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Blackfin Tuna (Thunnus Atlanticus) in the Venezuelan Fisheries

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Blackfin Tuna (Thunnus Atlanticus) in the Venezuelan Fisheries SCRS/2011/122 Collect. Vol. Sci. Pap. ICCAT, 68(3): 1253-1260 (2012) BLACKFIN TUNA (THUNNUS ATLANTICUS) IN THE VENEZUELAN FISHERIES F. Arocha1, A. Barrios1, J. Marcano2, and X. Gutierrez2 SUMMARY Catches from 1988 to 2009 for blackfin tuna (Thunnus atlanticus) from the Venezuelan industrial surface fleets and the small scale fisheries were reviewed, and catch and effort seasonality by fishery was analyzed. Standardized catch rates were estimated for the industrial surface fleets using Generalized Linear Model techniques. Size distribution from the industrial surface fleets by year and month were analyzed, and cumulative seasonal spatial distribution was discussed. RÉSUMÉ Les captures de thons à nageoires noires (Thunnus atlanticus) réalisées entre 1988 et 2009 par les flottilles industrielles de surface et les pêcheries de petits métiers du Venezuela ont été examinées, et la saisonnalité de la prise et de l'effort a été analysée par pêcherie. Des taux de capture standardisés ont été estimés pour les flottilles de surface industrielles à l'aide de techniques de modèle linéaire généralisé. La distribution des tailles des flottilles de surface industrielles par année et mois a été analysée, et la distribution spatiale saisonnière cumulative a été discutée. RESUMEN Se revisaron las capturas desde 1988 a 2009 para el atún aleta negra (Thunnus atlanticus) de las flotas de superficie industriales y de las pesquerías de pequeña escala de Venezuela, y se analizó la estacionalidad de la captura y del esfuerzo por pesquería. Se estimaron las tasas de captura estandarizadas para las flotas de superficie industriales utilizando técnicas del modelo lineal generalizado. Se analizaron las distribuciones por tallas de las flotas de superficie industriales por año y mes, y se debatió la distribución espacial estacional acumulativa. KEYWORDS Blackfin tuna, Thunnus atlanticus, catch rates, size structure, Venezuela 1. Introduction Blackfin tuna (Thunnus atlanticus) is commonly caught by industrial and artisanal Venezuelan fleets throughout the Caribbean Sea and adjacent waters. The Venezuelan purse seine fleet that operates in the Caribbean Sea consists of 10 vessels, mostly with a capacity of 600 t; while the baitboat fleet consists of about 8 vessels with a capacity that ranges between 50 and 250 t. This fleet operates most of the time in conjunction with the purse seine fleet. The artisanal fleets that fish for blackfin tuna are: the offshore small scale fleet that uses pelagic longline gear, and the coastal artisanal drift-gillnet fishery. In the artisanal fisheries, the target species are billfish species and dolphinfish, and catches of blackfin tuna are reported as ‘albacora’ by the fishers and has been confused with T. alalunga in the national catch statistics. In a recent sampling survey taken place in the Community of Playa Verde where an important artisanal drift-gill net fishery occurs (Arocha et al., 2009), it was confirmed that ‘albacora’ is actually blackfin tuna (T. atlanticus). 1 Instituto Oceanográfico de Venezuela, Universidad de Oriente, Cumaná 6101-Venezuela; email: [email protected] / [email protected] 2 INSOPESCA-Sucre, Cumaná 6101-Venezuela. 1253 The Venezuelan reported catch from the surface fishery to ICCAT was mostly of yellowfin tuna (YFT) and skipjack (SKJ), and minor quantities of bigeye tuna (BET) and albacore (ALB). However, in the early stages of the fishery (1980s) other tunas species, like frigate tuna (FRI) and blackfin tuna (BLF), have been consistently caught by the surface fisheries but have not been declared in the log-book data. In the late 1980s, a multispecific port sampling program was developed to estimate the species mix in the yellowfin tuna and skipjack reported landings from these fleets, it was found that blackfin tuna and frigate tuna were declared as yellowfin tuna or as skipjack (Pagavino et al., 1991). Further analysis indicated that the amount of blackfin tuna in the landings made up to ~5% of the tuna catch in the baitboat fleet and up to ~9% in the purse seine fleet (Gaertner et al., 1992). Since the early 1990s, blackfin tuna has been declared to ICCAT and reported in the logbooks from the surface fleets. Occasional reports have been made from the longline fishery, but are not clear if the data is accurate. Venezuelan artisanal fisheries have also reported blackfin tuna catches as ‘albacora’, not be confused with T. alalunga. The present report documents the available information on blackfin tuna (T. atlanticus) catches from all monitored fisheries in Venezuela, the catch rate trend, and the size structure of blackfin tuna from Venezuelan fisheries. 2. Methods Catch and effort statistics of blackfin tuna from Task I and Task II from the ICCAT database were compared with updated records from the Venezuelan national fisheries agencies (INIA and INSOPESCA). Catch and effort statistics from the artisanal fleets (drift gill-net and small scale longline), were obtained from research programs, the ICCAT Enhanced Program for Billfish that monitors the artisanal drift-gillnet fishery off La Guaira (Arocha et al., 2009) and INIA/INSOPESCA monitoring programs for the offshore small scale longline fishery (Arocha et al., 2006), reported as TASK II in the ICCAT data base. Standardized relative indices of abundance (CPUE) for blackfin tuna were estimated for the surface bait-boat fleet by Generalized Linear Modeling approach assuming a lognormal model distribution. CPUE indices for these fleets were prepared using TASK II catch and effort from the ICCAT database. Monthly landings (in weight) and effort (effective days of fishing) were aggregated for each fleet (PS and BB), and a separate standardized index was estimated for the bait-boat fleet only. For the standardization the only factor available other than year was season, defined here, for the purpose of the present analysis, as trimesters (Jan-Mar, Apr- Jun, Jul-Sep, Oct-Dec). Catch rates were transformed to log (cpue+10%mean) prior to the analysis. For sets that caught at least one blackfin tuna, estimated CPUE rates were assumed to follow a lognormal error distribution (lnCPUE) of a linear function of fixed factors. No standardization process was pursued in the purse seine fleet, only nominal CPUE are provided. Blackfin tuna size structure was obtained from the surface fisheries (BB, PS) based in Cumaná. Size data was recorded by port samplers during landings from these fleets. A total of 8020 blackfin tuna were measured between 1993 and 2009 (up to 2010 in PS). Observations of fish length (FL, measured with metric-tape) were analyzed by year and length, and by month (all years combined) and length. Lengths were sorted into 1-cm bins for the construction of box-plots; annual and monthly (all years combined) mean lengths, median, and range intervals were estimated. The seasonal (by quarter) spatial distribution of the size structure was analyzed for non- mature and mature fish based on an L50~50 cm FL (http://www.fishbase.org). 3. Results and discussion The total historical catch information available for blackfin tuna was revised because of the increased interest in the eastern Caribbean region regarding the fishery the species and because of the misidentification occurring in the artisanal fisheries. Upon the revision it was concluded by the Venezuelan tuna working group that the most reliable data on blackfin tuna catches were those available from TASK II information in the ICCAT database, as it corresponded to accurate port sampling and monitoring of >80% of the logbook data controlled by trained officials. Catch information from the artisanal fleets was included to account for the total catch of blackfin tuna from Venezuela, after confirmation that reported landings of ‘albacora’ (misidentified as T. alalunga) by the drift-gillnet fishery off La Guaira were in fact T. atlanticus (Figure 1). The confirmation of ‘albacora’ as T. atlanticus was possible with the assistance of a trained graduate student of IOV-UDO and ICCAT’s ERPB in Venezuela using ICCAT’s ID sheets for small tunas and ID sheets used by French scientists in Gualdalupe & Martinque (Lionel Reynal, IRD, personal comm.). Once confirmed with port sampling during 2011 and with 1254 local and traditional fishers, all ‘albacora’ (reported as T. alalunga) from the fishery was re-assigned as T. atlanticus (BLF). The highest catches were observed between 1998 and 2002, when in 2001 reached its peak of over 1700 t (Figure 2), since then the catch has remained at around 300 t, with the exception of 2004 and 2007 when catches increased to about 700 t. Although, the main blackfin tuna catch come from the Venezuelan industrial surface fleets, in the last year of the series the artisanal drift-gillnet fleet account for a substantial increase with respect to previous years. Blackfin tuna caught by the Venezuelan fishery showed a strong seasonal positive signal towards the end of the year in two of the fleets (purse seine and offshore small scale longline). While the artisanal coastal drift-gillnet showed a seasonal increase towards the beginning of the year (Figure 3), possibly indicating an offshore– inshore movement between the end and the beginning of the year. Standardization of catch rates were attempted for the industrial surface fleets using general linear model (GLM) techniques. However, only relative indices of abundance of the baitboat (BB) fleet were adequately standardized as indicated by the diagnostic plots (Figure 4). The model selected included year and season as factors, and the interaction year x season (Table 2). The standardized relative indices of abundance of blackfin tuna from the baitboat fishery show an uneven sustained declining trend beginning in 1997 (Figure 5 a), showing a minor recovery at the end of the time series. The nominal blackfin tuna catch rates from the purse seine (PS) fleet appear to be around or below 250 t/effective fishing days (EFF) during most of the time period, with three noticeable peaks in 1990, 1992-93, and 2001-02, of 1400 t/EFF, ~1000 t/EFF, and ~900 t/EFF, respectively (Figure 5 b).
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