F-32 



(so that the cpiantit.y increases as the prey stock increases). These correspond, somewhat 

 rou^ly, to predation hy, warm-blooded and cold-blooded predators respectively. 



On the assumption that krill production is independent of fishing, the yield from a 

 fishery will be largely at the expense of natural mortality as is indicated in equation (A) 

 below: 



dJJ 



= -FN - C - H N - M N (A) 



c d 



N •= Population Size 



Z = Coefficient of total mortality 



P = Coefficient of mortality due to fishing 



C = Consumption by vrarm blooded predators (N.B. dependent on consumer stock size. 

 Consumption/individual varying with consumer density) 



M = Coefficient of mortality due to other predators 

 c 



M = Coefficient of mortality leading directly to deccmposers 

 d 



Intuitively it is to be expected that fishing will first of all be at the expense of 

 M, and then reduce the proportion of M . Since C is for the most part dependent on the pre- 

 dator population size and the duration of the feeding season, this could be the least affected 

 by fishing. Althouf^ this conclusion is tentative it does indicate the need for considera- 

 tion of the effects of predators in a predictive model of a krill fishery. 



Just as predation is an important factor in considering a krill fishery, it is equally 

 important in studies of the predators themselves. This applies whether the predators them- 

 selves are likely to form the basis for a fishery (in v;hich case the need for such an 

 analysis is obvious) or not (in which case the information will be useful in understanding 

 ecosystem dynamics). In order to gain some insight and begin to understand the interspecific 

 interactions it is useful to consider the situation of an intensive krill fishery in one 

 limited area. The effect that this fishery has on the krill consumers will depend on the 

 size of the Unit Stock being fished. The hypothetical nature of much of the information on 

 the control of krill distribution shows that at this stage it is impossible to define more 

 than one management stock (Mackintosh 1973 indicates the possibility of there being several 

 although the information is not conclusive) and, bearing in mind that there may exist several 

 stocks it is worth considering the possible magiitude of any changes in relation to the stock 

 size. On the assumption that fishing would be restricted to one defined area a f^w of the 

 effects that it may be possible to quantify are listed in Table 6.16. The effects themselves 

 are described in subjective terms because the effects may differ from area to area. For the 

 purposes of the krill fishery these effects could indicate changes in the krill stocks which 

 would thus add weight to conclusions based on direct obseirvations. The changes in consumer 

 stocks also form part of a much largei? family of observations concerned with ecosystem model- 

 ling, a subject discussed in a later section. 



^^ lock Assessment 



Some Practical Considerations 



The fragile nature of euphausiids means that ajiy individuals which pass throu^ 

 the meshes of a trawl net are unlikely to survive so that irrespective of whether the krill 



