POPULATION BIOLOGY OF ADSPERSUS RATHKE, 1837 (, ) IN FORNELLS BAY, BALEARIC ISLANDS, WESTERN MEDITERRANEAN

BY

PABLO MANENT1) and JOSE ABELLA-GUTIÉRREZ Departamento de Medio Litoral, Grupo de Investigación del Bentos, Instituto Canario de Ciencias Marinas (ICCM), Ctra. Taliarte, s/n, Aptdo 56, E-35200 Telde, Gran Canaria, Spain

ABSTRACT

Growth and reproductive cycle of the prawn, Palaemon adspersus were studied in Fornells Bay from April 2002 to March 2003. Samples caught using a lift net showed that abundance is markedly seasonal, with peaks in autumn and winter. P. adspersus showed distinct sexual dimorphism, the females being larger than the males. The K coefficient of the VBGF (Von Bertalanffy Growth − − Formula) was higher in the females (2.065 year 1 for females and 1.076 year 1 for males) and the same applies to L∞ coefficient of the VBGF (47.8 mm in females and 34.14 mm in males). Ovigerous females were found from March to August. However, the low percentage of ovigerous females found in the study area suggests that reproduction and incubation do not occur at the study site. Also, as the ovary shows activity during the whole year cycle, it is not possible to distinguish a discrete reproductive period.

RESUMEN

El crecimiento y el ciclo reproductor de Palaemon adspersus fueron estudiados en la bahía de Fornells entre Abril de 2002 y Marzo de 2003. Las muestras obtenidas con una red por elevación indican una marcada variación estacional, con mayores abundancias en otoño e invierno. P.adspersus presenta dimorfismo sexual, siendo de mayor talla las hembras. Los parámetros de crecimiento calculados fueron superiores en hembras. El valor del coeficiente K de la VBGF (Fórmula de − − crecimiento de Von Bertalanffy fue 2.065 año 1 para las hembras y 1.076 año 1 para los machos. En el caso de L∞, fueron 47.8 mm en hembras y 34.14 mm en machos. Se han encontrado hembras ovígeras entre Marzo y Agosto. En cualquier caso, el bajo porcentaje de hembras ovígeras encontradas sugiere que la reproducción e incubación no se desarrollan en el área de estudio. Esto unido a que la actividad del ovario se mantiene durante todo el año no permite discernir un periodo reproductor claro.

1) Fax: +34.928132908; e-mail: [email protected] © Koninklijke Brill NV, Leiden, 2006 Crustaceana 79 (11): 1297-1308 Also available online: www.brill.nl/cr 1298 PABLO MANENT & JOSE ABELLA-GUTIÉRREZ

INTRODUCTION Palaemon adspersus Rathke, 1837 is distributed from the Baltic Sea, along the western coast of Norway, the Bay of Biscay, and the British Isles, to the Moroccan coast and the Mediterranean Sea, the Adriatic Sea, and the Black Sea (Zariquiey- Álvarez, 1968). It tolerates a wide range of temperatures and salinities, and has a preference for shallow waters, often under estuarine conditions (Berglund, 1982, 1985). It is a main component of the epifauna in the shallow meadows of the eelgrass, Cymodocea nodosa (Ucria) Ascherson in Alfacs Bay (Ebro Delta, western Mediterranean) (Fusté, 1988), and the target of small-scale fisheries in several places (Holthuis, 1980; Figueras, 1986). Contributions to our knowledge of the population biology of P. adspersus in the Mediterranean have been provided by Guerao et al. (1994), Guerao & Ribera (1995, 2000), and Guerao & Abelló (1996). In Minorca (Balearic Islands), the genus Palaemon is the most diverse among the palaemonids and some enter into brackish waters where they show remarkable physiological plasticity (Pretus & Real, 1999). P. adspersus was heavily fished until recent years and it was caught using bait with a traditional lift net, known as “ravol”. This study is part of a project to analyse the use of this lift net among recreational fishermen. It also aims to contribute new information about growth and reproduction of this species in a geographical area where it has not been studied before.

MATERIAL AND METHODS This study was carried out in Fornells Bay (40◦03N4◦07E), located on the northern coast of Minorca, Balearic Islands (fig. 1), specifically in the innermost part of the bay, where the bottom is covered with Cymodocea nodosa meadows. The ravol is a lift net. It consist of an iron loop with a diameter ranging from 28 to 40 cm, supporting a nylon net with a mesh size of 4-8 mm; over the net, a smaller net is sewn with a mesh size of less than 1 mm, in which the bait is attached. Starting from the loop, three ropes are fastened to a larger rope, shorter than 2 m, and a float is tied to its end to enable recovery with an adequate stick. In our case, it was baited with fish scrap and the whole was covered with a branch of the macrophanerophyte, Pistacia lentiscus (L.) before it was lowered into the water. This mastic tree, or lentisk, can grow into a small tree and is very abundant in the garrigues and under stories of the pine and holm oak forests. Monthly samplings took place from April 2002 to March 2003 (except for January, due to logistic problems). In each case, five fishing operations were realized during two consecutive days. Four of these took place during day time, one at night. Only one operation was realized in November. Each operation was