F-31 



Table 6.15 (continued) 



Of these components probably the most useful as far as providing a reliable index of 

 fishing effort are the power of the vessel combined with an indication of fishing time that 

 teikes account of "idle time" caused by gear/vessel saturation. 



At the moment there is no well-established method of monitoring the changes in abundance 

 of krill, in either relative or absolute terms, although suitable techniques are being dev— 

 eldped. Since it is a member of the zooplankton, krill can be caught by specially desired 

 quantitative nets such as the Rectangular Midwater Trawl with 8 square metre mouth (RMT 8) 

 developed by the Institute of Oceanographic Sciences (U.K.) and used with success by the 

 Federal Republic of Germany's Antarctic Expedition. This type of direct method is very 

 time consuming and expensive in ship's time and is therefore not an efficient method of esti- 

 mating standing stock over the whole region. Quantitative net hauls can be made to identify 

 eohosounder indications and thus form the basis of a remote sensing survey. The dense con- 

 centrations of krill when they occur near the surface may also be detected by satellite 

 photographs. Both the remote sensing methods mentioned above rely on the occurrence of 

 swaiTOB and since these are almost certainly varying in occurrence, depth and density with 

 season there is clearly a great deal of scope for biological investigations in this field. 



The next stage in resource assessment will almost certainly be the desi^ and applica- 

 tion of an analytical model with the intention of adding increased precision to the estimates 

 and predictions. The classical Beverton and Holt type model assumes constant recruitment 

 and also that the level of natural mortality remains constant and independent of fishing 

 intensity. For a short lived species such as krill it is very likely that there will be 

 large year to year fluctuations in recruitment caused by both physical and biological varia- 

 tion. (There is no evidence to either prove or disprove this suggestion - a situation that 

 is likely to remain until some reliable estimates for standing stock are available. ) 



Natural mortality is, for convenience, goiierally assumed to be more or less constajit 

 and independent oi' fishing. However, for a short-lived animal (with possibly only one year 

 class in the exploited population) in the centre of a food chain this assumption is almost 

 certainly not valid. 



Natural mortality can for convenience be attributed to two causes: consumption by 

 predators and natural death direct to decomposers. The mortality due to predation can be 

 further subdivided depending on whether a given stock of predators take a fixed quantity 

 (so that the rate decreasea as the prey stock increases) or exert a fixed rate of predation 



