1 SCRS/2008/088 STANDARDIZED CATCH RATES OF BLUEFIN TUNA, THUNNUS THYNNUS, FROM THE ROD AND REEL/HANDLINE FISHERY OFF THE NORTHEAST UNITED STATES DURING by Craig A. Brown 1 Sustainable Fisheries Division Contribution SFD-2008-??? SUMMARY Individual trip rod and reel/handline catch per unit effort data, collected through interviews with fishermen, were used to estimate standardized catch rates considering factors such as time of year, area fished, boat type, fishing method, fishery open/closed status, bag limits and target. Models were developed for all size categories of bluefin tuna (except for those < 66 cm sfl), implementing a delta-poisson approach in which catch rates are considered as a product of binomially distributed probabilities of a positive catch and Poisson distributed positive catch rates. Seven indices of abundance of bluefin tuna from the United States rod and reel fishery are presented. These indices are calculated separately by size category and for two distinct time periods and The indices for the early period include a series for small bluefin (< 145 cm SFL) for and for large bluefin (>195 cm SFL) for ; these are presented unchanged from previous analyses. Also presented unchanged are the indices for cm SFL bluefin and large bluefin (>195 cm SFL, ), which were not used for the last Western Atlantic stock assessment. For the period ,new models were developed and indices calculated for cm, cm, and >177 cm SFL bluefin. The distinct periods were defined because changes in survey data collection implemented in 1993 permitted separation of the catches into the smaller size intervals and because regulatory and management changes imposed different daily limits and fishery closures for those size categories. KEYWORDS Abundance indices, Atlantic bluefin tuna, Catch/effort, Catch rate standardization, Generalized linear model, Multivariate analysis, Pelagic fisheries, Rod and reel, Tuna fisheries 1 NOAA Fisheries, Southeast Fisheries Center, Sustainable Fisheries Division, 75 Virginia Beach Drive, Miami, FL, , USA.
2 1. INTRODUCTION Rod and reel fishermen have targeted bluefin tuna off the northeast coast of the United States for many decades. The National Marine Fisheries Service has monitored that fishery through the Large Pelagic Survey, data from which has typically been used to develop abundance indices for bluefin tuna. As a consequence of ICCAT recommendations which became effective in 1992, various regulatory changes were implemented. Those measures included daily catch limits on anglers and/or vessels and fishery closures for various size categories of bluefin. New size categories were defined: young small < 66 cm SFL also known as young school small small (SMSM) cm SFL also known as small school large small (LGSM) cm SFL also known as large school small medium (SMMD) SFL large medium (LGMD) cm SFL since 1992 sometimes known as giant large (LG) > 195 cm SFL also known as giant Large Pelagic Survey procedures to use the new size categories were fully implemented in These catch limits, closures and size categories were considered in recent calculations of indices of abundance (Brown 2007, Brown 2003, Brown 2002, Brown and Turner 2001, Ortiz et al. 1999, Turner et al. 1999, Brown et al. 1999). The purpose of this paper is to present abundance indices for all sizes of bluefin tuna, with the exception of young small bluefin tuna (which are illegal to land and which are infrequent in the catch). Due to the regulatory and survey methodology changes, these indices are calculated separately by size category and for two distinct time periods and ; the exception being an index for LG bluefin for Two additional years of data were available compared to the data available for the last analyses (Brown 2007), and new models are developed. The indices presented here can be separated into three categories: 1) two early period indices (SM and LG) which were left unchanged from previous analyses because there is no additional data to be considered, 2) two indices which were not updated because they were not used for the last rwo Western Atlantic stock assessments (SMMD and LG ), and 3) three late period indices (SMSM, LGSM, and LGMD-LG) which were developed incorporating data from ). The LG index assumes that the regulatory changes since 1992 have had no effect. The regulatory changes combined the LGMD and LG categories (Turner et al. 1999), which were separate regulatory categories prior to There are indications that these changes may have led to an alteration in how the bluefin size classes are perceived by the fishing community; both large medium and large are often jointly referred to as giants, an apparent change from the years prior to This suggests that there may be some mis-classification in reported size categories (Brown et al. 1999) as well as in reported targetting. For these reasons, indices of abundance for large medium and large bluefin (tunas >177 cm SFL) are presented as a group for the period (LGMD-LG).
3 2. MATERIAL AND METHODS The Large Pelagic Survey collects data on the catch and effort through interviews with fishermen at the dock and in the past has collected such information over the telephone. Information collected usually includes date, landing area, boat type (charter or private), fishing area, number of anglers fishing, number of lines in the water, hours fished, type of fishing (primarily trolling or chumming), fishing target, and catch by bluefin size category. The process of calculating the indices of abundance from this data involves the standardization of yearly changes in catch rate, accounting for the influence of those factors which have a significant influence. Factors which were considered as possible influences on catch rates included year, month (and BIMONTH, grouping successive months together), boat type (BOATTYPE), fishing method (FISHMETH), fishery specific fishing areas (sometimes combined into larger regions), open/closed status of the fishery, angler/vessel catch limits, target and interview type (dockside/telephone recall or DOCKRECL). Also considered were any vessel based catch limits for in effect for the trip, specific to each size category (e.g. VLIM_SSM refers to the vessel catch limit for SMSM bluefin tuna). Fishing effort was defined as hours fished as has been done in recent analyses for bluefin tuna (Brown 2007, Brown 2003, Brown 2002, Turner and Brown 1998). The applicable fishery closures and catch limits, allocated by regulatory categories Angling (npn-commercial) and General (commercial), have been documented by Ortiz et al and Brown et al. 1999, and Brown (2003, 2007); the present analyses incorporate these regulatory effects as well as the appropriate closures and limits from (Tables 1-4). All other restrictions imposed upon the data in recent analyses, such as target category or open/closed fishery status, are retained for the present analyses. The fishing areas used were the same as used for large tuna analyses for several years. For the SMSM and LGSM categories, the four (4) fishing areas were classified as off Virginia, Maryland-Delaware, New Jersey, and New York-Massachusetts. For the SMMD, LGMD, and LG analyses, the three (3) fishing areas were classified as off North Carolina-Delaware (NCDE), from New Jersey eastward to south of Cape Cod (STHN), and the Gulf of Maine (GOMA) (Turner and Brown 1998). REGION was defined as either SOUTH (from Delaware south) or NORTH (from New Jersey north). The Lo method (Lo et al. 1992) was used to develop standardized indices; with that method separate analyses are conducted of the positive catch rates and the proportions of the observed trips which were successful. Some recent analyses (Ortiz et al. 1999,Turner et al. 1999, Brown et al. 1999) used a delta-lognormal model approach; that approach used a delta distribution with an assumed binomial error distribution for the proportion of positive observations (trips), and assumed a lognormal error distribution for the catch rates on successful trips. The present analyses maintain the approach of Brown and Turner (2001) and Brown (2002, 2003, 2007), in which a delta-poisson model approach was used. This approach differs from the delta-lognormal approach in that a Poisson error distribution is assumed for the catches on successful trips. Parameterization of the model was accomplished using a Generalized Linear Model (GLM) structure: The proportion of successful (i.e. positive observations) trips per stratum was assumed to follow a binomial distribution where the estimated probability was a linearized function of fixed factors, such as year, month, area, region, fishing method, boat type, type of interview, and bag limits (expressed as combined vessel bag limit). The logit function linked the linear component and the assumed binomial distribution. Similarly, the estimated catch on positive trips was modeled as a function of similar fixed factors with an assumed Poisson distribution, the log function as a link, and an offset term based on the effort (natural log of the hours fished). A stepwise approach was used to quantify the relative importance of the main factors explaining the variance in catch rates. That is, first the Null model was run, in which no factors were entered in the model. These results reflect the distribution of the nominal data. Each potential factor was then tested one at a time. The results were then ranked from greatest to least reduction in deviance per degree of freedom when compared to the Null model. The factor which resulted in the greatest reduction in deviance per degree of freedom was then incorporated into the model, provided two conditions were met: 1) the effect of the factor was determined to be significant at at
4 least the 5% level based upon a χ 2 (Chi-Square) test, and 2) the deviance per degree of freedom was reduced by at least 1% from the less complex model. This process was repeated, adding factors one at a time at each step, until no factor met the criteria for incorporation into the final model. All potential effects were tested in this fashion, and then all possible two-way interaction effects were tested (including factors which were not retained in the model when tested by themselves). Factors were dropped from consideration during model-building if factors measuring the same parameters in greater detail were incorporated into the model. For instance, REGION would be dropped from consideration if AREA is incorporated into the model first, since multiple areas are grouped together to form regions. Once the set of fixed effects was specified, possible random effect year interaction s were evaluated. These random effects were tested for significance using the likelihood ratio taken as the difference of the 2*log likelihood estimator between the complete model (i.e. including the random variate) and the reduced model (i.e. dropping the random variate). The 2*log likelihood difference statistics follows a χ 2 distribution. Values greater than 3.84 (α=5, df=1) were considered significant. The final model then, included any significant fixed effects, two-way interactions and random effect (year*factor) interactions. The product of the standardized proportion positives and the standardized positive catch rates was used to calculate overall standardized catch rates. For comparative purposes, each relative index of abundance was obtained dividing the standardized catch rates by the mean value in each series. No additional data were available for the early period prior to 1993, so the SM ( ) and LG ( ) indices are reported here as derived in the previous analyses (Ortiz et al and Brown et al. 1999, respectively). For all other indices, new models were developed by Brown (2003), testing all potential factors, all potential two-way interactions, and all potential year interactions. For the LG index from , BOATTYPE was not considered as a potential factor as this variable was not recorded during the 1984 survey; including BOATTYPE in the model would prevent an index estimate for RESULTS AND DISCUSSION The final models for each index, including any random effect year interactions meeting the significance criteria, are shown in Table 5. The index values are shown in Table 6 and in Figure 1. Although two indices including the LG size class bluefin are presented, the LGMD-LG index may be more representative of the later time period. The probable mis-classifications of both catch and target by fishers since 1992 suggest that the LG index may be affected by at least some of the LGMED catch and effort after 1992, rather than strictly reflecting LG catch and effort. 4. LITERATURE CITED BROWN, C.A Updated standardized catch rates of bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States during Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 54(2): BROWN, C.A Updated standardized catch rates of bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States during Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 55(3):
5 BROWN, C.A Standardized catch rates of bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States during Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap.????. BROWN, C.A., AND S.C. TURNER Updated standardized catch rates of bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States during Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 52: BROWN, C.A., S.C. TURNER AND M. ORTIZ Standardized catch rates of large (> 195 cm) and large medium ( cm) bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States during Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 49(2): LO, N.C. L.D. JACKSON AND J.L. SQUIRE Indices of relative abundance form fish spotter data based on delta-lognormal models. Can. J. Fish. Aquat. Sci. 49: ORTIZ, M., S.C. TURNER AND C.A. BROWN Standardized catch rates of small bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States during Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 49(2): TURNER, S.C. AND C.A. BROWN Update of standardized catch rates for large and small bluefin tuna, Thunnus thynnus, in the Virginia - Massachusetts (U.S.) rod and reel fishery. Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 48(1): TURNER, S.C., C.A. BROWN AND M. ORTIZ Review of the available information on medium bluefin tuna, Thunnus thynnus, from the rod and reel/handline fishery off the northeast United States. Int. Comm. Conserv. Atl. Tunas, Col. Vol. Sci. Pap. 49(2):
6 Table 1. Daily trip limits for vessels fishing under Angling category regulations. Effective date area size category (cm SFL) Year from date through date area < < 145 < /1 12/1 all /1 12/1 all Table 2. Opening and closing dates for Angling category. Effective date area size category (cm SFL) Year from date through date area < < 145 < /1 12/31 all open /1 12/31 all open Table 3. General category limits per vessel per day. Effective date Size (cm SFL) Year date (m,d) > 177 > / /1 3 Table 4. Open dates for the General category fishery in all areas. Year June July August September October November 2006 open Open open open open open Open open open open open
7 TABLE 5. FINAL MODELS INDICES UPDATED USING ADDITIONAL DATA AND DEVELOPING NEW MODELS Size category size range (cm) year range MODEL (random effects in italics) small-small (SMSM) prop. pos.= year boattype pos CPUE= year boattype month area fishmeth year*area year*fishmeth large-small (LGSM) prop. pos.= fishmeth year boattype dockrecl pos CPUE= year month boattype fishmeth area vlim_lsm dockrecl region dockrecl*region year*month year*region year*boattype year*fishmeth large-medium (LGMD) and large (LG) > prop. pos.= year month dockrecl area dockrecl*area pos CPUE= year fishmeth lgmlgbag fishmeth*lgmlgbag dockrecl area dockrecl*area month dockrecl*month year*dockrecl
8 TABLE 6: BLUEFIN TUNA RELATIVE ABUNDANCE INDICES Size Category SM SMSM LGSM SMMD LG ( ) LG ( ) LGMD-LG <145 cm cm cm cm >195 cm >195 cm >177 cm year value c.v. value c.v. value c.v. value c.v. value c.v. value c.v. value c.v
9 RELATIVE INDEX FIGURE 1: Relative abundance indices for bluefin tuna with approximate 95% confidence intervals. 4.0 SMALL BLUEFIN TUNA <145 cm SMALL-SMALL BLUEFIN TUNA cm 5.0 LARGE-SMALL BLUEFIN TUNA cm 4.5 RELATIVE INDEX RELATIVE INDEX
10 RELATIVE INDEX RELATIVE INDEX FIGURE 1 (cont.): SMALL MEDIUM BLUEFIN TUNA cm 4.0 LARGE BLUEFIN TUNA >195 cm LARGE MEDIUM - LARGE BLUEFIN TUNA (> 177 cm) LARGE BLUEFIN TUNA >195 cm RELATIVE INDEX