are too large to be successfully attacked by most marine predators. Orcas are typically considered the only real predatory threat to large whales, and even they have to use teamwork to take down a young whale. Large sharks, which also sit near the top of the marine food web, are known to scavenge on whale carcasses as a nutritious and blubbery supplement to their usual diet of fishes and smaller marine mammals. However, evidence has been found that white sharks actually take a proactive approach to increasing the whale carcass supply by attacking live northern right whale calves. Now researchers in South Africa directly observed dusky sharks actively teaming up to bring down a humpback whale calf.
North Carolina is well known for both its distinctive barrier islands (making Pamlico Sound the largest lagoon in the U.S.) and highly productive fisheries. Both of these features exist in large part because North Carolina sits that the point where two of the largest ocean currents in the Atlantic meet. From the north, the Labrador Current meanders from the Arctic Circle along the Canadian, New England, and Mid-Atlantic shorelines and crashes into the Gulf Stream at Cape Hatteras, deflecting this warm current off its own shore-hugging course from the south and out across the Atlantic Ocean. Aside from literally defining the shape of the Outer Banks, the collision zone represents the boundary between temperate waters to the north and subtropical waters to the south. This presence of this border means that, depending on the time of year and local weather conditions, you can catch just about any marine fish native to the Northwest Atlantic Ocean off of the Outer Banks.
This satellite image of sea surface temperatures shows the Gulf Stream (warm red current coming from the south) meeting the Labrador Current (cold purple current coming from the north). Image from Woods Hole Oceanographic Institute (whoi.edu).
Though the fisheries news cycle has mostly been taken up by the 15-year anniversary of the Sea Around Us project (and some choice words between researchers), today also marked the official announcement of the 12-month finding on the petition to list dusky sharks on the U.S. Endangered Species Act. Long story short, the National Marine Fisheries Service has decided that the dusky shark population in the Northwest Atlantic doesn’t need ESA protection to avoid extinction. While it may be tempting to decry NMFS’ decision as falling short for a species that has long been a prime example of declining shark populations, what it actually means is that things are looking up for the dusky shark. Finding that out only takes a little reading into the decision documents.
This time of year, it’s appropriate to think of things to be thankful for. This being an ocean-focused blog, I’d like to share something ocean-related that I’m thankful for, and hopefully spread a little Ocean Optimism in the process. What I’m thankful for is that Carcharocles megalodon is extinct. This may not seem like cause for optimism, but honestly the present-day ocean and Megalodon are better off without each other. And while we may not have 50-foot sharks around anymore (at least not the superpredatory kind), there are actually a lot of species we know and love that have either outlasted Megalodon or are only around because the big beast isn’t around anymore.
The 2010 Shark Conservation Act prohibits removal of fins at sea for all sharks landed in U.S. Waters, with a glaring exception for smooth dogfish, or smoothhound sharks. In an effort to ensure that fishermen aren’t performing the cruel practice of throwing a still-living but finless shark overboard, a fin:body ratio of 12% for smooth dogfish became law as part of this bill. This means that the total weight of smooth dogfish fins cannot be more than 12% of the total dressed weight of the bodies when the sharks are landed.
Some time ago I wrote a post questioning where this 12% ratio came from, especially since the best available published literature at the time suggested a ratio of only 3.5% for smooth dogfish. The Atlantic States Marine Fisheries Management Commission (ASMFC) responded, claiming that they had data backing up a find:body weight ratio of 7-12%. Now, thanks to the SEDAR stock assessment workshop for this species, the study conducted by the ASMFC is publicly available (albeit nearly four years after it was written into the law).
So where does this seemingly extremely high fin:body ratio come from? It depends on how you slice it.