Bottom trawl discards in the northeastern Mediterranean Sea

Fisheries Research 53 (2001) 181±195

Bottom trawl discards in the northeastern Mediterranean Sea A. Machiasa,*, V. Vassilopouloub, D. Vatsosa, P. Bekasb, A. Kallianiotisc, C. Papaconstantinoub, N. Tsimenidesa a

Institute of Marine Biology of Crete, P.O. Box 2214, GR 71003 Iraklion, Greece b National Centre for Marine Research, Hellinikon, 16604 Athens, Greece c Fisheries Research Institute, Nea Peramos, Kavala, Greece

Received 17 March 2000; received in revised form 25 July 2000; accepted 6 October 2000

Abstract Discard practices of trawlers in three main areas of the Aegean and western Ionian Seas were examined. Data collected on board commercial vessels during 3 years (1995±1998) of seasonal (Autumn, Winter, Spring) monitoring were used to estimate discarded quantities. About 44% (range 39±49%) of the total catch was discarded at sea (13 500±22 000 t annually); the main component of the discards was ®sh. Hauls were classi®ed into two main clusters using the discard quantities of ®sh, crustaceans, and cephalopods. The two clusters were discriminated by depth. Two sets of equations that classify new hauls in the clusters, using depths and duration of the hauls or marketable yield, were applied. The discarded yield of ®sh showed more precise relationships with their marketable yield in each season than did crustaceans and cephalopods. The latter were strongly affected by the speci®c characteristic of the different areas and clusters. # 2001 Elsevier Science B.V. All rights reserved. Keywords: By-catch; Discard quantities; Northeastern Mediterranean; On-deck sorting

1. Introduction We use the term discards for marine fauna brought onto the deck of a ®shing vessel and subsequently returned to the sea. Several abundant species are discarded, dead or dying, either because of their size or because of poor commercial value. Discarding at sea, which is a key issue in ®sheries and a major source of uncertainty in ®sheries management, may result from economic constraints or from legal and administrative obligations imposed by the managerial scheme employed (Saila, 1983). Alverson et al. (1994) estimated average global discards at 27 Mt *

Corresponding author. Tel.: ‡30-81-393-429; fax: ‡30-81-393-400. E-mail address: [email protected] (A. Machias).

(27% of the global catch), ranging from 17.9 to 39.5 Mt. Fishery biologists and management agencies have recognized the importance of reliable quantitative information on the discrepancies between landings and actual catches of a species (Alverson et al., 1994; Tsimenides et al., 1995; Stergiou et al., 1998; Stratoudakis et al., 1999a). Conclusions derived from stock assessment studies are clearly affected by the availability and reliability of information on the quantities of ®sh discarded at sea. So far, stock management has relied heavily on landings. However, estimates of ®shing mortality based on landings rather than catches (which include discards) are likely to be biased downwards, since discarded organisms do not generally survive, and represent a potentially signi®cant, economic loss (Chen and Gordon, 1997; Philippart, 1998; Stratoudakis et al., 1999b and references therein).

0165-7836/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 7 8 3 6 ( 0 0 ) 0 0 2 9 8 - 8

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Inclusion of discards, when these are signi®cant will reduce the inaccuracies (especially of recruitment) and improve the potential quality of assessments. Moreover, ®shing mortality of a population can be due to by-catch or death during ®shing; a process that stresses them, making them vulnerable to scavengers and other predators. Trawling therefore increases forage availability (direct or indirect) to several species in the system. The pay-off of biomass removal from the sea and/or return to the sea has an important effect on the shape and function of communities, which has so far been very little investigated (Jennings and Kaiser, 1998; Hall, 1999). The Mediterranean ®sheries are highly diverse in terms of species and ®shing gears used, and are not managed through TACs and quotas. Hence, discarding is perceived mainly as throwing away unmarketable species, or species and size groups of low commercial value, because of legal obligations imposed by regulation of minimum landing sizes. Although all types of gear produce discards, bottom trawling has the greatest impact (Stergiou et al., 1998). The small cod end used in the eastern Mediterranean might suggest a higher discard ratio than other regions (Alverson et al., 1994), but available information on catches discarded by ®shing vessels in this area is limited (Tsimenides et al., 1995; Stergiou et al., 1998; Vassilopoulou and Papaconstantinou, 1998). The main problem in the Mediterranean is the absence of monitoring of the discarded fraction of the catches. The Greek trawl ®shery has a total annual catch of about 20 000 t, representing some 15% of the total marine catch, 26% of its wholesale value (Stergiou et al., 1997a). An important feature of ®shing in Greek waters is that trawling is banned from 1 June to 30 September. In the present study, discarded trawl catches in the three main ®shing areas of Greece were considered using data obtained from seasonal monitoring of the operation of representative commercial vessels for three consecutive years. As the information on the discarding procedure in oligotrophic areas was very limited, the main features of this process were presented, the quantities of discarded ®sh, crustaceans and cephalopods were estimated, and the discard ratio compared with that of other areas. Moreover, the possibility of making up for the lack of monitoring by determining relationships between discards, depths

of the hauls, and landings was examined. Speci®cally, the hauls were discriminated in clusters according to their depths and the yield of marketable and discarded catches. Furthermore, quantitative relationships between discarded and marketable catches were calculated to determine the possibility of obtaining shortterm estimation of discards from landings. 2. Material and methods 2.1. Sampling procedure Commercial ®shing areas in the Ionian Sea, the Thracian Sea, and the Cyclades Islands (Fig. 1) were selected for the study because they are important ®shing grounds and representative of the major commercial grounds in Greece (Stergiou and Pollard, 1994). In these areas, the catches were recorded on board commercial vessels three times a year, from 1995 to 1998 in (A) October (Autumn), which coincides with the opening of the trawling ®shing season for the Greek trawlers and the recruitment period of most species (Stergiou et al., 1997b), (B) February (Winter), which represents the middle of the ®shing season for the Greek trawlers, and (C) May (spring), just before the closure of the ®shing season. The sampling was carried out using commercial trawl nets with a stretched mesh size of 28 mm (knot to knot). Data were collected from at least 20 hauls in each area, during each sampling period, using two commercial vessels (not the same in each sampling). Hauls were classi®ed according to three predetermined depth zones: stratum A: 0±150 m, stratum B: 150±300 m and stratum C: >300 m. Vessels were representative of those from the areas studied, in terms of size and construction characteristics, and have been operating routinely in the above-mentioned areas for several years. 2.2. Data collection Field work included estimating the total catch per haul, as well as recording the faunistic composition of the catch, listed as ®sh, cephalopods and crustaceans, for which the identi®cation was made to the species level; other by-catch organisms, such as crinoids, echinoids, bivalves, and other taxa were classi®ed

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183

Fig. 1. Map of the study areas: (A) East Ionian Sea; (B) Thracian Sea; (C) Cyclades islands.

as ``other invertebrates''. For comparison, total catches were transformed to hourly yields (g/h). After the marketable catch was sorted by the crew, the number of boxes was counted and their weight calculated. The discarded portion of the catch was also put in boxes, the number of which was counted and weighted. Discarded species were sorted out, the number of the individuals recorded and their length and total weight per species noted. In cases, where certain species numbered many specimens, a representative sample was examined. Furthermore, the following data were recorded for each haul: (A) the

position, minimum and maximum depths and duration of the haul, as well as the stratum at which the trawl operated at least for the 50% of the haul; (B) the marketable and discarded fractions of each species; (C) the percentage of the marketable and discarded fractions of ®shes, crustaceans, cephalopods and ``other invertebrates''. 2.3. Data analysis Usually, commercial vessels operated in more than one stratum during each haul. Consequently, there has

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been need for post-classi®cation of the hauls, according to their ®shery characteristic and the amount of discards. This post-classi®cation grouped the ®shing grounds according to the discarded yield. The data from all areas were combined to examine differences in patterns of abundance of discards determined for each season using cluster analysis. The dissimilarity between two trawls j and k was de®ned using the Bray±Curtis distance equation djk: Ps jYij Yik j djk ˆ Psiˆ1 iˆ1 …Yij ‡ Yik † where Yij and Yik are the discarded catch i in stations j and k, respectively (Field et al., 1982). Biomass was log-transformed before the analysis. The unweighted group-average method was used for clustering the trawls. The cluster analysis was performed using the discarded amount of ®shes, crustaceans, cephalopods and others. Since the information on the discarding procedure in the areas under study was very limited, the main features of this procedure were presented, and the quantities of discards in the framework of the sampling were estimated. Discarded catches per unit effort (CPUE) were standardized for differences among areas (A), seasons (S), clusters (C) and ®shing periods (Y) by means of general linear models (Gavaris, 1980; Kimura, 1980). The following model was used: log CPUE ˆ A ‡ S ‡ C ‡ Y ‡ AS ‡ AC ‡ AY ‡ SC ‡ SY ‡ CY ‡ c, where c is a constant. Separate models were applied to ®shes, crustaceans, cephalopods, and the total discarded fraction. In each model, only signi®cant parameters were included, at a signi®cance level of 0.05 using a stepwise backward method. The results of analysis of variance for the main effects of the model are also presented, followed by a posteriori pairwise comparisons using the honestly signi®cant difference (HSD) test. In the cases of interaction separate analysis of variance was adopted between the factors. Moreover, the possibility to make up for any lack of monitoring by using the relationships among discards, depths of the hauls and landings, was examined. First, to determine which were the characteristics that could distinguish the groups de®ned by the cluster analysis, discriminant analysis was applied to each of the seasons, using as variables (A) minimum, max-

imum depths, and duration of a haul, and (B) marketable yield. Discriminant analysis was applied on the discard yield to determine whether the station-groups differed in this respect. The distinctiveness of stationgroups was measured using: (A) the Wilks' l criterion and its correspondent F-statistic to test the signi®cance of the overall difference between group centroids (Tasuaoka, 1971; Rao, 1973), and (B) the squared canonical correlation for each discriminant function, which was interpreted as the part of the total variance in the corresponding discriminant function accounted for by the groups (Lebart et al., 1984). Moreover, the percentage of stations correctly assigned to groups by the analysis was computed and considered as an indirect measure of the adequacy of the classi®cation feature. Second, the standardized discriminant function coef®cients, assigned to the variables by the discriminant analysis, were interpreted as giving the relative contribution of each variable in the discriminant functions (Green, 1971; Tasuaoka, 1971). Moreover, the classi®cation functions were estimated for use in classifying the new observations. The system classi®es a new case by evaluating each function and assigning the case to the cluster corresponding to the highest value (Bolch and Huang, 1974). Third, in each season relationships were established among discarded yield and marketable yield, duration of the hauls, minimum and maximum depth, clusters (as dummy variables), and areas (as dummy variables). The relationships were estimated using backward stepwise multiple regression analysis with dummy variables for each cluster. Cluster analysis was performed using the statistical package Primer, discriminant analysis, and general linear models were performed using the statistical package SYSTAT and regressions were estimated using the statistical package SIMSTAT. All statistical inferences refer to the p < 0:05 signi®cance level. 3. Results 3.1. Description of discards The typical duration of a trip was 1 day and the haul duration ranged from 40 min to 7 h. Usually, a commercial vessel operated in more than one stratum

A. Machias et al. / Fisheries Research 53 (2001) 181±195

during each haul, covering a great range of depths (up to 200 m). A total of 95 commercial vessels operated in the areas investigated, having typical engine powers of 400±500 hp. Most importantly, trawl operations have no speci®c target species and the ®shermen commonly try to catch all available commercial species. The species collected during the nine sampling periods were classi®ed into three categories: (1) com-

185

mercial species those that consistently exhibited marketable and discarded fractions (46 ®sh, three crustaceans, 10 cephalopods); (2) local commercial species those that were not present in all areas, or in other areas had a marketable fraction and in others only a discarded fraction (67 ®sh, seven crustaceans, six cephalopods); (3) discarded species those that always had only a discarded fraction in all areas (82 ®sh , 52 crustaceans, eight cephalopods).

Table 1 List of commercial ®sh species Argentina sphyraenaa Arnoglossus laternab Aspitrigla cuculusb A. obscuraa Aulopus filamentosusa B. ocellarisa Boops boopsb B. salpaa Callionymus lyraa C. maculatusa C. phaetona Centracanthus cirrusa Centrolophus nigerb Centrophorus granulosusa Citharus linguatulab Conger congerb Dasyatis centrouraa D. pastinacab D. dentexa D. macrophthalmusa D. maroccanusb Dicologoglossa cuneataa Diplodus annularisa D. puntazzoa D. sargusa D. vulgarisa E. encrasicholusa Epinephelus guazab Esox luciusa Eutrigla gurnardusb Galeorhinus galeusb Galeus melastomusb Gobius nigera Helicolenus dactylopterusb Heptranchias perloa Hexanchus griseusa Lepidopus caudatusa Lepidorhombus bosciib L. whiffiagonisb a

L. cavillonea Lithognathus mormyrusa Lophius budegassab L. piscatoriusb Merlangius merlangus euxinusa Merluccius merlucciusb Microchirus variegatusa Micromesistius poutassoub Molva elongataa Monochirus hispidusa Mugil cephalusa Muliobatis aquillaa Mullus barbatusb M. surmuletusb Mustelus asteriasa M. mustelusa Ophidion barbatuma Pagellus acarneb P. bogaraveob P. erythrinusb Pagrus pagrusb Peristedion cataphractuma Phrynorhombus regiusa Phycis blennoidesb P. phycisb Pomatomus saltatrixa Pteromylaeus bovinusa Raja albab R. asteriasa R. circularisa R. clavatab R. miraletusb R. montaguia R. naevusa R. oxyrinchusa R. polystigmaa R. radulaa Sardina pilchardusb Sarpa salpaa

Scomber japonicusa S. scombrusb Scophthalmus rhombusa Scorpaena elongatab S. notatab S. porcusb S. scrofab Scyliorhinus caniculab S. stelarisa S. cabrillab Solea kleinia S. soleaa Sparus auratab S. sphyraenaa Spicara flexuosab S. maenab S. smarisb Spondyliosoma cantharusa Squalus acanthiasa S. blainvilleia Symphurus nigrescensa Synodus saurusa Torpedo marmorataa Trachinus araneusa T. dracob T. radiatusa Trachurus mediterraneusb T. picturatusb T. trachurusb Trigla lucernab T. lyrab Trigloporus lastovizab Trisopterus minutus capelanusb Uranoscopus scaberb Zeus faberb

local commercial species: the species that were not present in all areas, or in other areas had a marketable fraction and in others only a discarded fraction. b The species that consistently exhibited marketable and discarded fractions.

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Table 2 List of totally discarded ®sh species Acantholabrus palloni Alosa fallax Anthias anthias Antonogadus megalokynodon Aphia minuta Argyropelecus hemigymnus Arnoglossus imperialis Arnoglossus rueppelli Arnoglossus thori Atherina boyeri A. hepsetus B. tentacularis Bothus podas Buglossidium luteum Callanthias ruber C. fasciatus C. reticulatus C. risso Capros aper Carapus acus C. rubescens Chimaera monstrosa C. agassizi Chromis chromis C. coelorhynchus Coris julis Crenilabrus cinereus D. colonianus

D. quadrimaculatus Diaphus metopoclampus Diplodus cervinus Echelus myrus Echiodon dentatus Epigonus telescopus Etmopterus spinax Gadella maraldi G. argenteus Gaidropsarus mediterraneus Gnathophis mystax G. colonianus G. cruentatus G. geniporus Hippocampus hippocampus H. ramulosus Hoplostethus mediterraneus Hymenocephalus italicus Lappanella fasciata L. dieuzeidei Lesueurigobius friesii L. suerii Macroramphosus scolopax Maurolicus muelleri M. ocellatus Molva dipterygia macrophthalma Nezumia sclerorhynchus Ophichthus rufus

A total of 114 ®sh species had both marketable and discarded fractions, while another 82 had only a discarded fraction. Ten crustacean species and 16 cephalopod species had marketable and discarded fractions, while another 52 crustacean species and eight cephalopod species had only a discard fraction (Tables 1±3). The most interesting differences between areas concerned Blennius ocellaris which has a commercial value only in the Thracian Sea, and Engraulis encrasicholus that was totally discarded in the Cyclades Islands, due to the small quantities caught. Cluster analysis revealed that hauls could be divided into two main groups in all seasons (Fig. 2). The most abundant totally discarded species, in the ®rst cluster were Serranus hepatus (100±1000 specimens per hour), Lepidotrigla cavillone (60±200 specimens per hour), Cepola rubescens (50±100 specimens per hour), and Deltentosteus quadrimaculatus (100± 200 specimens per hour), while in the second stratum

O. rochei Oxynotus centrina Parablennius gattorugine P. tentacularis Parophidion vassali R. brachyura R. melitensis Rhynorhombus regius Sardinella aurita S. hepatus S. ocelata S. variegata Squatina aculeata S. oculata Stomias boa Symphodus cinereus S. mediterraneus S. roissali S. rostratus S. ligulatus Synchiropus phaeton Syngnathus acus S. typhle T. marmorata T. nobiliana Trachyrhynchus trachyrhynchus

the most abundant totally discarded species were Coelorhynchus coelorhynchus (100±400 specimens per hour), Gadiculus argenteus (300±3500 specimens per hour), Hymenocephalus italicus (100±400 specimens per hour), and Chlorophthalmus agassizi (400± 1500 specimens per hour). Furthermore, the estimated general linear models were Fish :

log CPUE ˆ A ‡ S ‡ AS ‡ AC ‡ c …R2 ˆ 0:321; p < 0:05†

Crustaceans :

log CPUE ˆ S ‡ AS ‡ AC ‡ SC ‡ c …R2 ˆ 0:322; p < 0:05†

Cephalopods : log CPUE ˆ A ‡ S ‡ C ‡ AS ‡ SC ‡ c …R2 ˆ 0:245; p < 0:05† Total :

log CPUE ˆA ‡ S ‡ AS …R2 ˆ 0:441; p < 0:05†

A. Machias et al. / Fisheries Research 53 (2001) 181±195 Table 3 List of commercial crustacean and cephalopods species, as well as the discarded crustacean and cephalopods species Crustaceans Commercial Calappa granulataa Homarus gammarusb Macropipus tuberculatusa Maja squinadoa Nephrops norvegicusb Palinurus elephasa Parapenaeus longirostrisb P. kerathurusa Plesionika edwardsiia Squilla mantisa

Discards Alpheus glaber Atelecyclus rotundatus C. granulata Chlorotocus crassicornis Dorippe lanata Eriphia verrucosa Ethusa mascarone Galathea strigosa Goneplax rhomboides Homola barbata Ilia nucleus Inachus comunissimus I. leptochirus I. parvirostris I. thoracicus Latreillia elegans Liocarcinus corrugatus L. depurator Macropodia longipes M. longirostris M. rostrata Medoripe lanata Microcassiope minor Munida irs var. rutlanti M. tenuimana Ophioderma longicauda

Table 3 (Continued ) Crustaceans

Abralia veranyi Alloteuthis media A. subulata Bathypolypus sponsalis P. tetracirrhus Rondeletiola minor Sepietta oweniana Sepiola intermedia Pagurus excavatus Parthenope macrocheles Pasiphaea sivado Pilumnus hirtellus P. spinifer Pisa carinimana P. muscosa P. nodipes P. antigai P. gigliollii P. heterocarpus P. martia Polycheles typhlops Pontocaris cataphracta P. lacazei P. spinosus Processa acutirostris P. canaliculata P. macrodactyla P. macrophthalma Rissoides desmaresti R. pallidus Scyllarus pygmaeus Solenocera membrancea

Cephalopods Typton spongicola Xantho incisus

Cephalopods Eledone cirrhosab E. moschatab Illex coindetiib Loligo forbesib L. vulgarisb Neorossia carolia Octopus salutiib O. vulgarisb Pteroctopus tetracirrhusa Rossia macrosomab Scaeurgus unicirrhusa S. elegansa S. officinalisb S. orbignyanab Todarodes sagittatusb Todaropsis eblanaea

187

a

Local commercial species: the species that were not present in all areas, or in other areas had a marketable fraction and in others only a discarded fraction. b The species that consistently exhibited marketable and discarded fractions.

where A is the area, S the season, C the cluster, and c the constant. The value of each parameter is presented in Table 4. Thereafter, a general linear model was used to estimate the discarded biomass per taxon in each sampling period: log CPUE ˆ A ‡ Y ‡ c (Fig. 3). The percent discard was 34±44% for ®shes, 48±91% for crustaceans, 11±31% for cephalopods, and 39±49% for total catch (Fig. 3B). The quantity of ®sh and cephalopods discarded was lower in Ionian Sea than other areas. Fish and crustaceans discarded were higher during Autumn in all areas, while cephalopods showed lower discard quantity in Ionian Sea, as well as during Summer. Detailed differences between area, season and stratum are presented in Table 5. 3.2. Standardization of discard quantities According to discriminant analysis, the above two clusters were divided according to depth, particularly their minimum depth. Furthermore, the two clusters were also distinguished as far as their crustacean and cephalopod marketable yield are concerned (Table 6). The hauls that were made mainly on the continental shelf were placed in the ®rst cluster, while those in deeper waters were placed in the second cluster (Table 7). Two systems of classi®cation functions were estimated by discriminant analysis for use in classifying the new observations in the clusters using: (A) minimum and maximum depth, and duration; (B) the marketable yield (Table 8). Regression analysis (Table 9) revealed that a signi®cant fraction of the variation in ®sh discards (32± 38%) was explained by the corresponding marketable yield, while another 9±10% was explained by the duration and the depth characteristic of the haul. Speci®cally, discarded yield was positively correlated with the marketable yield and depth, and was nega-

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with the duration of hauls. The latter trend appeared in cephalopods whose discard yield was highly unpredictable. Cluster variables were not introduced systematically in the models and in most cases they have smaller explanatory power than depths, so they were excluded from the models. 4. Discussion

Fig. 2. Cluster analysis results: (A) Autumn; (B) Winter; (C): Spring.

tively correlated with the duration of each haul. Crustaceans showed a weak relationship with the examined variables (explained 20±30% of the total variation). Their discard yield was related to the corresponding marketable yield (10±20%), but the differences between areas were more signi®cant (18±26%). Crustacean discards also showed a negative correlation

Fishing has signi®cant direct and indirect effects on the habitat, diversity, and productivity of communities. The main direct effect is that trawling is responsible for increasing the mortality of marketable as well as discarded species. Trawling is responsible for the bulk of the discards (Stergiou et al., 1998; Hall, 1999). There is a large proportion of unmarketable species discarded by the trawl ®shery; the category of marketable species discarded includes: (A) species of low commercial value, discarded despite their legal size due to their low price in the market, and (B) undersized specimens for which minimum landing sizes were considered unmarketable. As a result, the discard of commercial catches greatly affects the estimation of ®shing mortality which relies upon landings (Chen and Gordon, 1997; Philippart, 1998). Quantitative information on discarding also has a great importance in relation to the energy ¯ow of the marine ecosystem. Trawling provides two main sources of food for benthic scavengers: ®rst, as food falls that originate from discards not consumed by seabirds, and second, as ``non-catch'' mortality (Jennings and Kaiser, 1998) because demersal trawls damage or kill a proportion of the organisms in the path of the gear. No information is available on the survival of escapees in Mediterranean, but it is expected to be comparable to or lower than values from other areas (Stergiou, 1999). Despite its importance, long-term monitoring of the discarded yield is uncertain, because of the need for special effort and ®nancing. The proportion of discards in relation to the retained catch, derived from several studies, has been used to obtain discard estimates, from the available data on landings for several regions (Hall, 1999). Fisheries in the eastern Mediterranean have four distinct features compared to those of other areas (Stergiou et al., 1997b): (1) highly oligotrophic conditions; (2) a diversity higher than in other northern

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Table 4 The values of the estimated general linear models FISa Constant

9.223

CRb 5.413

Area

Cyclades Ionian Thracian

0.574 0.685 0.111

Season

Autumn Winter Spring

0.200 0.170 0.030

Cluster

CL-1e CL-2

Area  season

Cyclades  Autumn Cyclades  Winter Cyclades  Spring Ionian  Autumn Ionian  Winter Ionian  Spring Thracian  Autumn Thracian  Winter Thracian  Spring

0.200 0.197 0.002 0.418 0.384 0.034 0.218 0.186 0.031

0.363 0.817 0.454 0.701 0.288 0.413 0.339 0.528 0.866

Area  cluster

Cyclades  CL-1 Ionian  CL-1 Thracian  CL-1 Cyclades  CL-2 Ionian  CL-2 Thracian  CL-2

0.156 0.179 0.336 0.156 0.179 0.335

1.108 0.136 0.945 1.108 0.135 0.972

Season  cluster

Autumn  CL-1 Winter  CL-1 Spring  CL-1 Autumn  CL-2 Winter  CL-2 Spring  CL-2

0.283 0.552 0.269

CEc

Totald

5.72

9.869

0.207 0.661 0.454

0.34 0.997 0.657

0.752 0.621 1.373

0.187 0.187 0.001

0.306 0.306

1.811 1.113 0.697 1.811 1.113 0.699

0.246 0.258 0.504 0.208 0.111 0.097 0.039 0.369 0.407

0.112 0.010 0.102 0.35 0.454 0.104 0.238 0.445 0.206

0.508 0.507 0.889 0.381 0.381 0.889

a

Fish discarded yield. Crustacean discarded yield. c Cephalopods discarded yield. d Total discarded yield. e Cluster. b

temperate environments, but lower than tropical systems; (3) a number of marketable species much greater than in temperate environments, but lower than in tropical systems, (4) the small size of species, coupled with the very small mesh size of trawl net, increase the diversity and the number of discards (Stergiou, 1999). As a result, a far wider range of taxa is directly affected by ®shing, with 45% of the catches discarded. It could be said that it is a distinct transitional system between temperat and tropical systems.

In oligotrophic systems like that of the eastern Mediterranean, discards could be an important component of energy ¯ow. The situation is particularly complicated in mixed species demersal ®sheries where there is complex and dynamic discard behavior (Stratoudakis et al., 1999b). Moreover, energy ¯ow related to discards has different effects between taxa (®shes, crustaceans, cephalopods). It is clear that ®sh consume damaged or exposed animals in the trawl path, but there is no clear evidence yet that ®sh are the main scavengers of discards falling from the surface

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Fig. 3. Mean discard yields with standard errors, per hour of haul in each sampling: (A) mean discard quantities (in kg/h) of haul in each sampling; (B) discards in each sampling as percentage of the respective catch; A95, 96, 97 ˆ Autumn 1995, 1996, 1997; W96, 97, 98 ˆ Winter 1996, 1997, 1998; S96, 97, 98 ˆ Spring 1996, 1997, 1998.

(Jennings and Kaiser, 1998). On the contrary, invertebrates seem to be the main scavengers of discarded species, while ®sh (e.g., Triglidae) are usually secondary consumers of the invertebrates which concentrate to consume discards (Kaiser and Spencer, 1994).

According to the present study the abundance of Triglidae was high in the ®rst cluster, which could be attributed to their potential to exploit the concentrated invertebrate biomass. The differential survival of discarded organisms (Kaiser and Spencer, 1995), as

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Table 5 Results of ANOVA for the main effects of the modelsa Variable

F

p

Comparisonsb

Fish Area Season Area  season Area  cluster

53.095 5.504 5.891 5.281

0.000 0.004 0.000 0.005

I < T; I < Cy A > W; A > S I : A > S; W > S T : 1 > 2; Cy : 1 < 2; I : 1 < 2

Crustacea Season Area  season Area  cluster Season  cluster

4.695 6.919 31.706 47.457

0.010 0.000 0.000 0.000

A > W; A > S Cy : W < S T : 1 > 2; Cy : 1 < 2 A : 1 > 2; W : 2 > 1; S : 2 > 1

Cephalopods Area Season Cluster Area  season Season  cluster

14.228 38.358 7.666 2.918 14.755

0.000 0.000 0.006 0.021 0.000

I < C; T S < A; S < W 1>2 I: W>A S: 1>2

150.969 7.333 10.772

0.000 0.001 0.000

I < Cy < T A < W; W > S I: W>S

Total Area Season Area  season a

In the third column the results of a posteriori pairwise comparisons (HSD). In case of interaction the main differences estimated by separate ANOVA for each factor is indicated …p < 0:05†. b I: Ionian Sea; T: Thracian Sea; Cy: Cyclades Islands.

well as the use of energy ¯ow from mid-water and benthic scavengers, is expected to be re¯ected in the abundance and in their relationships between discarded and marketable fractions.

In investigated areas, the trawl discards were estimated at about 44% (range: 39±49%) of the total catches. Based on this ratio, the total discard quantities derived from bottom trawls ranged from 13 500 to

Table 6 Discriminant analysis on duration, minimum depth, maximum depth and discriminant analysis on Fish, Crustaceans, Cephalopods marketable yield Season

Autumn Winter Summer

Wilks' l

0.847 0.945 0.811

Significant level

p < 0:001 p ˆ 0:03 p < 0:001

Correct assignments (%) 72.15 67.69 71.25

SDFCa Duration 0.495 0.844 0.401 Fb

Autumn Winter Summer a

0.806 0.941 0.843

p < 0:001 p < 0:021 p < 0:001

Standardized discriminant function coef®cient. Marketable ®sh yield. c Marketable crustacean yield. d Cephalopods marketable yield. b

77.22 60.59 63.69

0.327 0.183 0.149

Minimum depth 1.521 1.737 0.757 Crc 0.289 0.787 0.863

Maximum depth 0.459 1.546 0.050 Ced 0.957 0.457 0.333

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A. Machias et al. / Fisheries Research 53 (2001) 181±195

Table 7 Mean values of each cluster with their standard errors Cluster 1

Cluster 2

Autumn

a

Minimum Maximuma

83.4  4.03 108.4  5.10

129:9  6:11 150:7  6:76

Winter

Minimum Maximum

101.8  4.72 128.0  5.76

129:6  6:66 157:9  9:16

Summer

Minimum Maximum

99.9  5.11 132.0  6.32

153:1  7:00 224:3  7:76

a

nean trawls is 28 mm for Greece (40 mm for other European Mediterranean countries) and the expected discard ratio >70% of total catch. This difference is obviously related to the differences between Mediterranean and northern ®sheries. Speci®cally, in the Mediterranean, codends used are small because the commercial species are different and have much smaller sizes than those of northern areas. Furthermore, the number of marketable species is greater but smaller in size. Consequently, a different trend line between the discard ratio and codend mesh size should be estimated for the area. In any case the discard percentages seemed to be lower than those of northern areas are. Hauls could be divided into two clusters on the basis of their discard catch. These clusters were discriminated according to their depth, as well as their crustacean and cephalopod discard quantities. Furthermore, new hauls could be assigned to these clusters a posteriori, either from their minimum, maximum haul depth and duration of haul, or from the estimated marketable yield. Greatest quantities of discarded (and marketable) yield were observed in Autumn, as a result of the recruitment for most species during this period, as well as of the absence of trawling in the previous 4 months (Stergiou et al., 1997b). Regression analysis revealed that about half of the variance of ®sh discard could be

Mean minimum and maximum depths (in m) for each cluster.

22 000 t annually (12% of the total landings). An equal quantity is expected to be the non-catch mortality during the trawl operation (Lindeboom and de Groot, 1998). The contribution of other ®shing gears to the total amount of discards is much lower (Stergiou et al., 1998; Tsimenides et al., 1995) than those of bottom trawls. As the bulk of discards is derived from bottom trawling, the total ratio of discards is also expected to be lower than the average global catches (Alverson et al., 1994). Alverson et al. (1994) estimated a negative trend in the ratio between discards and trawl codend mesh size, based on northwest Atlantic data. The estimated ratio in the present study was much lower than that predicted from this trend line. Speci®cally, the codend mesh size of MediterraTable 8 Discriminant analysisa Variables

Autumn Cluster 1

Haul features Durationb 1.221 Minimumc 0.030 0.021 Maximumd Constant 2.058 Log-marketable yield Fish 46.67 Crustaceans 0.90 Cephalopods 2.86 Constant 106.39

Winter

Summer

Cluster 2

Cluster 1

Cluster 2

Cluster 1

Cluster 2

0.941 0.054 0.027 2.962

0.771 0.028 0.014 1.751

1.018 0.043 0.024 2.890

0.554 0.016 0.002 1.551

1.078 0.023 0.003 4.658

47.81 0.89 1.89 108.67

29.78 0.31 3.66 68.48

29.50 0.57 3.36 66.99

40.07 3.14 3.57 93.51

39.57 3.73 3.09 91.40

a The classifying functions or discriminants for use in classifying new observations. The rows are the variables used and the columns, the clusters. The system classi®es a new case by evaluating each function and assigning the case to the cluster corresponding to the highest function value. b Duration (in h). c Minimum depth (in m). d Maximum depth (in m).

A. Machias et al. / Fisheries Research 53 (2001) 181±195

193

Table 9 Regression analysis per taxon, per season and for ®shing period (all seasons)a Equationb

R2-valuesc

Fish Autumn Winter Summer Fishing period

D ˆ 1:70 ‡ 0:82M 0:19T ‡ 0:012min D ˆ 2:76 ‡ 0:66M 0:15T ‡ 0:005min D ˆ 4:6 ‡ 0:50M 0:33T ‡ 0:01max D ˆ 3:14 ‡ 0:66M 0:24T ‡ 0:01min

Crustacea Autumn Winter Summer Fishing period

D ˆ 7:97 0:87A1 1:96A2 ‡ 0:14M 0:01min D ˆ 6:61 0:99A1 0:66A2 ‡ 0:20M 0:01min D ˆ 4:94 ‡ 0:17M D ˆ 40:81 ‡ 0:32M ‡ 0:19T 0:01min

0.346 0.306 0.182 0.208

Cephalopods Autumn Winter Summer Fishing period

D ˆ 8:24 D ˆ 7:38 D ˆ 5:15 D ˆ 6:94

0.115 0.103 0.306 0.131

Total discard Autumn Winter Summer Fishing period

D ˆ 2:84 ‡ 0:78M 0:25T D ˆ 4:34 ‡ 0:58M 0:24T ‡ 0:002max D ˆ 1:07 ‡ 0:93M 0:36T ‡ 0:004max D ˆ 118717:6 ‡ 18061:42M 7945:6T

0.446 0.316 0.585 0.412

Relationships between discard and marketable yield Fish Autumn Winter Summer Fishing period

D ˆ 1:56 ‡ 1:10M D ˆ 1:81 ‡ 0:76M D ˆ 1:86 ‡ 0:76M D ˆ 1:26 ‡ 0:83M

0.361 0.385 0.294 0.329

Crustacea Autumn Winter Summer Fishing period

D ˆ 6:31 ‡ 0:14M D ˆ 5:54 ‡ 0:15M D ˆ 4:94 ‡ 0:17M D ˆ 4:34 ‡ 0:26M

0.076 0.129 0.182 0.146

Total discard Autumn Winter Summer Fishing period

D ˆ 1:12 ‡ 1:06M D ˆ 2:56 ‡ 0:71M D ˆ 3:49 ‡ 1:32M D ˆ 230791:8 ‡ 26469:1M

0.347 0.201 0.431 0.128

0:24T 0:11M 0:26T 0:50T ‡ 0:01max ‡ 0:20M 0:26T ‡ 0:000007M

0:01max

0.453 0.455 0.424 0.423

a

Relationships between discarded and marketable yield presented separately. D: log-discard of catches (in g); M: log-marketable catches (in g); T: duration of the hauls (in h); min: minimum depth of the hauls (in m); max: maximum depth of the hauls (in m); A1: dummy variable with value 1 for data from the Cyclades islands and 0 for the other areas; A2: dummy variable with value 1 for data from Ionian Sea and 0 for the other areas. c Coef®cient of determination. The signi®cance level for all relationships is <0.05. b

explained by the available variables, while the relationships of crustaceans and cephalopods with marketable yield were very weak. The relationships between marketable and discard catches were power functions. In other words, the discarded fraction increased more

rapidly than the marketable fraction in each haul (positive relation) in all cases. Consequently, high catches resulted in higher discard rates than did low catches. The statistically signi®cant relationship between discarded and marketable yield could be

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A. Machias et al. / Fisheries Research 53 (2001) 181±195

related to homogeneity of the environment, owing to the intensive unselective trawling (Jennings and Kaiser, 1998). Hauls of long duration usually resulted in smaller catches (and discards) per hour, but at sites of higher productivity, hauls were short with higher discard fractions (negative relations with time in all cases). Deep hauls had higher discarded fractions of ®sh in the yield (larger but fewer individuals and less commercial species in deep waters). The main causes for the weak relationships in respect of crustaceans were that in shallow depths, the entire crustacean yield comprised discarded species with no marketable yield. Furthermore, interactions between areas, season and cluster were strong. Total discard yield showed the same trends with its main component, the ®sh, but exhibited a weaker relationship. Strong interactions between areas, seasons, and strata, appearing in the general linear models, revealed that the relationships became more accurate if data from each area and from each stratum were analyzed separately. The latter is especially true for the crustaceans and cephalopods. Furthermore, in a local estimation, additional parameters, such as prices of the marketable species, could be used for a more precise estimation of the discard yield. The latter parameter was dif®cult to use in a general model. For example, the prices of ®shery products in the Ionian Sea were usually ®xed from the beginning of the ®shing season according to agreements between each ®sherman and the merchants. In the other areas, the prices ¯uctuated throughout the ®shing period, depending upon market demand. The threat to species populations, the wastefulness of the activity, and the problem that undocumented discarding poses to stock assessment are all major issues (Hall, 1999). The regulations that govern ®sheries and the vagaries of the market place often create a complex web of incentives and disincentives that drive the discarding practice of ®shermen (Jennings and Kaiser, 1998). Increasing the selectivity of ®shing methods could reduce unwanted catch. These methods adopt one of two strategies: ®rstly, to exploit behavioral differences between the various species ®shed and, secondly, to approach exploitation based on different sizes of species (Hall, 1999). According to Stergiou et al. (1997a), the replacement of trawls with 28 mm codend mesh size, as presently used, by a mesh

size of 40 mm, will not be accompanied by any signi®cant commercial loss while the weight of discards would be signi®cantly reduced. Potential solutions might also include temporal and/or spatial closures or continuous monitoring of the ®shery and closure once a given by-catch quota has been reached (Hall, 1999).

Acknowledgements The present study was supported and funded by EU DGXIV, projects 94/065 and 95/061. We wish to thank Prof. K.I. Stergiou and Dr. G. Stratoudakis for their critical reading of an earlier version of this manuscript.

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