583 



Abstract.— Finfish bycatch taken by 

 the U.S. Gulf of Mexico shrimp fishery 

 is an important issue in the manage- 

 ment of fisheries resources given the 

 perceived high mortality of the differ- 

 ent fish stocks taken as bycatch in the 

 region. Bycatch data are characterized 

 by a high number of low catches, a 

 few high catches, and depending on 

 the finfish species, a significant propor- 

 tion of observations with zero bycatch. 

 An evaluation of the current general 

 linear model for generating bycatch 

 estimates indicates that the bycatch 

 data do not conform to the assumptions 

 of this model because bycatch estimates 

 depend upon choices within the model 

 that can significantly change the results 

 of the model. These choices include the 

 constant value added to catch-per-unit- 

 of-effort (CPUE) values prior to the 

 logarithmic transformation (to avoid 

 undefined logarithms with zero CPUEs ) 

 and the standard time-unit selection for 

 calculating CPUE values from catch in 

 numbers and variable tow times. Cur- 

 rently a value of one is added to ob- 

 served CPUE, and a constant time unit 

 of one hour has been used; however, 

 these choices are somewhat arbitrary. 



An alternative approach to model 

 bycatch data is to use a delta distribu- 

 tion that has two components. Compo- 

 nent one models the proportion of zeros, 

 and component two models the posi- 

 tive catches. In our study, we applied 

 the delta lognormal model to estimate 

 finfish bycatch in the shrimp fishery. 

 This model avoids the problems of 1) 

 the addition of a constant positive value 

 to log-transformed CPUEs, and 2) the 

 selection of a standard time unit for 

 CPUE calculations. Bycatch estimates 

 determined with the current general 

 linear model were compared with those 

 determined with the delta lognormal 

 model for Atlantic croaker iMicropogo- 

 nias undulatus). red snapper iLutJa- 

 nus campechanus). Spanish mackerel 

 iScomberomorus maculatus), and all 

 finfish from 1972 through 1995. Analy- 

 sis and evaluation of the performance 

 of the delta lognormal model indicated 

 that this model fits the bycatch data- 

 base better than the current general 

 linear model. 



An alternative method for estimating bycatch 

 from the U.S. shrimp trawl fishery 

 in the Gulf of Mexico, 1972-1995 



Mauricio Ortiz 

 Christopher M. Legault 



Nelson M. Ehrhardt 



Division of Marine Biology and Fisheries 



Rosenstiel School of Marine and Atmospheric Sciences 



University of Miami 



4600 Rickenbacker Causeway 



Miami, Florida 33149 



Present address (for M Ortiz): Miami Laboratory 



Southeast Fisheries Science Center 

 National Manne Fisheries Service, NOAA 

 75 Virginia Beach Dr, Miami, Florida 33149 



E-mail address (for M Ortiz) mauricio ortiz^i noaa gov 



Manuscript accepted 10 January 2000. 

 Fish. Bull. 98:583-599 (2000). 



In recent years shrimp bycatch has 

 become one of the most important issues 

 in fishery management in the south- 

 eastern United States, inclutJing the 

 U.S. Gulf of Mexico. In 1990, the U.S. 

 Congress requesteii a 3-year research 

 program to assess the impact of bycatch 

 by the shrimp fishery on federally man- 

 aged fishery resources along the south 

 Atlantic and the U.S. Gulf of Mexico 

 coasts (Public Law 101-627, secllOc'). 

 As a result, the National Marine Fisher- 

 ies Service (NMFS) created the Cooper- 

 ative Shrimp Bycatch Characterization 

 Project (NOAAM, a fonr-year program 

 which focused on 1) characterizing 

 onboard shrimp trawl bycatch, 2) devel- 

 oping and testing bycatch reduction 

 devices ( BRDs ), and 3 1 evaluating alter- 

 native bycatch management options. 

 Among the major objectives identified 

 in this project were those of updating 

 and expanding temporal and spatial 

 bycatch estimates (offshore and inshore 

 waters) (NOAAM. 



Since 1987, the NMFS has provided 

 bycatch estimates for several finfish 

 species in the Gulf of Mexico by using a 

 catch-per-unit-of-effort( CPUE) method, 

 where bycatch CPUEs are estimated fol- 

 lowing a general linear approach ( Nich- 

 ols et al.2). Briefly a bycatch CPUE rate 

 is estimated for each fish species by 

 year ( 1972-95 ), area, season, and depth- 

 zone stratum. These bycatch CPUEs 



are multiplied by an estimated annual 

 shrimping effort within the stratum, 

 and the total annual bycatch is the sum 

 of the bycatch for each stratum. To esti- 

 mate bycatch, the sample unit is defined 

 as the number offish of a given species 

 caught each net-hour during a tow. The 

 current general linear model was evalu- 

 uated by considering two main topics: 

 1) the assumptions entailed with using 

 the model and the theoretical basis for 

 generating the estimates, and 2) the 

 appropriateness of the available data 

 to the configuration and analysis of the 

 model. More specifically, we examined 

 the matrix structure used in the gen- 

 eral linear model, the logarithm usage 

 in the general linear model, and the 

 standardization of effort in the CPUE's 

 in the current general linear model. 



NOAA. 1995. Cooperative research pro- 

 gram addressing finfish bycatch in the Gulf 

 of Mexico and South Atlantic shrimp fisher- 

 ies: a report to congress. National Marine 

 Fisheries Service Southeast Regional 

 Office, 9721 Executive Center Dr N., St. 

 Petersburg, FL 33702. 

 ' Nichols S., A. Shah, G. J. Pellegrin, and 

 K. Mullin. 1987. Estimates of annual 

 shrimp fleet bycatch for thirteen finfish 

 species in the offshore waters of the Gulf 

 of Mexico. Report to the Gulf of Mexico 

 Fishery Management Council. The Com- 

 mons at Rivergate, 3018 U.S. Highway 301 

 N., Tampa, FL 33619. 



