FISHERY BULLETIN: VOL. 84, NO. 4 



17°40' 



■o 



3 



o 



Z 



I7°38' 



145-48' 



145°50' 

 East Longitude 



145°52' 



Figure 2.— Contour map of the Alamagan study site. Isobaths given in meters. Open circles repre- 

 sent set locations of standard traps (one set is composed of five traps); closed circles show sets 

 of the pyramid trap. The stipple border encloses the area of greatest fishing intensity. One minute 

 latitude = 1.85 km. 



concentrated in the area enclosed by stipple borders, 

 where 65% of all sets (44 of 68) occurred. This area 

 represents 312 horizontal ha of shrimp habitat. 



Intensive Fishing Experiment 



Although 20 standard shrimp traps were set daily 

 for 15 consecutive days, 29 traps were lost due to 

 entanglement on the bottom. This resulted in 271 

 effective trap-nights of standard fishing effort and 

 a gear loss rate of 9.7% (Table 1). The loss of shrimp 

 traps is not believed to have affected the outcome 

 of the intensive fishing experiment for two reasons: 

 First, the Pacific mackerel bait was rapidly ex- 

 hausted in the traps, as evidenced by its condition 

 after a single night's soak; second, large holes were 

 usually evident in traps if the fishing gear was suc- 

 cessfully retrieved after being fouled on the bottom. 



The catch of H. laevigatus was quite pure; only 

 trace amounts of H. longirostris, H. ensifer, and the 

 eel Synaphobranchus affinis co-occurred in the 

 traps. The latter species was observed to consume 

 individual H. laevigatus on occasion, but this had a 

 negligible impact on overall catch rates of the 

 shrimp. 



A total catch of 663.36 kg of H. laevigatus was 

 landed from standard traps, yielding an overall 

 CPUE of 2.45 kg/trap-night (Table 1). In addition, 

 another 112.77 kg were taken in eight sets of the 

 pyramid trap for an overall CPUE of 14.10 kg/trap- 

 night (Table 1). The larger commercial trap out- 

 performed individual standard Honolulu Laboratory 



Table 1 .—Summary of catch and effort statistics of the intensive 

 fishing experiment for Heterocarpus laevigatus at Alamagan Island, 

 9-24 January 1984. All catches in kilograms and effort in standard 

 trap-nights. CPUE includes only standard trap catches. 



traps by a ratio of 5.76 to 1. Thus, one set of the 

 large trap was roughly equivalent to one set of a 

 string of five standard traps, but the former was 

 much more variable in its performance. Overall, a 

 total of 776.13 kg of H. laevigatus were removed 

 from the study area during the 16-d experiment. 

 These averaged 28 g each (16 shrimp/lb). 



The data presented in Table 1 are arranged to be 

 fitted by the Leslie model (Ricker 1975). The CPUE 

 was computed each day based solely on standard 

 trap catch and effort statistics, although cumulative 



930 



