The Home of Blue Water Fish Rather than singly inhabiting the trackless ocean, pelagic fish species travel together in groups, which migrate between hidden, productive oases A. Peter Klimley, John E. Richert and Salvador J. Jorgensen ore than two decades ago, I (Klim- It was a wonder. But what left us side of the ocean have later been caught Mley) pressed my mask against my dumbfounded was the sudden erup- on the other side. However, these data face, took a deep breath and flipped tion of this multilayered community. do not tell marine scientists whether over the edge of a small Mexican fish- Just one week before, we had visited the individual moved alone or as part ing boat into the Gulf of California. The the same site and seen nothing. The of a school, as a single species or within spectacular vision I saw that day has difference between the visits was like an aggregation of many species. These shaped the questions that motivate my comparing an empty stadium to one unanswered questions are part of a research career in marine biology. crowded with tens of thousands of general ignorance that has hindered ef- I was looking for hammerhead sharks cheering fans. Had we witnessed the forts to maintain healthy populations of over the Gorda Seamount, a shallow arrival of a massive influx of oceanic pelagic fishes, many of which are in a underwater ridge at the mouth of the species to the Gulf of California? precipitous, worldwide decline because gulf between the Baja Peninsula and of over-harvesting. Consequently, many the western coast of Mexico. Wear- Marine Oases fisheries managers and conservationists ing a mask, snorkel and fins, a local Thanks to the popularity of nature now favor the creation of protected hab- colleague and I saw, through the dis- shows on television, most people itats, similar to game preserves, to ease persing bubbles from our entry, a stun- know that many terrestrial animals mi- pressure on hard-hit fish stocks. The ning sight—we were in the middle of grate from one place to another as the problem with these “marine protect- a swarm of fish, as if we had joined seasons change. For example, in Africa ed areas” is where to put them. What the piscine version of rush hour at every year hundreds of thousands of is the habitat of pelagic fishes such as a subway station. More than a hun- wildebeests, gazelles and zebras leave tuna, dolphinfish and mackerel? Is it dred hammerhead sharks, some close the southern plain of the Serengeti to the broad expanse of the oceans, which enough to touch, passed by us as we avoid the dry season. Without rain, cover four-fifths of the globe? floated above a seemingly endless, the lakes evaporate and the grass dies, As marine ecologists, we propose tightly packed school of flashing, sil- causing the base of the food chain to that pelagic species might instead pass ver and black skipjack tunas. Nearby, a collapse and forcing large herbivores quickly through the vast, mostly emp- cyclone-shaped school of gray-striped to walk hundreds of miles in search ty ocean yet stay longer at biotic oases mullet snappers, each almost a meter of forage. As they slowly make their to feed on locally abundant prey— long, swam slowly in a circle. Small long-distance trek, the herds linger analogous to the way terrestrial spe- green jacks and plate-shaped pom- at remaining water holes to sate their cies congregate at water holes along pano were everywhere, darting to thirst and feed on the lush riparian fo- their migratory path. If this hypoth- feed on tiny, shrimp-like krill and tail- liage. These oases are terrestrial biotic esis is true, policy makers could focus beating larvaceans. “hotspots” along a migratory route on sheltering some of these locations, with few other sources of food. When rather than the entire ocean, to improve A. Peter Klimley is an adjunct associate professor at the rains return, the animals go back to the health of pelagic populations. the University of California, Davis. In addition to their green pasture in the south. scientific articles, he has written texts and popular Biologists know much less about Intro to El Bajo nonfiction about pelagic fish behavior and ecology. the migration of marine species, par- Much of our research into the ecology John E. Richert and Salvador J. Jorgensen are Ph.D. ticularly those pelagic or free-swimming of pelagic fishes is based on observa- candidates at UC Davis. Richert studies food chains fish that inhabit the blue ocean far from tions in the Gulf of California, par- in the Gulf of California and manages the Pelagic the coast. Animals in the pelagic realm ticularly at the fertile Espíritu Santo Fish Research Group. Jorgensen studies the move- are typically independent of the bottom Seamount, which we have studied ments of pelagic fishes; he also co-founded Iemanya Oceanica, a nonprofit group dedicated to shark and are wide-ranging. For example, in concert with several Mexican col- conservation. Address for Klimley: Department of fish tagged in temperate waters during leagues. El Bajo Espíritu Santo or EBES Wildlife, Fish and Conservation Biology, University the summer have been recaptured in (literally, “shoal, or bank, of the Holy of California, Davis, One Shields Avenue, Davis, semitropical or tropical waters during Spirit” in Spanish) is a submarine CA 95616. Internet: [email protected] winter, and individuals tagged on one ridge that, in less than 2 kilometers, © 2005 Sigma Xi, The Scientific Research Society. Reproduction 42 American Scientist, Volume 93 with permission only. Contact [email protected]. Amos Nachoum/Corbis Figure 1. These skipjack tunas in the Gulf of California are pelagic fish, meaning their home is the open sea far from shore. With pelagic fish populations under threat, the size of their habitat presents unique challenges to conservation. Often neglected in the mathematics of fisheries management is an understanding of how tunas and other pelagic species interact with one another and the submarine geography as part of a dynamic ecosystem that changes as the fish undertake annual migrations between feeding grounds. rises steeply from a 1,000-meter basin and the ocean surface, providing more per second, which can occur during to within 18 meters of the surface. We drifting prey over time for predator fish spring tides, may cause twin eddies use a global positioning system to lo- lurking near the peak. as large as 1 kilometer on the down- cate the area, which is completely un- Many observers have noted high current side of EBES. This motion dis- derwater and invisible from the boat. biological productivity around sea- rupts the pycnocline, the boundary be- Shallow seamounts, such as Espíritu mounts and islands—a phenomenon tween the warmer, mixed surface layer Santo and Gorda, support rich stocks that oceanographers refer to as the and the colder, unmixed (but often nu- of pelagic fishes because of an abun- “island-mass effect.” Part of this abun- trient-rich) layers below. The nutrients dance of plankton that attracts consum- dance can be explained from a purely in the upper layer enhance the growth ers. So why is plankton (and the rest of physical perspective: Obstacles in the of phytoplankton (microscopic plants), the food web) enriched near EBES with path of a moving fluid usually cause and the eddies may also trap plankton respect to the surrounding ocean? We hydrodynamic disturbances—eddies in their reversing current flows. think that much of the answer, espe- and vortices—in the flow. Being situat- Rogelio Gonzalez-Armas stud- cially at Espíritu Santo, has to do with ed in an inland sea, EBES lacks a strong ied plankton dynamics at EBES as a the so-called “Venturi effect,” which de- unidirectional current, but it does have graduate student at the Centro de In- scribes how flow speed increases when a daily tide that oscillates around the vestigaciones Biológicas del Noroeste a fluid is forced through a narrow area. seamount like a cocktail swirled by a (CIBNOR), also in La Paz. By towing This physical law also explains the high fixed stirring rod. Armando Trasviña- a cylindrical net at six stations around winds through mountain passes. At El Castro, a physical oceanographer at the seamount, Gonzalez-Armas ob- Bajo Espíritu Santo, the same volume the Centro de Investigación Científica served a two- to seven-fold increase in of water carrying a given number of y de Educación Superior de Ensenada the concentration of copepods—minute plankters must flow through the more (CICESE) in La Paz, Mexico, estimates crustaceans that feed on phytoplank- constricted space between the seamount that tidal flows exceeding 0.5 meters ton—at the ridge, compared with sam- © 2005 Sigma Xi, The Scientific Research Society. Reproduction www.americanscientist.org 2005 January–February 43 with permission only. Contact [email protected]. to swim a 500-meter stretch of the ridge that includes its apex. During a typical Espíritu Santo dive we see loose schools of fish dash- Seamount ing after plankton, which fills the water. 18 Espíritu We also see oval-shaped jacks and bul- UNITED Santo 30 24 let-shaped mackerel. At the pinnacle of North STATES 30 the ridge, only 15 meters across, female 36 and male creolefish rush up from the 42 24 18 bottom to release enveloping clouds 48 of gametes. A flat plateau of sand and 54 rock separates the peak from a second 60 60 m high spot to the north, a 30-meter-wide Espíritu Santo mesa made of enormous, pillow-shaped stones. A tightly packed school of large 0 50 100 m South N snappers and a cadre of 15 to 20 ham- La Paz merhead sharks often hover nearby.