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

CO-OCCURRENCES OF SARDINE AND ANCHOVY LARVAEIN THE CALIFORNIA CURRENT REGION OFF CALIFORNIA AND BAJA CALIFORNIAELBERT H. AHLSTROMBureau of Commercial FisheriesFishery-Oceanography CenterLa Jolla, CaliforniaThis is a report on the co-occurrences of sardineand anchovy eggs and larvae in the collections ofthe California Cooperative Oceanic Fisheries Investigations(CalCOFI) during 1951-60. More important,it is also a study of the interaction between two speciesof fish-both filter-feeders on planktonic organismsoccupyingthe same trophic level. This study leadsfurther into the problem of whether there is a limitto the biomass of sardines plus anchovies that can beaccommodated in the environment and whether onespecies increases in abundance only at the expenseof the other.During the period of the CalCOFI surveys, whichbegan in 1949, the population of the Pacific sardine(flardinops caerulea) as determined from the distributionand abundance of eggs and larvae, decreasedmarkedly, especially since 1954. In contrast, the populationof the northern anchovy (Engraulis mordax),as determined from the distribution and abundanceof larvae, increased spectacularly. This relation bringsup the question of whether the anchovy is moving intothe ecological niche previously occupied by the sardine.CalCOFI survey cruises have been made off Californiaand Baja California for more than 15 years.Coverage was fairly intensive in 1949-60, whencruises were made at approximately monthly intervals.In 1961-64, cruises were spaced at quarterlyintervals. Temporal coverage, consequently, was muchbetter during the decade 1951-60, the years dealtwith particularly in this report.One of the difficulties in working up the observationson sardine and anchovy eggs and larvae isthe massiveness of the data. Even the data for the10-year period, 1951-60, are based on more than 16,000separate collections. Anchovy larvae occurred in 6,755collections, or 42.1 percent of the total, and sardinelarvae in 2,133. or 14.3 percent. These data can beexamined in many different ways. For example, thesardine and anchovy larvae from all collections aremeasured by 1-millimeter intervals, for abundance andsurvival studies. Was survival of larvae better in sampleswhere anchovy and sardine larvae co-occurred,or in samples where the larvae of one species occurredalone ? What T1-m the relation of co-occurrences tothe temporal and areal distributions of the larvae ofthe two species or to their relative abundances perhaul? What mas the influence of changing environmentalconditions on the frequency with which sardinesand anchovies eo-occurred ?The CalCOFI survey cruises initially were plannedto delimit and assay the distribution and abundanceof the pIanktonic eggs and larvae of the Pacific sar-,dine to determine indirectly the distribution andabundance of the adult population at time of spawningand to obtain information on the factors affectingthe survival of year classes. Sardine spawning wasfound to have an extensive and variable areal distributionand to take place during much of the year.especially off Baja California. Consequently, we triedto cover systematically a rather large area of theocean off California and Baja California.The CalCOFI station pattern is illustrated in Figure1. Inasmuch as I plan to discuss the distributionand abundance of eggs and larvae in different partsof the CalCOFI region, I have subdivided it into 7areas-three off California and four off Baja California.The station lines included in each area are asfollows :AreaStation LineNorthern California __......................... 40- 57Central California ............................Southern California __........................ 60- 77S0- 93Northern Baja California ..................... 97-107Upper central Baja California _______ ___________ 110-120Lower central Baja California ______ _____-___ ___ 123-137Southern Baja California ...................... 140-157Not all areas were covered on each cruise. Fourwere consistently worked-those lying between stationlines 80-137. The majority of cruises also includedthe central California area. Usually only 1or 2 cruises per year were made off northern Californiaand southern Baja California. With rare exceptions,only a single plankton haul was taken ateach station occupied during a cruise.In the course of obtaining information about sardineeggs and larvae, we also obtained informationabout many other fishes with planktonic young. Evenat the beginning of the surveys, sardine larvae wereoutnumbered by larvae of northern anchovy (Engraulismordax), hake (Merluccius productus), rockfish(Xebastodes spp.), and usually jack mackerel(Trachurzu symmetricus).The occurrence of numerous species of larvae ledto a decision to identify and enumerate all the kindsin the collections. This decision posed problems inidentification, but these were solved with perseverance.It became evident that surveys of eggs andlarvae constituted one of the indispensable methodsof resource evaluation. Most pelagic fishes haveplanktonic stages that can be sampled more simplyand quantitatively than the adults.Larvae of the Pacific sardine and northern anchovyoccur mainly off southern California and off most ofBaja California. The sardine also occurs throughoutthe Gulf of California. The northern anchovy, asomewhat more temperate species, does not occur inthe Gulf, and its larvae seldom occur south ofMagdalena Bay. It ranges farther north, however,than the sardine. In recent years there has been littlespawning off central California (north of Pt. Conception)by either anchovies or sardines, although