Analysis and standardization of landings per unit effort of red shrimp <em>Aristeus antennatus</em> from the trawl fleet of Barcelona (NW Mediterranean)

Abstract

Summary:Monthly landings and effort data from the Barcelona trawl fleet (NW Mediterranean) were selected to analyse and standardize the landings per unit effort (LPUE) of the red shrimp (Aristeus antennatus) using generalized additive models. The dataset covers a span of 15 years (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) and consists of a broad spectrum of predictors: fleet-dependent (e.g. number of trips performed by vessels and their technical characteristics, such as the gross registered tonnage), temporal (inter-and intra-annual variability), environmental (North Atlantic Oscillation [NAO] index) and economic (red shrimp and fuel prices) variables. All predictors individually have an impact on LPUE, though some of them lose their predictive power when considered jointly. That is the case of the NAO index. Our results show that six variables from the whole set can be incorporated into a global model with a total explained deviance (ED) of 43%. We found that the most important variables were effort-related predictors (trips, tonnage, and groups) with a total ED of 20.58%, followed by temporal variables, with an ED of 13.12%, and finally the red shrimp price as an economic predictor with an ED of 9.30%. Taken individually, the main contributing variable was the inter-annual variability (ED=12.40%). This high ED value suggests that many factors correlated with inter-annual variability, such as environmental factors (the NAO in specific years) and fuel price, could in turn affect LPUE variability. The standardized LPUE index with the effort variability removed was found to be similar to the fishery-independent abundance index derived from the MEDITS programme.Keywords: LPUE; standardized LPUE; Aristeus antennatus; generalized additive models; NW Mediterranean; deep-water fisheries.Análisis y estandarización de los desembarcos por unidad de esfuerzo de la gamba roja Aristeus antennatus por la flota de arrastre de Barcelona (Mediterráneo noroccidental)Resumen: Se llevó a cabo un análisis del volumen de desembarcos por unidad de esfuerzo (LPUE) de la gamba roja (Aristeus antennatus) de la flota de arrastre en el puerto de Barcelona (Mediterráneo noroccidental) mediante modelos aditivos generalizados (GAM). El conjunto de datos cubre un periodo de 15 años (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) y consiste en un amplio espectro de predictores: variables dependientes de la flota (el número de mareas efectuadas por cada embarcación y las características técnicas de estas, como el tonelaje bruto), temporales (variabilidad inter-e intra-anual), ambientales (índice de Ocilación del Atlántico Norte [NAO]) y económicas (precio de la gamba roja y precio del combustible). Todos los predictores a nivel individual tienen impacto sobre LPUE, pero algunos de ellos pierden su poder explicativo cuando se considéran conjuntamente con otros, como en el caso del índice NAO. Nuestros resultados muestran que seis variables del conjunto pue...

Structured additive distributional regression for analysing landings per unit effort in fisheries research

“…While this indicates an increased expectation, it also indicates a larger uncertainty which reflects the natural tendency that there is more variability for very successful vessels. Such effects have also been previously described by Maynou et al [39] and Mamouridis et al [32] .…”

“…These results are in line with those found by Vasilakopoulos and Maravelias [59] for the whole Mediterranean sea and may, to some extend be a result of fuel prices (or other price changes in resources required), see Guillen and Maynou [23] . -When interpreting the effect of the NAO, it is interesting to compare our results to those of [32] where only temporal variation was considered. The minimum in LPUE observed there for the period 1999 to 20 0 0 seems to be partially absorbed by modelling the NAO effect in our model.…”

“…Therefore, although the catches and landings may strongly depend on these variables, an index of abundance "at sea" should be independent from factors related to fishing. Also the associated economy can be very influential, either directly or indirectly [32] .…”

“…While LPUE should refer to the portion of the catch that is actually sold, the difference between catch and catch being sold can be considered small in case of a lucrative species such as the red shrimp [32] . Potential sources of losses could be, for example, deterioration of catch that is therefore discarded, onboard consumption or the traditional monthly distribution among the crew of a vessel.…”

“…For example, they are at least partially triggered by changes in the North Atlantic Oscillation [NAO,38 ] which can be explained by the seasonal availability of preys and their life cycle [10,50] . Furthermore, they are linked to the fishing activity which is in turn influenced by economic factors, such as fuel and red shrimp prices and therefore subject to the market demand [32] .…”