The standard tow from 1951 through 1968 was an oblique haul 

 to 140 m depth (to 15 m of the bottom in shallow areas) designed 

 to filter a constant amount of water per depth interval (ca. 

 3m 3 /m of depth) over the vertical range of most ichthyoplankters. 

 Hauls were made at a ship speed of 1.5-2.0 knots and initiated by 

 clamping the net line to the towing cable with the 4 5 kg terminal 

 weight about 10-15 m below the surface. The net was lowered to 

 140 m depth by paying out 200 m of wire over a 4 minute period 

 (35 m of depth/min.). After fishing at depth for 30 seconds, the 

 net was retrieved at 2 m/min. (14 m depth/min.). The angle of 

 stray of the towing cable was recorded every 3 seconds and 

 maintained at 45° (+3°) by adjusting the ship speed and course. 

 After reaching the surface, the net was washed down and the 

 samples preserved in 5% formalin buffered with sodium borate. 

 Flowmeter readings were made at the beginning and end of each 

 tow. Detailed descriptions of gear and methods are given by 

 Ahlstrom (1953), Kramer et al. (1972), and Smith and Richardson 

 (1977) . 



LABORATORY PROCEDURES 



Laboratory processing began with the determination of a 

 displacement volume for each sample (methods described in Staff, 

 SPFI, 1953 and Kramer et al., 1972). Zooplankton volumes 

 (including ichthyoplankton) of samples collected in 1957 are 

 listed in Thrailkill (1959) and presented graphically in Smith 

 (1971) . 



Sorting involved the removal of ichthyoplankton from the 

 sample and identification and separation of eggs and larvae of 

 selected species (see introduction) . Usually, each sample was 

 sorted completely; however, some of the samples were fractioned 

 into aliquots using a Folsom plankton splitter (McEwen et al., 

 1954) prior to sorting. Several criteria were used to determine 

 whether a sample was fractioned: samples containing an abundance 

 of thaliacians and coelenterates and exceeding 150 ml in total 

 plankton volume were fractioned (to 50%, 25%, 12.5%, or 6.25%) to 

 approximate a reduced volume of 50 ml for sorting; samples with 

 an excessive quantity of fish eggs and/or larvae were 

 occasionally fractioned to expedite the sorting process in order 

 to meet scheduled deadlines. If the identified fraction of an 

 aliquot yielded rare or interesting species of fish larvae, the 

 remaining fraction was frequently sorted and identified with the 

 intent of finding additional specimens. Aliquot percentages for 

 fractioned samples from 1957 are listed in Table 1 under the 

 "Percent Sorted" column. 



A "standard haul factor" (SHF) was calculated for each tow 

 to make them comparable and allow estimations of areal abundance. 

 This factor adjusts the number of eggs or larvae in a haul to the 



2 Personal communication, James R. Thrailkill, National Marine 

 Fisheries Service, Southwest Fisheries Center, La Jolla, CA. 



