FISHERY BULLETIN: VOL. 84, NO. 2 



Henceforth, this model will be referred to as the con- 

 stant catchability Leslie model. 



In a multispecies situation, competition between 

 species for baited hooks may produce a dominance 

 hierarchy where some species are more aggressive 

 feeders than others and effectively out compete the 

 less aggressive feeders for baited hooks. The catch- 

 ability of the species at the top of the dominance 

 hierarchy, is independent of the presence of more 

 subordinate species, while the catchability of those 

 species not at the very top of the hierarchy will vary 

 inversely with the abundance of the more dominant 

 species. A simple model which describes the catch- 

 ability of a subordinate species (q(s,t)) as a function 

 of the cumulative catch and initial population size 

 of the more dominant species, K(d,t) and N(d,0) 

 respectively is 



q(s,t) = q(s)(K(d,t)/N(d,0)) 



(3) 



where q(s) is the catchability of the subordinate 

 species in the absence of the dominant species. Com- 

 bining Equations (2) and (3) produces 



CPUE(s,0 = q(s)(K(d,t)/N(d,0)) 



x (N(s,Q) - K(s,t)) (4) 



where V( ) and E( ) represent the variances and 

 means, respectively. 



APPLICATION OF 



MULTISPECIES LESLIE MODEL TO 



SNAPPER INTENSIVE FISHING 



A 13-d intensive fishing experiment covering the 

 period 10-19 April and 5-7 May 1984 was conducted 

 at Pathfinder Reef (lat. 16°30'N, long. 143°05'E) in 

 the Mariana Archipelago. Pathfinder Reef is a cir- 

 cular pinnacle rising steeply from a depth of about 

 1,600 to 16 m beneath the surface At the 200 m con- 

 tour, the diameter is about 0.8 nmi (Fig. 1). The snap- 

 per population at Pathfinder Reef is a closed popula- 

 tion for purposes of the intensive fishing since the 

 closest bank is a small pinnacle 40 nmi to the north. 



Intensive fishing was conducted from the NOAA 

 ship Townsend Cromwell using four bottom hand- 

 lines on hydraulic gurdies targeting species in the 

 150-275 m depth range. Each day during the 13-d 

 experiment, fishing was conducted around the en- 

 tire perimeter of the bank. During the experiment 

 1,467 bottom fish were caught. Three lutjanids, 

 Pristipomoides zonatus, P. auricilla, and Etelis car- 

 bunculus, accounted for 1,317 fish or about 90% of 

 the catch (Table 1). Fishing effort was measured in 



and by defining K(ds,t) 

 q{s)(N(s,0)/N(dM^dB2 

 (3) becomes 



K(d,t)K(s,t), B\ = 

 q(s)IN(d,0) Equation 



CPUE(s,0 = BlK(d,t) - B2K(ds,t). 



Estimates of SI and .62 are obtained from multiple 

 linear regression and the estimates of N(s,0) and q(s) 

 are computed as 



N(s,0) = B1IB2, and q(s) = N(d,0)B2. 



The estimate ofN(d,0) is determined from the con- 

 stant catchability model. As is evident from Equa- 

 tion (4), the estimate of N(s,0) is independent of the 

 estimate oiN(d,0). Estimates of the variance of the 

 estimate of N(s,0) are obtained from estimates of 

 the means and variances of the estimates of 61, and 

 B2 and an exact expression for the variance of a 

 ratio (Frishman 1975). Thus, 



V(N(s,0j) = V01IB2) 



V(Bl)[E(B2)f - V(B2)[E(Bl)f 

 (E(B2)f [V(B2) + [E(B2)f] 



(5) 



Figure 1— Bathymetric chart of Pathfinder Reef showing the 

 segments of the 100-fathom (183 m) contour used to partition daily 

 fishing effort. 



424 



