GENERAL DISCUSSION 367 



there is no correlation between egg laying and recruitment, and the regula- 

 tion of numbers occurs in the post-recruitment phase. 



For a fisheries biologist these may come to the same thing — the number 

 of recruits need in neither case depend on stock size. 



M. Graham: There are species offish in which recruitment does depend 

 on the stock size: e.g. salmon and perhaps herring. Certainly to say they 

 are unrelated is not a vahd generahzation. 



J. G. Skellam: Stock density rather than the total number is the im- 

 portant factor in this connection. 



J. B. Cragg: In Holt's two models, in the first (Fig. 3^) is self-regulation 

 considered to be operating prior to recruitment, while in the second (Fig. 3/j) 

 is it active afterwards, bringing the population down to a steady state ? 



S. J. Holt: No. Regulation occurs in the early stages in both. The 

 damping down of oscillations, however, occurs in the post-recruitment stage 

 in the second case. Damping is not itself a regulatory process and may occur 

 elsewhere in the life history. 



E. D. Le Cren: The timing of density effects is important. Consider 

 two generations: in the first instance, as suggested by Nikol'ski", these may 

 have different adult numbers but lay the same number of eggs — regulation 

 here occurring by a variation in the level of fecundity. Alternatively, there 

 is the system in which regulation occurs in the pre-recruitment stages of 

 the larvae, leading to the same result but depending on another stage of the 

 Hfe cycle. 



S. J. Holt: Looking at the system in these two ways leads us to con- 

 template different regulatory mechanisms. Yet all result in the population 

 derived from eggs coming to a constant average level related to the available 

 niches. 



D. A. Hancock: Shellfish show very irregular recruitment. The 

 answer to poor oyster fisheries is to improve the stock by aiding settlement, 

 yet this does not ehminate the irregularity in the process. The cockle also 

 shows very variable settlement. Yet examples of regulation at the recruitment 

 level can be seen in invertebrates, starfish giving an example. In one instance 

 a big setdement of starfish was followed by good growth, and then to a 

 stoppage in growth or actual shrinkage. Mortality rose at this stage. After a 

 year the animals were about the size of a normal three-month-old starfish, 

 and these competed for food and prevented the estabhshment of the following 

 year group of recruits. 



G. V. Nikol'skii: Is the ampHtude of variation the same at all points 



