SMITH ET AL.: ZOOPLANKTON PATTERN ANALYSIS CalCOFl Rep., Vol. 30,1989 ANALYSIS OF THE PAllERNS OF DISTRIBUTION OF ZOOPLANKTON AGGREGATIONS FROM AN ACOUSTIC DOPPLER CURRENT PROFILER PAUL E. SMITH MARK D. OHMAN LAURENCE E. EBER National Marine Fisheries Service Marine Life Research Group, A-027 National Marine Fisheries Service Southwest Fisheries Center Scripps Institution of Oceanography Southwest Fisheries Center P. 0. Box 271 University of California, San Diego P. 0.Box 271 La Jolla, California 92038 La Jolla, California 92093 La Jolla, California 92038 ABSTRACT INTRODUCTION A test set of data of volume reverberation, mea- Distributional patterns in the pelagic habitat are sured from the R/V New Horizon, nominally from known to exist at several time and space scales 12-240-m depth in 4-m intervals, was taken in April (Haury et al. 1978; Greenlaw and Pearcy 1985). 1988 using a 307-kHz acoustic Doppler current pro- Some spatial scales have been evaluated for some filer (ADCP). A vertical profile was produced each planktivorous pelagic schooling fish (Graves 1977; minute for three days over the San Diego Trough Fiedler 1978; Smith 1978b), zooplankton (Star and off southern California. The results show kilome- Mullin 1981; Haury and Wiebe 1982; Pieper and ter-scale zooplankton aggregations with somewhat Holliday 1984), and mesopelagic micronekton larger gaps between them. The area profiles had (Greenlaw and Pearcy 1985). Two aspects of plank- about loo%coverage of zooplankton aggregations. ton sampling that must take account of these scales Aggregation target strengths were usually stronger are (1) estimation of the abundance of a species and during the morning and evening migrations. Mi- (2) an understanding of the interactions between gration rates were of the order of 5-8 cm s-’ during planktonic organisms and their immediate environ- ascent and 3-4 cm s-’ during descent. Zooplankton ment, principal predators, competitors, and food aggregations were of horizontal dimensions in- organisms. termediate between fish schools and groups of Vertical and horizontal integration can increase schools. the precision of estimates of abundance of “patchy” organisms in the plankton sample (Wiebe 1972). RESUMEN The integrated sample, however, may underesti- En abril de 1988, a bordo del buque de investiga- mate the local density of an organism by sampling ci6n New Horizon, se colect6 una serie de datos de volumes at inappropriate depths, or may include volumen de reverberacidn, haciendo us0 de un species with which the target species never inter- aparato de 307 kHz para perfilar la corriente doppler acts. Resolving the patchy dispersion ofprey within ac6stica (PCDA), abarcando una profundidad de 12 a fluid volume is important for estimating attack a 240 m en intervalos de 12 m. Se midi6 un perfil rates by predators (Ohman 1988a). vertical cada 1 minuto, por un period0 de 3 dias, en The space scales relevant to a particular popula- la Depresi6n de San Diego, frente a1 sur de Califor- tion or process are invisible in the pelagic habitat. nia. Los resultados indican agregaciones de zoo- Pilot sampling studies are necessary to help formu- plancton en escalas de kilbmetros, con espacios algo late and constrain the questions to be answered (An- mis grandes entre las mismas. Aproximadamente drews and Mapstone 1987). Acoustic methods can 10% del Area de 10s perfiles presentaron agrega- be useful for such preliminary surveys, as a means ciones de zooplancton. Las fuerzas de blanco de to obtain rapid, broad coverage, and as a tool to agregaciones fueron, en general, mis fuertes dur- direct other, invasive sampling methods. Acoustic ante las migraciones matutinas y vespertinas. Las methods are not generally useful for identifying or- tasas de migracidn fueron del orden de 10s 5 a 8 cm ganisms or precisely estimating their biomass. A seg-’ durante el ascenso y de 10s 3 a 4 cm seg-’ continuous acoustic record can be used to delineate durante el descenso. Las dimensiones horizontales spatial scales that must be encompassed or resolved de las agregaciones de zooplankton fueron inter- (Holliday 1985). medias entre card6menes de peces y grupos de car- Studies of the scale and intensity of fish schools d’umenes. have been used to design and interpret surveys of northern anchovy eggs (Smith 1970; Hewitt et al. 1976; Graves 1977; Fiedler 1978; Smith 1978a,b, [Manuscript received February 16.1989.1 1981; Smith and Hewitt 1985a,b). Two major find- %% SMITH ET AL.: ZOOPLANKTON PATERN ANALYSIS CalCOFl Rep., Vol. 30,1989 ings of this work that depended on acoustic tech- west of San Diego (figure 1). The equipment used nology are that (1) on the small scale, within a included a 307-kHz acoustic Doppler current pro- school, the biomass of fish is on the order of 15,000 filer (ADCP)', a multiple opening-closing net, and grams per square meter of horizontal surface, and environmental sensing system (MOCNESS, their consumption of plankton would be approxi- Wiebe et al. [1985]), and standard hydrographic mately 750 grams per square meter of fish school bottle casts. It is important to note that the ADCP horizontal surface per day; and (2) the fish schools had not been calibrated in the amplitude domain of tens of meters in diameter are assembled in con- and that the effect of methods of conditioning centrations of shoal groups extending for tens of the amplitude signal have not been studied. The kilometers. We do not know the rules of assembly Sea-Bird Electronics temperature sensor on the of shoal groups: one mechanism could be that MOCNESS frame was calibrated against deep-sea schools' swimming speeds decrease and turning be- reversing thermometers. Chlorophyll a and pheo- havior increases when the school is in the upper pigments retained on Gelman GF/F filters were ex- ranges of plankton biomass. The lower ranges of tracted in refrigerated acetone and analyzed on a plankton biomass could be a temporary result of Turner Designs Model 10 fluorometer. Dissolved foraging groups of schooled fish (Koslow 1981). oxygen was determined by the Carpenter (1965) In addition to the spatial complexity of the pelag- modification of the Winkler titration method. The ic habitat at several horizontal scales, rapid changes MOCNESS (20 nets, 1-m2 effective mouth area, in the vertical distribution oforganisms at dawn and 333-pm mesh) was towed at about 50-75 cm s-l. A dusk are well known (Enright 1977; Pieper and nighttime profile was made from 500 m to the sur- Bargo 1980). The impact of these migrators on the face in approximately 50-m intervals between 0131 distribution pattern of their prey must be added to and 0401 on April 6, 1988. A comparable daytime the direct effects of diel vertical migration on the profile was made from 1330 to 1510 on April 6. Vol- distribution of plankton. Initial time and space scale ume filtered per haul averaged 354 m3 (range 156- information from acoustical methods may be useful 927 m3). Samples were preserved in 10% Formalin in designing the most effective direct sampling pro- buffered with sodium borate. Select copepod spe- cedures for studies of fine-scale processes and of cies were enumerated after subsampling with a geographic-scale population abundance. Stempel pipette; when organisms were rare the en- Acoustic profiles may be used to estimate the tire sample was counted. scale and intensity of patchiness and to help specify the number, size, and placement of samples. The Acoustical Methods primary sampling design questions to be asked in The primary function of the ADCP is to sense pelagic studies are: current direction and speed as a function of depth. How many samples are needed? It accomplishes this by detecting the Doppler fre- How big should the sample units be? quency shift of backscattered sound from plank- At what separation distance can adjacent sam- tonic organisms, particles, and small-scale ple units be said to be effectively independent? discontinuities in the water column. This frequency Over what spatial and temporal scales are pro- shift is proportional to the relative velocity between cesses operating? the backscattering source and the transducer. The What are limits of the temporal and spatial change in frequency is measured by four trans- scales to be used for such studies? ducers. Unlike echo sounders having the transducer aimed downward, or sonars (sonically determined It is the purpose of this study to report the quali- azimuth and range) having the transducer directed tative spatial analysis from several days of continu- laterally at any angle with reference to the bow, the ous acoustical sampling. From these records, we four ADCP transducers are oriented at 30" from the will summarize the incidence of biological aggre- vertical: approximately forward, aft, right abeam, gations, their spatial dimensions, the dimensions of and left abeam (figure 2). This means that the axes the spaces between aggregations, the depth distri- of the complementary beams are as far apart as the bution of aggregations, and the rates of change in depth (2 X sin (30") = 1) below the transducer. In depth. - ~~~ .~~~~ ~ METH 0DS 'Manufactured by RDI Inc. San Diego. (Mention of manufacturer does not imply endorsement by the U.S. government or the University of California). The pilot study was conducted in a 5 by 15-n. mi. A more complete technical description is available in a manual from the area oriented SE NW about 25 n. mi. west north- manufacturer. See also Flagg and Smith (1989). 89 SMITH ET AL.: ZOOPLANKTON PATERN ANALYSIS CalCOFl Rep., Vol. 30,1989 33O30' 33000' \........ ': .. ,.... 2: ... , .. .. ' ...:: San Clemente Is. 32O30' '1 17OOO' Figure 1, Track of R/V New Horizon during ZB1 cruise. Data were collected during the entire cruise. See figure 8. this report we analyze the information content of The probability that the acoustic pulse will en- the echo amplitudes, rather than their frequencies.