194 K. E. F. WATT 



Such equations are discussed in more detail and applied elsewhere (Watt, 



1955, 1956, 1959^). 



Now such formulations of the exploitation problem will only be useful 

 and meaningful where biomass productivity can in fact be increased by a 

 programme of harvesting. In terms of basic ecological principles, this means 

 that where the rate of conversion of food into biomass by a species is being 

 kept down by intraspecific competition pressure, release of that pressure by 

 thinning can make the species more efficient. However, this state of affairs 

 can only occur where a species is hving in an environment that is favourable 

 for its natahty, growth and survival. 



Where a species is living in an area that is favourable only in certain years, 

 population density may never become great enough for there to be any 

 measurable effects of intraspecific competition. In such cases it is true that 

 competition is the ultimate population regulator, in that if all other regulating 

 factors fail, competition pressure will indeed come into operation. However, 

 where a species is living at the geographical boundary of its favourable range, 

 climate may in fact almost completely regulate numbers of the population. 



A case in point is the smallmouth black bass population studied by the 

 author (Watt, 1959^). In this population 94 per cent of the variation in 

 numbers of four-year-olds recruited to the fishable stock is due to temperature 

 in the summer the age-cohort was spawned. Summer temperatures also have 

 a profound effect on weight growth of individuals during their second and 

 third years of life. When we try to find the fishing schedule that will maxi- 

 mize productivity in such a population, applying calculus to the expanded 

 version of an equation such as (i), we fmd that any fishing at all only decreases 

 productivity. Yet we can fish the population without decimating it. In this 

 instance, the role of fishing in the dynamics of the natural population is to 

 compete for a chance of killing fish that would otherwise be wasted to 

 natural mortality caused ultimately by a rigorous chmate. 



Clearly equation (i) is not of sufficiently ubiquitous applicabihty to be of 

 general use. 



It should be noted that while smallmouth black bass populations are largely 

 regulated by temperature in north temperate zones, they are probably 

 regulated by intra- and interspecific competition, say, in Georgia or Alabama. 



Various other types of populations are found amongst the fishes. Pilchards, 

 sardines, smelt and alewives mature very rapidly, and reproduce within the 

 first two or three years of life or only slightly later. Recruitment must be 

 very sensitive to extreme weather conditions in the year of spawning. Also, 

 bio mas productivity must be very sensitive to weather conditions, because 

 mean length of life is not long enough for good growing seasons to compen- 

 sate for the effect of, say, two successive poor growing seasons. Lake trout 



