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Spätial Distribütion of Decapod Crustaceans in the Galician Continental Shelf (NW Spain) Using Geostatistical Analysis

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Spätial Distribütion of Decapod Crustaceans in the Galician Continental Shelf (NW Spain) Using Geostatistical Analysis ., International Council for Shellfish Committee the Exploration of the Sea ICES C.M.: 1991/K:4 Spätial distribütion of decapod crustaceans in the Galician continental shelf (NW Spain) using geostatistical analysis Juan Freire, Luis Fernandez & Eduardo Gonzalez-Gurdaran Departamento de Bioloxia Animal, Faeultade de Ciencias, Universidade da Coruna. E-15071 fA.,Coruna, Spain ABSTRACT " Geostatistieal methodology was applied to analyze spatial strueture and distribution of the epibenthic erustaeeans Liocarcinus depurator, Macropipus tubercu/atus, Po/ybius hens/owii, Munida intermedia, Munida sarsi, P/esionika heterocarpus and So/enocera • membranacea in the Galician eoniiriental shetf during three survey eruises earried out in 1983 and 1984. The experimental vadograms were ealeulated and fitted to spherieal models. The spatial strueture model was used to estimate abundanee and map the populations using kriging. The variograms have a variable strueture depending on speeies, population density and/or geographieal area. Spatial strueture beeomes well-defined as density inereases for L. depurator, M. tubercu/atus, M. illtermedia and M.sarsi, whereas P. hens/owii, P. heterocarpus and S. membranacea do not present a simple relationship. Range of spherieal models, pateh size, fluetuates between 7 arid 32 Km, and is Iinked both'to interspecifie differenees in spatial pattern, and, in some eases, to density. L. (Jepurator and M. tubereulatus are distributed over wide areas of relatively low average density, and witti variable loeation of the groupings. Patehes of P. hens/owii stay in a fairly eonstant loeation from one eruise to another, in spite of the graat fluetuations in density. Anomuran (M. intermedia and M. sars/) arid shrimp (P. heterocarpus and S. membranacea) species present relatively stable high density areas during the different eruises on EI medium seale, although the loeation of the patehes ehanges on a small seale. This suggests that there are stable physieal faetors that eontribute to determine how the speeies are distributed. Deptti is a Iimiting faetor on a large seale, whereas oceanographic coriditions, in particular upwelling processes and riutdent-rich water from the rias, make up the spatial strueture on a smaller seale in some species. " INTRODUCTION The use of geosÜltisties (CLARK, 1979; MATHERON, 1971) in marine biology was introduced during the second half of ttie 1980's (CoNAN, 1985). It is currently used in the assessment of harvested populations, mainly invertebrates (CONAN & WADE, 1989; CONAN ET AL., 1988; NICOLAJSEN & CONAN, 1987; PETITGAS & POULARD, 1989). The introduetion of this type ofanalysis to fisheries was motivated by ttie crltieism of the traditional methods of stock assessment (arithmetic mean or swept-area based in randorn or stratified random sampling), and in particular their application tO invertebrates (CONAN, 1984). These assessments are sUbjeet to estimalion errors, as the spatial distribution of the organisms is riot taken into aecount, nor, in the ca se cif many invertebrate speeies, is their limited or , , 1 .. ,i I ,I, .' non-existent mobility. Also, regionalized estimates are obtained with geostatistical methodology, and they are important for the analysis öf spatial fishery dynamics (CONAN, 1985). .,I . However, spatial analysis in ecology has ev~lved from the study of probability distributions of sampies considered to be independent (see HURlBERT, 1990 for a i'ecent critique), to autocorrelation and spatial structure analysis (ClIFF &ORD, 1981; JUMARS ET AL.~ 1977). In this sense geostatistical analysis allows us to analyze and model spatial variability which has traditionally becn avoided, an'd uses the spatial structure öf the population to enhance both mean and variance estimates (MATHERON, 1971). :, , Geostatistical analysis does not require a special sampling design albeit best rssults of vai'iograms, mapping and assessments are obtained for sampies taken along a regular grid (BURROUGHS, 1987). The present study analyzes data from three survey cruises carried out in the Galician continental shelf (NW Spain) using geostatistical techniques to describe and map the ablJndance and spatial structure of seven species of epibenthic decapod crustaceans (Liocarcinus depurator, Macropipus tube'rculatus, Polybius henslowii, Munida inteiinedia, Munida 5arsi, Plesionika heterocarpus a'nd Solenocerci membranacea). The ".,. results will aHow lJS to analyze 1) the feasibility of using geostatistics for existing data collected according to traditional methods, and 2) the feasibility of enhancing such methodology. ! • I . I MATERIAL ANO METHOOS. I Sampling The sampling is described in detail by GONzAi...EZ-GuRRIARAN & OLASO, 1987. We anaiyze data of three cruises that took place in tha Galician continental shert: CARIOCA 83 (C83, September 1983), ICTIO-NW 84 (184, May 1984) and CARIOCA 84 (C84, August-September 1984). During each cruise a randomly stratified sampling was carried out (up tö 500 m deep), in which the shelf was divided into ihres geographical areas (Miiio-Fisterra, Fisterra-Estaca de Bares and Estaca 'de Bares-Ortega!), considering two strata to be divided bythe isobath öf 200 m(Fig. 1). Baka type trawl was lJsed, with each tow lasting between 30 and 60 minutes. For data an'alysis, the densities of ihe. different speciss were standardized to 60 minute trawls. : ' j Oata analysis I • f\.. , ,.. '.'! \ ,.' <".' •. " i ; • In geostatistical methodology (CLARK, 1979; CONAN, 1985; MATHERON, 1971), the covariance of the parameter studied is analyzed and modelIed in terms of the distance between sampling units (variogram), and the optimumweights are calculated for aach sampie in order to estimate the population density as weil as the variance of tha estimate~ whether at a point (point kriging) or a block (block kriging). I I , The variogram represents the semivariance T(h) (variance between independent s8mples minus the ciovariance between sBmples sep~raied by adistance h): I t • .' . N.... .. r(h) = 1/2n I [Z(xj )- Z(xj +h)]2 1-' I, wllere' Z(x.) and Z(x.+ h) are ttie density 8t point "J arid 'in the sampies located at a distance h (lag) from X;, ri is the number of pairs of stations sampled, ärid N the number of sampling p~n~. I , 2 ! I A theoretical modei is fitted to the experimental variogram. We used t~e sphericai model (the most common in the analysis of marine populations arid in geostatisties in general): vithere Co is the nugget effect, due to the variability between replicates, the microstructure whieh remains undetected because of the sampie size, or errors in measurement or loeation; C represerits the sill minus the nugget effect, where the sill is the asymptotie "alue of semivarianee, reaehed with a "alue of h = a,ealled range, whieh represents the . maximum distanee at whieh spatial effeets are deteeted. Variograms were calculated for the overall sampling area and for two geographieal zones of the shelf (Fig. 1): North, from Fisterra to Ribadeo, with a SW-NE shoreline orientation; and South, from Mino to FistEma, with N-S orieritation and a great influence from the Rias (in the C84 eruise experimental variograms for the southern area were not ealeulated beeause tt1e number of sampling points was too smalI). Results presented " correspond to isotropie variograms; anisotropy was not. studied in detail, although anisotropie variograms ealeulated in the direetion of the shoreline (riot shown) have a similar strueture to isotropic variograms for each area. Point kriging was used for • estimating values at the nades of a 5 x 5 Km grid eovering a survey area extending from the coast to the 500 rri isobath. Variogram models fitted for the overall samplirig area were used for kriging. The data analysis was earried out using GEOMIN software modified by G. Conan and E. Wade (Marine Biology Research Centra, Universite de Moneton, Canada) and GEO­ EAS software (ENGLUND & SPARKS, 1988). , .., RESULTS Table 1 shows data on eatehes for eaeh speeies and eruise as weil as parameters of variogram models. Figs. 2-7 present experimental and model variograms and point kriging based isocontour density maps. Liocarcinus depurator. In C83 in the southern area a-spatial covariance with a range of 14­ Km is deteeted; and in C84, when this speeies reaehes greater densities, the variogram points to a spatial strueture having a praetieally non-existent nugget effeet aild a range of around 20 Km. In this cruise, L. depurator oeeupies an extensive area of relatively high density to the north of Fisterra, and maximum values found in shallow waters (approximately 100 m). Macropipus tubereulatus. A spatial eovariance in· the distribution of this species is detected in all three data sets analyzed. The range of the variograms fluctuates between 10 and 28 Km. The nugget. effeet is importäni only iri C84, suggesting the existerice cf groupings with less than 28 Km in size. M. itiberculatus appears mainly in tha northern area, although the centres of greatest density are relativelyvariable in the different eruises, and very widespread, generally loeated at depths of over 200 m. PolYbius henslowii. Variograms show ranges betwaen 12-20 Km in eruise C83 and 28 Km in C84. C83 variograms have an important nugget effeet, suggesting undeteeted mierostruetures. Incruise 184~ withverylow densities, spatial eovarianee is present oniV in a short range (7.5 Km) and no pattern is apparent analyzing riort.hern arid sOlJtherri areas sepäratelY. In the three eruises, maximum densities are loeated in zones opposite1:he Rfas Baixas and in the Fisterra-Estaca zone, with maximum density values (> 2000·t1our-') .. 3 j found in coastal areas. The patch structure in C84 is less complex than in othe'r crujses. I I Munida intermedia. In C83 spatial covariance is undetected. In 184 the variograms are noisy although they show two maximums of semivariance ai 13 and 22 Km. In cruise C84, which the highest densities, spatial cevariances'range up to 25 Km (20 Km in the North). Variograms showing spätial covariance do not'preseni nugget effects. Maximum catches of M. intermedia are loeated in the deepesi zone of the Fisterra-Estaea area and out of the Rras Baixas, riear the eoäst.
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