CalCOFI Reports, Vol. 11, 1967 - California Cooperative Oceanic ...

THE PREDATION OF GUANO BIRDS ON THE PERUVIAN ANCHOVY(ENGRAULIS RllNGZNS JENYNS)ROMULO JORDANlnstituto del Mar del Per6lima, Per6INTRODUCTIONThe Perurian anchovy is regularly preyed uponby a variety of animals belonging to different ordersof the animal kingdom. Among those that thrive onits abundance are invertebrates, such as medusaeand squids, and a multitude of carnivorous fishes,birds and mammals. Two great industries owe theirexistence to the stock of anchovy: the guano industry,based on the collection of the bird droppings,which have served as a fertilizer of the soilfrom ancient times and the fishmeal industry, man’sexploitation of the anchovy resource, a recent innovation.Although the majority of the birds that inhabitthe Peruvian islands and coasts prey upon the anchovyto some extent, the list of the principal predatorsis not extensive. The main species are thefollowing :Family : SpheniscidaeXpheniscus hziinboldti Meyer-“Peruvianpenguin ”Family : ProcellariidaePufinus griseus ( Gmelin) -‘ ‘ Sooty shear-water”Family : PelecanidaePelecanus occidentalis thagus Molina-“Peruvian pelican’ ’Family : SulidaeXicla variegata (Tschudi)-“ Gannet”Xula nebouxi Milne-Edwards “Bluefootedbooby”Family : PhalacrocoracidaePhalacrocorax bougainvillii Lesson-“ Cormorant”Phalacrocorax gainaardi (Lesson)--“Red-footed shag”Phalacrocorax brasilianus (Humboldt)-“Bigua cormorant”Family : LaridaeLarus pipixcan Wagler (migratoria)-‘ ‘ Franklin’s gull ’ ’Family : SternidaeLarosterna inca (Lesson)-‘ ‘ Inca tern”Three of these species, the “guan;ty” (cormorant),the ipiquero 7 (gannet), and the 6 alcatraz 7 7 (pelican),are the main consumers, because of their greatnumbers. These are the species that transform theanchovy into the valuable fertilizer, which is de-posited in more than 40 islands and headlands alongthe Peruvian coast.Because of their overwhelming importance, thepresent discussion on the predation of birds on thestock of anchovy is limited to these three species.SPECIES COMPOSITION AND GEOGRAPHICALDISTRIBUTION OF THE MAIN POPULATIONCENTERSIn the order of importance the main guano producersare cormorants, gannets, and pelicans. Theepicenter of their distribution lies between lat. 6’S. and lat. 12” S. The birds occupy this zone duringspring and summer, when they form their densestconcentrations ; subsequently, during autumn andwinter, the birds are more dispersed and their areaof distribution is greatly enlarged, extending fromlat. 1” N. to lat. 38” S. The quantities and locationsof the main bird colonies along the Peruviancoast are shown in Figure 1.The number of adult birds, based on graphical censusesmade in December, 1962 and February, 1963,was estimated as being about 18 millions (JordAnand Fuentes, 1964), with the following frequenciesof species:Millions PercentCormorant ................................ 14.89 82.4Gannet __________________________________ 2.76 15.3Pelican _____ _ ____________________________ 0.42 2.3During the last 20 years this relative frequency doesnot seem to have changed substantially.FOOD AND FEEDINGThe anchovy constitutes 96% of the food of thecormorant (JordSn 1959) and at least 8Q% of the foodof the gannet and the pelican. During periods of lowavailability of the anchovy, due to changes in theoceanic climate of the Peru current or other causes,the birds suffer from malnutrition which eventuallyleads to cachexia and death. This is the reason why thebirds are completely dependent on the availabilityof the anchovy.The feeding habits of the three species differ, bothas regards fishing methods and time of food collecting.The cormorant dives and swims underwater in thepursuit of its prey to depths that exceed 12 m. Thesearch begins at 6 AM during the reproductive pe-105 1