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Ecology and fishery of the deep-water shrimp, Aristeus antennatus (Risso, 1816) off Algeria (south-western Mediterranean)

Life-history traits and exploitation of the red shrimp, Aristeus antennatus (Risso, 1816) (Decapoda, Aristeidae) on the Algerian slope (south-western Mediterranean) have been studied through monitoring of the commercial trawl fishery between 1999 and 2006 at two different ports on the west coast, as well as through monthly biological sampling of these catches from April 2005 to September 2006. This information was completed with data from experimental bottom trawls, carried out in the area during 2003 and 2004. A continuous increase of the trawl fleet and the landings of red shrimp during the study period were detected. Although this fishery takes place only in a very limited part of the distribution range of the species (around depths of 300-525 m), experimental data gave a continuous increase of red shrimp density from 400 to 700 m. The Algerian fishery showed seasonal variation in yields, with a maximum during summer and autumn, although the average daily yields were clearly higher than those from other western Mediterranean fisheries. Experimental trawls detected two distinct demersal assemblages on the slope grounds of the study area, separated approximately by the 400 m isobath. Decapod crustaceans constituted the dominant group in these assemblages, with the pink shrimp (Parapenaeus longirostris (Lucas, 1846)) and the red shrimp as their most important and abundant species, and either shallower, or deeper than that isobath, respectively. Both species of shrimp showed one of the highest abundance indices estimated in the Mediterranean yet. Fishes were also important in these assemblages, the composition of which suggested high species diversity, with complex and extensive trophic webs. Monthly length frequency distribution of commercial catches showed that the carapace length (CL) of female shrimps ranged between 18 and 65 mm, while males ranged from 16 to 41 mm CL. Females represented more than 70% of the individuals caught, although the sex-ratio by length showed a predominance of males for individuals ≤18 mm CL and of females for individuals ≥25 mm CL, while their monthly development also showed a predominance of females from April to September and a similar proportion of sexes from October to March. Ripening gonads of females were mostly found from June to August, and the gonadosomatic index also showed a clear peak during this period. The length at which 50% of females were mature was estimated as 27.7 mm CL. The length-weight relationship showed a strongly negative allometry for both sexes (b = 2.4), while the Von Bertalanffy growth parameters differed greatly between them, with values for maximum theoretical size and for the growth coefficient of females (L∞ = 70.6 mm CL and k = 0.5 year−;1, respectively) being higher than those for males (L∞ = 43.0 mm CL; k = 0.3 year−;1). The growth performance index (Φ) for females (3.4) was also higher than that for males (2.7). Se ha estudiado la estrategia vital y la explotacion de la gamba roja Aristeus antennatus (Risso, 1816) (Decapoda, Aristeidae) en el talud de Argelia (Mediterraneo sudoccidental), a partir del seguimiento de la pesqueria comercial de arrastre entre 1999 y 2006 en dos puertos de la costa occidental y del muestreo biologico mensual de las capturas entre Abril 2005 y Septiembre 2006. Esta informacion se completo con datos procedentes de pescas experimentales de arrastre de fondo, realizadas en el area en 2003 y 2004. Durante el periodo de estudio, se ha detectado un incremento continuo de la flota de arrastre y los desembarcos de gamba roja. Aunque esta pesqueria se lleva a cabo solamente en un rango muy limitado de la distribucion batimetrica de la especie (aproximadamente entre 300 y 525 m), la informacion procedente de las pescas experimentales mostro un continuo incremento de la densidad de gamba roja desde 400 hasta 700 m de profundidad. La pesqueria argelina mostro variaciones estacionales en los rendimientos, con maximos durante verano y otono, si bien sus valores medios diarios fueron claramente superiores a los de otras pesquerias del Mediterraneo occidental. Las pescas experimentales de arrastre mostraron la presencia de dos comunidades demersales distintas en los fondos de talud del area de estudio, separadas aproximadamente por la isobata de 400 m. Los crustaceos decapodos constituyeron el grupo predominante de estas comunidades, siendo la gamba blanca (Parapenaeus longirostris (Lucas, 1846)) y la gamba roja sus especies mas importantes y abundantes a 400 m de profundidad, respectivamente. Ambas especies mostraron unos de los mayores indices de abundancia estimados en el Mediterraneo. Los peces fueron tambien importantes en estas comunidades, con una diversidad de especies elevada y redes troficas amplias y complejas. La distribucion mensual de tallas de las capturas mostro que la longitud cefalotoracica (LCT) en las hembras oscilo entre 18 y 65 mm, mientras que en los machos entre 16 y 41 mm LCT. Las hembras representaron mas del 70% de los individuos en la captura, aunque la proporcion de sexos mostro una predominancia de machos en individuos ≤18 mm LCT y de hembras en individuos ≥25 mm LCT, mientras que su evolucion mensual tambien mostro una predominancia de hembras desde Abril hasta Septiembre y una proporcion similar de ambos sexos entre Octubre y Marzo. Se encontraron hembras en maduracion principalmente desde Junio hasta Agosto, y su indice gonadosomatico tambien mostro un maximo durante este periodo. La talla a la cual el 50% de las hembras fueron maduras se estimo en 27,7 mm LCT. La relacion talla-peso mostro una acusada alometria negativa en ambos sexos (b = 2,4), mientras que los parametros de crecimiento de Von Bertalanffy mostraron claras diferencias entre sexos, con valores de talla maxima teorica y coeficiente de crecimiento en hembras (L∞ = 70,6 mm LCT y k = 0,5 ano−1, respectivamente) mayores que en machos (L∞ = 43,0 mm LCT; k = 0,3 ano−1). El indice de crecimiento (Φ) en hembras (3,4) fue tambien mayor que en machos (2,7).