STANDARDIZED AGE SPECIFIC CATCH RATES FOR ALBACORE, Thunnus alalunga, FROM THE SPANISH TROLL FISHERY IN THE NORTHEAST ATLANTIC,

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1 SCRS/00/114 STANDARDIZED AGE SPECIFIC CATCH RATES FOR ALBACORE, Thunnus alalunga, FROM THE SPANISH TROLL FISHERY IN THE NORTHEAST ATLANTIC, V. Ortiz de Zárate 1, J. Cramer 2 and M. Ortiz 2 SUMMARY Abundance indices by age group of albacore (Thunnus alalunga) caught by the Spanish troll fleet in the north east Atlantic were standardized using catch rate data from 4069 trips collected over the period Standarized CPUE s for age groups 1 to 4 were estimated through the General Linear Modeling approach by applying the log-normal error distribution model. RÉSUMÉ Les CPUE par groupe d âge du germon (Thunnus alalunga) capturé par la flottille de ligneurs espagnols de surface sont standardisées à partir de sorties répertoriées entre 1981 et Les indices d abondance relative pour les groupes d âge 1 à 4 ont été estimés au moyen de l approche du Modèle linéaire généralisé postulant une distribution log-normal d erreurs dans les modèles. RESUMEN Las CPUE por grupo de edad de atún blanco(thunnus alalunga) capturado por la flota de superficie española de cacea se han normalizado a partir de 4069 mareas recogidas para el período Los índices de abundancia relativa para los grupos 1 a 4 años se estimaron por medio del procedimiento de Modelo Lineal Generalizado asumiendo una distribución log-normal de errores en los modelos. INTRODUCTION Data collected of nominal catch rates by age of individual trip from spanish surface commercial fleet have been used to develop relative abundance indices by age of young albacore from the North Atlantic stock (Mejuto and Garcia, 1997; Ortiz de Zárate and Cramer, 1999, Ortiz de Zárate et al. 2000). In the absence of other independent sources of information, the estimated catch rates derived by means of the General Linear Modeling approach was used as abundance indices to tune the VPA analysis of the north Atlantic stock. The SCRS has used standardized catch rates from the troll fishery as an index of abundance for juvenile albacore ages 2 and 3 of north Atlantic stock (Anonymous, 1997; Anonymous, 1999) 1 Instituto Español de Oceanografía Apdo. 240, Santander 39080, España 2 2 NMFS, Southeast Fisheries Science Center 75 Virginia Beach Drive, Miami, Florida 33149, U.S.A

2 The purpose of this paper is to update the relative catch rates indices by age for albacore surface troll fishery from 1981 to Standarized age-specific catch rates (ages 1 through 4) were estimated by means of the GLM approach assuming the log-normal error distribution model in the analysis. MATERIAL Information on trips from commercial trollers was recorded at landing ports through interviews of skippers. The following data was obtained for each trip: date of landing, number of fishing days, area of effort (5x5 degrees), catch in number, catch in weigth (kg), as well stratified length sampling to the nearest centimeter (FL= cm range ) was carried out for catch commercial categories landed by single trips recorded. Age length keys (Santiago, 1999) obtained from length distribution analysis of north atlantic albacore stock catch by size for the period done by means of Multifan length frequency analysis were used used to size the catch at age composition by trip for albacore age groups : 1, 2, 3 and 4. Age length key for 1997 was carried over 1999 catch at size distribution. METHOD The seasonal migration of inmature albacore to the northeast Atlantic waters and Bay of Biscay during summer months determines the performance of the fleets according to the species annual behaviour. In 1999 the distribution of catches for the troll fleet has remained in the traditional grounds (figure 1). Therefore the stratification of fishing areas used in the GLM analyses is the same as in previous analyses. Figure 2 shows the spatial divisions established for the model effect area specifications. The temporal strata month agreggation was done in order to have enough observations. As in previous analysis no changes were detected in the last two years when the following strata were chosen: May, June and July (T1), August and September (T3) and October and November (T2). The present analyses used the following factors: Year, Zone and Quarter to model age-specific cpue from 1 to 4 age groups for troll fleet. The final model set was: LOG (cpue) = µ+ Yi + Zk + Ql + ε ikl where µ= overall mean Y = effect year: Z = effect zone: NW,NE,SW,SE in figure 2. Q = effect time, three artificially created quarters: T1, T2, T3. ε ikl = log-normal error distribution The fitting of the final model was conducted by GLM procedure of S-PLUS statistical software (Ver.4.5, release 2). RESULTS AND DISCUSSION Total number of data observations included in the analyses was 4,069 trips. Table 1 shows the distribution of observations by the classed predictor variables for the troll fleet. First years in the time series show very few number of observations for troll fleet specially for quarter T2 which corresponds to autumn months.

3 Examination of zero observations by age group is represented in table 2. As observed the percentage of null observations is less than 10 % for ages 1, 2 and 3 and over for age group 4 in the data set. Then log (cpue) was used to predict standardized least squares means by age only on the positive observations. Results of the analyses of variance for the four models fitted from 1 to 4 age group cpue s are included in table 3. All the effects explaining the dependent variable show significance tested by the analysis of variance as seen in the F-values obtained. The variance explained by the different models varies between 22.3% and 44.4% for the target groups 2 and 3 years old. The model which explains the most variability is the one fitted for age 2. The relative indices estimates by age, their standard error and 95 % confidence intervals are presented in table 4a for ages 1 and 2 and in table 4b for ages 3 and 4. Figures 3 and 4 show the plots of standardized residuals, predicted log cpue s and qq plots for the catch rates by age (1 to 4) estimated by the logcpue model with normal error distribution. Trends of relative indices of abundance obtained for ages 1, 2, 3, and 4 for the troll fleet are shown in figure 5. For the last year 1999, the age 2 group continues some decline that begun in 1996, as for age 3 after the decline in 1996 a slight upward trend is seen for this age group being very similar in the last 3 years. Age 1 is not a group size targeted by this fleet, however it could be interpreted as an indication of year recruitment strength.. Ages 2 and 3 are targetted by both fleets therefore could be more accurate indicators of relative abundance indices. Age 4 is very rare in troll catches and more present in baitboat catch, mainly in the autumn months in the Bay of Biscay fishing ground, although fluctuates on annually bases. There are some new predictor variables that could further be investigated such an environmental factors for which albacore seems to be specially sensitive and fleet structure that could be tested if explanatory variables are defined from available information to model troll cpue trends. REFERENCES ANONYMOUS, SCRS Detailed Report on Albacore. ICCAT, Col. Doc. Sci. vol XLVI(3). ANONYMOUS, SRCS Detailed Report on Albacore. ICCAT, Col. Doc. Scient. vol XLX (1). MEJUTO, J., B. GARCIA, Standardized age specific catch rates for albacore (Thunnus alalunga) from the spanish surface fleets in the northeast atlantic, years ICCAT Coll. Vol. Sci. Pap. Vol. XLVI (3) pp: ORTIZ DE ZARATE, V. and J. CRAMER Standardized age specific catch rates for albacore (Thunnus alalunga) from the spanish surface fishery in the northeast Atlantic, ICCAT Coll. Vol. Sci. Pap. Vol. XLIX (4) pp: ORTIZ DE ZARATE, V., M. ORTIZ and J. CRAMER, Updated standardized age specific catch rates for albacore (Thunnus alalunga) from the spanish surface in the northeast Atlantic, ICCAT, Coll. Vol. Sci. Pap. Vol. LI, SCRS/99/115. SANTIAGO, J North Atlantic albacore catch- at- age estimates for the period ICCAT Coll. Vol. Sci. Pap. Vol. XLIX (4) pp: S-PLUS Professional edition for Windows. Copyright MathSoft, Inc.

4 Table 1. Observations (Nº trips) by year, area and quarter for TROLL fleet, TROLL ZONE YEAR QUARTER Total general Total Total Total Total Total Total Total Total Total Total Total Total Total Total Total Total Total Total Total Total general

5 Table 2. Proportions of null observations by age from spanish TROLL fleet from 1981 to TRO LL T o ta l T r ip s % n u ll observation s A g e A g e A g e A g e Table 3. Analysis of variance ANOVA for the 4 CPUE s models estimates by age group. Troll fleet Summary of fits GEAR AGE R 2 MSE F-STAT CV TR 1 O TR TR TR Type III Sum of Squares for Model TR AGE1 Df Sum of Sq Mean Sq F Value Pr(F) YEAR ZONE QUARTER Residuals Type III Sum of Squares for Model TR AGE 2 Df Sum of Sq Mean Sq F Value Pr(F) YEAR ZONE QUARTER Residuals Type III Sum of Squares for Model TR AGE 3 Df Sum of Sq Mean Sq F Value Pr(F) YEAR ZONE QUARTER e-014 Residuals Type III Sum of Squares for Model TR AGE 4 Df Sum of Sq Mean Sq F Value Pr(F) YEAR ZONE QUARTER Residuals

6 Table 4.a. Estimated parameters, relative CPUE S and 95% confidence intervals for albacore age groups 1 and 2 for Spanish TROLL fleet, Least Squares Means for LOG(CPUE1) TR fleet YEAR lsmean 1 st.error LCI CPUE1 UCI Least Squares Means for LOG(CPUE2) TR fleet YEAR lsmean 2 st.error LCI CPUE2 UCI

7 Table 4.b. Estimated parameters, relative CPUE S and 95% confidence intervals for albacore age groups 3 and 4 for Spanish TROLL fleet, Least Squares Means for LOG(CPUE3) TR fleet YEAR lsmean 3 st.error LCI CPUE3 UCI Least Squares Means for LOG(CPUE4) TR fleet YEAR lsmean 4 st.error LCI CPUE3 UCI

8 55º 50º 45º 40º 35º 30º 30º 25º 20º 15º 10º 5º 0º 5º 10º Figure 1. Locations of albacore operations for Troll fishery in º 50º 45º NW SW NE SE 40º 35º 30º 30º 25º 20º 15º 10º 5º 0º 5º 10º Figure 2. Area divisions for East Atlantic albacore troll fishery used in the analysis.

9 Deviance Residuals Deviance Residuals LGCPUE1 6 6 Pearson Residuals -4 LGCPUE2 6 Pearson Residuals Quantiles of Standard Normal Quantiles of Standard Normal A B Figure 3. A) Plots of residuals, the predicted logcpue age 1 and QQ plot for the predicted cpue age 1. Troll B) Plots of residuals, the predicted logcpue age 2 and QQ plot for the predicted cpue age 2. Troll

10 Deviance Residuals Deviance Residuals LGCPUE3 Pearson Residuals LGCPUE4 Pearson Residuals Quantiles of Standard Normal Quantiles of Standard Normal A B Figure 4. A) Plots of residuals, the predicted logcpue age 3 and QQ plot for the predicted cpue age 3. Troll B) Plots of residuals, the predicted logcpue age 4 and QQ plot for the predicted cpue age 4. Troll

11 ALB CPUE AGE 1 TROLL ALB CPUE AGE 3 TROLL Nº fish/fishing day Nº fish/fishing day YEARS A GE 1 CPUE 95% UCI 95% LCI YEARS AGE 3 CPUE 95% UCI 95% LCI ALB CPUE AGE 2 TROLL ALB CPUE AGE 4 TROLL 100 Nº fish/fishing day YEARS Nº fish/fishing day YEARS A GE 2 CPUE 95% UCI 95% LCI AGE 4 CPUE 95% UCI 95% LCI Figure 5. Albacore standardized catch rates indices by age in number fish/day for TROLL fleet in the North Atlantic, years