AN APPROACH TO YIELD ASSESSMENT FOR UNEXPLOITED RESOURCES 

 WITH APPLICATION TO THE DEEP SLOPE FISHES 



OF THE MARIANAS 



Jeffrey J. Polovina and Stephen Ralston 1 



ABSTRACT 



A comprehensive approach to estimate the maximum sustainable yield (MSY) for a tropical multispecies 

 resource which lacks catch and effort data is presented. This yield assessment approach was used to 

 design a fishery resource assessment survey of the Mariana Archipelago. An application of the method 

 is presented to estimate the MSY for a multispecies bottom fish resource, based on data collected during 

 the survey. The annual MSY for the deep slope fishes (primarily snappers and groupers) of the Mariana 

 Archipelago is estimated to be 109 t, which for comparative purposes is equivalent to 222 kg/nmi of 200 

 m isobath or 0.3 t/km . 



Assessment of tropical resources has always created 

 major problems in fisheries research (Saila and 

 Roedel 1979; Pauly and Murphy 1982). This has 

 been largely due to three factors: technical dif- 

 ficulties in aging, a high species diversity in tropical 

 communities, and what is typically a multiplicity of 

 artisanal gears used in these fisheries. The latter 

 problem has been especially difficult to surmount, 

 making it difficult to determine not only the level 

 of fishing effort but sometimes even the total 

 catch. Without these data many standard fish- 

 eries techniques such as stock-production methods 

 are inapplicable (but see Csirke and Caddy 

 1983). 



In recent years, however, new methods and 

 modifications of existing methods have been pro- 

 posed to estimate growth and mortality parameters, 

 standing crop, and yield for fish stocks in the 

 absence of a time series of commercial catch and ef- 

 fort data (Beddington and Cooke 1983; Pauly 1983; 

 Polovina 1986a; Wetherall et al. in press). We will 

 show that several of these techniques can be com- 

 bined, producing an integrated approach to yield 

 assessment designed specifically for tropical 

 fisheries resources in situations where catch and ef- 

 fort data are lacking. The approach is then applied 

 to data gathered in a fishery survey of the Mariana 

 Archipelago to estimate maximum sustainable yield 

 (MSY) for a multispecies resource of deep slope 

 snappers and groupers. 



'Southwest Fisheries Center Honolulu Laboratory, National 

 Marine Fisheries Service, NOAA, 2570 Dole Street, Honolulu, HI 

 96822-2396. 



YIELD ASSESSMENT 



The equilibrium yield assessment is presented 

 schematically in Figure 1. This approach assumes 

 that growth follows the deterministic von Berta- 

 lanffy curve with parameters K and L x , that the 

 mortality of fish above the smallest length fully 

 represented in the catch (L c ) occurs at a constant 

 instantaneous rate (Z), and that recruitment is con- 

 stant with R recruits entering the first vulnerable 

 age class annually. It is also assumed that the 

 resource is essentially pristine, such that an estimate 

 of the biomass recruited to the fishery in the absence 

 of exploitation (B^) can be obtained from a catch- 

 per-unit-effort (CPUE) survey and an estimate of 

 catchability. In the discussion section, the effect of 

 relaxing some of these assumptions will be con- 

 sidered. 



For each species under consideration, the data re- 

 quired for this program, at a minimum, consist of 

 a large length-frequency sample, otolith data and/or 

 a time series of length-frequency data, a systematic 

 CPUE survey, and an estimate of catchability, such 

 as that obtained from an intensive fishing experi- 

 ment. The large length-frequency sample is used to 

 jointly estimate the asymptotic length (L m ) and the 

 ratio of total instantaneous mortality (Z) to the von 

 Bertalanffy growth parameter (K) based on the 

 following relationship: 



= ZIK = (L m - 1)1 (T - L c ), 



where L c is a parameter defined above and I is the 

 mean length of all fish greater than L r (Beverton 



Manuscript accepted February 1986. 

 FISHERY BULLETIN: VOL. 84, NO. 4, 1986. 



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