The Home of Blue Water Fish

Home , Green jack, Halfbeak, Mackerel scad

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. Although some species are year- MEXICO Gulf of round residents of EBES, others change Pacific Ocean Mexico with the seasons. We have identified at Mexico City least 24 species of pelagic fishes at different times. They fall into five groups: sharks, billfish, tunas, jacks and snap- Figure 2. In the Gulf of California, between the Baja peninsula and the western coast of Mexi- pers. The billfish family, Istiophoridae, co, rising from a 1,000-meter basin less than two kilometers to the west, El Bajo Espíritu Santo is an example of the diversity at this is an underwater mountain that lies 18 meters below the surface at its highest point. site. Four species, three marlins and a sailfish, frequent the seamount during ples taken 10 kilometers to the north just under the surface as they gorge the day. Unlike marlins, which have or 8 kilometers to the south. Likewise, on concentrated plankton. This throng relatively small dorsal fins, sailfish have he found three times as many chaeto- of fish splits and merges abruptly as a dorsal fin that stretches along two- gnaths—another type of zooplankton predators such as dolphinfish or wahoo thirds of the animal’s body. Striped that eats copepods—in the water over chase after them. marlin and sailfish frequent EBES from EBES. This empirical evidence supports spring through fall, but the larger, rarer the conclusion that zooplankton are Go Fish blue and black marlins usually arrive concentrated by the Venturi effect and For the past five years, two of us (Jor- in late summer. They tend to loiter retained within tidal eddies near the gensen and Richert) have studied the close to the surface, maintaining their seamount. Together, these physical pro- ecological relations among the fish as- position by slowly sculling with their cesses result in more food at the lower semblage at El Bajo Espíritu Santo for pectoral fins. They also tend to be un- levels of the food chain, which in turn our Ph.D. research. Our work focuses friendly: On encountering a diver, a supports larger populations at higher on the movements of fishes that cause billfish will often put on an aggressive trophic levels. In fact, as you approach seasonal changes in the mix of species display by extending its fins to appear the seamount, the texture of the ocean (Jorgensen) and the feeding, or trophic, bigger, opening and closing its jaws and surface changes from perfect flatness relations among these fishes (Richert). shaking its rostrum to emphasize its to rippling wavelets caused by huge, We make many of our observations us- formidable armament. At this point it diffuse schools of small fishes that dart ing SCUBA equipment, which allows us is prudent for a diver to swim away at

Figure 3. Espíritu Santo is a mecca for pelagic fishes because plankton flowing over the seamount is concentrated by the Venturi effect, which speeds the flow of water in the narrow space between the peak and the sea surface. Lurking predator fish take advantage of the greater number of planktonic prey in the swifter currents near the pinnacle. (Plankton are not shown to scale.)

© 2005 Sigma Xi, The Scientific Research Society. Reproduction 44 American Scientist, Volume 93 with permission only. Contact [email protected]. Figure 4. Visual census data are collected during regular dives at Espíritu Santo and other nearby seamounts. In this photograph, two of the authors (Jorgensen and Richert) are using self-contained rebreather SCUBA equipment, which does not emit bubbles like convention- al open-circuit SCUBA. Bubbles tend to scare the fish and make an accurate census impos- sible. (Photograph courtesy of Jessica Taylor.)

the maximum angle of escape to avoid a charge from the fierce fish. Our (primarily Jorgensen’s) observations at EBES point to two different assemblages that change over the year. In the summer, when the water temperatures range from 24 to 26 degrees Celsius, a great number of fish ap- pears, including green jacks, scalloped summer assemblage abundance hammerhead sharks, dolphinfish and yellow snapper. The sailfish and mar- green jack scalloped hammerhead shark lins are also members of the summer assemblage, observations that are con- firmed by catch data from the fishing community. However, none of these species is present during the winter, when the water cools to between 16 and 20 degrees. During these months, yellow snapper dolphinfish yellowtail, amberjack and red snapper colonize the seamount. Using the identities and residence patterns of fish species as a starting point, we next sought to describe the “interactions” (a euphemism for who eats whom) among the living compo- winter assemblage abundance nents of this ecosystem. Most of what yellowtail amberjack red snapper scientists know about the feeding habits of deep-water fishes comes from the study of individual species that range over a wide area—an approach that overlooks the ecological connections between species. Our research (and Richert’s Ph.D. thesis) has tried to fill in some of these gaps by examining 100 30 trophic links between multiple pelagic summer assemblage average predators and prey that live in close winter assemblage average Figure 5. Different fish species live at Espíri- 80 tu Santo at different times of the year, but the seasonal residents tend to segregate broad- temperature (Celsius) 25 ly into two groups, or assemblages, based on water temperature. During the summer, 60 green jacks, yellow snapper, dolphinfish and scalloped hammerhead sharks are prevalent. The winter assemblage includes yellow- 40 tail, amberjack and red snapper. These line graphs show the relative abundance of each 20 species (scaled to an annual maximum) plot-

relative abundance (percent) ted over a 13-month period indicated on the 20 large graph. The heavy red line represents the average for the four summer species; the average of the three winter species is shown as a blue line. The black line indicates daily 0 15 water temperature at 25 meters. Note that in August November February May August the spring of 2000, hammerhead sharks did 1999 1999 2000 2000 2000 not return to the seamount.

creolefish

macroplankton yellowfin tuna leopard grouper flying fish

zooplankton flatiron herring yellowtail red snapper phytoplankton mackerel scad

skipjack tuna amberjack nanoplankton jumbo flying squid

plankton planktivores omnivores predators summer community food web

green jack

macroplankton creolefish sailfish flying fish flatiron herring yellow snapper zooplankton dolphinfish

striped mullet pelagic blue marlin phytoplankton red crab skipjack tuna yellowfin tuna mackerel scad

nanoplankton jumbo flying squid scalloped hammerhead shark

common halfbeak plankton planktivores omnivores predators apex predators

Figure 6. The authors’ hypothesis is that the summer assemblage, in which more species are present in greater numbers, represents a longer food chain than the winter assemblage. They are currently testing the idea by identifying the contents of fishes’ stomachs, which reveals what species they have been eating, and by analyzing the ratio of nitrogen isotopes in the muscle tissue of the fish, which indicates that individual’s trophic position. The ratio of naturally occurring nitrogen-15 to nitrogen-14 increases with each successive link in the chain. proximity within a single pelagic habi- Ultimately, these trophic studies temporary eddies that cannot support tat, such as EBES. Most of the data for will encompass not only vertebrates multiple generations. Thus, the Ventu- this project come from comparing the but also planktonic invertebrates. The ri effect probably causes the increased gut contents of fish caught at seamounts traditional belief is that seamounts zooplankton concentration that sus- in the southern Gulf with those of fish support longer food chains than the tains the diversity and abundance of from the open water between pinnacles. surrounding open water because of secondary consumers. We can also assess the animal’s trophic higher primary production (photosyn- position in the food web by the ratio thesis by phytoplankton). In the case of Fish Tracks of nitrogen isotopes within its muscle El Bajo Espíritu Santo, this assumption A visual census and dietary analysis tissue: The ratio of nitrogen-15 to the may not be true. In general, primary are traditional ecological tools for in- more common nitrogen-14 rises by a production is limited by the availabil- vestigating fish communities. Howev- discrete amount with each successive ity of nutrients, and at seamounts with er, given the limitations of a visual cen- link in the food chain. This method al- a constant current, long-term eddies re- sus (relatively poor visibility, mobile lows us to test the hypothesis that more tain nutrients, thereby enabling phyto- subjects), we also use ultrasonic telem- trophic levels exist at seamounts during plankton to feed, grow and reproduce etry. Our group, in collaboration with the summer (when more species are through multiple generations. How- fisheries ecologist Arturo Muhlia-Melo present) than during the winter. ever, at EBES the reversing tides make from CIBNOR, keeps two electronic

© 2005 Sigma Xi, The Scientific Research Society. Reproduction 46 American Scientist, Volume 93 with permission only. Contact [email protected]. listening stations at the seamount to rolls of 35-millimeter film because and Richert) periodically make a SCU- detect fish carrying beacons implanted unique vandals in the pelagic realm— BA dive to remove each monitor and within their bodies. Each beacon emits bioluminescent squid—are attracted temperature logger, carry them to the an ultrasonic signal with a unique to the intermittent flash of the light- surface and connect them to a laptop code, which lets us describe the resi- emitting diode and will bite through computer to download the files of fish dence patterns of individual members the clear plastic cover.) Each outpost attendance and water temperature. of the assemblage. is weighted with an anchor (we use Our goal is to eventually tag fishes The monitors, which are moored at a heavy, metal hawser salvaged from from all trophic levels, including con- each end of the ridge, include a hydro- a freighter) connected by a chain to a sumers, predators and apex predators, phone (underwater microphone), an mid-water set of buoys and another, in both the summer and winter assem- ultrasonic receiver and an electronic higher, cluster of buoys placed where blages. The tagging process, for some recording device within a cylinder they can be seen by a swimmer at the fishes at least, is fairly straightforward. about the size of a wine bottle. We surface. The device records the identity For example, the green jack, a second- also attach small temperature record- of every beacon within 500 meters, so ary consumer, and the yellowfin tuna, ers to the monitors to log hourly water between the two stations we can de- a predator, can be caught by hook and temperatures. (We protect these detect any tagged individual swimming line. In less than two minutes, we can vices with the small black cases from across the seamount. We (Jorgensen make an incision in a fish’s ventrum

coded ultrasonic beacon

monitor monitor

fish 26

fish 25 0000 fish 25 2200 0200 fish 26

2000 0400

1800 0600

1600 0800

monitor 1400 1000 1200

Figure 7. As a complement to taking a seasonal census, the authors also collect data on the daily residence habits of individual fish using ultrasonic beacons. Most fish, such as green jacks and yellowfin tunas, can be caught by hook and line. After landing the fish, the biologist inserts a transmitter into a small incision in the fish’s belly, then sutures the wound and returns the fish to the water. Two underwater listening stations, one at each end of the ridge, record the presence of all tags within a 500-meter radius. Periodically, a member of the team dives down to retrieve the telemetry recorder and downloads the data to a laptop computer onboard the ship. The circle graph displays the record of signals from two green jacks at Espíritu Santo over a 24-hour period.

2000 0400

1800 0600

1 3 5 7 1600 monitor 9 0800 undeployed 11 13 15 1400 17 1000 1200

Figure 8. Unlike the other fish at Espíritu Santo, scalloped hammerhead sharks do not congregate there in order to feed at the seamount—their fa- vorite food lives in deeper water. To solve the riddle of their presence at this site, one of the authors (Klimley) tracked the movements of individual hammerheads. The study revealed a pattern of dusk departure and pre-dawn return (circular graph). Following individual sharks during the night, the author discovered that the sharks traveled long distances with uncannily straight headings (shown by the three compass plots), in the absence of celestial or geographic cues, only to return by the same precise route (one individual’s path is shown in the series of linked arrows). Comparing the routes to a geomagnetic survey of the sea bottom (red lines) showed that the hammerheads, with their ability to perceive faint magnetic fields, were using Espíritu Santo as a sort of transit hub, following the geomagnetic signatures of old basaltic lava flows to reach preferred feeding grounds. (belly), insert a transmitter into its body cavity, close the opening with sutures and release the fish back into the sea. Green jacks feed on macroplankton (such as chaetognaths), krill and larval or juvenile fishes. Although this species can be found at EBES at all times of the day or night, ultrasonic track- ing data have also shown more than one individual leaving and returning to the seamount at the same time of day, indicating that they traveled together as part of an aggregation of fishes. We have also seen yellowfin tuna near the ridge at any given hour, and several tunas also moved in and out of range at the same time, showing that they were part of their own school. The period during which the greatest number of tunas were present (roughly 7 a.m. to 7 p.m.), overlaps with our prelimi- nary data on the peak residence times of green jack fishes (about 10 a.m. to 8 p.m.). Are the tuna preying upon the jacks during these overlapping periods? Or do the species share some migratory Figure 9. To record a shark's movements, the authors manually tag individuals with coded association? By integrating our analyses transmitters. This task requires them to dive down to a school of sharks, sometimes on a sin- of feeding habits and residence patterns gle breath from the surface, and use a pole spear to shoot a small barb, connected by a tether to for multiple species, we hope to answer the ultrasonic beacon, into the fish’s back. (Photograph courtesy of the authors.) these important questions.

© 2005 Sigma Xi, The Scientific Research Society. Reproduction 48 American Scientist, Volume 93 with permission only. Contact [email protected]. Jumping the Shark the same path. This feat was especially ecology of these species partly so that Compared with the relative ease—even remarkable considering the animal re- management efforts can be guided by leisure—of reeling in jacks and tunas to mained 200 meters below the surface a more comprehensive vision of pelagic tag, placing beacons on hammerhead (eliminating celestial cues) and 800 me- fish assemblages. If policy makers know sharks is a daunting challenge. Unlike ters above the bottom (obscuring topo- how trophic webs and migration pat- the other two species, scalloped ham- graphical landmarks). Indeed, when terns operate at and between seamount merheads do not feed at the seamount, that shark was at its greatest distance hotspots, they will be better equipped preferring to bide their time in small from the seamount, the consistency of to design marine protected areas that schools at greater depths. For that its navigational headings (indexed by a are more likely to sustain the size and reason, we have to swim down to the so-called “coefficient of concentration”) diversity of pelagic populations, thereby sharks and use a pole spear—a long, measured 0.996—a deviation of only 0.4 benefiting the fish, and the fishers, that flexible rod powered by a stretchy percent from a perfectly straight course. depend on them. loop of rubber tubing—to attach the In short, this hammerhead was swim- tags. If the sharks are in less than 25 to ming with the precision of an automo- Bibliography 35 meters of water, one of us (Klimley) bile driving down a highway—without Gonzáles-Armas, R., R. Polomares-Garca and can take a deep breath at the surface, being able to see the striped lines or R. De Silva-Dávila. 2002. Copepod and free-dive to the school, pick out a large even the road itself. macrozooplankton distribution associated to El Bajo Espíritu Santo Seamount. Contri- individual and release the spear so Knowing the unique sensory capa- buciones al Estudio de los Crustáceos del Pací- that a small, barbed dart on the end bilities of the hammerhead (it detects fico Este 1:1–11. penetrates the tooth-like skin of the faint bioelectric fields, which help it find Klimley, A. P., S. J. Jorgensen, A. Muhlia-Melo shark’s back. The dart is connected by hidden prey), we later surveyed the geo- and S.C. Beavers. 2003. The occurrence of a short tether to the ultrasonic beacon, magnetic field around the seamount and yellowfin tuna (Thunnus albacares) at Espíri- tu Santo Seamount in the Gulf of California. which floats unobtrusively just above discovered that the paths of the sharks Fishery Bulletin 101(3): 684–692. the back. When the sharks are in deep- coincided with magnetic valleys (inten- Klimley, A. P. 1993. Highly directional swim- er water or smaller schools, we use a sity minimums) and ridges (intensity ming by scalloped hammerhead sharks, type of SCUBA called a closed-circuit maximums) leading away from El Bajo Sphyrna lewini, and subsurface irradiance, rebreather, which doesn’t emit bubbles Espíritu Santo like spokes from a hub. temperature, bathymetry, and geomagnetic to spook the fish, and wait silently for These magnetic traces were likely pro- field. Marine Biology 117:1–22. the sharks to approach. Once tagged, duced by the flow of magnetite-impreg- Klimley, A. P., and S. B. Butler. 1988. Immigra- tion and emigration of a pelagic fish assem- the shark usually accelerates away for nated basalts from the seamount during blage to seamounts in the Gulf of California a moment, but quickly returns to the volcanic eruptions. Taken together, the related to water mass movements using group, indicating that the stress from evidence suggests that this predator, un- satellite imagery. Marine Ecology Progress the procedure is minimal. like some others, does not congregate Series 49:11–20. As is true for the other tagged species, at the seamount because it is a biotic Klimley, A. P., and D. R. Nelson. 1984. Diel movement patterns of the scalloped ham- some individual sharks appear to move hotspot, but because the area provides merhead shark (Sphyrna lewini) in relation independently and others swim togeth- a network of “paths” that enables it to to El Bajo Espiritu Santo: A refuging central- er as a school. However, the similarities find a constellation of productive feed- position social system. Behavioral Ecology end there. On the days that we tracked ing grounds in the surrounding waters. and Sociobiology 15:45–54. their movements, all of the hammer- Muhlia-Melo, A., P. Klimley, R. González-Ar- heads at El Bajo Espíritu Santo left the A Fishy Future mas, S. Jorgensen, A. Trasviña-Castro, J. Rodríguez-Romero and A. Amador-Buen- ridge between 6:00 and 8:00 p.m., and The astounding biodiversity at Gorda rostro. 2003. Pelagic fish assemblages at the they all returned between 4:00 and 7:00 Seamount that initially motivated our Espíritu Santo seamount in the Gulf of Cali- a.m. the next day. The uniformity of this research is rare today. Although the fornia during El Niño 1997-1998 and non- behavior led us to wonder where these Gulf remains fertile, destructive fish- El Niño conditions. Geofísica Internacional predators were going. To answer that ing practices and poor fisheries man- 42(3): 473–481. Trasviña-Castro, A., G. Gutierrez de Velasco, question, we used an ultrasonic monitor agement have jeopardized many fish A. Valle-Levinson, R. González-Armas, on the boat to follow behind some of the populations. A school of hammerheads, A. Muhlia and M. A. Cosio. 2003. Hydro- tagged hammerhead sharks at night. It for example, often included hundreds graphic observations of the flow in the vi- turned out they often made extensive of individuals in the 1980s. But after cinity of a shallow seamount top in the Gulf trips to other nearby areas, probably to intensive gillnetting at shark nursery of California. Estuarine, Coastal and Shelf Sci- ence 57:149–162. eat the squid found in deeper water. We areas over the past two decades, it is based this inference on a pattern of slow now unusual to see schools of even 20 swimming at great depths when farthest sharks. We often visit traditional habi- from the seamount, plus the fact that the tats and do not see a single one. squid in their stomachs often possessed It is easy, but incorrect, to blame only light-emitting photophores, which are the fishers for declining fish populations. For relevant Web links, consult this found in benthic species. We realize that fish stocks are an impor- issue of American Scientist Online: During these excursions, the ham- tant resource for coastal communities merheads showed an extraordinary and that fisheries management must http://www.americanscientist.org/ sense of direction. One shark swam for also reflect social and economic needs. IssueTOC/issue/681 20 kilometers in a direct line away from Our Pelagic Fish Research Group, which the seamount, abruptly turned 180 de- includes scientists from the U.S. and grees and came back to EBES along Mexico, is striving to understand the