1. CLIMATE VARIABILITY AND COMMON OCTOPUS POPULATION CHANGES IN GALICIAN SHELF WATERS (NE ATLANTIC)
Otero, J.1*, González, A.F.1, Álvarez-Salgado, X.A.1, Gilcoto, M.1, Guerra, A.1, Rocha, F.1, Groom S.2
1Marine Research Institute (IIM, CSIC), Vigo, Spain. 2Plymouth Marine Laboratory, Plymouth, UK.
Introduction
Cephalopods are short-life cycle organisms with interannual fluctuations mainly caused due to the recruitment strength. Presumably, though heavily harvested, the environmental constraints on early life phases have a mayor effect on recruitment and lately in the fishable biomass. Atmospheric and oceanographic forcing is mainly investigated on pelagic species (González et al., 1997). Common octopus, Octopus vulgaris Cuvier, 1797, is one of the most important resources in the small-scale fishery context of Galicia (NW Spain). It is a simultaneous terminal spawning species with a hatching peak centred in autumn months (González el al., 2005). The pelagic paralarvae spends almost four months in the water column before settlement and recruitment is considered to occur in the onset of summer being available to the fishery the next year. Galicia constitutes the northern boundary of the Canary upwelling system. The wind patterns determine the seasonal upwelling, with favourable northerly winds from March to September and southerly winds the rest of the year (Álvarez-Salgado et al., 2002). In this area the wind regime is a direct result of the atmospheric pressure gradient reflecting the climatological coupling of the thermohaline changes in the central water upwelled in the eastern north Atlantic (Pérez et al., 2000). In this study, we investigate the links between the physical forcing and their effects on the biomass changes in the common octopus population during the last decade. We use the North Atlantic Oscillation (NAO) as a proxy to the climate interannual changes at the Northeast Atlantic scale. We also test the responses of the abundance with the local hydrodynamics.
Material and Methods
The Galician coast is divided in eight areas according to their geographical distribution (Fig. 1). Common octopus total catch data (tons) from the artisanal sector between 1998 and 2004 were obtained from the official web site Data for the years were obtained from the early Fisheries Information Service, SIP. Data on effort (total days fishing) were only available for the period Taking into account the high correlation between the CPUE (kg day-1) and catches (kg) in those years (r=0.91, p=0.002), we assume that such catch data reflects the abundance of the exploited population for the entire period. We use the mean index of the North Atlantic Oscillation (NAO) for the period September-March as a proxy to the climate interannual changes at the Northeast Atlantic scale. The NAO is based on the difference of normalized sea level pressures (SLP) between Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland since 1865, and it was obtained from the web site We also, used data on upwelling index obtained from the wind field located in the 43º 11º, data on primary production obtained from satellite (SeaWiFS, Joint et al., 2002) and data on mesozooplankton abundance as local variables defining the hydrography and food-web dynamics of the Galician coast. Relationships between variables were investigated by Pearson correlation analysis, Spearman rank correlation and multivariate techniques. Autocorrelation function (ACF) was calculated to test for the presence of temporal autocorrelation for fishery data, and spectral analysis was applied to NAO data..
Fig. 1. Map of the study area indicating the main geographic regions.
Results
Two-year moving average of catch trends and one-year lagged autumn-winter NAO index (sep-mar) were negatively correlated during the last decade r=-0.92, p=0.000; Sr=-0.95 p=0.000 (Fig. 2). The NAO index was related with the local wind, by means of percentage of days blowing northwards (r=-0.69, p=0.02). Moreover, local wind was also related with the catch data during the same period (r=0.78, p=0.005; Sr=0.77 p=0.005). Fishery data and NAO index seems to be in phase depicting cycles of 6 years, in fact the ACF of the catch data showed the highest correlation at lag 3 years (Zt+3= Zt r2=0.8, p=0.000). Spectral analysis of NAO index (sep-mar) from 1865 to 2002 showed that the dominant oscillatory component of the index occurred at a period of 5.8 years. In Galician waters, annual (Fig. 3) and monthly (r=0.98, p=0.000, n=12) trends on primary production and mesozooplankton were closely correlated. On the other hand, both levels of the food web were significantly described by the abiotic variables:
PPshelf= (-Qx)-0.061(NAO) r2=0.42, r=0.65, p=0.000 n=63
Log (Mesozoo)= (-Qx)-0.065(NAO) r2=0.26, r=0.51, p=0.000 n=63
Moreover, primary production and secondary producers, as expected, are related to each other:
Log (Mesozoo)= (PPshelf) r2=0.45, r=0.67, p=0.000 n=72
Fig. 2. Two-year moving average of both total catch data (tons) and one year lagged Sep-Mar NAO index.
Discussion
The influence of the NAO on different levels of the marine ecosystem is widely known (Stenseth et al., 2004), however, studies regarding cephalopod species are scarce (Dawe et al., 2000). Our results, showed that higher catch levels come from lower NAO index from the previous autumn-winter when the hatching peak occurs (González et al., 2005). In the other hand, it seems that the fishable biomass describes cycles of six years in phase with the NAO data. Tunberg and Nelson (1998) showed a cyclical pattern of 7-8 years in the macrobenthic communities on the Swedish west coast influenced by the climatic oscillations. In the Galician coast, low NAO values imply moderate upwelling intensity (Álvarez-Salgado et al., 2003), in fact, the wind regime is a direct result of the atmospheric pressure gradient reflecting the coupling of climatological patterns and thermohaline changes (Pérez et al., 2000). The influence of the NAO in this area affecting pilchard recruitment was shown by Guisande et al (2001). Lower trophic levels (primary and secondary producers) are well modelled using both wind and NAO data, suggesting the favourable conditions of moderate upwelling intensity and low NAO which might positively affect the paralarvae phase. Thus, we suggest a bottom-up control of the benthic population of O. vulgaris shown in Fig. 4.
Fig. 3. Annual trends of primary production in the shelf and mesozooplankton abundance from Jan 1998 to Dec 2003.
NAO (-)
Moderate upwelling (-Qx)
Primary production (+)
Secondary producers (+)
High paralarvae survival
High catches
Fig. 4. Proposed mechanisms relating NAO to common octopus fishery for the Galician coast.
Acknowledgements
This research was founded by the CYCIT project REN
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