1999 

 OUR LIVING OCEANS 



fort, which inckides the amount, type, and cffcc- 

 ri\'cnc,ss of fishing gear and the time spent fishing. 

 Catch per unit of effort (CPUE) is .in index show- 

 ing the ratio of a catch of fish, in mimheis or in 

 weight, and a standard measure of the fishing ef- 

 fort expended to catch them. Intermittent high 

 fishing effort is termed pulse fishing. 



Surplus production (or production) is the to- 

 tal weigiu of fisii that can be removed by fishing 

 without changing the size of the population. It is 

 calculated as the sum of the growth in weight oi 

 individuals in a population, plus the addition of 

 biomass from new recruits, minus the biomass of 

 animals lost to natural mortality. 



The production rate is expressed as a propor- 

 tion of the poptilation size or biomass. The pro- 

 duction rate can be highly variable owing to envi- 

 ronmental fluctuations, predation and other bio- 

 logical interactions with other populations. On av- 

 erage, production decreases at low and high popu- 

 lation sizes, and biomass decreases as the amoimt 

 of fishing effort increases. This means there is a 

 relationship between average production and fish- 

 ing effort. This relationship is known as the pro- 

 duction function. 



Production functions are the basis for certain 

 important concepts used in this report: long-term 

 potential yield, current potential yield, and recent 

 average yield. In addition, the term stock-level is 

 employed as a biological reference for determin- 

 ing resource status relative to the level which would 

 on average support the long-term potential yield. 

 Recent average yield also is reported in order to 

 allow comparison of the current situation to long- 

 term potential. 



Many other reference levels are used as bench- 

 marks for guiding management decisions. A num- 

 ber of these are expressed as fishing mortality rate 

 levels that would achieve specific results from the 

 average recruit to the flsherv if the stock were sub- 

 jected to fishing at those rates indefinitely. Some 

 of these benchmarks are used to index potential 

 fisherv production, and others are used to index 

 potential reproductive output. F^^^ is the fishing 

 mortalit)' rate that maximizes the yield obtained 

 from the average recruit. Growth overfishing oc- 

 curs o\er the range of fishing niort,ilit\', at which 

 the losses in weight from total mortality exceed 

 the gain in weight due to growth. I his range is 



defined as beyond F . F^, is a rate that results in 

 almost as much yield per recruit as F does, but 

 can be much lower — and thus more conservative — 

 than F (at F^^ , only a 10% increase in yield per 

 recruit occurs following an additional unit of fish- 

 ing effort compared to the yield per recruit pro- 

 duced by the first unit of effort on the unexploited 

 stock). Benchmarks used to measure reproductive 

 potential usually express an amount of spawning 

 output relative to the amount expected under no 

 fishing. For example, Fj„._ and Fj^.^ are the rates 

 that would reduce spawning biomass per recruit 

 to 20 or 30% of the unflshed level, tespectively. 

 This percent.ige of the unfished level is also known 

 as the spawning potential ratio (SPR). 



Long-term potential yield (LTPY) 



LTI'Y is the maximum long-term average yield 

 that can be achieved through conscientious stew- 

 ardship, by controlling F through regulating fish- 

 ing effort or total catch levels. LTPY is a reference 

 point for judging the potential of the resource. 

 However, it is not necessarily the goal of fishery 

 managers to always set the maximum yield. Other 

 factors influence the choice of a management ob- 

 jective, such as socioeconomic considerations or 

 conservation and eco.system concerns for other ma- 

 rine life indirectly affected by fishery harvests. Fhe 

 methods of estimating LTPY, and LTPY itself, may 

 be controversial. Nevertheless, NMFS scientists 

 have used their best professional judgment to pro- 

 vide these figures as a gauge of long-term produc- 

 tion potential whenever possible. 



Current potential yield (CPY) 



C;P^', the current potential catch that can be 

 safely taken, depends on the current abundance 

 offish and poptilation dynamics of the stock. It is 

 usually estim.ited by applying the F associated with 

 LTPY (e.g. target fishing effort) to the current 

 popiilaiion size. This yield may be either greater 

 than or less than LTPY. CPY is the amount of catch 

 that will maintain the present population level 

 (biomass) or, for o\erutilized stocks, stimulate a 

 ttend toward recovery to a population size that 

 will produce the I I'PY. lor stocks at high biom- 

 ass levels, the CVY m.iv be Lusher than the L'FPY. 



282 



