Small-Scale Distribution Characteristics of Munida Spp. Populations (Decapoda: Anomura) Off the Catalan Coasts (Western Mediterranean)
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Journal of Sea Research 53 (2005) 283–296 www.elsevier.com/locate/seares Small-scale distribution characteristics of Munida spp. populations (Decapoda: Anomura) off the Catalan coasts (western Mediterranean) Carme Hugueta,b, Francesc Maynoua, Pere Abello´ a,* aInstitut de Cie`ncies del Mar (CSIC), Passeig Marı´tim de la Barceloneta 37–49, 08003 Barcelona, Spain bNetherlands Institute for Sea Research (NIOZ), Department of Marine Biogeochemistry and Toxicology, PO Box 59, 1790 AB Den Burg, Texel, The Netherlands Received 22 December 2003; accepted 29 November 2004 Available online 5 February 2005 Abstract The small-scale distribution characteristics of three species of the anomuran genus Munida, namely Munida rutllanti, Munida intermedia and Munida tenuimana, have been studied on the lower shelf and upper-middle continental slope near the coasts of Tarragona and the Ebro delta region in the western Mediterranean. The differential bathymetric distribution showed that a fair degree of overlap exists between M. rutllanti and M. intermedia, with the former being mainly found at shallower depths. The size population structure of the three species showed that polymodality, as indicative of a longer life-span and of a higher degree of population structuring was more evident in the deeper-living species, whereas unimodality, as an indication of a recruitment dependent population, was more evident in the shallowest-occurring species. Geostatistics have been applied to determine the degree of spatial variability occurring in M. intermedia abundance, the better-sampled species. The Mantel test has been used to assess the significance of the overlap between the three species distributions as well as the relation between certain population parameters. A discussion on the population characteristics of M. rutllanti is made in relation with its expansion along the western Mediterranean in the last decades. D 2004 Elsevier B.V. All rights reserved. Keywords: Spatial distribution; Geostatistics; Population structure; Habitat partitioning; Munida; Western Mediterranean 1. Introduction A´ lvarez, 1968; D’Udekem d’Acoz, 1999), three of which are usually found on soft-bottom areas: Munida Five species of the squat lobster genus Munida are rutllanti Zariquiey A´ lvarez, 1952, Munida intermedia known in the western Mediterranean (Zariquiey A. Milne Edwards and Bouvier, 1899 and Munida tenuimana G.O. Sars, 1882. The shallower-occurring * Corresponding author. species M. rutllanti, mostly concentrates on the shelf E-mail address: [email protected] (P. Abello´). and upper slope, M. intermedia is mainly distributed 1385-1101/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.seares.2004.11.001 284 C. Huguet et al. / Journal of Sea Research 53 (2005) 283–296 from the shelf-break to the middle slope, whereas M. carbonate content characteristics. They presented a tenuimana mostly occurs on the middle and lower high carbonate content and the texture ranged between slope, with its distribution extending down to over mud and muddy-sands with various sorting levels, 2000 m depth (Abello´ et al., 1988, 2002; Cartes, from well sorted areas to poorly sorted areas corre- 1993a). The biology and ecology of the different sponding to the Ebro river outflow influence area. species of the genus Munida in the Mediterranean and The hydrography of the study area is characterised NE Atlantic waters have been studied by various by the rich Ebro river outflow and by a permanent authors, among whom Hartnoll et al. (1992), Freire et south-westward slope current that flows over the al. (1992), Gramitto and Froglia (1998), Sanz-Brau et continental shelf (Font et al., 1988). The water column al. (1998), and Company et al. (2003). The presence of can be divided into three layers: a superficial water several congeneric species over a depth gradient mass down to ca. 200 m that has considerable suggests the occurrence of a certain degree of seasonal temperature and salinity fluctuations (Hop- ecological exclusion or habitat partitioning among kins, 1985; Font et al., 1988), the Levantine inter- them. mediate water between 200 and 600 m, which shows Geostatistical analysis has been applied in ecology little seasonal variation, and the Mediterranean deep to the study of the spatial organisation of populations water below 600 m with almost constant temperature or environmental variables (Cressie, 1991; Rossi et (12.8 8C) and salinity (38.4 PSU). al., 1992; Webster and Oliver, 2001). It allows an accurate analysis of the spatial distribution of geo- 2.2. Survey referenced variables, such as density, size structure or any other biological variables, producing distribution A trawl survey (GEODELTA) was conducted over maps of the variable under study (Maynou et al., muddy bottoms off the Tarragona and Ebro delta 1996). Within this context, this work was designed to continental shelf and slope (Fig. 1)(Maynou and study the small-scale distribution and population size Sarda`, 1997). A regular grid 1 by 2 nautical miles was structure of M. rutllanti, M. intermedia and M. set parallel to the coast and a start location for each tenuimana using spatial correlation tests to detect tow was randomly selected within each cell. The any possible interspecies and intraspecies interactions. sampling was conducted between 7 and 15 April 1994 Geostatistical analysis was applied only to M. and a total of 72 stations were sampled. The depth of intermedia data since this was the best-sampled the sampling locations varied from 83 to 713 m. species, with the aim to detect small-scale spatial The experimental sampling gear was a specially structures. The population variables were also inves- designed single-warp otter trawl (dMaireta System tigated in relation to physical and ecological param- TrawlT; Sarda` et al., 1998). The codend stretched eters that could explain the differential distribution of mesh was 12 mm so as to retain small individuals not the three Munida species in the study area. normally available to the commercial fishing gear. The actual opening of the trawl was measured using an acoustic system (SCANMAR) and stabilised at 2. Material and methods 13.4 m width by 2.0 m height. During the survey, tows were made parallel to the depth contours. The 2.1. Study area duration of each tow was set to exactly 15 min (time of effective trawling). The towing speed varied The geomorphology of the study area is charac- between 2.3 and 2.7 knots (mean 2.5 knots). Start terised by a wide continental shelf off the Ebro delta, and end locations for each tow were taken by GPS. with a steeper slope in the southern sector than in the The actual surface covered by each tow was computed northeast. Two canyons are also found indenting the from the GPS and the SCANMAR readings. shelf (Fig. 1). The sediments in the area were studied The total catch of the three species of Munida was by Maynou and Sarda` (1997), showing three distinct counted, weighed, sexed and measured. In juvenile areas based on redox potential, organic matter, sedi- individuals and in individuals infested by rhizocepha- ment temperature, grain size and distribution, and lan parasites, sex was determined by the position of C. Huguet et al. / Journal of Sea Research 53 (2005) 283–296 285 41.25 Tarragona 41.00 40.75 Delta Ebre m m 00 40.50 00 8 4 m 0 0 100 m 6 m 0 0 2 0.75 1.00 1.25 1.50 Fig. 1. Map of the study area with locations of trawl hauls. the gonopores; otherwise, males were recognised by tions was assessed by determining the normal their first and second modified pleopods, and females components of the distribution with the subroutine by the occurrence of well-developed pleopods in all NORMSEP, fed with the results of the previously abdominal segments. The density of each species was applied Bhattacharya’s method, included in the soft- computed as number of individuals haÀ1 from the ware package FiSat II available from the FAO total surface covered by each tow. The distribution of website. the Munida species with depth was calculated as the The presence of a rhyzocephalan parasite was mean number of individuals haÀ1 by 50 m depth noted for M. intermedia to assess the rates of infection intervals. in this species and the spatial structuring of the Carapace length (CL) was measured in mm from infection. the posterior margin of the right-hand side orbit to the posterior border of the carapace. Sizes of individuals 2.3. Geostatistics of the three different Munida species were grouped in 1 mm classes and were plotted separately for males The fundamentals of geostatistics, with emphasis and females. Polymodality in size frequency distribu- on the methods employed here, can be found 286 C. Huguet et al. / Journal of Sea Research 53 (2005) 283–296 elsewhere (e.g. Cressie, 1991; Maynou, 1998; Another aspect of geostatistics is the possibility of Webster and Oliver, 2001). Basically, the density estimating the density of the variable of interest at of each Munida species can be considered as the points not sampled or over areas where a global À1 random variable Z(xi): Number of individuals Ha estimate is needed (Cressie, 1991). This is known as at location xi, whose properties are relatively kriging and can be regarded as spatial prediction or constant over the area at the spatial and temporal estimation. In order to proceed with kriging, the scales of study. These assumptions can be verified experimental variogram has to be fitted to a theoretical in practice by an exploratory data analysis previous variogram model (some models are given in Cressie, to geostatistical analysis (Webster and Oliver, 2001). 1991 and Webster and Oliver, 2001). When the Under these conditions, the structure of spatial experimental variogram has been computed correctly variability of Munida species abundance can be and there exists a clear underlying spatial structure, it is studied by computing an appropriate structure func- often easy to choose and fit a theoretical model.