SCRS/2010/071 Collect. Vol. Sci. Pap. ICCAT, 66(3): 1204-1215 (2011)

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INITIAL INVESTIGATIONS OF ENVIRONMENTAL INFLUENCES

ON ATLANTIC BLUEFIN TUNA CATCH RATES IN THE SOUTHERN GULF OF ST. LAWRENCE

A.S.M. Vanderlaan1, B.A. Block2, J. Chassé3

M.E. Lutcavage, A. Hanke1,

4, S.G. Wilson2

and J.D. Neilson1

SUMMARY

The standardised catch per unit effort (CPUE) for Atlantic bluefin tuna in the southern Gulf of St. Lawrence (sGSL) reveals an increasing trend in recent years. Although standardised for factors known to influence catch rates, the influence of environmental variables on CPUE has yet to be fully explored. The main objective of this study is to examine the role that environmental factors, with a focus on the cold intermediate layer (CIL), may have on the distribution of the tuna and their associated CPUE. Data from pop-up satellite archival tags for bluefin tuna tagged in 2007 and 2008 in the sGSL reveal differences in time-at-temperature and time-at-depth between September and October. Data from annual research surveys in September were used to examine water mass characteristics and revealed considerable spatial and temporal variation. This study is in its preliminary stages and future work will include the estimation of annual habitat availability (volume), based on the 3 oC isotherm that defines the upper bound of CIL, to determine if CPUE is some function of the habitat volume that is available to tuna.

RÉSUMÉ

La capture par unité d’effort (CPUE) standardisée du thon rouge de l’Atlantique dans le Sud du Golfe du St Laurent dégage ces dernières années une tendance à la hausse. Bien qu’elle soit standardisée pour des facteurs connus pour influencer les taux de capture, l’influence des variables environnementales sur la CPUE doit encore être complètement explorée. L’objectif principal de la présente étude vise à examiner le rôle que des facteurs environnementaux, l’accent étant mis sur la couche intermédiaire froide (CIL), pourraient avoir sur la distribution des thonidés et leur CPUE associée. Les données de marques-archives pop-up reliées par satellite apposées sur des thons rouges en 2007 et 2008 au Sud du Golfe du St Laurent révèlent des différences entre le temps de maintien en température et le temps passé en profondeur entre septembre et octobre. Les données obtenues des campagnes de recherche annuelles menées au mois de septembre ont servi à examiner les caractéristiques de la masse d’eau et ont révélé une variation spatio-temporelle considérable. Cette étude est à son stade préliminaire et les futurs travaux porteront sur l’estimation de la disponibilité annuelle (volume) de l’habitat, basée sur l’isotherme de 3ºC qui définit la limite supérieure de la couche intermédiaire froide, afin de déterminer si la CPUE est une fonction du volume de l’habitat qui est disponible pour les thonidés.

RESUMEN

La captura por unidad de esfuerzo (CPUE) estandarizada del atún rojo del Atlántico en el Golfo de San Lorenzo meridional (sGSL) presenta una tendencia ascendente en años recientes. Aunque se ha estandarizado mediante factores que se sabe que influyen en las tasas de captura, la influencia de las variables medioambientales no ha sido aún plenamente explorada. El principal objetivo de este estudio es examinar el papel que pueden desempeñar los factores

1 Fisheries and Oceans Canada, St. Andrews Biological Station, 531 Brandy Cove Road, St. Andrews, New Brunswick, Canada. Email address of lead author: Angelia.Vanderlaan@dfo-mpo.gc.ca 2 Tuna Research and Conservation Center, Stanford University, Hopkins Marine Station, 120 Oceanview Boulevard, Pacific Grove, California, United States of America 3 Fisheries and Oceans Canada, Gulf Fisheries Center, 343 University Avenue, Moncton, New Brunswick, Canada. 4 Large Pelagics Research Center, University of Massachusetts Amherst, 108 East Main Street, Gloucester, Massachusetts, United States of America.

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medioambientales en la distribución de los túnidos y sus CPUE asociadas, prestando especial atención a la capa media fría (CIL). Los datos de las marcas archivo pop-up por satélite del atún rojo marcado en 2007 y 2008 en el sGSL presentan diferencias entre septiembre y octubre en el tiempo que pasan a distintas temperaturas y el tiempo que pasan a distintas profundidades. Los datos de las campañas anuales de investigación en septiembre se utilizaron para examinar características de la masa de agua y revelaron una variación espacial y temporal considerable. Este estudio se encuentra en sus etapas preliminares y el trabajo futuro incluirá la estimación de la disponibilidad anual del hábitat (volumen) basado en la isoterma de 3 oC que define el límite superior de la CIL para determinar si la CPUE es alguna función del volumen del hábitat que está disponible para los túnidos.

KEYWORDS

Tuna fisheries catch rates, environmental factors, seasonal variation

1. Introduction Atlantic bluefin tuna (Thunnus thynnus; hereafter tuna) are a highly migratory species with an extensive range throughout the North Atlantic and inhabit coastal and pelagic waters (Block et al. 2005, Wilson and Block 2009). The tuna tolerate a large temperature range (e.g. Teo et al. 2007a, Walli et al. 2009) and are endothermic (Carey and Teal, 1969). Many tagging studies have focused on foraging areas off North Carolina and in the Gulf of Maine as well as in spawning areas in the Gulf of Mexico (e.g. Stokesbury et al. 2004, Teo et al. 2007, Schick and Lutcavage 2009). Lawson et al. (2010) provided a detailed analysis of diving behaviour in relation to oceanographic variables in the Gulf of Maine and on the Scotian Shelf and found that diving and distribution patterns were influenced by water column structure. From a tagging perspective, less is known about the tuna that inhabit the southern Gulf of St. Lawrence; however, recent tagging studies provide an opportunity to examine the behaviour of bluefin tuna in this region (Block et al. 2009, Wilson et al. 2011). It is in this area that the standardised catch per unit effort (CPUE) reveals an increasing trend that is in contrast to a low but stable CPUE trend in southwest Nova Scotia and a decreasing CPUE in the U.S.A. fishery (Neilson et al. 2007 and 2009). Ravier and Fromentin (2004) found a negative relation between tuna trap- catch rates and temperature and although the western CPUE series are standardised for factors known to influence the catch rates (e.g. month, area, gear, type, fleet), the influence role of environmental variables has not yet to been fully explored. The influence of environmental variables on CPUE may be particularly important in Canadian waters where the species is near the northern extent of its distribution. Of particular interest in the Gulf of the St. Lawrence is the regular occurrence of the cold intermediate layer (CIL). The CIL is one of the most striking features of the water mass characteristic in the Gulf of St. Lawrence with temperatures of as low as -1oC typically located 60 m below the surface in mid-August (Gilbert and Pettigrew 1997). It is reasonable to hypothesise that the CIL may limit the vertical distribution of tuna in the southern Gulf of St. Lawrence given that globally there few observations of tuna occupying water masses for extended periods where temperatures are <3oC (Walli et al. 2009, Lawson et al. 2010). Consequently, it becomes axiomatic that the CIL may also limit the extent of foraging habitat available to tuna in the southern Gulf of St. Lawrence. These hypotheses are explored here using temperature data from tagged tuna and from annual research surveys in the southern Gulf of St. Lawrence. The results presented here are intended as a progress report concerning this ongoing investigation. 2. Methods Atlantic bluefin tuna were tagged in the southern Gulf of St. Lawrence as part of two research programmes undertaken by the Tag-A-Giant (TAG) programme (Block et al. 2001, 2005, 2009, Wilson et al. 2011) and the Large Pelagics Research Centre (LPRC) initiative at the University of New Hampshire (Galuardi et al. 2010). Tagging methodology and the geo-location estimates derived from 14 pop-up satellite archival tags are described in Block et al. (2009) and Wilson et al. (2011). We also use data from an additional five tuna-tags deployed in the southern Gulf of St. Lawrence where details are documented in Galuardi et al. (2010). All tags were deployed in 2007 and 2008 with the LPRC tagging tuna in August and September and the TAG programme

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tagging tuna in late October (Table 1; Figure 1). Due to the differences in sampling intervals set by the two research groups, the LPRC data were reprocessed to provide a sampling frequency to match that of the TAG program, and we focus on geo-location, time-at-temperature, time-at-depth, and temperature-at-depth. The average proportion of time at a temperature anomaly (˚C), relative to sea -surface temperature (SST), was calculated from the temperature data recorded by the tags. For each tuna, the daily temperature at 0 m was used to estimate the average SST experienced by each tuna (SSTi, i = 1, 2, …, 19). These averages were then averaged across all tags deployed in the same month and year to estimate the overall average SST for the region (SSTj; j = 1, 2, 3, 4) by month and year that is equally weighted among tuna. Similarly, the average time-at-temperature (recorded daily by the tags) was calculated for all tuna (TATi), and averaged again across all tags deployed in the same month and year (TATj). The time-at-temperature anomaly (TANj) was then estimated as the difference between the median of each time-at-temperature class and the median of the associated month and year SSTj . A mean of the proportion of time at temperature anomaly was then calculated across the 4 deployment periods (September and October of 2007 and 2008). Since 1971, Fisheries and Oceans Canada has conducted an annual research survey in the southern Gulf of St. Lawrence in September. These surveys provide data from conductivity, temperature and depth (CTD) profilers that were used here to examine inter-annual variations in water mass characteristics as well as in 2007 and 2008. The number of CTD casts varied annually and ranged from 83 to 213 casts among the data presented here (174 ± 34, annual mean ± SD). Temperature profiles were used to estimate the minimum temperature of the CIL and the depth at which the CIL occurs. As in Gilbert and Pettigrew (1997), we defined the CIL as the layer where temperature is < 3°C, although other definitions have been used (c.f. Brownan et al. 2000, Galbraith et al. 2010). This definition does not affect the minimum temperature estimate (Tc) of the CIL. The definition does affect the estimate of the amount of the water column available to tuna as it will determine the vertical extent of the CIL. However, as there are very few observations of tuna occupying waters at < 3°C, and given our goal of determining the spatial and inter-annual variability of the CIL in relation to the temperature ambit of tuna, the < 3°C definition is appropriate. When defining and determining the characteristics of the CIL, only those temperature profiles that contained at least some data between 30 and 120 m were used. The depth and extent of the CIL was defined by the depths of the upper (Zi) and lower (Zf) 3°C isotherms that bracketed the Tc estimate (Figure 2). Available habitat for the tuna was defined by the Zi at each CTD cast location and linear interpolation was used to map depths available to tuna across the entire southern Gulf of St. Lawrence region, assuming a > 3°C thermal ambit. 3. Results and discussion Data from the southern Gulf of St. Lawrence (Table 1) shows that tagged tuna, averaging 394 kg (±52 SD) in size, are resident in the region over highly variable periods ranging from 1 to 18 days post-tagging and averaged 6.2 days (± 4.5 SD). There does not appear to be a difference between the post-tagging residence period in the Gulf for those tuna tagged in September (4.2 days ± 3.7 SD) and those tagged in late October (6.8 days ± 4.7 SD). This may be partially due to sample size differences. The absolute residence period cannot be estimated, as we do not know when the tagged tuna entered the southern Gulf of St. Lawrence. The proportion of time-at-depth in the southern Gulf of St. Lawrence was similar in 2007 and 2008 but there were differences in the proportion of time-at-depth among fish tagged in September relative to those tagged in late October within a given year. Tuna tagged in September spent the majority of time in the 5 to 10 m stratum and were not observed at depths below 50 m (Figure 3). This contrasts with those tuna tagged in late October that spent, on average, more time in the 50 to 100 m stratum (Figure 3). These monthly differences in time-at depth might be attributable to differences in tagging locations, as tags deployed in August and September were released NW of Prince Edward Island and tags deployed in October were released SW of Cape Breton Island (Figure 1), although all tuna were tagged in waters <50 m deep. Due to the errors associated with estimating geo-locations subsequent to tagging (Teo et al 2007, Silber et al. 2006) it is difficult to determine a bottom depth for the estimated geo-locations to test this hypothesis. As with the time-at-depth estimates, there was little difference in the time-at-temperature in 2007 and 2008, however there were large differences between months within a year. On average, the majority of the tagged tuna spent extended periods in waters characterised by a 16 to 18°C temperature range in September of 2008. In October, the majority of the tuna had an ambit in waters characterised by a 10 to 12°C range. The above differences in time-at-temperature and time-at-depth may be due to differences in water mass characteristics in the southern Gulf of St. Lawrence. For example the depth of the thermocline was generally deeper in October,

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especially in 2007 and 2008 (Galbraith et al. 2010) and may allow tuna access to more habitat as the presumed thermal barrier (<3oC) is at greater depth due to mixing in the surface layer. Temperature data derived from tags has been used to define preferred habitat and thermal preferences for tuna (e.g. Block et al. 1998, Walli et al. 2009). However, the time-at-temperature profiles for tunas tagged in the southern Gulf of St. Lawrence may simply reflect habitat availability or habitat use rather than habitat ‘preference’ wherein the later may be artifact (i.e. a product of human conception as opposed to an inherent element; Neuheimer & Taggart 2010). The variations in the average time-at-temperature among years and months may simply be due to environmental conditions of the southern Gulf of St Lawrence. When examining the average proportion of time spent at a temperature anomaly (average of TANj), and regardless of the year or month a tuna was tagged, the majority of time was spent within ±3°C of the SST (Figure 4). There is considerable spatial and temporal variation in water mass characteristics in the southern Gulf of St. Lawrence (Figures 5 and 6). Spatially there was a >11°C difference in SST across the southern Gulf of St. Lawrence in 2007 and in 2008 the average SST was 2.5°C warmer than the average SST in 2007 (16.1 ± 2.3°C and 13.6 ± 2.3°C, respectively). The average depth to the CIL, a measure of available habitat for tuna, in 2007 and 2008, were similar, though again there was considerable spatial variability (Figures 5 and 6). The average minimum temperature of the CIL (Tc) was colder in 2008 (0.018 ± 0.84oC) relative to 2007 (0.68 ± 0.64 oC) and the median temperature of the CIL in 2008 was <0oC. The longer time series of the average temperature of the CIL (Tc) ranged from -0.017 to 0.93oC and the average depth ranged from 29 to 39 m (Figure 7). When the depth to the CIL is shallow, the vertical ambit of tuna may be equally shallow (upper ~30 meters of the water column in September). To date there appears to be no indication of any major change in the average temperature of the CIL and the depth to the CIL (Figure 7) that could be positively contributing to the increase in bluefin catch rates. Water mass characteristics in the southern Gulf of St. Lawrence are not the only environmental factors that vary temporally. There has been a shift in the fish-species assemblage of southern Gulf of St. Lawrence (Benoît and Swain 2008) that is associated with the decrease in the average CIL temperature. There may be several factors contributing to the observed increase in tuna CPUE and this analysis has only begun to explore them. 4. Future work This analysis is in its preliminary stages, and there are many environmental factors to be examined. Additional tagging data will be used to investigate the characteristics of the habitat the tuna are using in the southern Gulf of St. Lawrence and a large-scale tagging program is planned for this year. The volume of available habitat in the southern Gulf of St. Lawrence will be calculated annually, based on the 3oC isotherm, to determine if there is a relation between the volume of habitat available for tuna and CPUE. The distribution and availability of prey, e.g. herring and mackerel, will also be examined. Due to the spatial variation of water mass characteristics across the southern Gulf of St. Lawrence, the analysis of water mass characteristics may be spatially restricted to the area that defines the majority of the catch. We propose examine the coherency, or lack thereof, among the minimum temperature of the CIL, the amount of available habitat, and the standardised catch rates. Those factors that are coherent with CPUE can then be incorporated into CPUE standardisation techniques to determine their contribution to explained variation in catch rates. 5. Acknowledgements We thank Captain D. Cameron, First Mate S. Gillis, Captain B. Chisholm, S. MacInnis, P. Sutherland, B. Keus, M. Jenkins, K. Bruce, E. Murray Jr., E. Clark, and the crews of all participating vessels We also thank Benjamin Galuardi, Mike Castleton, Stacey Paul, Sean Smith, and Tobias Spears for assistance with data. This work was funded by Fisheries and Oceans Canada, National Oceanic and Atmospheric, the Tag-A-Giant Foundation and the Monterey Bay Aquarium Foundation.

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6. Literature Cited Benoît, H.P. and Swain, D.P. 2008, Impacts of environmental change and direct and indirect harvesting effects

on the dynamics of a marine fish community. Can. J. Fish. Aquat. Sci. 65(10): 2088-2104.

Block, B.A, Dewar, H., Farwell, C., and Prince, E.D. 1998, A new satellite technology for tracking the movements of Atlantic bluefin tuna. Proc Natl Acad Sci USA 95(16): 9381-9389.

Block, B.A., Dewar, H., Blackwell, S., Williams, T., Prince, E., Farwell, C., et al. 2001, Migratory movements, depth preferences, and thermal biology of Atlantic bluefin tuna. Science 293(5533): 1310.

Block, B.A., Lawson, G.L., Boustany, A.M., Stokesbury, M.J.W., Castleton, M., Spares, A., Neilson, J.D. and Campana, S.E. 2009, Preliminary results from electronic tagging of bluefin tuna (Thunnus thynnus) in the Gulf of St. Lawrence, Canada. Collect. Vol. Sci. Pap. ICCAT, 64(2): 469-479.

Block, B.A., Teo, S., Walli, A., Boustany, A., Stokesbury, M., Farwell, C., et al. 2005, Electronic tagging and population structure of Atlantic bluefin tuna. Nature 434(7037): 1121-1127.

Brownan, H.I., Rodriguez, C.A., Béland, F., Cullen, J.J., Davis, R.F., Kouwenberg, J.H.M., Kuhn, P.S., McArther, B., Runge, J.A., St-Pierre, J. and Vetter, R.D. 2000, Impact of ultraviolet radiation on marine crustacean zooplankton and ichthyoplankton: a synthesis of results from the estuary and Gulf of St. Lawrence, Canada. Mar Ecol Prog Ser 199: 293-311.

Carey, F.G. and Teal, J.M. 1969, Regulation of body temperature by bluefin tuna. Comp Biochem Physiol 28(1): 205-213.

Galbraith, P.S., Pettipas, R.G., Chassé, J., Gilbert, D., Larouche, P., Pettigrew, B., Gosselin, A., Devine, L. and Lafleur, C. 2010, Physical Oceanographic Conditions in the Gulf of St. Lawrence in 2009. DFO Can. Sci. Advis. Sec. Res. Doc. 2010/035. iv + 73 p.

Galuardi, B., Royer, F., Golet, W. and Logan, J. 2010, Complex migration routes of Atlantic bluefin tuna (Thunnus thynnus) question current population structure paradigm. Can J Fish Aquat Sci 68(6): 966-976.

Gilbert, D., and Pettigrew, B. 1997, Interannual variability (1948–1994) of the CIL core temperature in the Gulf of St. Lawrence. Can J Fish Aquat Sci 54(Suppl 1): 57-67.

Lawson, G.L., Castleton, M.R. and Block, B.A. 2010, Movements and diving behavior of Atlantic bluefin tuna Thunnus thynnus in relation to water column structure in the northwestern Atlantic. Mar Ecol Prog Ser 400: 245-265.

Neilson, J.D., Smith, S., Ortiz, M. and Lester, B. 2009, Indices of stock status obtained from the Canadian bluefin tuna fishery. Vol. Sci. Pap. ICCAT 64(2): 380-404.

Neilson, J.D., Paul, S.D. and Ortiz, M. 2007, Indices of stock status obtained from the Canadian bluefin tuna fishery. Collect. Vol. Sci. Pap. ICCAT 60(3): 976-1000.

Neuheimer, A.B., and Taggart, C.T. 2010, Can changes in length-at-age and maturation timing in Scotian Shelf haddock (Melanogrammus aeglefinus) be explained by fishing? Can J Fish Aquat Sci 67(5): 854-865.

Ravier, C. and Fromentin, J-M. 2004, Are the long term fluctuations of Atlantic bluefin tuna (Thunnus thynnus) population related to environmental changes? Fisheries Oceanography 13(3): 145-160.

Silber, J.R. Lutcavage, M.E., Nielsen, A., Brill, R.W., and Wilson, S.G. 2006. Interannual variation in large scale movements of Atlantic bluefin tuna (Thunnus thynnus) determined from pop-up satellite archival tags. Can J Fish Aquat Sci 63(10): 2154-2166.

Schick, R.S. and Lutcavage, M.E. 2009. Inclusion of prey data improves prediction of bluefin tuna (Thunnus thynnus) distribution. Fisheries Oceanography 18(1): 77-81.

Stokesbury, W.J.W., Teo, S.L.H., Seitz, A., O’Dor, R.K. and Block, B.A. 2004, Movement of Atlantic bluefin tuna (Thunnus thynnus) as determined by satellite tagging experiments initiated off New England. Can J Fish Aquat Sci 61(10): 1976-1987.

Teo, S., Boustany, A., Dewar, H., Stokesbury, M., Weng, K., Beemer, S., et al. 2007, Annual migrations, diving behavior, and thermal biology of Atlantic bluefin tuna, Thunnus thynnus, on their Gulf of Mexico breeding grounds. Mar Biol 151(1): 1-18.

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Walli, A., Teo, S., Boustany, A., Farwell, C., Williams, T., Dewar, H., et al. 2009, Seasonal Movements, Aggregations and Diving Behavior of Atlantic Bluefin Tuna (Thunnus thynnus) Revealed with Archival Tags. PLoS ONE 4:1-18.

Wilson, S.G., and Block, B.A. 2009.,Habitat use in Atlantic bluefin tuna, Thunnus thynnus inferred from diving behavior. Endang Species Res 10, 355-367.

Wilson, S.G., Lawson, G.L., Stokesbury, M.J.W., Spares, A., Boustany, A.M., Neilson, J.D., and Block, B.A. 2011. Movements of Atlantic bluefin tuna from the Gulf of St. Lawrence, Canada to their spawning grounds, Collect. Vol. Sci. Pap. ICCAT, 66, this issue.

Table 1. Data summary for 19 Atlantic bluefin tuna tagged in the southern Gulf of St. Lawrence detailing individual fish identification (ID) and estimated mass, date (d/m/y) and location of tag and release, and post-release residence period in the Gulf region.

ID Mass (kg)

Date Location Post-release residence (day) ˚W Longitude ˚N Latitude

14216 320 16/09/2007 63.51 46.73 2 5107036 19/10/2007 61.52 46.17 1 5107037 475 19/10/2007 61.52 46.18 5 5107038 19/10/2007 61.50 46.20 12 5107042 360 24/10/2007 61.59 46.17 7 5107043 430 24/10/2007 61.59 46.17 7 5107046 440 25/10/2007 61.53 46.13 15 5107047 395 25/10/2007 61.51 46.16 4 5107048 340 26/10/2007 61.52 46.15 11

36217 340 03/09/2008 63.97 47.18 1 36215 340 04/09/2008 63.95 47.16 5 36216 410 04/09/2008 63.95 47.17 10

5108017 430 20/10/2008 61.54 46.20 18 79160 385 31/09/2008 63.95 47.17 4

5108020 500 25/10/2008 61.49 46.29 16 5108021 430 25/10/2008 61.47 46.27 4 5108022 385 25/10/2008 61.46 46.26 4 5108023 385 25/10/2008 61.40 46.29 14 5108024 340 25/10/2008 69.49 46.21 7

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Figure 1. Bathymetric (100 and 200 m isobath) chart of the southern Gulf of St. Lawrence illustrating locations where Atlantic bluefin tuna were tagged and released in August and September (triangles) and October (open circles).

Figure 2. Example of a CTD temperature profile in the southern Gulf of St. Lawrence in September 2008 illustrating the vertical position of the 0, 1, and 3oC isotherms that could be used to definite the vertical extent of the cold intermediate layer (CIL). The depth and extent of the CIL defined here is noted by the depth of the upper (Zi) and lower (Zf) 3°C isotherms that bracketed the minimum temperature of the CIL (Tc).

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Figure 3. The average proportion of time that tagged Atlantic bluefin tuna spent at depth in the southern Gulf of St. Lawrence in September 2008 (upper panel) and late October 2008 (lower panel).

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Figure 4. The average proportion of time that tagged Atlantic bluefin tuna spent at temperature in the southern Gulf of St. Lawrence in September 2008 (upper panel) tag, in late October 2008 (middle panel), and the average proportion of time spent at a temperature anomaly (lower panel). The average sea-surface temperature (dashed vertical line) is indicated in the upper two panels.

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Figure 5. Bathymetric (100 and 200 m isobath) charts of the southern Gulf of St. Lawrence illustrating the sea surface temperature (upper panel) and depth available to tuna (depth to cold intermediate layer; lower panel) in September 2007 with diamonds representing the locations of the CTD casts during the annual research survey.

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Figure 6. Bathymetric (100 and 200 m isobath) charts of the southern Gulf of St. Lawrence illustrating the sea surface temperature (upper panel) and depth available to tuna (depth to cold intermediate layer; lower panel) in September 2008 with diamonds representing the locations of the CTD casts during the annual research survey.

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Figure 7. A time series of the average (± 1 SD) depth (Zi; upper panel) and minimum temperature (Tc, lower panel) of the cold intermediate layer in the southern Gulf of St. Lawrence.