598 



Fishery Bulletin 98(3) 



-0.5 



-1 



-1.5 



Finfish 



• a... Delta lognormal model 

 -• — Bycatcti GLM model 



72 74 76 78 80 82 84 86 



Red snapper 



90 92 94 



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



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 



Year 



Figure 7 



Normalized plots of bycatch estimates by species from the current general linear model and the delta lognormal model. 



expected, both models agree better in the case of the fin- 

 fish bycatch estimates where the proportion of zero CPUE 

 values is the lowest (less than SV^ ). 



Besides the problems related to zero observations, sev- 

 eral other considerations must be addressed to generate 

 annual bycatch estimates: 



1 The matrix structure of area, season, year, and data 

 source is inadequately covered by observations. This 

 is true for any model that uses these same factors 

 but in particular for the period 1985-90, when commer- 

 cial observations were not available. It may be benefi- 

 cial to limit the analysis to years, areas, and seasons 

 where there are data from both commercial and research 

 sources. This change, however, will require the redefini- 

 tion of the objectives of the bycatch estimation proce- 

 dure because the estimated annual bycatch will not be 

 possible for the 1972-95 period. 



2 Another important requirement is the standardization 

 of the CPUE units for both the commercial and research 

 observations. We feel that these CPUEs represent dif- 



ferent units for each type of observation for each par- 

 ticular species and that a single linear relationship is 

 not adequate. This standardization will require a thor- 

 ough analysis of each fleet and additional information in 

 order to convert the effort units from nominal to effec- 

 tive units for each fleet prior to bycatch estimation. It 

 has been suggested that the more recent data obtained 

 by the Bycatch Characterization Project (NOAAM could 

 be used for this type of analysis. As an alternative, we 

 modified the delta lognormal model to incorporate the 

 observed catch (i.e. numbers of fish) as the dependent 

 variable, and we used the tow time (i.e. hours fishing) 

 as a covariate in the systematic linear component of the 

 delta lognormal model. With this modification, the total 

 deviance explained by the model increased for red snap- 

 per However, we would recommend standardizing the 

 nominal CPUE instead of simply adding more variables 

 to an already unbalanced matrix and avoid considering 

 only goodness-of-fit as an indicator. 

 In the analysis of bycatch by species, it is presently 

 assumed that estimated bycatch in number offish belongs 



