Effects of the North Atlantic Oscillation on Spanish catches of albacore, Thunnus alalunga, and yellowfin tuna, Thunnus albacares, in the North–east Atlantic Ocean

Effects of the North Atlantic Oscillation on Spanish catches of albacore, Thunnus alalunga, and yellowfin tuna, Thunnus albacares, in the North–east Atlantic Ocean.— Tuna are highly migratory pelagic species (HMPS) with great importance in commercial fishing. Several authors have highlighted the effect of climatic oscillations such as the North Atlantic Oscillation (NAO) on HMPS. This paper analyzes the effects of the NAO on two HMPS: albacore, Thunnus alalunga, and yellowfin tuna, Thunnus albacares. Fishing data from the Spanish fleet operating in the North Atlantic area were obtained from the International Commission for the Conservation of Atlantic Tunas (ICCAT) database. The results show a positive correlation between the NAO index and the Catch per Unit Effort (CPUE) for both albacore and yellowfin tuna, depicting a potential effect on their capturability.


Introduction
Tuna species are an important fishing and commercial resource and have been exploited since antiquity. Their migratory and gregarious behavior makes them especially susceptible to some fishing gear. Two tuna species, skipjack tuna, Katsuwonus pelamis, and yellowfin tuna, Thunnus albacores, are currently among the top 10 fish catches in the world (FAO, 2010).
The International Commission for the Conservation of Atlantic Tunas (ICCAT) is responsible for the conservation and management of tuna and tuna-like fishes in the Atlantic Ocean and contiguous waters. ICCAT promotes the collection of biological and fishery data concerning tuna and tuna-like species and the analysis of statistical information related to the state of conservation and trends in abundance of these fishery resources exploited in the ICCAT competence area. According to available ICCAT data, albacore, Thunnus alalunga, and yellowfin tuna, Thunnus albacares, constitute the major part of tuna catches by the North Atlantic Spanish fleet.
Several authors have discussed the effect of climatic oscillations on migratory species (Robinson et al., 2009), particularly on various tuna species, such as the Atlantic bluefin tuna, Thunnus thynnus (Macías et al., 2012;Báez et al., 2013). For this reason, some authors propose modeling the response of migratory species to large-scale climatic phenomena integrating weather conditions in large areas, such as the North Atlantic Oscillation (NAO), rather than studying the effect of local conditions (Forchammer et al., 2002;Robison et al., 2009). The NAO is considered the largest source of variability -both seasonal and interannual-affecting the climate of the North Atlantic area: Europe, North Africa and North America (Hurrell, 1995). The NAO refers to an oscillation between the anticyclone of the Azores and a low-pressure area near Iceland, which redistributes air mass from the Arctic to the subtropical Atlantic (Hurrell, 1995). Part of the Atlantic and adjacent seas (particularly the Mediterranean Sea) respond quickly and locally to the NAO by varying the surface sea temperature, the depth of the ocean mixed layer, the ocean heat content, the thickness of the ocean ice shelf, surface current circulation, and the intensity and direction of the prevailing winds (Visbeck et al., 2001).
We hypothesized that the NAO could have an effect on tuna catches. The aim of the present study was to analyze the effect of the NAO on Spanish albacore and yellowfin fishery captures in the North Atlantic.

Material and methods
ICCAT compiles capture data reported by member states by fishing gear and area (called task I); fishing effort data are also sorted by fishing gear and area (called task II). These data are freely downloadable from the ICCAT website. Fishing gear used to catch tuna in the ICCAT area mainly consists of baitboat, purse-seine and surface longline. Cases with available historical data series of more than ten years according to type of fishing gear (baitboat, purse seine, troll and longline) and species were albacore caught using purse seine and baitboat, and yellowfin tuna caught using purse seine and baitboat. Albacore is a medium size tuna. The ICCAT recognizes two stocks of albacore in the North Atlantic region, one in the Atlantic sensu stricto and another in the Mediterranean Sea. The present study includes only data from the Atlantic stock because only Spanish Atlantic fleet catches were taken into consideration.
Yellowfin tuna is a cosmopolitan species distributed in open waters of tropical and subtropical areas of the three oceans, not being present in the Mediterranean. It is assumed that there is a single yellowfin tuna stock for the whole Atlantic Ocean.
NAO data were collected from the National Oceanic and Atmospheric Administration (NOAA) website: http://www.cpc.ncep.noaa.gov. We used the NAO average data per year for the analyses. However, as many authors (e.g., Hurrell, 1995) emphasize that the NAO has its maximum effect between November and April, we also used the NAO average of these We performed linear and non-linear regression between the CPUE (Catch Per Unit Effort by species, fishing gear and year, retrieved from the ICCAT task II data) as the dependent variable, and climate indexes (NAO and NAOw) as independent or explanatory variables. In all cases, the normality of the data was verified using the Kolmogorov-Smirnov test.

Results and discussion
We obtained a positive significant linear and non-linear regression between the CPUE (the unit of effort being hours at sea) for the Atlantic albacore and NAO (table 1, fig. 1A) and between the CPUE of the North Atlantic Spanish baitboat fishery and NAO (table 1, fig. 1B).
Atmospheric oscillations modify the intensity of several meteorological phenomena, such as wind, rain and storms. They increase the run-off and nutrient input to the sea, changing response to the trophic levels of marine ecosystems (Drinkwater et al., 2003) and exploited resources. These meteorological phenomena may, in turn, affect fishery resources in at least two aspects: i) the fishing effort value, and ii) the variability of the pelagic ecosystem. When referring to fishing effort, climatic oscillations at sea, such as a severe storm, might reduce the fishing effort. Likewise, occurrences promoting favorable weather conditions (e.g., a period of stability and calm sea) might increase the fishing effort in a certain area. However, we did not observe a significant correlation between NAO and fisheries effort   1. La CPUE del atún blanco (A) y del rabil (B) para la pesca con redes de cerco en la costa atlántica de España representada gráficamente y la NAO como variable independiente. Se observa una tendencia positiva con respecto a los valores positivos de la NAO. Se muestra la relación lineal y no lineal más significativa (junto con su función). La línea continua representa la tendencia lineal (la función se muestra aen la derecha) y la línea discontinua representa la tendencia no lineal (la función se muestra en la izquierda).  Moreover, meteorological phenomena could affect the biology and behavior of large migratory species of commercial interest such as those considered here, either affecting the trophic resources available, their migration period, or their distribution and/ or local abundance. Changes in the depth of the thermocline could also affect the availability of fish (i.e., catchability) (Lehodoy, 2000).
Our analyses of Spanish catches of purse seine albacore catches and baitboat yellowfin tuna catches in the North Atlantic showed a positive, significant regression of CPUE with the NAO as an independent variable. However, we did not find a significant correlation between fishing effort and atmospheric oscillations in any of the fisheries studied. These results suggest that the NAO effects on the variable CPUE mainly occur through effects on the pelagic ecosystem, affecting the abundance of individuals (or their catchability) of the Spanish fishery target species, albacore and yellowfin tuna, in a given area.
The relation between CPUE and NAO was positive in both species. This can be explained by the fact that, during positive NAO phases, western winds that could shift the species shoals eastward towards European and African coasts -the area where the Spanish fleet operatesl-increase. Furthermore, during positive NAO phases, the number and intensity of North Atlantic storms increase, contributing to the occurrence of phytoplankton blooms (Martínez-García et al., 2010;Baez et al., 2014), which in turn may attract potential prey of tuna species. According to Consoli et al. (2008), albacore tuna can be considered a top carnivore, and small pelagic fish (e.g., Aulopiformes and Clupeiformes) and cephalopods are its potential prey. In this line of reasoning, Báez & Real (2011) found a significant relationship between monthly landings of anchovy, Engraulis encrasicolus, and the NAO around the Gulf of Cádiz. It is therefore possible that favorable conditions for small pelagic fishes also attract the top predators in certain areas.
Our findings suggest it would be desirable to incorporate NAO predictive models into current models used by ICCAT experts for fisheries management so as to enable more efficient management of large migratory pelagic fisheries.