FISHERY BULLETIN: VOL. 75, NO. 1 



Plankton Samples 



All samples were preserved immediately in 10% 

 seawater Formalin 3 buffered with marble chips. 

 Samples were transferred to 5% buffered Forma- 

 lin after they had been stored in the laboratory for 

 1 mo. Houde and Chitty (1976) have discussed 

 methods used to determine plankton volumes. All 

 fish eggs and larvae were sorted from each 505-/xm 

 mesh net plankton sample under a dissecting mi- 

 croscope for later identification and enumeration. 



Eggs and larvae of round herring are distinctive 

 and easily identified (Houde and Fore 1973). 

 Round herring eggs from each station were enu- 

 merated; larvae were enumerated and measured 

 with an ocular micrometer in a dissecting micro- 

 scope. 



Temperatures and Salinities 



Temperature and salinity profiles of the water 

 column at each station were obtained on all 

 cruises. 4 Usually a mechanical bathythermo- 

 graph cast was made to describe the vertical tem- 

 perature profile. This was followed by a hydrocast 

 consisting of from two to seven 1.7-liter Niskin 

 bottles with reversing thermometers. Samples for 

 salinity were brought to Rosenstiel School of 

 Marine and Atmospheric Science for analysis. On 

 cruises IS 7308 and IS 7320 a salinity- 

 temperature depth unit was used in place of the 

 Niskin bottles to obtain temperature and salinity 

 data. Round herring egg and larva data were 

 examined in relation to temperatures and 

 salinities at stations where they were collected. 



Determining Egg and Larvae Abundance 



Catches of round herring eggs and larvae at 

 each station were standardized to give abundance 

 in numbers under 10 m 2 of sea surface: 



n. 



c j 2 j 



10 



(1) 



where n, = the number of individuals (eggs or lar- 

 vae) at station j under 10 m 2 of sea 

 surface 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



••Temperature and salinity data for these cruises can be re- 

 trieved from the MAFLA file at the National Oceanographic 

 Data Center, Washington, D.C. 



c = the catch of eggs or larvae at station^' 

 Zj = the depth of tow (in meters) at station/ 

 Vj = the volume filtered by the net (in cubic 

 meters) at station j. 



Both total larval abundance under 10 m 2 and lar- 

 val abundance in each 1.0-mm length class under 

 10 m 2 were determined. 



Numbers of eggs or larvae also were estimated 

 in the area represented by each station. These 

 areas were determined by the polygons described 

 by the perpendicular bisectors of lines from the 

 station in question to adjacent stations (Sette and 

 Ahlstrom 1948): 



Pj 



CjZj 



Aj 



(2) 



where p • = the estimated total number of eggs or 

 larvae in the area represented by sta- 

 tion j 

 Cj, Zj, and Vj are defined in Equation (1) 

 Aj = the area (in square meters) rep- 

 resented by station j . 



Total larvae and larvae by 1.0-mm length classes 

 were estimated for each station area. Most sta- 

 tions represented areas ranging from 0.75 to 3.15 

 x 10 9 m 2 . 



The estimated total number of eggs and larvae, 

 as well as larvae by 1.0-mm length classes, was 

 estimated for the entire area represented by each 

 cruise: 



P, = I 



(3) 



7 = 1 



where P, = the cruise estimate (i.e., the total 

 number of eggs or larvae estimated in 

 the area represented by cruise i) 

 k = the number of stations sampled dur- 

 ing cruise i 

 Pj is defined by Equation (2). 



Variance estimates on the abundance of eggs 

 were obtained for each cruise using a combination 

 of methods outlined by Cushing (1957) and Taft 

 (1960). Only stations at which round herring eggs 

 had been collected at least once during the 1971-74 

 survey period were included in obtaining these 

 estimates. Other stations were considered to be 

 outside the area of spawning, because round her- 

 ring eggs were never collected there. These usu- 

 ally were the three stations on each transect that 



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