The Relation Between the Distribution of Zooplankton Predators and Anchovy Larvae
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ALVARINO: DISTRIBUTION OF ZOOPLANKTON PREDATORS AND ANCHOVY LARVAE CalCOFI Rep., Vol. XXI, 1980 THE RELATION BETWEEN THE DISTRIBUTION OF ZOOPLANKTON PREDATORS AND ANCHOVY LARVAE ANGELES ALVARINO National Oceanic and Atmospheric Administration National Marine Fisheries Sewice Southwest Fisheries Center La Jolla. CA 92038 ABSTRACT dores y las correspondientes muestras de plancton han Monthly CalCOFI cruises of 1954, 1956, and 1958 demostrado que cuando abundaban en el plancton cope- were analyzed for abundance of populations of species of podos y eufausidos, 10s depredadores ingerian menos Chaetognatha, Siphonophorae, Chondrophorae, Medusae, larvas de peces. and Ctenophora. Data were also noted on other abun- dant zooplankters in the samples (copepods, euphausiids, INTRODUCTION decapod larvae, pteropods, heteropods, polychaetes, salps, Mortality of pelagic marine fish larvae can result from doliolids, and pyrosomes). Information was grouped into a variety of causes, both biotic and abiotic. Among the three categories of abundance of anchovy larvae per stan- more important biotic causes are starvation, predation, dard haul (more than 241 anchovy larvae, from 1 to 240, parasites, and disease; among the abiotic causes are and absence of larvae). In general, concentration of pre- storms, currents, ultraviolet radiation, temperature, sa- dators was inversely related to aggregations of anchovy linity, oxygen, and pollution. It was the consensus of larvae. Absence of anchovy larvae coincided with pro- participants in a Colloquium on Larval Fish Mortality chordates, decapod larvae, pteropods, heteropods, and Studies, held in La Jolla during January of 1975, “that polychaetes, and abundance of anchovy larvae concurred the major causes of larval mortality are starvation and with abundance of copepods and/or euphausiids. This predation, and that these may interact” (Hunter 1976). It habitat can be designated “anchovy water.” was noted in the report that most research emphasis had Gut content analysis indicated that predatory pres- been placed on starvation and relatively little work existed sure on fish larvae was weaker when there was abundance on predation. Observations on predation of invertebrate of other food animals, e.g. copepods and euphausiids in plankton organisms on fish larvae appear in Lebour the waters, as shown by the plankton collections. (1922, 1923, 1925), Bigelow (1926), Fraser (1969), Dekhnik et al. (1970), and others, as discussed in Alva- RESUMEN riiio (1976, 1977). Las poblaciones de especies de Quetognatos, Sifon6- Several recent laboratory studies have dealt with in- foros, Condroforos, Medusas y Ctenoforos han sido vertebrate predation on newly hatched larvae of the analizadas en las colecciones de plancton obtenidas northern anchovy: copepods as predators (Lillelund and durante 10s cruceros mensuales de CalCOFI en 1954, Lasker 1971) or the euphausiid Euphuusiu pucz3cu in 1956 y 1958. Conjuntamente se anotaron datos sobre la Theilacker and Lasker (1974). These predators are most abundancia de otros zooplanctones presentes en esas effective on the relatively passive yolk-sac larvae of the muestras de plancton (Copepodos, Eufausidos, larvas de anchovy and less so on actively swimming larger anchovy Decapodos, Pteropodos, Heteropodos, Poliquetos, Salpas, larvae. The principal planktonic predators on large an- Doliolos y Pirosomas). La informacion obtenida sobre la chovy appear to be Chaetognatha, Siphonophorz, Chon- abundancia de las especies correspondientes, se ha drophorae, Medusae, and Ctenophora. Predation by these agrupado en tres categorias, en relacion con la cantidad zooplankters has been observed frequently by planktolo- de larvas de anchoa en cada arrastre (mas de 241 larvas, gists, who find fish larvae in various stages of digestion desde 1 hasta 240, y ausencia de larvas). En general, la inside the guts of these predators. Despite such observa- concentracion de depredadores y larvas de anchoa apa- tions, no one has attempted a thorough analysis of such recia en relacion inversa. Se observo con frecuencia, que predation on fish larvae. en las zonas de surgencia no aparecian larvas de anchoa. It is the purpose of this contribution to study the rela- La ausencia de larvas de anchoa coincidia con la presen- tion between occurrence and abundance of anchovy lar- cia de Procordados, larvas de Decapodos, Pteropodos, vae (Engradis rnordux) to the other elements in the Heteropodos, y Poliquetos, y la abundancia de larvas de ’plankton, with emphasis on potential predators, and also anchoa concurria con gran cantidad de Copepodos y to characterize the assemblages of plankters in relation to Eufausidos. Este habitat podria denominarse “agua de anchovy larva abundance. The monthly CalCOFI collec- anchoa.” tions made off California and Baja California in 1954, Los analisis del contenido estomacal de 10s depreda- 1956, and 1958 were selected for this study. These three I50 ALVARINO: DISTRIBUTION OF ZOOPLANKTON PREDATORS AND ANCHOVY LARVAE CalCOFI Rep., Vol. XXI, 1980 years were respectively, slightly colder, colder, and Chaetognatha, Siphonophorae, Medusae, Ctenophora; the warmer than the long-term average for the California presence of copopods, euphausiids, pteropods, hetero- Current region. One reason for reanalyzing field collec- pods, decapoda larvae, polychaetes, tunicates; and infor- tions of plankton rather than doing experimental studies mation on the volume of water strained per haul. in the laboratory on Chaetognatha, Siphonophorae,Chon- During the analysis of the collections of the three drophopae, Medusae, and Ctenophora as predators is that years, it was observed that approximately 36% of the these organisms are diffkult subjects for experiments. chaetognaths had food in their stomachs, and all speci- They lack the capacity for food reserve storage, for exam- mens of the two largest species, Sagitta hexupteru and S. ple, hence must feed continuously to avoid starvation. The scrzppsue, contained food in many collections. Some reason for limiting the study to anchovy larvae while ig- chaetognath stomachs contained prey not present in the noring occurrence and abundance of anchovy eggs is that corresponding plankton samples. the eggs are less exposed to predation by these predators, Data also were taken on the relative abundance of the which respond primarily to movement. various constituent groups in each sample. The groups included, in addition to the five groups of predators dis- METHODS cussed above, were copepods, euphausiids, decapod lar- The plankton collections analyzed were from the vae, pteropods, heteropods, polychaetes, salps, doliolids, monthly cruises of the California Cooperative Oceanic and pyrosomes. The determinations were based on dom- Fisheries Investigations (CalCOFI) for 1954,1956, and inance of a group or groups in the collections analyzed. 1958. The area covered was the California Current re- Although initially it was proposed to analyze all sam- gion off California and Baja California, with usual cover- ples taken during the three years, 1954,1956, and 1968, age between San Francisco, off central California, to this soon proved to be too large a task. Instead, emphasis Cape San Lazaro, off southern Baja California (Cal- was placed on the areas that contained anchovy larvae in COFI lines 60 to 137). The plankton was collected with a some abundance, in order to determine areal and tem- bridled net, 1 m in diameter at the mouth, about 5 m in poral coverages that would bear most directly on the length, constructed of heavy-duty bolting grit gauze of problem of predator-anchovy interactions. The collections 5Oxxx mesh. The nets were equipped with flow meters to used in the following analyses were 849 for 1954,3 16 for measure the volume of water strained during each tow. 1956, and 899 for 1958. The hauls were made obliquely from about 140 m to the Anchovy abundance was divided into three categories surface, depth of water permitting. on the basis of abundance in the tows: high, low, and zero. The total plankton sample was analyzed for the five The standard haul values of anchovy larvae in the high groups of predators: Chaetognatha, Siphonophorae, Chon- category corresponded to 241 larvae or more per haul. drophorae, Medusae, and Ctenophora. It should be pointed The standard haul value is the estimated number of an- out that Ctenophora are often destroyed by preservation, chovy larvae under 10 mz of sea surface. The count of low hence the numbers recorded are underestimates of their abundance included all positive hauls with counts lower true abundance in the collections. In all five groups, speci- than 24 1 larvae per standard haul. Inasmuch as the aver- mens present in each sample were identifed to species and age CalCOFI haul sampled to about 140 m depth, the counted by species. Only occasionally, when a species actual volume of water involved under 10 mz of sea occurred in large numbers, were counts made on an surface is approximately 1,400 m3. Although the number aliquot of the total sample. Numbers of each species were of anchovy larvae is not ordinarily expressed as the standardized to the number in 1,000 m3of water strained. number in 1,OOO m3 water strained, the unit of volume In addition, the size of each specimen in mm was re- used for zooplankters would be about 10/14 that of corded, together with stage of sexual maturity and/or anchovy larvae on the average, or approximately 172 stage of life cycle. Also, the number of specimens with larvae or more for the high category