Copepods in a hurry: Sustained high‐speed upward migration1

Replicated data on the vertical migration of the copepod Metridia pacifica indicate that this 2-3-mm animal can maintain upward swimming velocities of from 30 to 90 m h-l for durations of more than an hour. Internal evidence in the data suggests that much of the variability within this range reflects differences in motivation of the animals, rather than sampling error. These data are apparently the greatest sustained swimming speeds ever substantiated for any copepod and are larger by a factor of from two to ten than several previous field estimates for copepods. There are various reasons for an interthis question, since at certain times of day est in the speed at which small aquatic their -population movements are strongly organisms propel themselves through the unidirectional. Past studies of the vertical water, including, for example, evaluation migration of small organisms, however, of the energetic costs of their normal based on time-series samples of the popuswimming activity (Vlymen 1970; see also lation distributions in the water column, Klyashtorin and Yarzhombek 1973). In have often been handicapped by the relapart because of such concerns, a number tively long time intervals between succesof different laboratory techniques have sive sampling series. The result is that been applied to the problem, ranging from one probably has an overestimate of the direct observation of the animals in a coltime actually required to achieve a given umn of water (see Hardy and Bainbridge 1954) to the evaluation of moving-picture films of copepods prodded to evoke highspeed escape responses (Vlymen 1970). There is always uncertainty, however, in extrapolating laboratory data of these sorts to the oceanic environment. Undisturbed animals in a tube of water may not be adequately motivated to show -fully change in distribution pattern. Another source of uncertainty common to many field studies is associated with the diffuse vertical distribution of the populations in the water column, often leaving no choice but to assume that observed changes in the position of maximum density reflect the extent of movement of the individual animals; but it may, instead, indicate some normal speeds; and escape responses, at sort of more complex reshuffling of indithe other extreme, cannot be used as an viduals within the water column. index of average long term performance. Ultimately, then, confirmation of the labMcs%O~ oratory estimates under field conditions is essential for the final evaluation of what the organisms ordinarily do in a natural environment-what their normal “cruising speed” might be. In principle, vertically migrant organisms are ideally suited for field study of ’ Research was supported by NOAA grant 2-35208 and NSF grant GB 36750. a I thank II-W. Honeggcr, who supervised collcction of the data, and K. Gopalakrishnan and V. Gopalakrishnan, who counted the plankton samples. Suggestions of C. Miller and L. Haury assisted in revision of the manuscript. In the course of an oceanic sampling program designed for entirely different objectives, a fortunate combination of circumstances has now provided data which permit unusually reliable, replicate estimates of long term average swimming velocity by a species of planktonic copepod under field conditions. These data demonstrate remarkably high average speeds, compared with most past laboratory and field estimates. The study of the time-ofday of vertical migration, which led to these data, involved sampling the upper LIMNOLOGY AND OCEANOGRAPHY 118 JANUARY 1977, V. 22( 1) Copepods in a hurry 119 150 m of the 800-m water column, about 33 km offshore from La Jolla, California, at half-hourly to hourly intervals, during the late afternoon and early evening hours. Three sampling programs, each of 3-days’ duration, were undertaken in April, May, and July 1971. The sampling design involved treatment of the upper water column as a two-stratum system; the two strata, sampled by vertically towed nets, were from 150 m to the base of the thermocline, and from the latter point to the surface. Each of the paired nets (Brown 1975) had a mouth 50 cm in diameter; the nets were 2 m long, with 505-p mesh. Towing speed was 0.5 m s-l. The numbers of copepods in the two nets have been summed, for present purposes, to give abundances of animals per 0.39 m2 for the given depth intervals. The differences in numbers between the paired nets fell within expected Poisson-distribution (random) variability for the great majority of the sampling times. The entire population of the species of primary interest, Calanus helgolandicus ( pacificus), was apparently confined to depths of less than 150 m. The vertical migration of this species was therefore reflected by movement of animals from the lower into the upper stratum, and, of course, no estimates of velocity of movement are possible from such data. The second most abundant copepod in the samples, however, Metridia pacifica, had a daytime distribution almost entirely deeper than 150 m, and usually this species passed through the lower stratum sampled, to enter the surface stratum at night. The late-afternoon movement of the animals into the lower stratum was usually not a gradual diffuse trickle, but instead resembled the dramatic passage of an advancing front, and entry into the upper stratum was equally precipitous. These data therefore provide unusually precise estimates of the time at which the animals were at-or had passed through-two defined depths, permitting, then, an estimate of the average velocity of the copepods through the stratum from 150 m to the thermocline. If extensive reshuffling of the population during transit occurred, or if movement was not purely vertical, these estimates represent minimum values.