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Consumption and Growth Rates of Chaetognaths and Copepods in Subtropical Oceanic Waters1

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Consumption and Growth Rates of Chaetognaths and Copepods in Subtropical Oceanic Waters1 Pacific Science (1978), vol. 32, no. 1 © 1978 by The University Press of Hawaii. All rights reserved Consumption and Growth Rates of Chaetognaths and Copepods in Subtropical Oceanic Waters1 T. K. NEWBURy 2 ABSTRACT: The natural rates of food consumption and growth were cal­ culated for the chaetognath Pterosagitta draco and the copepod Scolecithrix danae in the Pacific Ocean near Hawaii. The chaetognath's consumption rate was calculated using the observed frequency of food items in the stomachs of large specimens from summer samples and the digestion times from previous publications. The natural consumption rate averaged only one copepod per 24 hr, or about 2 percent of the chaetognath's nitrogen weight per 24 hr. The growth rates of both P. draco and S. danae were calculated with the temporal patterns of variations in the size compositions of the spring populations. The natural growth rates averaged only 2 and 4 percent of the body nitrogen per 24 hr for, respectively, small P. draco and the copepodids of S. danae. These natural rates were low in comparison with published laboratory measurements of radiocarbon accumulation, nitrogen excretion, and oxygen respiration of subtropical oceanic zooplankton. THE RATES OF FOOD CONSUMPTION, metabo­ concentrations; little growth and poor sur­ lism, and growth have been determined for vival are obtained in such cultures. Experi­ zooplankton in some regions of the oceans. ments are usually run with no food or with Temperate and coastal rates have been abundant food, which yield basal rates and described by Mullin (1969), Petipa et al. maximum rates because the rates offunction­ (1970), and Shushkina et al. (1974). Sub­ ing ofmost zooplankton are dependent upon tropical neritic rates have been measured by food concentration (Ikeda 1976, Mullin, Beers (1964), Mayzaud and Dallot (1973), Stewart, and Fuglister 1975, Reeve 1970). Newbury and Bartholomew (1976), Reeve There is a need for methods ofdirect measure­ (1970), and Reeve and Baker (1975). Less ment of the natural rates of zooplankton. information is available on the rates of Two techniques for the measurement of subtropical oceanic zooplankton, and this natural functioning rates of subtropical oce­ information is reviewed below in detail. anic zooplankters were used for this study. Perhaps the main reason for the lack of One technique involved the calculation of determinations of the functioning rates of the food consumption rate of a chaetognath subtropical oceanic zooplankton is that the population. The natural frequency of food methods of measurement are difficult and/or items in the chaetognaths' stomachs was indirect. Laboratory culture is difficult with determined with field samples carefully col­ natural oceanic foods at low environmental lected with the quickest possible tows (30 min) throughout a whole diel period, as explained below. The diel mean consumption 1 This research was supported by National Science Foundation grant no. GA-36820. Manuscript accepted rate was then determined using the calculated 1 October 1977. frequency of food items in the chaetognaths' 2 This study was begun while the author was with the stomachs and the digestion times of pl"ey, Oceanography Faculty at the University of Hawaii, which have been reported previously in the Honolulu, Hawaii. The author's present address is: United States Department of the Interior, Alaska-Outer literature. Continental Shelf Office, Box 1159, Anchorage, Alaska The other technique involved the calcula­ 99510. tion of growth rates from the patterns of 61 62 PACIFIC SCIENCE, Volume 32, January 1978 temporal variations in the populations' size The copepod Scolecithrix danae is a or stage compositions. This graphical method calanoid with a mature body length of about assists in the identification of groups or 2 mm. It is omnivorous (Timonin 1971), cohorts ofindividuals in natural populations. feeding on large diatoms, blue-green algae, In temperate latitudes, the graphical method radiolarians, and crustaceans (Mullin 1966, has been used recently to calculate the Petipa et al. 1971). The species is epipelagic growth rate of chaetognaths (Sameoto 1971) (Roe 1972, Vinogradov 1968) and is broadly and tunicates (Heron 1972). distributed in warm oceanic waters. It is one Year-round investigations of the micro­ of the ten most abundant calanoids in the nekton in the subtropical Pacific near Hawaii Equatorial Pacific (Grice 1962) and in the have demonstrated spring-summer repro­ southern part of the North Pacific Central ductive periods and the subsequent recruit­ Gyre waters (Park 1968) adjacent to Hawaii. ment of small, but definite, cohorts of immature animals into the populations (Clarke 1973, 1974; Walters 1976). These MATERIALS AND METHODS variations in the populations' size com­ positions were observed regularly in the water The samples were collected at two stations that moves past the Hawaiian Islands. The near Hawaii in the southern part of the regularity ofthe variations indicates synchro­ North Pacific Central Gyre. Station 1 (21 ° N, nous patterns of reproduction and develop­ 158°20' W) is about 30 km southwest of the ment for the populations in the Central Gyre island of Oahu; station 2 is located in the of the North Pacific. The patterns were same region, about 10 km southwest ofOahu. considered regular and distinct enough to The water depths at stations I and 2 were 3000 initiate this study of the temporal variation and 1000 m, respectively. The local water during spring in the populations' size temperature in the upper 300 m (the habitat compositions of some smaller zooplankton of the studied species) changes from 25° C species. The field sampling program was at the surface to 12° C at 300 m. The local designed for the preliminary calculation of eddies circulated the surface water at the the growth rates and for development of stations toward the Hawaiian Islands, ac­ the necessary modifications of the graphical cording to both dynamic height calculations method for subtropical, oceanic populations. (Seckel 1955) and buoy observations at Two zooplankton species, the chaetognath station 1 (R. R. Harvey, personal com­ Pterosagitta draco and the copepod Scole­ munication). The larval fish in the samples cithrix danae, were chosen for the study. The were typically oceanic (1. Leis, personal chaetognath's taxonomy, habitat, and dis­ communication). Neritic meroplanktonic or­ tribution have been described by Aivariiio ganisms were not found in the samples or in (1965), Bieri (1959), and Sund (1959). Ptero­ the chaetognaths' stomachs. These observa­ sagitta draco is epipelagic and relatively short tions on currents and species compositions (maximum body length about 8 mm); these indicate that the sampled organisms probably two characteristics were generally associated had no interaction with the neritic community with the species that had rapid consumption of the Hawaiian Islands. rates in the Black Sea (Petipa et al. 1970). To minimize any sampling bias on the Pterosagitta draco is cosmopolitan in sub­ populations' size compositions due to avoid­ tropical, oceanic water (Alvarifio 1965), and ance, all the samples were collected with is one ofthe four mostabundantchaetognaths 70-cm diameter Bongo nets, and settled around Hawaii (Bieri 1959, Hida 1957). volumes of only about 0.3 liter were filtered Because of- the-species' - abundance-and during each tow. l'h<il sam€ D€tS, with 183­ possible rapid rate of consumption, P. draco and 202-j.lm mesh, were always used next to was expected to be one of the dominant each other on the Bongo frame. During each carnivores in the planktonic food web. sampling period, equal numbers of samples Consumption and Growth Rates of Subtropical Zooplankton-NEWBURY 63 were collected with each mesh size, so the copepods, were found to comprise a minor combined results for each time period contain proportion of the copepods in the stomachs no temporal bias due to mesh size. of P. draco. (3) The sizes of the copepods in The Pterosagitta draco and Scolecithrix the stomachs of P. draco were primarily danae populations were sampled by towing smaller than the size range of copepods in the nets steadily and obliquely through the the net with P. draco. Therefore, the frequen­ whole depth range of both populations. The cy of food items in the stomachs of P. draco depth range that has been reported for P. was probably changed little by the sampling. draco in the study region is the upper 200 to The frequency of food items in 600 speci­ 300 m (Alvariii.o 1964, Ko10sova 1972), mens of Pterosagitta draco was determined though some specimens of P. draco were for two size ranges of the animals (5-6 and reportedly at 600 m in the eastern tropical 6-7 mm), because the size and frequency of Pacific (Sund 1961). The S. danae depth food items consumed changes during the range is the upper 200 m (Heinrich 1961, development of P. draco. All the P. draco in Hida and King 1955, Park 1968), but one each sample were examined. Food items in report has been given of S. danae at 500 m in the mouths were not included in the data. the equatorial Pacific (Grice 1962). Hori­ The food items were identifiable through all zontal opening-closing tows were made at stages of digestion, even when packed into 350 m in the study area during March; the pellets for defecation (see photographs of nets caught no P. draco or S. danae. For the chaetognath's fecal pellets in Cosper 1973). present study, all the samples had a constant The pattern of temporal changes in the rate of descent and ascent through the upper size compositions of the populations was 300 m, as measured with a depth-distance analyzed with a series of 20 spring samples recorder (a recording depth meter and flow (see Table 2) collected at station 1 during meter). The depth-distance records were also the nights of 11 and 30 April, 21 May, and used to determine the exact volumes of 4 June 1973.
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