POLOVINA and RALSTON: MSY FOR DEEP SLOPE FISHES 



of three cruises. Two sites, Pagan Island and 

 Esmeralda Bank, were sampled on each of the six 

 cruises to establish a time series of length-frequency 

 data. 



The NOAA ship Townsend Cromwell was used as 

 the fishing vessel for all the cruises. The fishing was 

 conducted from four hydraulic gurdies equipped 

 with 365 m of braided 90 kg Dacron 2 line. The ter- 

 minal rig consisted of four hooks spaced about 1 m 

 apart and of 2 kg weight. 



At each island and bank, an attempt was made 

 to perform a systematic fishing survey of the bot- 

 tom fish community along the 200 m contour. Fish- 

 ing was conducted while the vessel drifted and 

 targeted the 125-275 m depth range. Fishing effort 

 was measured in line-hours, defined as the product 

 of the number of lines fished with the length of time, 

 in hours, that they are fished. 



Seven species— one jack, Caranx lugubris, and six 

 snappers, Pristipomoides zonatus, P. auricilla, P. 

 filamentosus, P.flavipinnis, Etelis carbunculus, and 

 E. coruscans— accounted for about 92% of the catch 

 (Polovina 1986b). Large length-frequency samples 

 were collected for all seven species, primarily from 

 the unfished islands and banks, and were used to 

 jointly estimate MIK, the ratio of instantaneous 

 natural mortality (M) to the growth parameter of 

 the von Bertalanffy growth curve (K), and the 

 asymptotic length (LJ by regressing a sequence of 

 mean lengths on minimum lengths (Wetherall et al. 

 in press). Otoliths were collected for all seven species 

 and the growth coefficient K was estimated by fit- 

 ting a von Bertalanffy growth curve to otolith data 

 with L fixed at the value estimated from the 



oo 



length-frequency analysis (Ralston and Williams 3 ). 

 Once K and the ratio of MIK were estimated, an 



estimate of M was obtained from their product. The 

 size of entry to the fishery was estimated as the in- 

 tegrated midpoint of the ascending limb of the size- 

 frequency distribution (Gulland 1969). This size was 

 then converted to an age of entry into the fishery 

 (t c ) by application of the von Bertalanffy growth 

 curve. The values of L m , K, M, t m , and t c for the 

 seven species which are required by the yield 

 analysis are given in Table 1. The exponent of the 

 length-weight equation (b) for most of the species 

 is not significantly different from 3.0, so to simplify 

 the computation, it will be taken as 3.0 for all the 

 species (Ralston in press). 



An estimate of the catchability of the bottom 

 fishes which was used to convert CPUE into stand- 

 ing stock was derived from an intensive fishing ex- 

 periment conducted at Pathfinder Reef (Polovina 

 1986a). Thirteen days of intensive handline fishing 

 with the Townsend Cromwell at Pathfinder Reef 

 produced a substantial and significant decline in 

 CPUE. Application of the Leslie model (Ricker 

 1975), which regresses CPUE against cumulative 

 catch, produced estimates of catchability for three 

 species— Pristipomoides zonatus, P. auricilla, and 

 Etelis carbunculus (Polovina 1986a). While interest- 

 ing differences in catchability among species were 

 found, the estimate of the total unexploited biomass 

 for the three species obtained from the species 

 specific Leslie model was not significantly different 

 from the total unexploited biomass computed from 

 the Leslie model applied to the catch and CPUE data 

 pooled over all three species. Catchability from the 

 pooled Leslie model is estimated to be 0.0066 nmi/ 

 line-hour. This value was used as an estimate of total 



2 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



3 Ralston, S., and H. A. Williams. Age, growth, and mortality 

 of deep slope lutjanid fishes from the Mariana Archipelago. 

 Manuscr. in prep. Southwest Fisheries Center Honolulu Labor- 

 atory, National Marine Fisheries Service, NOAA, Honolulu, HI 

 96822-2396. 



Table 1.— Population parameters for the seven major species caught by handlining 



in the Marianas. 



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