A study on the variability of albacore (Thunnus alalunga) longline catch rates in the southwest Pacific Ocean KARINE BRIAND,* BRETT MOLONY,   AND PATRICK LEHODEY à Oceanic Fisheries Programme, Secretariat of the Pacific Community, BP D5, 98848 Noumea, New Caledonia ABSTRACT Relationships between albacore tuna (Thunnus ala- lunga) longline catch per unit effort (CPUE) and environmental variables from model outputs in New Caledonia’s Exclusive Economic Zone (EEZ) were examined through generalized linear models at a 1° spatial resolution and 10-day temporal resolution. At a regional (EEZ) scale, the study demonstrated that a large part of albacore CPUE variability can be ex- plained by seasonal, interannual and spatial variation of the habitat. Results of the generalized linear models indicated that catch rates are higher than average in the northwestern part of the EEZ at the beginning of the year (January) and during the sec- ond half of the year (July–December). In the north- western region of the EEZ, high CPUEs are associated with waters <20.5° in the intermediate layer and with moderate values of primary produc- tion. Longline CPUE also appeared to be dependent on prey densities, as predicted from a micronekton model. Albacore CPUE was highest at moderate densities of prey in the epipelagic layer during the night and for relatively low prey densities in the mesopelagic layer during the day. We also demon- strated that the highest CPUEs were recorded from 1986 to 1998, which corresponds to a period with frequent El Nin ˜o events. Key words: albacore tuna, catch per unit effort, El Nin ˜o, generalized linear model, habitat, longline fishery, micronekton, New Caledonia Exclusive Economic Zone INTRODUCTION Considerable interannual variability in albacore (Thunnus alalunga) catch rates are observed in domestic fisheries all over the Western and Central Pacific Fisheries Commission area (http://www.wcpfc. int) and in New Caledonia in particular (Fig. 1). Much of this variability is attributed to climatic and prevailing oceanographic conditions (Langley, 2006). Understanding oceanographic spatio-temporal vari- ability and its impacts on fish population dynamics and fisheries is important for fishermen and fishery man- agers, due to the biological, economic and social consequences. This knowledge can be useful for the development of national fishery management plans, economic planning, and the consideration of man- agement options to address specific issues. Previous studies (Grandperrin, 1978; Bour et al., 1981; Leh- odey, 2000; Lehodey et al., 2006) have suggested that climate-related changes in the marine ecosystem influence the vertical and horizontal distribution of tunas, migration and movement patterns, larval sur- vival and recruitment strength and therefore impact on fisheries performance, including catch rates, at various scales. The most obvious interannual driver of oceano- graphic variability in the central and eastern Pacific Ocean is the El Nin ˜o Southern Oscillation (ENSO). ENSO is an irregular, low-frequency, climatic oscilla- tion, fluctuating between warm (El Nin ˜ o) and cold (La Nin ˜a) states, evolving under the influence of the atmosphere–ocean dynamic interaction with an irregular frequency of 4–7 yrs (Fig. 1). During ENSO events, the temperature structure of oceanic waters changes substantially in space and time as a result of atmospheric pressures, sea level, wind and current patterns (Philander, 1990; Trenberth, 1997). Climatic phenomena such as ENSO are known to cause inter- annual changes in local tuna abundance and distri- bution, thereby impacting tuna fisheries in the tropical *Correspondence. e-mail: karineb@spc.int   Present address: WA Fisheries and Marine Research Labo- ratory, Department of Fisheries, Western Australia, PO Box 20 North Beach WA 6920, Australia. à Present address. Marine Ecosystems Modelling and Moni- toring by Satellites, CLS, Space Oceanography Division, 8-10 rue Hermes, 31520 Ramonville, France. Received 19 April 2010 Revised version accepted 13 July 2011 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 20:6, 517–529, 2011 Ó 2011 Blackwell Publishing Ltd. doi:10.1111/j.1365-2419.2011.00599.x 517