The NAO index and the long-term variability of Nephrops norvegicus population and fishery off West of Ireland

Nephrops landings from Spanish trawlers in Porcupine bank (West of Ireland) have decreased a 90% in the last 25 years. With the aim of studying the causes of this situation, the relation between CPUE (1979–2005) and several environmental and population factors is investigated through a monthly time series approach. Discards in the fishery are negligible.As LPUE time series exhibits a seasonal pattern, a seasonal decomposition of the CPUE is carried out showing a decreasing trend from 1982 to 2003.The LPUE series is modelized resulting in an autoregressive and moving average model (ARMA) that highlights a monthly positive and seasonal negative autodependence.Population and environmental parameters (i.e. recruits per unit effort “RPUE”, sex ratio, ovigerous females’ proportion, North Atlantic Oscillation index “NAO” and Atlantic Multidecadal Oscillation index “AMO”) were tested at different lags as independent variables of the ARMA model. The outcoming multivariated dynamic ARMA model states that CPUE is positively related to sex ratio lagged by 8 years and negatively related to the NAO index from 6.5 years before.In accordance with those results 1978, 1983, 1986, 1989, 1992, 1995 and 1996 Porcupine Bank stormy, warm and wet conditions would be linked to the 1984, 1989, 1993, 1996, 1998, 2001 and 2003 Nephrops CPUE low values.Those results show an expectable model in relation to Nephrops density in which environmental conditions would have a relevant role.