SCIENTIFIC PRINCIPLES AND TERMS 



INTRODUCTION 



A POPULATION IS a 

 group of animals that are 

 genetically related owing 

 to interbreeding. Ideally, 

 populations should be 

 considered distinct 

 groups for fishery 

 management purposes. 

 But it is difficult to 

 determine which 

 individuals of a species 

 form a population, and it 

 may not be practical to 

 manage them as a 

 population. Thus, this 

 report uses the term 

 "population" to identify 

 interbreeding biological 

 groups. The term "stock" 

 is used to identify groups 

 of animals for 

 management purposes. 



Fish abundance or population size can be 

 expressed as either the number of fish or 

 the total fish weight (or "biomass"). Pish 

 abundance is determined by growth of the 

 individual fish and the addition or recruit- 

 ment of new generations of young fish 

 (i.e., "recruits") to the population. Those 

 gains must then be balanced against re- 

 movals from the population by fishing 

 (called Ashing mortality) and natural 

 causes such as predation, starvation, and 

 disease (called natural mortality). In 

 stock assessment work, fish removals are 

 commonly expressed in terms of rates 

 within a time period. The fishing mortality 

 rate is a function of fishing effort (the 

 amount of fishing gear and the time spent 

 fishing). 



Surplus production (or just "produc- 

 tion") is the weight (biomass) of fish that 

 can be removed by fishing without causing 

 a change in population size. It is calculated 

 as the sum of the growth in weight of indi- 

 viduals in a population, plus the addition of 

 biomass from new recruits, minus the bio- 



mass of animals lost to natural mortality. 

 The production rate is expressed as a 

 proportion of the population size or bio- 

 mass. The production rate is highly vari- 

 able owing to environmental fluctuations, 

 predation, and other biological interactions 

 with other populations. On average, pro- 

 duction rate decreases at low and high 

 population sizes. Thus, surplus production 

 tends to be low at the extremes of popula- 

 tion size (i.e., where biomass or production 

 rate is low). It is more likely to be high at 

 some intermediate level of population bio- 

 mass. But, on average, biomass decreases 

 as the amount of fishing effort increases. 

 This means there is a relationship between 

 average production and fishing effort. The 

 relationship is known as the production 

 function. A hypothetical production func- 

 tion is shown in Figure 1 . Production func- 

 tions are the basis for three important 

 terms used in this report: Long-term Po- 

 tential Yield (LTPY), Current Potential 

 Yield (CPY), and Recent Average Yield 

 (RAY). 



LONG-TERM POTENTIAL 

 YIELD (LTPY) 



In the best professional judgment of NMFS 

 scientists, LTPY is the maximum long-term 

 average yield (catch) that can be achieved 

 through conscientious stewardship, by 



controlling the fishing mortality rate to 

 maintain the population at a size that would 

 produce a high average yield or harvest. 



CURRENT POTENTIAL 

 YIELD (CPY) 



The yield or catch that can be taken during 

 any particular period depends on the exist- 

 ing fish population size and current produc- 

 tion rate. The yield may be either greater 

 than or less than LTPY, and this report uses 

 the term "current potential yield." In the 

 best professional judgment of NMFS scien- 



tists, CPY is the yield that will maintain the 

 current population level (biomass) or stim- 

 ulate a trend toward a population that will 

 produce the LTPY. CPY is frequently esti- 

 mated by applying the fishing mortality 

 associated with LTPY to the current popu- 

 lation size. 



RECENT AVERAGE 

 YIELD (RAY) 



To document the actual fish catches, this 

 report employs the term "recent average 

 yield" (RAY). This is the reported fishery 



landings averaged for the 3-year period, 

 1988-90. 



