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Estimates of Shrimp Trawl Bycatch of Red Snapper (Lutjanus Campechanus) in the Gulf of Mexico

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Estimates of Shrimp Trawl Bycatch of Red Snapper (Lutjanus Campechanus) in the Gulf of Mexico Fishery Stock Assessment Models 817 Alaska Sea Grant College Program • AK-SG-98-01, 1998 Estimates of Shrimp Trawl Bycatch of Red Snapper (Lutjanus campechanus) in the Gulf of Mexico B.J. Gallaway LGL Ecological Research Associates, Inc., Bryan, Texas M. Longnecker Texas A&M University, Department of Statistics, College Station, Texas J.G. Cole LGL Ecological Research Associates, Inc., Bryan, Texas R.M. Meyer Meyers Chuck, Alaska Abstract Estimation of red snapper bycatch in the shrimp trawl fishery of the Gulf of Mexico has been a contentious issue. Estimates are generated by the National Marine Fisheries Service (NMFS) using a general linear model which establishes a relationship between resource trawl survey data and catch data from the fishery obtained by observers on shrimp fishing vessels. The more complete time series of resource trawl data is then used to predict commercial vessel CPUE which is multiplied by total fishing effort to determine bycatch. The estimates are characterized by exceptionally low R2 values and highly skewed residuals (70% of the catch observations were zeros). We have attempted to improve the estimates by using fewer and larger time-space cells, pooling catch and effort data to reduce the number of zeros contained in the analysis, incorporating significant inter- actions, and using epochs to guard against nonstationarity. The R2 values for the revised models are 2 to 3 times higher than the R2 for the base case, and the distribution of the residuals is greatly improved. The re- vised estimates in recent years average on the order of 30 to 47% lower than the NMFS estimates. Nevertheless, bycatch levels are high (26 to 32 818 Gallaway et al. — Shrimp Trawl Bycatch of Red Snapper million per year) and are increasing due to increasing abundance of juve- niles. However, the age of structure of the bycatch may consist of a much larger fraction of age-0 fish and fewer age-1 fish than has been thought. Bycatch Reduction Devices (BRDs) do not effectively exclude age-0 red snapper. Thus, the existing stock recovery policy based on NMFS GLM bycatch estimates and using BRDs to reduce shrimp trawl mortality of juvenile red snapper may be ineffective. Introduction The fishery for red snapper (Lutjanus campechanus) in the Gulf of Mexico began over 150 years ago off Pensacola, Florida (Goodyear 1995) and by 1872 it had developed as a separate industry. The stocks offshore of Pensa- cola were greatly depleted between 1865 and 1883, causing the fishery to shift first to the Florida middle grounds (1883 to 1885), and then farther southward along the west Florida coast during 1885 to 1910 (Camber 1955). In 1892, the fishery also expanded to (1) newly discovered red snapper grounds in the Campeche Banks off Mexico, and (2) to the western Gulf of Mexico between the mouth of the Mississippi River to about Galveston, Texas. In the eastern U.S. gulf and Campeche, Mexico, subsequent geo- graphic expansions of the fishery were driven by dwindling stocks in the areas previously fished. The U.S. fishing fleet was excluded from Mexican waters in the early 1980s and as a result the effort was redirected at the remaining stocks in U.S. waters. By this time, the U.S. stock was essentially restricted to the western Gulf part of the range from Mississippi/Alabama to Texas. This area has been fished commercially since 1892, and also lies in the heart of the Gulf of Mexico shrimping grounds. Many juvenile (age-0 and age-1) red snapper are taken as bycatch in the shrimp fishery (Nichols et al. 1987, 1990; Nichols 1990; Nichols and Pellegrin 1992; Nichols 1996). Although there is some debate (e.g., Rothschild et al. 1997), most con- sider the gulf red snapper stock to be, at present, severely overfished (Goodyear 1995, MRAG Americas, Inc. 1997). Management actions began in the mid-1980s and a stock rebuilding plan has been developed. Some of the management measures implemented between 1984 and 1996 include size and bag limits for the recreational fisheries, commercial and recre- ational quotas, prohibition of traps and longline gears in certain areas, and prohibition of commercial sale of red snapper from shrimp trawls. Also, Turtle Excluder Devices (TEDs) were mandated for use in the gulf shrimp fishery in 1990. These mechanical separation devices likely ex- clude large fish as well as turtles, and may have some effect on reducing take of juvenile red snapper (unpublished data). Collectively, these actions appear to have had positive effects on the stock as reflected by increases in both stock and recruitment and, possibly, the increased size of harvested fish (Schirripi and Legault 1997, Rothschild et al. 1997). A key component of the stock rebuilding plan, yet to be im- Symposium on Fishery Stock Assessment Models 819 plemented, is to reduce mortality from shrimp trawl bycatch of age-0 and, in particular, age-1 red snapper through the use of bycatch reduction de- vices (BRDs). BRDs are more effective at excluding age-1-sized than age-0- sized red snapper (Nichols et al. 1995, NMFS 1996). Quantification of bycatch levels and the age-0 and age-1 fractions rep- resented in this incidental catch are necessary for stock assessment and rebuilding evaluations. Estimates of bycatch are provided by the National Marine Fisheries Service (NMFS) as described in Goodyear (1995). In sum- mary, the NMFS bycatch estimates are generated from a general linear model (GLM) applied to two datasets generated from shrimp trawl catches of red snapper. One dataset consists of catch-per-tow data which are pro- vided from resource surveys conducted by NMFS, predominantly in sum- mer and fall of each year (Nichols and Pellegrin 1989, Goodyear 1995). Features of this program since 1985 include a semi-synoptic sampling of the entire western Gulf of Mexico in summer and fall based on a random sampling design and the use of a standard shrimp trawl. Although the scope and design of the resource surveys have varied through time, con- tinuous data are available for the fall season offshore Louisiana since 1972. For simplicity, we shall refer to these data as SEAMAP data, and include results from the Fall Groundfish and Summer SEAMAP programs. The second dataset comes from records of finfish catch and fishing effort compiled on an individual tow basis by observers placed on shrimp fishing vessels specifically to quantify the bycatch including red snapper. Observer data are collected year round, but observer programs are not conducted every year. Even when conducted, only a small fraction of the fleet is sampled. Observer data are available for 1972 to 1982, and 1992 to 1996 periods. We shall refer to these data as Observer (OBSR) data. The structure of the GLM model used by NMFS to estimate commercial catch per unit effort (CPUE) for a single net is: Log (CPUE + 1)ijklmn = mean + dataseti + yearj + seasonk + areal + depthm + eijklmn for an array of space (4 areas × 2 depths) – time (three 4-mo seasons or trimesters) cells over the 24-y period, 1972 to 1996. In effect, the GLM “calibrates” shrimp vessel catch rates and resource trawl surveys during the periods and areas that had observations in common, and then uses the resource trawl data to index shrimp trawl bycatch (Nichols et al. 1987, 1990; Nichols and Pellegrin 1992). The GLM-based estimators of the mean log (CPUE+1) are transformed to an unbiased estimate of the commercial CPUE. These estimates are then multiplied by 2 (the assumed average num- ber of nets) times the effort estimated for that time-space cell in hours fished. The catch estimates are then summed to provide an overall by- catch estimate. This GLM approach has been selected because observer programs have not been conducted in each space-time cell and the stock assessments require annual estimates of bycatch mortality. Use of the 820 Gallaway et al. — Shrimp Trawl Bycatch of Red Snapper SEAMAP data enables an estimate for each year as well as an update of the previous year’s estimates. We believe the structure of the NMFS GLM model is problematic and that improvements can be made. The basic problems are that (1) on the order of 70% of the tows in each dataset have zero catch of red snapper, (2) less than 50% of the space-time cells have been sampled overall (29% of the OBSR cells and 46% of the SEAMAP cells), (3) interactions between the five factors in the GLM model are ignored, and (4) there is no consider- ation of possible effects of nonstationarity (effects of explanatory vari- ables changing over time). Our approach toward addressing these problems is to (1) combine catch and effort over several tows to reduce the number of zeros, (2) use fewer but larger strata to reduce the number of empty cells, (3) consider significant interactions, and (4) conduct separate mod- els for early and late epochs to protect against nonstationarity. Our hypothesis was that the above described changes would result in a better fit of the revised models (higher R2 and lower residual error) as compared to models using the NMFS GLM structure, and a more normal distribution of residuals. The findings of the revised approach are pre- sented and then discussed in terms of their ramifications with respect to management measures designed to recover red snapper stocks. The Data As noted above, the basic data used in the analysis are of three types: observer data, resource trawl survey data, and shrimp fishing effort data. The observer data are available for only two periods: 1972 to 1982 (histor- ical) and 1992 to 1996 (modern). The historical observer data and what we have defined as the SEAMAP data were obtained from S.
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