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  • Swimming Speed and Metabolic Rate during Routine Swimming and Simulated Diel Vertical Migration of Sergestes similis in the Laboratory1
  • David L. Cowles

Sergestes similis (Hansen, 1903) is a common mesopelagic vertically migrating shrimp common in the temperate and subarctic North Pacific Ocean. The species is a diel vertical migrator, although it remains primarily above the oxygen minimum layer in regions such as off California where the layer is well developed. This shipboard study with a computer-controlled swim tunnel provided the first continuous examination of this species' swimming behavior and metabolism over a 24-hr cycle. Sergestes similis swam at a routine speed of around 4.4 to 4.95 cm sec-1. Burst speeds ranged from 14 to >20 cm sec-1. Swimming speeds during the day, at low temperatures simulating those at daytime depths, were similar to those at night at the higher temperatures characteristic of the surface. Night metabolic rates were higher than in the day, especially during the early night when most feeding activity may take place. Swimming speeds during times of simulated vertical migration averaged slightly faster than those of routine day or night swimming, averaging 6.2 cm sec-1 during the time of upward migration and 5.4 cm sec-1 during simulated downward migration, but the difference was not significant. Downward migration is not accomplished by passive sinking. Calculations based on observed swimming activities and metabolic rates indicate that vertical migration confers a clear metabolic energy savings to S. similis over remaining resident in surface waters, though this result may not be applicable to other vertical migrators and is likely moderated by decreased feeding opportunities at depth.

Diel Vertical Migration is a widespread phenomenon among midwater species, the adaptive value of which has generated considerable discussion since discovery of the phenomenon. Postulated adaptive values have included predator avoidance, optimal foraging strategies, horizontal dispersal, growth and fecundity effects, metabolic advantages of changes in temperature, niche segregation, avoidance of photo-damage, group selection effects, and various combinations of these (Kerfoot 1970, 1985, Lampert 1989, 1993, Bevelhimer and Adams 1993, Loose and Dawidowicz 1994). Most of these hypotheses can be grouped into two major categories: that vertical migration confers some sort of metabolic advantage, or that it serves to reduce mortality by some mechanism such as predator avoidance (Lampert 1993).

Sergestes similis (Hansen, 1903) is a mesopelagic vertically migrating penaeid shrimp found in abundance along the North Pacific coast from Japan to the Gulf of California, off Chile, and in the eastern South Atlantic (Butler 1980, Farfante and Kensley 1997). It inhabits primarily the region above the oxygen minimum layer (Figure 1). Daytime depths for the species off the Oregon and California coasts, where it is one of the most abundant midwater micronektonic species (Barham 1956, Genthe 1969, Pearcy and Forss 1969, Omori 1979), range from 200 to 650 m, and nighttime depths range from 50 to 200 m (Pearcy and Forss 1966, Omori and [End Page 215] Gluck 1979, Schlining et al. 1997). Eighty percent of the population can be found between 10 and 400 m depths off California (Childress 1975).

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Figure 1.

Depth distribution of Sergestes similis off California and Oregon, with vertical profiles of temperature and oxygen level. Error bars depict the day and night depth distributions reported in the literature. Solid bars indicate the depth range in which 80% of the population is found; dashed lines indicate the full reported depth range. See text for references. Temperature and oxygen profiles are from 32.05° N, 120.65° W in Rhines (1994). Depth distributions were extracted from Pearcy and Forss (1966), Omori and Gluck (1979), Schlining et al. (1997), and Childress (1975).

As with most mesopelagic species, little is known about vertical migration in S. similis other than the range of depths in which it is found during the day and night. Information is scant, for example, about its behavior or activity levels at daytime and nighttime depths, on the costs and benefits of vertical migration to the species, or even whether downward migration is accomplished by active swimming or by passive sinking. Recent in situ studies of the species from a remotely...


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