588 



Fishery Bulletin 98(3) 



2,000 

 1,600 ° 



Finfish 



Atlantic croaker 



72 74 76 78 80 82 84 86 88 90 92 94 



Red snapper 



72 74 76 78 80 82 84 86 88 90 92 94 



Spanish mackerel 



10.0 



72 74 76 78 80 82 84 86 



72 74 76 78 80 82 84 



90 92 94 



Year 



Figure 3 



Estimated total bycatch with the general linear model when usmg different tow-time standard units. The current general linear 

 model uses a one-hour tow time (base scenario). 



the logarithmic transformation of the CPUE, the c values 

 must be selected for each species independently. Therefore, 

 the same c value might not be appropriate for different 

 bycatch species, and if new bycatch data are added, then 

 the c value must be re-evaluated. 



Standardizing effort in the general linear model 



The general linear model predicts bycatch CPUE by cell 

 in units of number offish caught in one shrimp trawl net 

 per hour. Because actual observations of bycatch are the 

 number of fish caught in a shrimp net during a tow and 

 because tow times are variable, observations are converted 

 to a standard unit of one hour tow time. This standard- 

 ization procedure implies a direct linear relation between 

 number of fish caught and tow time for all observations 

 (i.e. if 10 fish were caught in a 30-min tow. the CPUE 

 would be 20 fish per hour). However, the average tow time 

 and the tow time distribution from commercial observa- 

 tions are considerably different from those from research 

 observations. Most of the commercial tows range from 1 

 to 7 hours and have a mode of approximately 4 hours; a 

 few tows are over 12 hours. In contrast, research tows are 

 predominantly of 10-minute duration (737^ ), and the rest 

 last 1 hour or less. 



Given these differences in fishing and sampling time- 

 effort between commercial and research obsei-vations, we 



estimated total bycatch by using different time units to 

 convert the observed catch to CPUE values. We selected 

 10-, 30-, and 240-minute time units instead of the cur- 

 rently used one-hour unit. These were chosen on the basis 

 of the most frequent tow time for research observations 

 (10 min), the mean tow time of research observations (30 

 min), and the mode tow time for commercial observations 

 (240 min). The predicted CPUEs were then multiplied by 

 the shrimping effort per cell in the modified time units. 

 Shrimping effort was given in 24-hour-day fishing effort. 

 Therefore, if the predicted CPUE units were 0.5 hour (30 

 min), the 24-hour shrimping effort would be multiplied by 

 2. The c value was 1.0 for all these calculations. 



Modifying the time unit for calculating CPUE values also 

 had an effect on the annual estimates of shrimp bycatch 

 from the general linear model (Fig. 3). Similar to the results 

 of the evaluation of the logarithmic constant, the changes 

 of estimated bycatch were different for each species and 

 varied in the direction of the change. For example, for 

 finfish and Atlantic croaker, a time unit of 10 minutes 

 decreased estimated annual bycatch (5'/; and 68'''i on aver- 

 age, respectively). By contrast, red snapper and Spanish 

 mackerel estimated bycatch increased with the 10 minute 

 unit {I27r and 78"^, respectively). With the commercial 

 mean tow time (240 min), bycatch estimates of Atlantic 

 croaker increased on average 300';, and lO'/r for finfish. 

 For red snapper, estimates changed only in the most recent 



