Reproductive patterns in Norway lobster (Nephrops norvegicus L., Crustacea Decapoda Nephropidae) of different Mediterranean areas

Abstract

The main aspects of reproduction - namely times of ovarian maturation, brooding and hatching, sizes of maturation and fecundity - were studied in female Norway lobsters in seven areas of the Mediterranean biogeographic subprovince: the Algarve Sea, the Alboran Sea, the Catalan Sea, the Ligurian Sea, the Northern Tyrrhenian Sea, the Adriatic Sea and the Gulf of Euboikos. These included both shelf and slope populations within a latitudinal range of 8°. The reproduction showed clear seasonality with some small differences between areas: maturation delayed by 1-2 months in northern areas (Ligurian, N. Tyrrhenian and Adriatic Seas) compared to southern areas; absence of embryos for periods of 4-5 months in slope females and for less clearly defined periods in shelf females. The case study of the Ligurian Sea was used to illustrate the ovarian cycle and to describe the pre- and post-puberal growth of the female. First mating occurs at an average size of 32 mm CL, which corresponds to age 3+ of the female and the hatching of the first brood corresponds to age 4. Differences were registered in reproductive sizes in the study areas, with a range of 23-30 mm CL for the smallest berried females and a range of 30-36 mm CL in the 50% maturity size. These differences were ascribed to local factors, whereas ages of the basic steps of reproduction were assumed to be unchanged. Females of age 2 formed the most abundant share of the fished stock. An estimate of fecundity was made in terms of brood size (embryos were counted at an early stage of development): the range was 1119-1834 embryos in a standard female of 35 mm CL. Reproductive potential, calculated on the basis of a virtual population analysis, was strongly influenced by the size structure of the female population in each area and there was a ratio of about 14:1 between the least (Ligurian) and the most exploited (Catalan) fishing grounds.