F-23 



The only information available on diurnal feeding cycles is that of Pavlov (1969) who 

 related the occurrence of svjarms, feeding and food availability, (see section 6.5). It is 

 not however clear to what extent thcoe results are typical of the Southern Ocean as a whole 

 or how they vary seasonally. Since the presence of certain diatoms in the guts of krill can 

 have adverse effect on the quality of products made from them there is clearly a need for 

 more study in this field. 



6.7 Production and Biomass 



The extreme variability in the density of krill caused mainly by its swarming habit 

 has presented enormous problems in estimating biomass and production. There are quite a 

 large number of papers giving estimates calculated both by direct and indirect means and 

 these will be considered along with the major sources of error. 



Based on visual observations from the deck of niiS William Scoresby and assuming a den- 

 sity of one individual per cubic inch in the top yard of the water column, Marr (I962) 

 estimated a mean biomass of 2.5 g /m'^ for krill. In the same paper he recalculates the 

 biomass using the results of Hoyerdahl (1932) for mean weight and gets a figure of 29-28 g/m^. 

 The possible error in these estimates is enormous since the area under consideration is 500 

 yards on either side of the ship's track and swarms are assumed to be one yard deep. These 

 rough calculations presented with great reserve, as I>Iarr puts it, may be» he thinksirepre— 

 sentative of the Sist Wind and Weddell Drift zones. Assioming that this area is about half 

 that described by Mackintosh and Brown (1956) as south of the Antarctic convergence the total 

 biomass is 521 million tons (based on 29-28 g/m^) or 44-5 million tons (based on 2.5 g/m^). 

 Itorr, basing his reasoning on the probable consumption of krill by whales is of the opinion 

 that the hi^er figure is the more reasonable. This is the only direct estimate I have so 

 far come across that is supported by calculation and yet even in this case the figures are 

 presented within very broad limits since they are calculated by extrapolation from a rela- 

 tively very small area that was at the time seen to be unusual. 



In an analysis based on echosounder survey and fishing catches, Neraoto _et al. (in press) 

 have estimated standing stock as being from 0.5 kg/m-^ to 30 kg/m-* althou^ they do not indi- 

 cate how these figures apply to the total Southern Ocean. 



Makarov and Shevtsov (1972) quote a range of 953 to 1 350 million tons for the bicmass 

 of krill (abstract only seen) while Gulland (197O) estimated the total zooplajikton biomass 

 to be about 10 g dry wei^t per square metre or I50 million tons for the whole Antarctic and 

 assumes that 50^^ of this is krill. Based on Gulland' s estimates the krill biomass is there- 

 fore 75 million tons dry weight, or perhaps 750 million tons wet wei^t. The figure of ^0% 

 for the proportion of krill in the zooplankton is open to question as Voronina and Naumov 

 (1968) found that Euphausiids made up only 7-6^ of their zoopleinkton catches. However, since 

 their nets may well have missed a significant proportion of the Euphausiid population this 

 figure cannot be taken as firm even though it is based on a large number of observations. 



To estimate production from these biomass figures, a conversion rate of 1:1 has been 

 suggested (Gulland 1970, Hempel 1970) based on a mean life span of one year. Allen (1971 ) 

 has questioned this ratio and by comparison with mortality rates in other Euphausiids (there 

 being no infoimation available for Ej_ superba ) he suggests that the production is between 1.8 

 and 2.1 times the biomass with the lower figure the more likely. On this basis the krill 

 production is 940 million ton/year from Marr's figures and 1 350 million ton/year based on 

 Gulland' s figure. 



Indirect estimates of biomass and production have been made based on both higher and 

 lower trophic levels. First of all considering estimates based on primary production, Hempel 

 (1970) quotes results from localities all round the Antarctic continent for annual carbon 

 fixation of 43 g C/m2 (Currie I964), 84 g c/m2 (El-Sayed 1967) and 100 g c/m^ (Ryther I963) 

 and suggests that since the measurements tended to be in the richer areas a figure of 

 50 g c/m2 is a reasonable approximation. Taking a conversion ratio of carbon to fresh wei^t 



