BRINTON: POPULATION BIOLOGY OF EUPHAUSIA PACIFIC A 



Previous Investigations 



In addition to the observations on the life 

 history of E. pacifica (Nemoto 1957; Ponomareva 

 1963; Smiles and Pearcy 1971), aspects of the 

 energy budget and physiology of this species have 

 been studied. Lasker (1964, 1966) measured 

 moulting frequency, feeding rates, respiration 

 and carbon utilization by specimens maintained in 

 the laboratory, and observed growth rate in 

 juveniles and adults. Fowler et al. (1971) con- 

 sidered effects of temperature and size on moult- 

 ing. Small et al. (1966) measured respiration at 

 different temperatures and discussed energy flow, 

 while Small (1967) further examined energy flow. 

 Paranjape (1967) made observations on moulting 

 and respiration. Aspects of depth-habitat and 

 pressure in relation to respiration were considered 

 by Small and Hebard (1967), Pearcy and Small 

 (1968), and Childress (1971). Gilfillan (1972) studied 

 oxygen uptake in relation to laboratory controlled 

 temperatures and salinities. 



Total oocytes in a large female were counted by 

 Ponomareva (1963). Clutch size estimates and the 

 vertical distribution of different age groups were 

 given in Brinton (1962b and 1967a, respectively). 



limitations of the Study 



Understanding the population biology of an 

 oceanic species depends in large part upon the 

 extent to which a representative part of the 

 population can be representatively sampled. In the 

 planktonic environment, currents not only tend to 

 transport the organisms across an observer's 

 horizon, but also cause relative horizontal dis- 

 placement of life stages because, in many species, 

 the various stages of development live at different 

 depths and experience different horizontal trans- 

 port. This is true of euphausiids. Species under- 

 going both ontogenetic and daily vertical migra- 

 tions, such as E. pacifica, are further subject to 

 differential horizontal transport. Thus, water 

 movement is a variable which complicates any 

 plan for temporal continuity in sampling a 

 population. The area covered and the time spent in 

 carrying out an assessment of a population does 

 not need to be great if the waters are restricted 

 geographically and if growth and development of 

 the population is measurable between successive 

 assessments. Clearly, a gyre of circulation, such as 

 the eddy lying off southern California, may be 

 expected to harbor elements of a population that 



persists locally. This study area has proven prac- 

 tical in size according to the logistics of CalCOFI. 



MATERIALS AND METHODS 



Samples were obtained by oblique tows, 0-140 m 

 depth (except where the water was shallower), 

 using the CalCOFI standard net, 1-m mouth 

 diameter and 0.55-mm mesh width (Ahlstrom 

 1948). The mesh width of the cod end and of a 

 40-cm section in front of it was 0.25 mm. The 

 volume of water strained through a net was 

 determined with a TSK (Tsurumi-Seiki Kosaku- 

 sho) flowmeter.- Most volumes were in the range 

 of 300-400 m^ The net was towed at about 75 

 cm/s. The 1953-56 cruises provided month-to- 

 month data, including more frequent surveys off 

 southern California in late 1955 (four in Sep- 

 tember, three in November). Station positions and 

 collecting data together with displacement 

 volumes of the plankton samples are from annual 

 listings of CalCOFI plankton sampling 1953-56 

 (South Pacific Fishery Investigations 1954, 1955, 

 1956; Thrailkill 1957). 



Specimens smaller than 3 mm in length are able 

 to pass through the meshes of the net and there- 

 fore were not representatively sampled. Smaller 

 specimens (2 mm) are nevertheless retained by the 

 fine meshes of the cod end of the net and counts of 

 these are included as indicative of the presence of 

 the small calyptopis larvae. Free floating eggs of 

 E. pacifica are not retained by this net. Estimates 

 of egg production are derived from examination of 

 the ripe females sampled, as described in the 

 discussion of fecundity below. 



A total of 819 samples from 48 cruises, 5301 

 (January 1953) through 5612 (December 1956), 

 were examined (Figure 4d). Only nighttime sam- 

 ples were used since juveniles and adults are not 

 representatively sampled in the daytime, owing to 

 vertical migration and avoidance of the net 

 (Brinton 1967a). Between 7 and 43 nighttime 

 samples were collected in the study area during 

 each cruise. "Night" was considered to be the 

 period from 1 h after sunset to 1 h before sunrise. 

 A few sunrise and sunset samples were analysed if 

 they were collected under overcast skies. A sample 

 marginal to, but outside of, the area was studied 

 when such a sample was from a locality nearer to 

 the closest boundary of the area than any of the 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



735 



