1^-10 



6. 3 Growth and Life Span 



Several authors have published curves of size at age for E^ superba (e.g., Ruud 1932, 

 Bargnann 1945i Marr 1962) (Pig 6.5) which indicate a seasonal pattern of growth during a two 

 year life span to a maximum size of about 6 cm. More recently Mackintosh (1972) has produced 

 curves of "local apparent growth" based on the catches from specific regions. His analysis 

 does not include a consideration of the origin of the krill S.t a given location but relies 

 purely on the size of individuals at the time and place of capture. 



The simple two year life span has been questioned by several authors (Marr 19^2, Ivanov 

 1970, Hakarov 1971, Mackintosh 1972) who were all v;orking from infoimation from size frequency 

 distributions. Recently Hakarov (1975) has demonstrated the ability of female Ej_ superba to 

 spawn more than once thus indicating a longer life span than two years. 



I>larr (1962) because of the presence of a few specimens of abnormal size in his samples 

 suggested that under certain conditions full sexual maturity and spawning may be delayed 

 until the third year. This phenomenon has been reported in the related Ej_ triacantha (Baker 

 1959)* The degree to which the odd size group vra.s present in Marr' a samples was very small 

 indeed, Hovjever, Ivanov (1970) with a much smaller number of samples detected an intermediate 

 size group in a large proportion of samples from the Scotia Sea. He discussed this in detail 

 sLnd concluded that the intermediate size group represented an additional year class. 



At the moment there is insufficient evidence available to conclusively prove that this 

 intermediate size group does not represent an additional year class. Relevant information 

 for both points of view will therefore be considered. 



At the same time that Ivanov was preparing his results Mackintosh (1972) was in the 

 process of reanalysing in detail the Discovery material. As a result of his analyses he 

 showed the presence of an intermediate size class in most of the areas of dense distribution. 

 For those areas where there are sufficient samples Mackintosh has drawn an expected growth 

 curve which shows the intermediate group as being midway between the previously accepted 

 first and second year groups. Mackintosh argues that since the previously accepted one and 

 two year classes follow on in a smooth curve with fast growth in spring and slow growth in 

 winter, the cause of the intermediate size group must be something other than an additional 

 year in the normal life liistory. On the assumption that the intermediate size fproup is real 

 and not an artefact of sampling. Mackintosh considers the possibility of am early or late 

 spawning producing the effect.' On balance he favours the theory that the intermediate group 

 results from a second spawning later in the year (perhaps in May) but it should be emphasized 

 that this conclusion is based on extrapolation from the start of the intermediate growth 

 curve back for over a year. Makarov (1975? 1976) has shown tha^ a significant proportion of 

 the population spawn more than once, first time spawners release their eggs during the later 

 part of the summer and second time spawners in the early part of the summer. The release of 

 eggs at more than one time during the season of maximtun primary production (and therefore 

 growth) is likely to produce considerable blurring of the peaks in any size frequency distri- 

 bution. Compared to _Ej_ triacamtha the size frequency distributions for E. superba show 

 relatively indistinct peaks (see Baker 1959» Marr 1962, Barpjnann 1945t Ivanov 1970, etc.). 



Mackintosh (1972) detected a correlation between the presence of the intermediate group 

 and an above average temperature in the preceding half year (but not earlier). Al thou^ it 

 is unlikely that a warmer year would cause a second spawning, it is possible that a warm year 

 could result in a greater disparity in the growth of larvae resulting from spawnings at dif- 

 ferent times in the production season. 



Hakarov (1971 ) in a paper that also considers the conclusions of Ivanov (l970) is of the 

 opinion that the extra size group does not represent an extra year class, but is a result of 

 the mixing of krill from different sources. This suggestion is supported by the fact that 

 the Bransfield Strait and South Geor,gia, areas of possible mixing, show the phenomenon quite 

 frequently. Against the idea is the fact that the Bist Wind Drift zone also contains an 



