An exploratory analysis of limb loss incidence and length-weight relationship in Norway lobster (Nephrops norvegicus L.) from the Strait of Sicily

Norway lobster (Nephrops norvegicus L.) catches gathered in fifteen experimental bottom trawl surveys conducted in the Strait of Sicily (Central Mediterranean Sea) from Spring 1990 to Autumn 1998 have been analysed to quantify the incidence of limb (chelipeds) damage and estimate length (CL, mm) weight (total, W, g) relationships (LWR). Surveys specifications are presented in table A. The proportion of broken (PB) (i.e., without chelipeds) Norway lobsters by survey were significantly higher in females (0.70-0.87) than males (0.54-0.81). A synoptic representation was organised by each "xi" survey (where xi denotes the progressive number, from 9th to 23rd, assigned by IRMA). In particular, the proportions of "broken" (PB) specimens by size (carapace length, CL; 2 mm, class width) were computed and presented both in table (xi.1) and plots (xi.1). PB referring to class size with more than 49 specimens were linearly regressed to size in order to appreciate any eventual rough trend. The length frequency distributions for broken and unbroken specimens were also computed for each sex (Fig. xi.2). Finally, on the unbroken specimens the BW at CL scatterplots were organised and the a and b parameters were estimated by the expected LWR power function: W=aCLb (Fig. xi.3). The exploratory analysis of the synoptic tables and figures showed the following results: 1)no systematic pattern was found between the PB and size, whereas an higher, although variable, incidence of damaged specimens was detected within the intermediate size class; 2)a preliminary analysis of the LW relationships of unbroken specimens did not evidenced any discontinuities and hence a unique line was computed by each sex and survey. Comparing the single surveys, a positive allometry (b>3) resulted the rule for males and the most frequent situation for females (Table B). A seasonal effect (low b values in spring) was detected in females LWR (Table C and Fig. A). The overall mean values of the coefficients (a-b) were 0.000440-3.133 and 0.000576-3.073 for males and females respectively (Table C). The sex related differences in both PB and LWR likely reflect the interaction between the reproductive cycle (ovarian and extruded eggs maturation and developing) and the changes in the availability to the bottom gear of the Norway lobster females. The results of the present analysis will be used to build up a formal test of these hypotheses.