Ortiz et al,: Estimates of bycatch from the shinmp trawl fisfiery in the Gulf of Mexico 



597 



Finfish 



400 



200 







Delta lognormal model 

 - Bycatcfi GLM model 



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



Red snapper 



t . , : ,  — 



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



Year 



Atlantic croaker 



74 76 78 80 82 84 86 88 90 92 94 



Spanish mackerel 



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



Figure 6 



Estimates of total annual bycatch from the delta lognormal model and the current general linear model, for finfish (millions of 

 pounds), Atlantic croacker, red snapper, and Spanish mackerel (millions offish) 1972-1995. 



Conclusions and recommendations 



Analyses of the total finfish bycatch and the bycatch of 

 Atlantic croaker, red snapper, and Spanish mackerel show 

 that the delta lognormal model estimates differ both in 

 magnitude and trends from those generated by the cur- 

 rent general linear model. However, these differences are 

 not consistent among species. In terms of absolute magni- 

 tude, they are substantially different for Atlantic croaker 

 and Spanish mackerel over all years (1972-95), whereas 

 for red snapper differences are greater in the most recent 

 years of the time series (1987-95). Total finfish bycatch 

 estimates are more similar in magnitude and trend for 

 both models. Although the trends of bycatch in the time 

 series from 1972 to 1995 are similar for the species exam- 

 ined, the absolute estimated values are highly variable. 

 Because these estimates are included as additional catch 

 (usually for age and 1) in the stock assessments of 

 directed fisheries, the uncertainty of the bycatch estimates 

 will impact the results of these assessments. Further, this 

 uncertainty will extend to management policies adopted 



from these assessment results for species like Spanish 

 mackerel, king mackerel, and red snapper (Ehrhardt and 

 Legault, 1997: Goodyear"). 



As presented before, the general linear model estimates 

 depend on choices about the constant added to the CPUE 

 values prior to logarithmic transformation and on the stan- 

 dard time unit chosen for calculating CPUE values. These 

 problems emerge from the noncompliance of the bycatch 

 data with the assumptions associated with the general 

 linear model. In particular, the observed CPUEs are not 

 lognormally distributed owing to the significant propor- 

 tion of zero observations within the data. In contrast, the 

 delta lognormal model conforms better with the structure 

 of the data and avoids the problems of choosing a c value 

 for catches in the logarithm transformation and of select- 

 ing a standard time unit for the CPUE calculations. As 



Goodyear, C. P. 1995. Red snapper in U.S. waters of the Gulf 

 of Mexico. Contribution report MIA 95/96-05, 171 p. Miami 

 Laboratory, Southeast Fisheries Science Center, NMFS, NOAA, 

 75 Virginia Beach Dr., Miami, FL 33149. 



