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

INVESTIGATIONS OF FOOD AND FEEDING HABITS OF LARVAE AND JUVENILESOF THE ARGENTINE ANCHOVY ENGRAULIS ANCHOITA'JANINA DZ. DE ClECHOMSKllnstituto de Biologia MarinaMar del Plata, ArgentinaINTRODUCTIONUp to the present time no papers on the feeding ofthe larvae of marine fishes have been produced inArgentina. The present contribution is the first studyof this nature referring to this area of the SouthwestAtlantic and it deals with problems related to thefeeding habits of the larvae and juveniles of theArgentine anchovy.The purpose of this paper is to study the food andfeeding habits of the anchovy larvae from the time atwhich the larvae begin to be able to feed, to the timewhen the juveniles approach the adult stage. Thisstudy has been based upon a quantitative and qualitativedetermination of the food components, to obtaina better assessment of the contribution of the differentgroups of organisms to the diet of the larva(. andjuveniles of the anchovy. The characteristics of thefeeding of the young forms of the anchovy at differentstages of development were studied in relation to themorphological changes ~vhicli take place throughoutthe growth of the individuals. The morphologlcalcharacters most closely related to the feeding processin fishes are: the digestire tract, the gill rakers andthe dimensions of the mouth. Therefore, in the presentwork, special emphasis has been given to a detailedstudy of these morphological elements. Also, an analysisof the changes in the feeding of the larvae andjuveniles of the anchovy during different seasons ofthe year has been attempted.Together with the anchovies, juvenile indivudals ofanother species, Aicstroatlzerina incisn, were gathered.The individuals of this species share the habitat of,and are found mixed with those of the anchory. Thepurpose of collecting other material was to study theintestinal content of the individuals of this otherspecies with the aim of obtaining comparative materialas a frame of reference for the anchovy, taking especiallyinto account the competition for their foodsupply. The results of the analysis of the feedinghabits of the juveniles of A. incisa will be found inanother paper by this author (Ciechomski, in press).MATERIALS AND METHODSThe larvae and juveniles of anchovy which wereused as material for this study were collected fromcoastal maters off Mar del Plata. The juveniles wereobtained from every month of the year. The larvae ofsizes 3.0-4.9 mm and 5.0-22.0 mm were obtained onlylThis paper has heen iireparerl through the sponsorship of theConselo Wncioiinl 17s Invrst?gncio?les CientafLcns y Tecwwas ofAvr/eiittua.during the summer. A total of 1,705 larvae andjuveniles has been studied, ranging in size from3.0 to 90.0 mm. From these, 503 individuals containedfood, the remainder having their digestive tractsempty.For the elaboration and presentation of the data, themonths of September, October, and November havebeen considered spring, December, January, and Februarywere considered as summer, March, April, andMay autumn, and June, July, and August winter.The material to be studied was fixed on the spot,right after its collection, with formaldehyde, in orderto stop the digestive processes, since this was convenientin view of the study made of the intestinalcontent. Further treatment of the material was continuedin the laboratory.Each individual was measured and its digestivetract was separated under a magnifying glass. Theexamination of the intestinal content was performedunder the magnifying glass and the microscope. Allof the components vere separated, counted and measuredby means of a micrometric eyepiece. For assessingthe weight of the individuals, a list of meanweights was made in which the larvae and juvenilesof anchovy were classed in total length classes with2 mm intervals.Plankton samples were obtained together with theanchovies, and the predominant forms were noted.Although this appraisal was not made in a quantitativemanner and was not expressed numerically, it wasconsidered useful as an indication of the feedingselectivity of the fishes which were studied.In the quantitative treatment of the data, one ofthe methods used was the determination of the frequencyof occurrence. The calculation of the weightof the total food ingested and of the percentage compositionof the various food components was basedon the mean live weight of the planktonic organismswhich make up the diet of the anchovies. Since atthe present time no data are available on the weightof these planktonic organisms, which are the basicfood supply of the larvae and juveniles and, in somecases, of adult planktofagous fishes, for the most ofthe planktonic components the volume was determinedby the method of volume shift. In other cases, whendealing with very small organisms of irregular shape,such as some diatoms (Coscinodiscus, Triceratizm,etc.) the volume determination was accomplishedgeometrically. Multiplying the volume by the correspondinggroup index (Hagnieier, 1961) the meanweights of the organisms were obtained. The following