Diversity Patterns of Exploited Demersal Fish Communities ... · Tracian Sea, 150 species occurred...

American Fisheries Society Symposium 38:651–667, 2003© 2003 by the American Fisheries Society

Diversity Patterns of Exploited Demersal Fish Communitieson the Continental Shelf and Slope in the Greek Seas

(Northeastern Mediterranean)

MARY LABROPOULOU*, COSTAS PAPACONSTANTINOUNational Centre for Marine Research, Agios Kosmas, Hellinikon, 16 604 Athens, Greece

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More recent studies of fish community struc-ture have focused on patterns of spatial and tem-poral variation in composition, abundance, anddistribution of demersal fish assemblages of thecontinental shelf and slope at several latitudes(Markle et al. 1988; Bianchi 1991, 1992a, 1992b;Fujita et al. 1995; Fariña et al. 1997b; García et al.1998; Moranta et al. 1998). Changes in speciescomposition and size structure of demersal fishcommunities in response to fishing have beendemonstrated (Haedrich and Barnes 1997; Sains-bury et al. 1997; Zwanenburg 2000). One keychallenge appears to be to incorporate ecosystemobjectives within fisheries management, includingmeasurable indicators such as ecosystem diversity,species diversity, and ecologically dependentspecies (Gislason et al. 2000).

Information about soft-bottom fish assem-blages is particularly scarce in the eastern Mediter-ranean region, where demersal fishes are heavilyexploited as principal targets or as bycatch. InGreek waters, demersal fishes of the continentalshelf and slope are subjected to an intensive fisherycarried out by trawl, gill-net, and longline fleets.The gill nets and longlines catch a few species,whereas the trawls exploit a multispecies fishery,targeting several demersal and benthic species. Theresults of experimental trawl fishing in the Greekseas indicate that commercially important demer-sal and inshore stocks suffer from overfishing. As aresult, commercial catches consist of mainly imma-ture individuals and various noncommercialspecies that are discarded (Stergiou et al. 1997).

In the present study, trawl catch data obtainedseasonally (summer 1983 to autumn 1993) fromthe continental shelf and the upper slope of a broadgeographical area in the Greek seas (northernAegean, Thracian, and Ionian seas) were analyzedto determine the structure of the demersal fishcommunity. The main objectives were to definethe faunal composition, spatial structure, main fau-nistic assemblages, and diversity patterns in thearea and to explore whether there were any general

Reductions in catch rates and the mean size ofcaught fish is well documented in world fisheries(Pitcher 1996). Consequently, new approaches tothe study of exploited populations have been sug-gested, including the study of the fish assemblagestructure in relation to environmental variablesand the characterization of seasonal changes toimprove management practices. Despite thisprogress, basic descriptions of demersal fish faunascomprising both commercially exploited speciesand nontargeted components are not yet widelyavailable for many coastal and offshore regions,although their value with respect to understandingpossible fishing effects may be high (Rogers et al.1999). As pointed out by Caddy and Sharp (1988),this type of study is a necessary step toward under-standing the dynamics of multispecies stocks. Suchwork then can be extended to descriptive commu-nity dynamics to find general patterns that may beassociated with particular environmental condi-tions and fishing effort.

Changes in species composition with depth oncontinental shelf and slope are well established formany areas (e.g., Haedrich et al. 1980; Carney etal. 1983; Abelló et al. 1988; Hecker 1990; Cartesand Sardá 1993; Koslow 1993; Smale et al. 1993;Cartes et al. 1994; Sardá et al. 1994; Gordon et al.1995; Fariña et al. 1997a). Physical and biologicalfactors have been discussed as causes of faunalzonation with depth. Hydrographic conditions, thesteepness of the continental slope, and substratetype are among the major physical factors consid-ered. Resource availability, predator–prey relation-ships, and interspecies competition are the mostimportant biological factors reported. However,most studies have examined megafaunal assem-blages, and little attention has been paid to thestructure of the highly exploited demersal fishassemblages, apart from the degree to which differ-ent fish species are zoned with depth.

*Corresponding author. Telephone: 210 9822557 Fax: 210 9811713;E-mail: [email protected]

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trends in the distribution of fish fauna relative todepth, season, or geographical location.

MethodsStudy area and sampling procedure

The northern Aegean Sea is an almost rectangularbasin, separated from the southern Aegean Sea bythe archipelago of the Cyclades islands. The bot-tom topography of the northern Aegean Sea ischaracterized by a series of deep trenches andtroughs (with depths reaching 1,600 m) separatedby shallow sills and shelves. The northern part, theThracian Sea, lies over an extended shelf and isseparated on its west from the Gulf of Salonika(Thermaïkos Kólpos) by the Chalcidice (Khalkidi-ki) peninsula. The North Aegean Trough, whichextends in a southwest–northeast direction, sepa-rates the Thracian Sea from the rest of the north-ern Aegean Sea (hereafter referred to as the North-ern Aegean Sea), which is characterized by a high-ly irregular coastline, the presence of many islands

and a very narrow continental shelf (Figure 1). TheNorthern Aegean Basin is the region where theeastern Mediterranean receives the Black Sea out-flow. Water from the Black Sea is the major sourceof fresh and brackish water for the region; the con-tribution of all the rivers flowing into the AegeanSea is smaller than that of the Black Sea by at leastone order of magnitude (Poulos et al. 1997).

The Northern Aegean Sea is also influencedby nutrient inputs from outflowing Black Seawaters as well as by freshwater runoff along itsnorthern rim. Therefore, it is characterized byhigher nutrient and plant pigment concentrationsthan the rest of the eastern Mediterranean, whichis known to be the one of the most oligotrophicmarine regions of the world (Stergiou and Pollard1994). Environmental parameters (e.g., tempera-ture, salinity) are fairly constant along the slopebelow 200 m (13.5–14°C and 38.0–39.0‰, respec-tively; D’Onghia et al. 1995), while muddy sedi-ments generally predominate on the shelf and

LABROPOULOU AND PAPACONSTANTINOU

Figure 1. Sampling stations on the continental shelf and upper slope of the study area: Thracian Sea (■),Ionian Sea (▲), and Northern Aegean Sea (●).

653DIVERSITY PATTERNS OF EXPLOITED DEMERSAL FISH COMMUNITIES

slope, from 100 to 1,000 m deep (Lykousis andCollins 1987).

Surrounded by Italy, Greece, Libya, andTunisia, the Ionian Sea is the largest sea of the east-ern Mediterranean (Figure 1). The Greek (eastern)part of the Ionian Sea is characterized by the exis-tence of the deep Greek Trench, lying along thewestern and southwestern Greek coast and theislands of the Cretan Arch. The Ionian Sea is influ-enced by three main water masses: modifiedAtlantic water, Levantine intermediate water, andintermediate water of the eastern MediterraneanSea (Theocharis et al. 1993). The area under studyextends beyond the Gulf of Patras to the Gulf ofCorinth to the east and to the Ionian Sea to thewest, at a maximum depth of 390 m. River runoff issignificant because two main rivers—Achelous(Akhelóos) and Evinos—discharge water into thearea, greatly affecting the salinity of the surfacelayer, which ranges from 35.5 to 38.0‰. However,at depths below 60 m, temperature and salinity areconstant, at about 14–14.8°C and 38.3–38.7‰,respectively (Friligos et al. 1985).

A total of 717 hauls were taken during 28experimental bottom-trawl survey cruises on a sea-sonal basis from standard depth stations between16 and 500 m (Figure 1). Specifically, 216 haulswere taken in the Ionian Sea from summer 1983 tospring 1985, 215 in the Northern Aegean Seabetween summer 1990 and spring 1992, and 286 inthe Thracian Sea from fall 1991 to winter 1993.Sampling stations were selected randomly withdepth stratification. The otter trawl used (footrope65.7 m long, headline 1.5 m high) was equippedwith a cod-end bag liner of 16-mm stretched mesh.Samples were collected in daylight, between 0800and 1700 hours. The duration of each trawl (bot-tom time) was 30–60 min, and trawling speed fluc-tuated between 2.5 and 3.0 knots depending on thedepth and the nature of the substrate. The catchfrom each haul was sorted and identified by species,and each species was enumerated and weighed sep-arately on board. Part of the catch was frozen orpreserved in formalin for later study. Because allhauls were carried out using a commercial trawlvessel and the same fishing gear, gear selectivitywas assumed to be constant.

Data analysis

Species abundance was calculated for each haulafter standardization of the data to a 1-h tow, mak-ing it possible to allow comparisons between sam-pling stations. Those species regarded as markedlypelagic in behavior were excluded from the analy-

ses because they had not been quantitatively sam-pled. To identify zonation patterns, cluster analysiswas applied to the species abundance matrix. Clus-ter analysis (group average), using the Bray–Curtissimilarity index (Field et al. 1982), was applied tothe standardized abundance values of the speciesusing the PRIMER algorithms (Plymouth MarineLaboratory). To normalize the data and avoidskew, a square-root transformation was applied tothe abundance data prior to cluster analysis (Fieldet al. 1982).

Multidimensional scaling (MDS) ordinationanalysis was performed with the same configurationas in cluster analysis with respect to similarityindex and transformation. The typifying and dis-criminating species of the cluster of stations weredetermined using the SIMPER procedure (Clarke1993), which indicates the average contribution ofeach species to the similarity (typifying species)and dissimilarity (discriminating species) betweengroups of samples. Variation in relative abundanceof species was also examined by using the graphicalrepresentations of species cumulative frequencydistributions (k-dominance curves [Lambshead etal. 1983]). Relative abundance of demersal specieswas superimposed using the ABC method of War-wick (1986) to provide information on the size ofthe most dominant species.

We calculated the ecological parameters ofabundance, number of species (S), species diversity,Shannon–Wiener index H′ (Hurlbert 1978)

nH′ = Σ (pi)(ln pi)

i=1

species richness d (Margalef 1968)

s − 1d = ——logN

evenness J (Pielou 1966)

H′J = ———Hmax

and dominance or Simpson’s index D (Krebs 1989)

D = Σ pi2

for each of the station-groups defined by clusteranalysis, where pi is the proportion of total samplebelonging to ith species, N is the number of indi-viduals of the entire sample, and Hmax = log(S). Fur-thermore, analysis of variance (ANOVA) was usedto determine how these attributes changed withwater depth, and the a posteriori Tukey’s test was

used to locate the source of any differences. Beforeusing parametric tests, the assumptions of normali-ty and homoscedasticity were tested, and whenthese assumptions failed, the data were log(× + 1)-transformed (Sokal and Rohlf 1981; Zar 1984). Allstatistical inferences were based on a significancelevel of 0.05.

ResultsOverall catch

A total of 179 demersal fish species belonging to 61families were collected from 717 trawls. In theNorthern Aegean Sea, 136 species were found overthe continental shelf (


Figure 2. Classification (a) and ordination (b) of sampling stations based on species abundance from theNorthern Aegean Sea.

656 LABROPOULOU AND PAPACONSTANTINOU

Figure 3. Classification (a) and ordination (b) of sampling stations based on species abundance from theThracian Sea.


Figure 4. Classification (a) and ordination (b) of sampling stations based on species abundance from theIonian Sea.


Table 1. Dominant fish species by area, based on abundance rank for each station group identified by clusteranalysis. Densities (%N) are averaged over all samples in each group. Cum.%, average contribution of eachspecies to the similarity (typifying species) and dissimilarity (discriminating species) between groups of samples.

Group no. Species %N Cum.%

Thracian Sea

Group I, 16–28 m (avg similarity 67.8; SD 4.9)Arnoglossus laterna 13.55 12.37Serranus hepatus 8.75 23.29Diplodus annularis 4.24 32.27Gobius niger 6.74 40.72Mullus barbatus 30.75 48.11Trisopterus minutus capelanus 1.21 53.63Spicara flexuosa 5.43 59.14Trigla lucerna 1.61 64.48Merlangius merlangus euxinus 3.54 69.45Scorpaena notata 0.45 74.15Merluccius merluccius 0.76 78.50Gobius paganellus 4.45 82.18Solea vulgaris 4.45 85.36Cepola rubescens 0.65 88.03

Group II, 30–90 m (avg similarity 73.8; SD 7.1)Serranus hepatus 17.36 7.85Trisopterus minutus capelanus 17.59 15.12Mullus barbatus 7.44 21.29Arnoglossus laterna 10.12 27.00Merluccius merluccius 3.21 32.08Spicara flexuosa 6.92 37.10Lepidotrigla cavillone 2.75 41.71Cepola rubescens 2.34 46.23Deltentosteus quadrimaculatus 5.35 50.73Callionymus maculatus 3.40 55.06Scyliorhinus canicula 2.45 58.74Citharus linguatula 1.67 62.33Lophius budegassa 0.90 65.76Serranus cabrilla 0.84 68.74Symphurus ligulatus 0.58 71.51Gaidropsarus sp. 0.42 74.05Raja clavata 0.15 76.18Arnoglossus thori 0.75 78.27

Group III, 100–190 m (avg similarity 73.6; SD 4.4)Trisopterus minutus capelanus 27.74 10.17Merluccius merluccius 10.16 18.29Argentina sphyraena 2.04 23.95Lophius budegassa 2.26 29.31Lepidorhombus boscii 1.38 34.50Arnoglossus laterna 2.46 39.37Scyliorhinus canicula 2.18 44.11Lepidotrigla cavillone 4.05 48.77Callionymus maculatus 2.54 53.41Cepola rubescens 1.59 57.88Serranus hepatus 6.99 62.10Capros aper 0.88 66.07Phycis blennoides 0.31 69.75Aspitrigla cuculus 1.68 73.02Trigla lyra 0.33 76.27Mullus barbatus 4.53 79.52


Table 1. continued


Thracian Sea

Group IV, 200–500 m (avg similarity 72.3; SD 7.8)Hymenocephalus italicus 14.71 11.03Gadiculus argenteus argenteus 24.52 21.95Lepidorhombus boscii 5.30 31.81Micromesistius poutassou 26.42 40.85Coelorhynchus coelorhynchus 2.41 47.97Phycis blennoides 1.06 54.59Lophius budegassa 1.14 61.12Argentina sphyraena 14.07 67.63Merluccius merluccius 6.88 74.03Galeus melastomus 0.50 78.95Trigla lyra 0.37 82.62Capros aper 0.16 86.12

Ionian Sea

Group I, 25–38 m (avg similarity 73.6; SD 6.7)Mullus barbatus 18.95 6.92Spicara flexuosa 16.61 13.80Merluccius merluccius 6.33 19.88Serranus hepatus 3.58 24.83Lepidotrigla cavillone 1.59 29.72Pagellus erythrinus 4.68 34.50Diplodus annularis 5.72 38.97Pagellus acarne 9.52 43.37Spicara smaris 10.11 47.05Arnoglossus laterna 2.57 50.42Trachurus trachurus 4.38 53.65Boops boops 1.79 56.62Cepola rubescens 1.35 59.55Trisopterus minutus capelanus 0.17 62.35Trachurus mediterraneus 0.54 65.13Sparus aurata 0.26 67.79Lophius budegassa 0.19 70.45Citharus linguatula 0.08 73.06Scorpaena notata 0.27 75.20Blennius ocellaris 0.04 77.34

Group II, 42–96 m (avg similarity 75.0; SD 5.4)Merluccius merluccius 18.68 7.07Spicara flexuosa 15.51 14.02Trisopterus minutus capelanus 8.50 19.94Lepidotrigla cavillone 5.21 24.88Mullus barbatus 11.57 29.43Serranus hepatus 0.03 33.78Trachurus trachurus 2.74 37.52Cepola rubescens 1.17 40.80Boops boops 1.30 44.07Arnoglossus laterna 1.69 47.27Spicara smaris 8.27 50.44Lophius budegassa 0.59 53.40Eutrigla gurnardus 1.34 56.33Trachurus mediterraneus 0.59 59.02Argentina sphyraena 0.52 61.52Pagellus erythrinus 2.18 63.99


Table 1. continued


Ionian Sea

Group II, 42–96 m (avg similarity 75.0; SD 5.4) (continued)Deltentosteus quadrimaculatus 0.51 66.39Citharus linguatula 0.41 68.65Blennius ocellaris 0.16 70.75Diplodus annularis 2.45 72.79Conger conger 0.14 74.72

Group III, 101–194 m (avg similarity 69.1; SD 7.2)Merluccius merluccius 47.14 12.43Lepidotrigla cavillone 1.76 17.30Argentina sphyraena 6.30 22.00Serranus hepatus 1.49 26.53Trachurus trachurus 1.64 31.04Trisopterus minutus capelanus 4.65 34.76Mullus barbatus 0.51 38.21Cepola rubescens 0.52 41.15Arnoglossus laterna 0.25 44.06Callionymus maculatus 0.71 46.94Gadiculus argenteus argenteus 4.74 49.74Raja clavata 0.15 52.49Capros aper 6.19 55.23Lophius budegassa 0.15 57.94Spicara flexuosa 1.07 60.60Citharus linguatula 0.24 63.11Aspitrigla cuculus 0.24 65.50Micromesistius poutassou 18.30 67.84Boops boops 0.14 70.07Eutrigla gurnardus 0.19 72.25

Group IV, 203–390 m (avg similarity 62.3; SD 5.7)Gadiculus argenteus argenteus 85.99 19.66Argentina sphyraena 2.47 29.28Merluccius merluccius 0.76 36.58Phycis blennoides 0.19 41.88Coelorhynchus coelorhynchus 1.02 46.76Lepidorhombus boscii 0.16 50.97Micromesistius poutassou 1.20 55.12Scyliorhinus canicula 0.66 59.23Lophius budegassa 0.06 63.25Capros aper 2.83 67.07Gaidropsarus sp. 0.02 70.35Lepidotrigla dieuzeidei 1.54 73.11Raja clavata 0.09 75.85Molva dipterygia macrophthalma 0.03 78.30

Northern Aegean Sea

Group I, 25–32 m (avg similarity 72.7; SD 4.8)Mullus barbatus 62.32 8.68Serranus hepatus 7.08 15.87Spicara flexuosa 3.50 22.52Pagellus erythrinus 6.42 27.82Pagellus acarne 0.84 32.45Trachurus mediterraneus 3.59 36.62Boops boops 0.39 40.50Merluccius merluccius 2.65 43.77


Table 1. continued


Northern Aegean Sea

Group I, 25–32 m (avg similarity 72.7; SD 4.8) (continued)Serranus cabrilla 0.28 46.99Citharus linguatula 0.11 49.95Arnoglossus laterna 1.12 52.87Cepola rubescens 0.32 55.76Gobius niger 0.11 58.36Diplodus annularis 4.58 60.92Lepidotrigla cavillone 0.01 63.15Raja milaretus 0.02 65.37Uranoscopus scaber 0.16 67.45Trigloporus lastoviza 0.01 69.40Mullus surmuletus 0.01 71.32

Group II, 44–98 m (avg similarity 72.5; SD 5.3)Merluccius merluccius 6.88 9.93Trachurus trachurus 7.99 14.27Trisopterus minutus capelanus 7.25 22.70Mullus barbatus 23.45 30.22Citharus linguatula 7.94 33.67Arnoglossus laterna 2.31 39.94Spicara flexuosa 7.34 42.86Serranus hepatus 11.72 45.63Lesueurigobius friesii 1.15 48.20Lepidotrigla cavillone 1.97 50.48Cepola rubescens 1.29 52.66Lophius budegassa 0.77 54.81Deltentosteus quadrimaculatus 0.54 56.97Gaidropsarus sp. 0.31 59.02Callionymus maculatus 0.97 60.94Zeus faber 0.17 62.82Symphurus ligulatus 0.30 64.69Pagellus erythrinus 1.23 66.42Trachurus mediterraneus 0.73 68.13Trigla lyra 0.47 69.75

Group III, 100–195 m (avg similarity 69.2; SD 6.2)Argentina sphyraena 16.55 10.90Mullus barbatus 6.07 18.15Lepidotrigla cavillone 5.99 25.35Trachurus trachurus 7.73 32.45Merluccius merluccius 9.06 36.08Capros aper 10.44 38.75Scyliorhinus canicula 4.70 41.20Trigla lyra 2.06 43.65Aspitrigla cuculus 2.37 46.10Trisopterus minutus capelanus 3.81 48.48Serranus hepatus 2.60 50.75Raja clavata 0.47 52.84Lophius budegassa 0.69 54.90Lepidotrigla dieuzeidei 1.19 56.79Citharus linguatula 1.89 58.66Mullus surmuletus 1.63 60.50Arnoglossus laterna 0.49 62.34Eutrigla gurnardus 0.15 64.05Callionymus maculatus 0.34 65.71


other bottom and oceanographic characteristicsplay a role, at least for structuring assemblages onthe continental shelf between the three areas. Sev-eral factors appear to contribute to this geographi-cal differentiation: the gradient in eutrophy, freshand brackish water runoff, temperature and salinitydifferences along a north–northwest to south–southeast axis, and differences in the extent andthe bottom type of the continental shelf. Never-theless, the most important quantitative boundaryfor all areas was located around 200 m, a depth sep-arating the species of the continental shelf fromthose of the upper slope extending down to 500 m.These results are in agreement with the findings ofSnelgrove and Haedrich (1985), who stated thatdeep assemblages tend to have a much broaderdepth range than do shallow assemblages. At thisdepth, the species richness, abundance, and bio-mass decline markedly. Below 200 m, some factorsof the benthic environment were remarkably sta-ble. Sediment structure consisted of a dominantsilt–clay fraction, and temperatures above thesesediments were constant. This may be the majorreason for the high degree of similarity among thespecies composition of the upper slope in theNorthern Aegean, Ionian, and Thracian seas.

Table 1. continued


Northern Aegean Sea

Group III, 100–195 m (avg similarity 69.2; SD 6.2) (continued)Raja milaretus 0.16 67.37Lepidorhombus boscii 2.38 69.01Trachurus mediterraneus 0.47 70.64

Group IV, 215–490 m (avg similarity 63.8; SD 5.9)Gadiculus argenteus 28.72 7.06Lepidorhombus boscii 5.71 13.43Coelorhynchus coelorhynchus 6.72 19.68Hymenocephalus italicus 9.29 25.56Micromesistius poutassou 12.58 31.40Merluccius merluccius 2.68 36.05Argentina sphyraena 8.22 40.66Phycis blennoides 0.87 45.22Trigla lyra 1.12 49.71Lophius budegassa 0.50 53.67Molva dipterygia macrophthalma 0.53 57.45Scyliorhinus canicula 1.58 61.14Peristedion cataphractum 0.19 64.31Capros aper 0.41 67.39Galeus melastomus 0.41 69.50Raja clavata 0.22 71.56Chlorophthalmus agassizi 1.43 73.37

DiscussionFour distinctive demersal fish assemblages wereclearly associated with the topography of the studyareas. A shallow assemblage that reached to about30 m represented coastal shallow bottom fauna ofthe continental shelf. Two assemblages correspond-ed to the middle (30–95 m) and deeper (100–200m) parts of the continental shelf, and a deep assem-blage extended beyond that depth, representingthe upper slope. The continental shelf assemblagesexhibited greater abundance and contained speciesof larger size and commercial interest, such as Euro-pean hake Merluccius merluccius, striped goatfish,red mullet Mullus surmuletus, common pandoraPagellus erythrinus, and poor-cod Trisopterus minutuscapelanus. The upper slope assemblage is character-ized by the presence of species such as silvery pout,European argentine Argentina sphyraena, glassheadgrenadier Hymenocephalus italicus, hollowsnoutgrenadier Coelorhynchus coelorhynchus, androughtip grenadier Nezumia sclerorhynchus, whichare small and not commercially important.

The main determining feature associated withthe structure of the demersal fish assemblages isdepth, because it reflects the changes from the con-tinental shelf to the continental slope. However,


Shifts in the abundance and occurrence ofdemersal fish species were associated with a depthgradient, even though the species were found overrelatively broad depth ranges in each of the studyareas. Species abundance varied significantly withdepth, but whenever depth ranges of these speciesoverlapped, the depth of maximum abundancechanged. Brown (1984) suggested that for ecologi-cally similar species, those that attain the highestlocal population densities tend to occur in a greaterproportion of sampling sites and tend to have awider spatial distribution. Hecker (1990) suggested

Figure 5. Comparison of k-dominance curves for demersal fish communities between each station-group inthe three locations.

that the changes in species composition betweendifferent megafaunal assemblages are due to thesubstitution of the dominant and subdominantspecies throughout the depth gradient by a contin-uous faunistic turnover.

Fish in the inshore zone tend to undertake anontogenetic migration into deeper water (Mac-Pherson and Duarte 1991; Warburton and Blaber1992; Blaber et al. 1995). Caddy (1993) hypothe-sized an offshore movement of older fish in severaldemersal species (e.g., common seabream Pagruspagrus, common pandora, red mullet, and striped


red mullet Mullus surmuletus), possibly contributingto the continuing good recruitment and stockrecovery in many areas. The preponderance of thepositive size–depth relationships in fish species mayreflect a fundamental aspect of fish life history(Cushing 1975). The occurrence of smaller,younger individuals in shallower water and themovement toward deeper waters during ontogenymust involve a substantial advantage. It has beensuggested that, by migrating to deeper waters, adultscould benefit from a reduced basal metabolic rateand increased life expectancy at lower temperatures(MacPherson and Duarte 1991). The observed pat-tern of gradual species replacement along the depthgradient is based primarily on the ontogenetic habi-tat shifts for some of the species and indicates thathabitat selection may be based on the interactionbetween density-dependent food resources and den-sity-independent environmental factors.

Regardless of the actual balance between biot-ic and abiotic factors in determining the demersalfish assemblages, the occurrence of relativelyhomogeneous areas of species composition has rel-evance to multispecies management. Mixed catch-es within the area of a given assemblage offer aredundancy in terms of species composition and

relative abundance. Such information can be ofvalue in dealing with bycatch and provide generalguidelines for overall rational planning and man-agement (Gomes et al. 1992).

Community classifications should take tempo-ral persistence into account. However, most studiesof the zonation of demersal fish fail to considertemporal variation (Markle et al. 1988; Bianchi1991, 1992a, 1992b; Moranta et al. 1998). In thepresent study, multivariate analyses did not detectany annual changes in the overall structure of theassemblages in each area. However, results suggestthat changes in the composition of assemblages areweakly associated with seasonal changes. There-fore, the demersal fish communities persisted overtime and species rankings remained constant, atleast on our cruise dates. Nonetheless, to detecttemporal change in groundfish assemblages, thelength of the time series must be considered. Someinferences are possible only with a long-term dataseries. For example, in a study lasting from 1978 to1991 on the Newfoundland continental shelf,Gomes et al. (1995) found that even though stocksizes had been declining steadily, changes in speciesassemblages could be detected only after severalyears of decline.

Table 2. Ecological parameters by area and depth zone and summary of statistical tests for the demersal fishcommunities in the Greek seas. For abundance, mean and SE were calculated using log(× + 1)-transformeddata. *P < 0.001.

Depth zone Depth (m) Abundance No. of species S Richness d Diversity H′ Evenness J′ Dominance D(fish/h)

Thracian Sea

1 22.5 (16–28) 3.63±0.7 25.3±1.38 2.86±0.17 1.87±0.08 0.59±0.02 0.27±0.022 57.3 (30–90) 3.36±0.3 32.7±0.54 4.26±0.10 2.31±0.05 0.67±0.01 0.19±0.013 122.1 (100–190) 3.24±0.4 32.8±0.82 4.10±0.07 2.23±0.03 0.64±0.01 0.19±0.014 303.4 (200–500) 3.12±0.5 25.3±0.90 3.21±0.11 1.76±0.05 0.55±0.01 0.30±0.02F* (3, 280) 7.36 23.63 32.82 27.46 14.13 14.49

Ionian Sea

1 35.4 (25–38) 3.14±0.1 23.2±1.07 3.06±0.16 1.93±0.13 0.62±0.04 0.23±0.052 62.4 (42–96) 2.98±0.4 21.6±0.43 3.00±0.06 1.84±0.05 0.60±0.02 0.28±0.023 115.1 (101–194) 3.22±0.6 21.7±0.61 2.88±0.09 1.65±0.08 0.54±0.02 0.36±0.034 284.2 (203–390) 3.52±0.6 15.2±0.59 1.79±0.09 0.92±0.08 0.32 (±0.02) 0.61±0.03F* (3, 212) 18.88 32.11 47.46 36.71 36.14 33.59

Northern Aegean Sea

1 32.0 (25–32) 4.16±0.1 28.3±1.92 2.86±0.23 1.42±0.12 0.42±0.04 0.43±0.032 67.6 (44–98) 3.19±0.7 29.7±0.55 3.89±0.07 2.21±0.03 0.66±0.01 0.22±0.023 140.1 (100–195) 3.21±0.3 27.7±1.05 3.63±0.12 2.07±0.06 0.63±0.02 0.18±0.014 359.1 (215–490) 3.23±0.4 24.7±0.59 3.19±0.07 1.92±0.04 0.61±0.01 0.24±0.01F* (3, 211) 19.81 12.59 21.01 21.38 15.23 18.58


Overall, in the Greek seas, 447 fish speciesbelonging to 129 families have been reported(Papaconstantinou 1988). Thus, the number ofspecies recorded in the present study represents40.1% of the total marine ichthyofauna in Greece.However, species composition differs over the areasand depths examined. High species richness anddiversity characterized the continental shelf, butboth variables decreased markedly with depth,whereas the reverse was true for species dominance.On the other hand, evenness did not change withdepth, indicating little variability in the numericalcodominance of species over the depth range exam-ined. The disparities in these general trends forgroup I in each area may be attributable to the morevariable environments in shallow coastal waters.The k-dominance analysis suggested that the spatialtrend in diversity and dominance was a strong fea-ture of the species assemblages under study, withdominance being the highest in group IV. Bianchiet al. (2000), who investigated whether changes indiversity and dominance could be related to fishing,concluded that the largest changes in diversityappeared to be due to changes in evenness, speciesrichness, or both, often leading to an increase indiversity in response to heavy exploitation.

The overall structure of the demersal fish com-munities studied here stresses once more the multi-species character of the Greek fisheries. Manage-ment should take into account this diversity,because fishing activities in these areas are notspecies selective. Despite the enforcement of regu-lations for demersal and inshore fisheries in Greekwaters (i.e., closed seasons and areas, limited issueof licenses, minimum legal landing size, and meshsize regulations), the fishing effort in the NorthernAegean, Ionian, and Thracian seas—as well as inthe rest of the Greek waters—is very high (Ster-giou and Pollard 1994). Studies in other areas,based on extended time series during which majorincreases in fishing effort took place, indicate thatfishing led to a decrease in catches and to increas-es in noncommercial species (Overholtz and Tyler1985; Rothschild 1992). Furthermore, evidenceindicates that the size structure of demersal fishcommunities is affected by fishing. The overalltrend is a reduction in large fish and a relativeincrease in small fish (Bianchi et al. 2000; Zwa-nenburg 2000).

In the present study, the fish assemblagesunder consideration have suffered a long history offishery exploitation. Therefore, overfishing hasaffected the population structures and densities ofthe demersal fish communities—at least at depths

up to 200 m, where most of the fishing activity isfocused. It is possible that the organization of thedemersal fish assemblages analyzed are determinedto a great extent by unidirectional trend inducedby fishing, bottom topography, and oceanographicfeatures of the study areas.

AcknowledgmentsThe authors thank all those involved in the dem-ersal trawl surveys carried out by the FisheriesDepartment of the National Center for MarineResearch in the northern Aegean, Thracian, andIonian Seas for their valuable help.

This work was partially supported by the Euro-pean Union Directorate-General (EU DG) XIV,contract no. MA-1-90, and the Greek GeneralSecretariat for Research and Technology.

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Diversity Patterns of Exploited Demersal Fish Communities ... · Tracian Sea, 150 species occurred...

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