484 



Abstract— Bycatch. or the incidental 

 catch of nontarget organisms during 

 fishing operations, is a major issue in 

 U.S. shrimp trawl fisheries. Because 

 bycatch is typically discarded at sea, 

 total bycatch is usually estimated by 

 extrapolating from an observed bycatch 

 sample to the entire fleet with either 

 mean-per-unit or ratio estimators. 

 Using both field observations of com- 

 mercial shrimp trawlers and computer 

 simulations, I compared five methods 

 for generating bycatch estimates that 

 were used in past studies, a mean- 

 per-unit estimator and four forms of 

 the ratio estimator, respectively: 1) 

 the mean fish catch per unit of effort, 

 where unit effort was a proxy for sample 

 size, 2) the mean of the individual fish 

 to shrimp ratios, 3) the ratio of mean 

 fish catch to mean shrimp catch, 4) the 

 mean of the ratios offish catch per time 

 fished (a variable measure of effort), 

 and 5) the ratio of mean fish catch per 

 mean time fished. For field data, differ- 

 ent methods used to estimate bycatch 

 of Atlantic croaker, spot, and weakfish 

 jaelded extremely different results, with 

 no discernible pattern in the estimates 

 by method, geographic region, or spe- 

 cies. Simulated fishing fleets were used 

 to compare bycatch estimated by the 

 five methods with "actual" (simulated) 

 bycatch. Simulations were conducted by 

 using both normal and delta lognormal 

 distributions of fish and shrimp and 

 employed a range of values for several 

 parameters, including mean catches 

 of fish and shrimp, variability in the 

 catches of fish and shrimp, variability 

 in fishing effort, number of observa- 

 tions, and correlations between fish and 

 shrimp catches. Results indicated that 

 only the mean per unit estimators pro- 

 vided statistically unbiased estimates, 

 while all other methods overestimated 

 bycatch. The mean of the individual fish 

 to shrimp ratios, the method used m the 

 South Atlantic Bight before the 1990s, 

 gave the most biased estimates. Because 

 of the statistically significant two- and 

 3-way interactions among parameters, 

 it is unlikely that estimates generated 

 by one method can be converted or cor- 

 rected to estimates made by another 

 method: therefore bycatch estimates 

 obtained with different methods should 

 not be compared directly. 



Manuscript approved for publication 

 17 December 2002 by Scientific Editor 



Manuscript received 3 April 2003 at 

 NMFS Scientific Publications Office. 



Fish. Bull. 101:484:-500(2003). 



Estimation of bycatch in shrimp trawl fisheries: 

 a comparison of estimation methods 

 using field data and simulated data 



Sandra L. Diamond 



Department of Biology 



Box 3131 



Texas Tech University 



Lubbock, Texas 79409 



E-mail address: Sandra Diamond'fflttuedu 



Bycatch, as used in the present study, 

 is the incidental catch of nontarget 

 organisms that occurs to some extent in 

 almost all commercial fisheries (Alver- 

 son, 1994). Some of these incidentally 

 caught organisms may be protected spe- 

 cies — such as marine mammals, marine 

 turtles, and seabirds — or they may be 

 fish or invertebrates that are either 

 harvested as target species by other 

 fisheries, or species that fishermen call 

 "trash fish" because they have little or 

 no economic value. Bycatch in most 

 commercial fisheries has only been a 

 major issue since the 1980s — primarily 

 because individuals caught as bycatch 

 have historically been discarded at 

 sea, leaving fishery managers and the 

 general public unaware of the extent of 

 bycatch mortality. For many organisms, 

 bycatch may be a significant source of 

 mortality, and inclusion of bycatch mor- 

 tality in stock assessments or manage- 

 ment plans may be critical for effective 

 management. 



Because bycatch species are not usu- 

 ally landed, quantifying bycatch poses 

 a very different problem from that of 

 quantifying the catch of a target spe- 

 cies. Several methods of quantifying 

 bycatch have been tried, including the 

 requirement that fishermen record 

 catch and bycatch in logbooks (Walsh 

 and Kleiber, 2001), use of research ves- 

 sel surveys to model commercial fishing 

 (Nichols et al.'), and the placement of 

 observers aboard fishing vessels (Julian 

 and Beeson, 1998). Although direct ob- 

 servation is the most accurate method, 

 unless observer coverage of the fleet is 

 complete, estimation of bycatch from 

 observation data requires sampling of 

 the fleet and then extrapolating from 

 the samples (the observations) to the 



entire fleet using statistical estimators. 

 Two types of statistical estimators are 

 used: mean-per-unit estimators and ra- 

 tio estimators. In both types of estima- 

 tors, the observed catch of the bycatch 

 species iy) is linked to an auxiliary vari- 

 able (x) for which the population total 

 is known (Cochran, 1977). In mean-per- 

 unit estimators, the auxiliary variable 

 is a measure of fishing effort such as 

 tow, day, trip, etc., where each unit of 

 effort is the same as one observation. 

 In ratio estimators, the auxiliary vari- 

 able is a variable that is correlated with 

 the catch of the bycatch species, such 

 as the catch of the target species or the 

 number of hours fished (Cochran. 1977). 

 The major difference between these two 

 types of estimators is that the auxiliary 

 variable in the mean-per-unit estimator 

 is a substitute for the number of obser- 

 vations rather than a mean value with 

 a variance, while the auxiliary variable 

 in the ratio estimator is the mean value 

 of a quantity that varies from sample to 

 sample. Although the statistical proper- 

 ties of these two types of estimators are 

 well known, the choice of which estima- 

 tor to use in bycatch research is often 

 based on the ease of collecting fleet 

 information on the auxiliary variable, 

 and not on any inherent properties of 

 the estimators themselves or on any 

 specific information about the relation- 

 ship between the catch of bycatch spe- 

 cies and the auxiliary variable. 



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

 K. Mullin. 1990. Updated estimates of 

 shrimp fleet bycatch in the offshore waters 

 of the US Gulf of Mexico, 22 p. Pascagoula 

 Laboratory, Southeast Fisheries Science 

 Center, NMFS, PO Drawer 1207, Pasca- 

 goula, MS 39568-1207. 



