Notes From the Bering Sea
This week, we’re bringing you something a little different: a guest post by Abigail McCarthy, a Seattle-based fisheries biologist who just finished a stint in the Bering Sea and Gulf of Alaska. Enjoy!
I work for the MACE (Midwater Assessment and Conservation Engineering) group at the Alaska Fisheries Science Center, located, oddly enough, in Seattle. As a fisheries biologist for MACE, I have the opportunity to spend a fair amount of quality time in the Eastern Bering Sea during summer, and in the Gulf of Alaska and SE Bering Sea during February and March. We come out here to look for Walleye Pollock, a singularly unattractive fish that makes up the greater part of the commercial catch in the US, and lately, in the rest of the world as well.* But we often find another odd-looking fish known as a Myctophid, or lanternfish.
Myctophidae are a family of fishes that make up much of the deep scattering layer worldwide. They’re called lanternfish because all but one species in the family has photophores, or light-emitting organs, on their bodies.** They use these organs to attract prey, as counter-illumination (a type of camouflage) and possibly to attract mates.
Here in the Gulf of Alaska and southern Bering Sea, we see several species of myctophids offshore in deep water from about 200m to 400m in depth. They are estimated to be one of the most populous families of fish in the world, and are important prey for everything from seabirds and whales to sharks and large, valuable game fish like tuna, as well as pollock.
Lanternfish are incredibly numerous, but tough to study. We need to do deep trawl sets to collect these deep water fish, but being caught and pulled up from the deep in a trawl net can be brutal—oftentimes, lanternfishes’ unique features, like photophores on their sides and foreheads, are difficult to distinguish. This means that figuring out exactly how many there are is a tricky proposition.
Finding—and counting—pollock is a lot easier. We use a combination of acoustic technology and old-fashioned trawl nets. First, we traverse our survey area in a pattern called a boustrophedon—a Greek word describing the pattern farmers used to use to sow and plow their fields. During these traverses we collect data on the ocean below our ship using echosounders. The principal of an echosounder is that when energy in the form of sound waves is sent towards the sea floor a certain amount of it is reflected back towards the ship from whatever it hits. The magnitude and form of that energy differ depending on what it hits first—the sea floor or a school of fish. The simple fish-finders found aboard commercial and recreational fishing boats use this same principle, but our fancy fish-finders use five different frequencies of sound and they can keep track of what we see. Ours also cost about a half million bucks, which is a bit more than your average recreational fisherman is willing to spend.
As we collect acoustic data we keep an eye on what the returned signals, or the echosign, looks like. Different patterns in depth and density, as well as differing returns from the five frequencies, can give us a good picture of which species or combinations of species, might be down there. We can then confirm those educated guesses by fishing on the echosign and carefully measuring and identifying that catch.
Again, lanternfish are a little trickier than other fish. For some mysterious reason, myctophids show up very well on one of our acoustic frequencies (38 kHz) even though they are completely absent from the other four frequencies. In science, mysterious reasons lead to lots of questions. And lots of questions lead to great potential for interesting research. Where exactly do these fascinating fish live, and why? What enables the same species of fish to live both in the waters off the coast of California and in the Shumagin Islands in the NW Gulf of Alaska? Why do so many species of larger fish eat them? Is it simply because they’re there, or do their photophores make them a more attractive snack? With the toolbox of acoustics at our fingertips, we can begin to answer these questions and to think of even more!
*While the focus of this survey is Pollock, we catch other species as well, many of which are returned to the sea unharmed.
**The brownsnout spookfish, a Mauka to Makai cool critter, also has photophores, but it is not a member of the Myctophidae family.