MULLIN and CONVERSE: EUPHAUSIID AND ZOOPLANKTON BIOMASSES 



a nocturnal tow. We fractionated each sample by 

 sieving a subsample (<0.5 of the sample) 

 through a 4 mm screen, and counted and man- 

 ually placed the euphausiids (and sergestids) re- 

 tained on this mesh on a preweighed glass fiber 

 filter. We then rinsed all or part of the same 

 subsample sequentially through 1 mm and 0.5 

 mm meshes, using recirculating seawater, and 

 then rinsed the plankton passing 1 mm but re- 

 tained by 0.5 mm onto another preweighed glass 

 fiber filter. We then rinsed both filters and their 

 catches with 67c ammonium formate, dried them 

 overnight at 50°-60°C, cooled them in a desic- 

 cator, and reweighed them. Blank filters (no 

 zooplankton) were treated similarly. The dry 

 weight of the sample, corrected for initial filter 

 weight and any weight change of the blank fil- 

 ters, times the subsamphng factor and divided 

 by the volume of water filtered by the original 

 net tow, is the biomass of "euphausiids" or of 

 "small zooplankton" (that retained by the 0.5 mm 

 mesh) for that station, and the weight of the 

 euphausiids divided by their number is the dry 

 weight per euphausiid. 



With particular regard to the Pacific whiting 

 fishery, which started in 1966 in the northern 

 part of the California Current, and the e.xpecta- 

 tion that biomass of whiting food might increase, 

 we tested the following null hypotheses (stated 

 in the "one-tailed" forms appropriate for our ex- 

 pectations). 



Hoi — Absolute biomass of euphausiids in the 

 northern area was not greater from 1966 

 onward than that before 1966. 



Ho2 — The ratio of biomasses (euphausiids/small 

 zooplankton) in the northern area from 

 1966 onwards was not gi-eater than a) this 

 ratio before 1966, or b) that in the south- 

 ern area. [The small zooplankton biomass 

 is used to correct for the expected north- 

 to-south differences, overall changes in 

 the biomass of zooplankton, or change in 

 samphng techniques, throughout the Cali- 

 fornia Current for reasons other than the 

 whiting fishery.] 



Ho3 — The dry weight per euphausiid in the 

 northern area was not gi-eater from 1966 

 onward than before 1966. 



Hypothesis Ho^a, for example, was examined 

 by a one-tailed rank sum (Mann-Whitney U) test 

 of whether the median of all ratios at northern 

 stations before 1966 was statistically indistin- 

 guishable from the median for all such stations 



beginning in 1966. The alternative which would 

 be consistent with an effect of the whiting fish- 

 ery would be, pre-1966 median <1966-and-later 

 median. In some cases, analogous ^tests were 

 also performed on data normalized by log-trans- 

 formation to determine whether transformed 

 means differed. In this approach, all stations 

 within one block of geography and time are 

 treated as equally valid estimates of the overall 

 median, independent of location of the stations 

 within the block. 



There are significant inshore/offshore gradi- 

 ents in biomass (see below), and comparisons 

 between years or between areas could be con- 

 founded by differences between gi'oups being 

 compared in the inshore/offshore locations of 

 usable samples. We took several precautions to 

 prevent this; fii'st, we tested for such gi-adients 

 in our own samples by comparing medians from 

 inshore and offshore subsets of stations by rank 

 sum tests. We also tested hypotheses H01-H03 

 using only the inshore subsets of the stations 

 (Fig. 1); this was done partly because the fishery 

 for whiting is generally conducted in areas shal- 

 lower than 500 m. 



We tested analogously data from a transect off 

 Oregon which was repeatedly sampled from 1962 

 to 1967 (see below). 



Hypotheses Hoi and Ho2a were tested further 

 by another approach which acknowledges that 

 within each major space/time block, stations may 

 differ systematically because of geography (e.g., 

 an inshore/offshore gradient), so that the iden- 

 tity of each station should be retained in the test. 

 The data from each northern California station 

 which was sampled at least thrice before 1966 or 

 in 1966-69, and at least once during the con- 

 trasting period, were ranked separately as for a 

 rank sum test, but the summed ranks were then 

 combined for testing against the expectation 

 from the null hypotheses. There were 7 stations 

 used in this test, and 21 data points from each 

 period. 



Falsification of these hypotheses implies that 

 there was a significant increase in euphausiid 

 biomass (and/or individual size) coincident with 

 the whiting fishery and that this increase was 

 unlikely to be caused by other factors affecting 

 zooplankton in the whole California Current, in- 

 cluding those types on which adult whiting do 

 not feed. 



In order to validate the assumption which 

 underlies H02 — that small zooplankton and 

 euphausiid biomasses vary in parallel in re- 

 sponse to the California Current's climate — we 



635 



