Investigating relationships between albacore tuna (Thunnus alalunga) CPUE and prey distribution in the Bay of Biscay

Abstract

The Bay of Biscay in the northeast Atlantic is an important feeding zone for juvenile albacore tuna (Thunnus alalunga) during their summer migration northwards. Spatial distribution and abundance of their potential prey [planktonic organisms, anchovy (Engraulis encrasicolus) and other small pelagics] were investigated in the southeast Bay of Biscay during acoustic surveys in autumn from 2003 to 2005. The relationships between albacore tuna catch per unit of effort (CPUE), and prey abundance and sea surface temperature (SST) were studied at different spatiotemporal scales. We observed positive and significant correlations between albacore tuna CPUE and anchovy abundance and total prey abundance, at different spatial scales. However, in 2003, a year characterised by extreme temperatures compared to the other years of this study, the relationship between CPUE and prey abundance was much weaker. Instead, we found a significant negative correlation with SST.

Introduction

Ecological systems exhibit heterogeneity over a broad range of scales, with marine organisms having an aggregated, patchy distribution on a wide variety of space and time scales from centimetres to thousands of kilometres (Wiebe, 1970, Steele, 1976, Haury et al., 1978, Mackas and Boyd, 1979, Mackas et al., 1985, Frontier, 1987, Russel et al., 1992, Davis et al., 1991, Bertrand et al., 2002b). Studying interactions between predator and prey implies choosing an appropriate space–time scale. This choice is difficult because populations and ecosystems cannot be described at a single scale (Levin, 1992). Processes at regional scales are more regular; its study being key to our understanding of trophic interactions among populations (Rose and Leggett, 1990). The relationships between tuna and their prey are generally studied at regional (1000s of km), meso (100s of km) and local (km) scales (Josse et al., 1998, Bertrand, 1999, Bertrand et al., 2002b, Goñi et al., 2009). The range of tuna distribution is known to be limited mainly due to hydrological conditions (Sharp, 1978, Sund et al., 1981, Brill, 1994, Bard, 2001, Bertrand et al., 2002a, Bertrand et al., 2002b); however, within areas of suitable abiotic conditions, tuna tend to be more abundant where prey are concentrated (Sund et al., 1981, Roger, 1994, Bertrand, 1999, Bertrand et al., 2002b).

Every summer, albacore tuna (Thunnus alalunga) perform seasonal feeding migrations into the Bay of Biscay. This area constitutes a hot spot for juvenile tuna in search of prey and sustains a traditional seasonal fishery, targeting albacore tuna from June to October. Catches occur off the shelf (but adjacent to the shelf break) from the Cantabric Coast (Fig. 1) up to 48°N (Santiago, 2004). So far, no effort has been put into producing combined studies of the spatial distribution of albacore tuna catches and their potential prey, predominantly small pelagic fish and euphausiids (Goñi, 2008).

Since 2003, annual acoustic surveys are undertaken, in autumn, in the Bay of Biscay (JUVENA Program, Boyra and Uriarte, 2006, Boyra et al., 2006). Although these surveys are executed to assess the abundance of juvenile anchovy and are under constraints imposed by management requirements, they partially overlap with the period and location of the albacore tuna baitboat and trolling fishing season in the Bay of Biscay. This makes JUVENA an important source of information, with acoustic data as a key tool, for the observation of the distribution and spatial–temporal structure of the pelagic community in the area.

The aim of this study was to examine how the spatial distribution and abundance of albacore tuna prey impact CPUE in the Bay of Biscay. Data come from the annual acoustic surveys (2003–2005) and CPUE logbook records from commercial fisheries. We used the description of interannual variation in the patterns of spatial distribution of small pelagic fish and euphausiids abundance as an indicator of food availability for albacore tuna. The relationships between presence and abundance of prey and albacore tuna were examined at both regional (study area) and local (44, 56 and 111 km) scales.