This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Editor's Note: William Gilly, a professor of biology at Stanford University's Hopkins Marine Station, embarked on new expedition this month to study jumbo squid in the Gulf of California on the National Science Foundation–funded research vessel New Horizon. This is his third blog post about the trip.
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SEA OF CORTZEZ— During the night we moved west, sampling acoustic data the whole time, to an area east of Isla Tortuga. Today we followed a routine that was established yesterday and will continue for the rest of the cruise. We glide along at five knots sampling acoustic data in a box-like pattern, and every so often stop the ship to deploy an instrument that continuously measures properties of the seawater under the ship as a winch lowers the device to the desired depth. These properties include electrical conductivity (from which salinity is derived), temperature and depth, and the device is therefore known as a CTD. Our CTD is also equipped with an oxygen sensor, because this environmental feature is relevant to much of our work here.
Humboldt squid, like most animals that we are familiar with, require oxygen for the biochemical reactions that generate energy and support life. But Humboldt squid have the peculiar ability to be able to tolerate low levels of oxygen that would be lethal to many organisms, particularly large, athletic ones. Not only can the squid tolerate low oxygen, they actually spend much of their time in a midwater region of low-oxygen known as the oxygen minimum zone (OMZ). The OMZ can be defined as the depth range where oxygen is less than one-tenth its concentration at the sea surface (where oxygen in water is basically in equilibrium with oxygen in air). In this part of the Gulf, the upper boundary of the OMZ is typically 200-300 meters, and the lower boundary is about 1,200 meters. From a terrestrial point of view, 10 percent oxygen saturation would occur at an altitude far higher than the top of Mt. Everest, where saturation is about 40 percent. The OMZ is indeed a hostile environment.
How the OMZ comes to exist in some of the most productive oceanic ecosystems on the plane is explained in a bit more detail in a previous post. To make a long story short, excess organic matter that sinks from the productive surface layer is metabolized by microbes on the way down through the water column, and microorganisms thus shape this vast midwater, or mesopelagic, world. At OMZ depths it is also cold, and there is little or no sunlight, so most animals living in this strange world are small, move slowly and use oxygen very efficiently. Many of them also produce their own light, including myctophid lanternfishes (a favorite food of Humboldt squid) and the silvery hatchet fish.
We sample these midwater organisms by using several types of trawl-nets. This additional part of our daily routine shows us the most abundant prey items for Humboldt squid in the local environment, and we can then compare this menu to what the squid are actually eating based on analysis of stomach contents of captured squid. Trawls at this site produced many myctophids, deep-water shrimp and other species of small squid—a diverse buffet for a growing jumbo squid.
Many of the organisms that inhabit the OMZ do not penetrate far into its core, where oxygen can reach nearly zero, and instead remain near the upper. These organisms can be extremely abundant, and in the daytime they form a dense layer that can be detected using the acoustic-sonar gear employed by Kelly Benoit-Bird's team. We use sonar to target our trawls to the same depth as this acoustic deep-scattering-layer (DSL). This allows us to selectively sample the creatures there, and we can evaluate the oxygen and temperature at the same depth using the CTD. In this case pictured, the sonar on the drifting vessel recorded the CTD as it passed through two distinct scattering layers at 300 and 400 meters depth.
As sunset approached, the sonar revealed another classic property of the DSL—or depending on your point of view, of the behavior of organisms that comprise the layer. A daily migration towards the surface occurs at night, and this allows these small mesopelagic animals to feed on plankton in surface waters under the relative security of darkness. The return migration to the upper OMZ at dawn leads to a dark, cold, hypoxic refuge from active visually oriented predators. But Humboldt squid can tolerate the conditions in the upper OMZ quite well, and the DSL sashimi-bar remains available to them all day long. And at night the squid can feed in near-surface waters. This non-stop feeding capability is likely to be part of the explanation as to how they can grow so large so rapidly.
Nightfall again saw us perched along the rail, fishing rods poking into the blackness and pulling up squid for our daily sampling. Tonight we quickly captured our desired sample of 30 squid, and then the bite turned off. These animals were a bit smaller than those from the last two nights, and there were more immature animals.
As we cleaned up the deck, we began noticing the slow rocking of the ship, despite the lack of wind and waves. Five or six hundred miles to the south, Hurricane Adrian was raging in the Pacific, and this category 4 storm was sending a significant ocean swell halfway up the Gulf of California. We turned in hoping that this storm would turn west and track into the open Pacific.
The next day we continued our box-transect in the same area as yesterday until early afternoon and then started toward Isla San Marcos and Santa Rosalia. In a normal year there would be 200 small, open boats (pangas) with outboard motors engaged in the local fishery for big squid—but this is not a normal year, and fishermen have not been able to find any large squid here for the last year. They are anxious to learn when the squid will return. Rumors have reached us that thirty or so boats recently started fishing out of Santa Rosalia, but now it is Saturday night, and fishermen here traditionally take this night off.
Soon Chad Waluk from Oregon State once more revealed the wonders beneath the surface with sonar. Shortly before sunset he thought that he saw a dense aggregation of squid as the ship passed over the ridge of seamounts that runs north from Isla San Marcos. Schools of some kind of fish appeared to be beneath the squid—the dense, red ball-like structure is characteristic of fish with swim bladders that produce a strong acoustic signal. Squid lack swim bladders but nevertheless produce a good signal that often look like vertical tick-marks on a display like that in the photograph. Squid signals also have a characteristic signature based on comparing different frequencies of sound, and the sonar we use has four frequencies. This feature permits using this approach to judge whether a given target represents squid.
With the sonar image in mind, we started jigging for squid and collected our sample of 30 individuals within 15 minutes or so. In addition, we were able to collect an equal number for Oscar, our cook, because he requested some fresh squid tubes (mantle not cut open) for another dish he was planning. We were more than happy to oblige, and these small squid were a perfect size.
During the fishing period, we witnessed squid attacks directed against each other many times. If a squid was being pulled up from beneath the ship, half a dozen others would follow along, attacking the one on the jig. All attacks were directed against the individual on the jig, a most unnatural situation, and it is unclear to what extent cannibalism contributes to natural diet. In the opinion of Unai Markaida, our collaborator from Colegio Frontera Sur, Campeche, it might account for 10 percent or more. But in seeing how effective the attacks were on their hooked cohorts, it is hard to believe that this behavior is not part of the natural foraging repertoire of these awesome predators. Years ago, when we were raising California market squid in the lab, one of the first behaviors displayed by the tiny hatchlings (about the size of a rice grain) was to steal food from another squid that had successfully captured a copepod prey. The miniature squid sensed the capture event and rushed over to investigate. The resulting struggles over the contested copepod were generally quite violent, with much twisting and rolling. Taking advantage of a developing situation, often at another's expense, is perhaps characteristic of squids in general. You just can't trust them at all.
At this stop on the Santa Rosalia fishing grounds, nearly all of the squid were small, only 25-30 centimeters mantle length, and immature. This was a big difference from 2010, when squid of this size were fully mature and spawning. We collected statoliths of small immature squid, but determination of age and comparison with the 2010 data will have to wait until we return home.
Images:
CTD deployment: W. Gilly
Lantern fish and hatchet fish: Ian Wilson, Colorado State University
Trawl deployment: Ian Wilson, Colorado State University
Acoustic images with CTD and San Marcos squid school: Chad Waluk, Oregon State University
Nighttime squid fishing: Ted Uyeno, Northern Arizona University
Map: Google Earth, adapted by W. GIlly