ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on March 3, 2008
ICES Journal of Marine Science: Journal du Conseil 2008 65(4):539-550; doi:10.1093/icesjms/fsn023
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Seasonal and temporal trends in metrics of fish community for otter-trawl discards in a Mediterranean ecosystem
1 Hellenic Centre of Marine Research, PO Box 2214, 710 03 Iraklion, Crete, Greece
2 Department of Biology, University of Crete, PO Box 2208, 71409 Iraklion, Greece
Correspondence to A. Machias: tel: +30 2810 337832; fax: +30 2810 337822; e-mail: amachias{at}her.hcmr.gr
Tsagarakis, K., Machias, A., Giannoulaki, M., Somarakis, S., and Karakassis, I. 2008. Seasonal and temporal trends in metrics of fish community for otter-trawl discards in a Mediterranean ecosystem. – ICES Journal of Marine Science, 65: 539–550.Trends in discard to marketed ratios, size spectra, diversities, and trophic levels of the demersal fish community were examined using data from a seasonally closed commercial trawl fishery in the eastern Mediterranean Sea (Ionian Sea), over a period of about 10 years. Trends were also examined for the artificial fractions derived from the discarding process (the marketed, the discarded, and the non-marketed clusters of the catch), as well as for the "big" and "small" fractions (defined by the size at which 50% of all specimens were discarded). The ratio of the discarded/marketed catch fluctuated greatly. Two commercial (Merluccius merluccius, and Mullus barbatus) and two non-marketed species (Lepidotrigla cavillone, and Argentina sphyraena) were the characteristic species of the fractions. A declining trend with time was observed for the examined time-series for species richness (S), Margalef’s d, and average taxonomic distinctness (
+), whereas the variation in taxonomic distinctness (
+) increased. The composition and/or trophic level of discards in relation to the marketed catch seemed to be indicative of the exploitation state of the demersal community: differences between the discarded and marketed fractions were high at the beginning of the fishing season (autumn), but the values of the indices converged at the end of the fishing season (spring). These changes could be attributed to alternative discarding strategies for certain species in response to increased cumulative fishing mortality towards the end of the period.
Keywords: bycatch, community metrics, trophic level
Received 8 October 2007; accepted 25 January 2008; advance access publication 3 March 2008.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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Discards are defined as marine fauna brought onto the deck of a fishing vessel and subsequently returned to the sea (Alverson et al., 1994; Hall, 1999; Allen et al., 2001). Kelleher (2004) estimated that on average global discards were some 7.3 million tons annually, representing about 8% of the global catch for the period 1992–2001. The reasons for discarding are numerous and involve legal, economic, environmental, and biological issues (Alverson et al., 1994; Stratoudakis et al., 1998; Allen et al., 2001; Rochet and Trenkel, 2005). Discards include species of no commercial value (even rare/endangered/protected species) and commercial species. The latter are represented by specimens smaller than the legal landing size and species/sizes discarded for their low market value.
Discarding has direct and indirect effects at population and ecosystem levels. At a population level, discarding may have an important effect on population dynamics and the yield of target and non-target species, representing a loss in terms of production and future economic returns (Jensen et al., 1988; Erzini et al., 2002). Discarding at sea is a key issue in stock assessment, and fishery biologists and management agencies have recognized the importance of producing reliable quantitative information on the discrepancies between landings and the actual catches of a species (Alverson et al., 1994; Stergiou et al., 1998; Stratoudakis et al., 1998).
Community and ecosystem level impacts of discarding, particularly the effects on biodiversity, community structure, trophic interactions, and stability are poorly understood (Borges et al., 2001; Erzini et al., 2002). Discarding of non-target species may have negative consequences for both commercial and non-commercial species owing to the effects on species interactions and cascading effects throughout the trophic web (Monteiro et al., 2001, and references therein). In the context of the rising emphasis on multispecies and ecosystem-based approaches to fisheries management, it becomes increasingly necessary to evaluate discarding practices with the intention of understanding their impact at population, community, and ecosystem levels (Borges et al., 2001).
The discarding process artificially splits the community into two fractions (or clusters): the commercial/marketed, and the discarded/non-marketed. It is likely that the quantities, size structure, and species composition of the discarded fraction change in response to the levels of fishing pressure. For example, at the start of a fishery, when high-priced (and/or large) fish are relatively abundant, species diversity and mean size/trophic level of discards might be relatively high. When quantities of highly valued (and/or large) fish have decreased through overfishing, discards may be less diverse and of smaller mean size/trophic level because more species/sizes are retained. Hence, a comparison of diversity and size structure between the marketed and discarded fraction of otter-trawl catches might be useful in indicating the exploitation status of the demersal community.
Discarding is affected by numerous factors (e.g. fisher strategies, market conditions, and fish community composition.). It would be difficult to study in detail the effect of each of these factors on a sufficiently large dataset, and there would probably be a high level of uncertainty given the volatility of the factors related to human preferences and decisions. An integrated approach using long-term monitoring data may overcome this difficulty, resulting in a reliable assessment of the combined effect of these factors on fish communities or the (possibly interacting) "fractions" defined by exploitation.
A comprehensive list of hypotheses and assumptions on variability of discarding has been compiled recently by Rochet and Trenkel (2005). Those authors showed that some methods of estimating discards are based on the hypothesis that "species composition of communities or length structure of populations determine what is discarded", although there have been no studies examining this hypothesis, i.e. "by attempting to relate community species composition to the species discarded".
Among different fishing gears, the trawl is responsible for most fisheries discards (Stergiou et al., 1998; Hall, 1999). In the Mediterranean, the discarded fraction of otter-trawl catches ranges from 20 to 70% by weight, mainly depending on the area and the depth of trawling (Carbonell et al., 1998; Stergiou et al., 1998; Machias et al., 2001; D’Onghia et al., 2003; Kelleher, 2004). From a total of 300 species caught in the eastern Mediterranean, only 
10% are consistently marketed and 30% are occasionally retained, depending on the sizes caught and market demands, whereas >60% are always discarded (Machias et al., 2001).
The aim of this study was to examine temporal and seasonal trends in various diversity metrics and trophic levels in the whole fish community, as well as in the artificial fractions derived from the discarding process of the trawl fishery in the eastern Ionian Sea. Data covering 10 years (371 hauls in 180 fishing days) were used to detect between-fraction differences as well as related changes associated with season and year. The artificial fractions used were (i) marketed fish, (ii) discarded fish, and (iii) non-marketed species. Additionally, we examined the fractions consisting of "small" and "big" fish, based on the estimated length at which 50% of the fish were discarded. The key question is whether inter-fraction differences and trends of partial fractions provide information on the exploitation status of the community, outlining the integrated effect of the multiple factors responsible for the variability of discards.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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We used data collected in the eastern Ionian Sea (Mediterranean Sea, Greece) from October 1995 to May 2005 (Figure 1), where the otter-trawl fishery is a typical Mediterranean multispecies one. The fishing period extends from October to May (there is a closed season from 1 June to 30 September), and codend mesh sizes are small (stretched mesh 28 mm) because of the small sizes of the species being exploited. The available information, based on data collected by the Hellenic Centre for Marine Research (HCMR; Anon., 2001), showed that for the examined period in the survey area, fishing effort, expressed as days at sea as well as days at sea multiplied by gross tonnage (Figure 2), indicated no significant trend within seasons (not shown here) or within years. Days at sea by gross tonnage increased in 2003 and 2004.
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Catches were recorded on board commercial vessels three times per year, in (i) October (autumn), which coincides with the opening of the fishing period for the Greek trawlers and the recruitment period of most fish species (Stergiou et al., 1997), (ii) February (winter), which represents the middle of the trawl fishing period, and (c) late May (spring), just before the fishing season was closed.
The presence of on-board observers was not considered to have influenced fisher behaviour because no significant differences were observed when comparing the relative biomass per unit effort of commercial species from the present work with landings data collected by HCMR (Anon., 2001) for the same area and period (Table 1). Moreover, in the same discard surveys, many undersized fish (i.e. smaller than the minimum allowed landing sizes) were eventually landed (Machias et al., 2004), indicating that skippers’ discarding practices were relatively unaffected by the presence of observers.
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In each sampling period, at least two commercial vessels, representative of the studied area in terms of vessel size and construction, were randomly sampled for several days/trips (Table 2). The duration of hauls ranged from 3 to 8 h (with mean and s.d., 5.3 ± 1.3 h; Table 2), and trip duration was always one day. All hauls analysed were carried out over the continental shelf at depths >60 m (Table 2). In all, 22 seasonal surveys were carried out and at least 10 hauls (one or two hauls per day) were sampled in each period, a total of 371 hauls in 180 days of fishing (Table 2). No sampling was carried out in spring and autumn 2000, or between autumn 2001 and spring 2003. More detailed information on sampling, sorting, and discarding practices, landed species, and species caught in the area are provided elsewhere (Machias et al., 2001, 2004).
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Fieldwork included estimating the total catch of each haul, and recording the faunal composition of the catch, for which identification was made to species level. The duration of each haul was recorded and total catches were standardized to hourly yields (g h–1) or abundances (n h–1). After the marketed catch had been sorted by the crew, the number and weight of each species was recorded. The discarded portion of the catch was put into boxes and weighed. Fish were sorted into species, and the number of individuals as well as their total lengths (to the nearest mm) and total weights (to the nearest g per species) was recorded. In cases where certain species were in large number, the length frequency was calculated from a representative sample of at least 50 fish. Pelagic species were excluded from the analysis following the standard practice in surveys with similar objectives (Greenstreet and Hall, 1996; Myers and Worm, 2003).
Catches were standardized per day (trip) and per haul-hour. Both approaches gave similar results. However, we present here only the results from the first approach (per day) because generally this is the best sampling unit for discard estimates (Borges et al., 2005).
Discarded/marketed ratio
In the first step of the analysis, the ratio of the discarded to the marketed fraction of the catch, for each sampling period, was estimated. The ratio (
) of the discarded fraction was calculated using the following formula (Cochran, 1977):
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Size spectra
Subsequently, we examined whether there were significant changes in size spectra during the period of the study. Pope and Knights (1982) observed a linear relationship between log numbers per size class and fish size, and the slope has been used often as an indicator of exploitation (Murawski and Idoine, 1992; Greenstreet and Hall, 1996; Rice and Gislason, 1996; Gislason and Rice, 1998; Bianchi et al., 2000; Jennings et al., 2002; Daan et al., 2005). Although the sampling was not designed for this, a trend of size spectra could reveal a change in the exploitation status of the fishing grounds.
Size spectra of fish species were constructed for each fishing period by plotting the natural logarithm of the total number of individuals caught per hour by 5-cm length class against the natural logarithm of the middle of each length class. Length range used was 10–50 cm, because catch efficiency of the otter trawl was poor for specimens <10 cm and fish >50 cm were rarely caught. The slopes and intercepts of the linear relationships were plotted against time. Differences in productivity should appear as differences in intercepts, whereas differences in transfer efficiencies and mortality rates should appear as differences in slopes (Bianchi et al., 2000, and references therein). In addition, an improved method described by Daan et al. (2005) was used to detect trends in slopes and intercepts.
Definition of characteristic marketed, discarded, and non-marketed species
In a subsequent step of the analysis, the trawl catch was divided into three artificial fractions, (i) marketed, (ii) discarded, and (iii) non-marketed, and for each of these fractions characteristic species were defined. The marketed (M) fraction was those specimens and species that were landed, the discarded (D) fraction those specimens and species that were discarded (including species that contributed and some that did not to the marketed fraction), and because most marketed species included a discarded fraction, we also defined a non-marketed (NM) fraction, defined here as the individuals of those species that were discarded without any marketed fraction. The NM fraction included those species that are not intentionally removed from the community as a result of fishing. In this context, it could be expected that the dynamics of those species differ substantially from those which suffer greater and more systematic fishing pressure.
The main species of marketed, discarded, and non-marketed fractions of the multispecies otter-trawl catch were defined by applying the target species approach proposed by Stergiou et al. (2003). Briefly, characteristic species were defined as those contributing a percentage of Bray–Curtis similarity equal to that at which the different fishing operations (fishing days) during the whole study period formed one cluster (group-average clustering) in terms of log-transformed values of biomass catch per day (Stergiou et al., 2003). The contribution of each species to the average Bray–Curtis similarity within the group of marketed, discarded, or non-marketed catch was identified with SIMPER analysis (Clarke and Warwick, 1994).
Diversity indices and mean trophic level of the catch
We also examined species diversity and the mean trophic level for the total fish catch, as well as for the three artificial (discarding related) fractions (M, D, and NM). The diversity indices we estimated were the widely used species richness S, Margalef’s d (in order to standardize S against the number of individuals in the sample), and the Shannon–Wiener H‘ (log2) and Simpson’s 1–
, which take into account the relative abundance of each species and provide an estimate of diversity and evenness, respectively. We also used two biodiversity indices (+, +) based on taxonomic distinctness that have been proved to be less sample-size dependent and more sensitive in detecting natural disturbances in several cases (Clarke and Warwick, 1998, 2001). Average taxonomic distinctness (+) is the mean path length through the taxonomic tree connecting every pair of species in the tree (Clarke and Warwick, 1998). It is calculated by adding the path lengths through a taxonomic tree between every pair of species in the list, and dividing by the number of paths. Variation in taxonomic distinctness (+) is the variance of the pairwise path lengths between every pair of species in the list, reflecting the unevenness of the taxonomic tree (Clarke and Warwick, 2001). The indices were calculated with PRIMER-5 software (Clarke and Warwick, 1994). The dependence of diversity metrics on sample size was examined, and variance levelled out after the fifth day, whereas 85% of the species were caught with ten hauls.
The trophic level of each fish species, reviewed by Stergiou and Karpouzi (2002) and Froese and Pauly (2004), was weighted by the abundance of each species, in order to calculate the mean trophic level of each catch per day (Pauly et al., 2001):
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Mean values for diversity indices and the mean trophic level for each sampling period (season of a particular year) were calculated. These were plotted against time and their trends were estimated by linear regression analysis. The effect of year and season on each diversity index and the trophic level was analysed using generalized linear models of the form I=k+k*Se+k*Y, where I is the variable examined (diversity index or trophic level), k the fractions compared each time, Se the effect of season, and Y the effect of year. For all indices, comparisons were made between marketed and discarded (M vs. D), and marketed and non-marketed (M vs. NM) subsets.
Community-wide lengths-at-discarding
In addition to the aforementioned M, D, and NM artificial fractions, trends, and comparisons of species diversity were also examined for two more community clusters based on size, i.e. small and big fish. The definition of these community clusters was based on the overall estimate of the length at which 50% of the total catch was discarded (L50), treating the catch as if it was a single species. The demersal fish community was therefore divided into fractions consisting of small (individuals <L50) and big fish (individuals >L50). For this task, L50 was calculated by pooling the entire dataset to make sure that there was no change in membership in the big and small groups over seasons and years. More specifically, the length frequencies of the overall marketed and discarded fractions, used to estimate the length at which 50% of individuals were discarded (L50), were derived by pooling individual species length frequencies, after weighting by species abundance. Ophioid species (e.g. Ophidion rochei, Cepola macrophthalma, Lepidopus caudatus, Echelus myrus, Gnathophis mystax) were excluded from this analysis, because their allometric growth differs substantially from the other species, resulting in an overestimation of L50. The L50 was determined from the logistic relationship between percentages P of fish discarded at length class L (Machias et al., 2004):
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Finally, following the procedure described by Petrakis and Stergiou (1997), the standard errors and the 95% CIs of L50 were calculated.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Of a total of 135 fish species caught (see Table 3 for the 40 most abundant), 88 had, at least once, a marketed fraction, and 106 had a discarded fraction. There were 101 non-marketed species, and 127 and 123 species, respectively, fell in the big and small community clusters.
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The ratio of the discarded fraction fluctuated (0.07–1.21), but with an average value of 0.61, and there was no significant trend over time (p > 0.05; Figure 3). Similarly, the slope and the intercept of the size-spectra relationships estimated for each sampling period failed to show a significant trend over time (p > 0.05). The intercept of the relationship using all data was 17.09, and the slope –4.48 (F = 711.47, p < 0.001, r2 = 0.99; Table 4), whereas following the approach of Daan et al. (2005), the intercept was 1.82 and slope was –0.17 (F = 173.97, p < 0.001, r2 = 0.97; Table 4).
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Using the criteria of Stergiou et al. (2003), SIMPER analysis showed that for the marketed fraction of the catch, two species (Merluccius merluccius, and Mullus barbatus) contributed most to total similarity (Table 5). The discarded and non-marketed fractions were characterized by three species: Lepidotrigla cavillone, Argentina sphyraena, and juvenile M. merluccius (Table 5).
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The community-wide length at discarding (L50), independent of season, was 13.6 cm (Table 6). The highest L50 was in winter and the lowest in autumn. L25 and L75 values were generally close to L50.
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Species richness (S) showed a significantly declining trend over time for the discarded and non-marketed fractions as well as for the small fish fraction (Table 7). Average taxonomic distinctness (
+) declined for all but the big fraction, and + (variation in taxonomic distinctness) showed an increasing trend for the total, marketed, and big fractions. The remaining diversity indices showed no significant trends over time (Table 7).
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Comparisons of diversity indices revealed significant differences between fractions, especially in Simpson’s 1–
, Shannon’s H‘, and variation in taxonomic distinctness (+) (Table 8, Figures 4–6). The effect of season (the k*Se interaction) was significant for 1– and H‘ indices (M vs. D: 1–, H’; M vs. NM: 1–, H‘; TL; B vs. S: 1–, H’). The effect of year (k*Y) was weakly significant in one case only (Table 8, Figures 4–
6). When a significant effect of season was present, the values of the index compared between fractions seemed to converge or even reverse during spring, i.e. towards the end of the fishing season. This effect seems to be attributed to a "transfer" of specimens and species from the discarded to the marketed fraction of the catch from the beginning to the end of the fishing season. Specifically, for a group of 11 species (Aspitrigla cuculus, Diplodus annularis, Micromesistius poutassou, Pagellus bogaraveo, Raja clavata, Spicara maena, Scorpaena notata, S. porcus, S. scrofa, Trigla lyra, Trisopterus minutus capelanus), the abundance ratio for marketed: discarded increased by at least 100% from autumn to spring and, concurrently, the landed average abundance increased and the discarded average abundance decreased. For a group of ten other species (Boops boops, Citharus linguatula, M. barbatus, P. erythrinus, R. miraletus, Serranus cabrilla, Symphurus nigrescens, Trigloporus lastoviza, Uranoscopus scaber, Zeus faber), only the ratio increased by at least 100% from autumn to spring. The opposite trend (a decrease in the marketed: discarded ratio) was observed for just five species (Dentex maroccanus, Lepidorhombus boscii, M. merluccius, Phycis blennoides, Trigla lucerna).
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Mean trophic levels did not show any significant trend over time (Table 7). The trophic level of the marketed fraction was higher than those of the discarded and non-marketed fractions (Table 8, Figures 4 and 5).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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We examined the effect of trawl fisheries in the eastern Ionian Sea on the demersal fish community as well as on the fractions that resulted from the discarding process (i.e. marketed, discarded, non-marketed, big, small), using metrics of community structure.
No declining trend in the slope of the size spectra was observed over the time period we analysed. This may reflect either the short timespan investigated (just 10 years), or the more general feature described by Stobberup et al. (2005) that size-based indicators are not suitable for tracing fishing effects in ecosystems characterized by fast growth rates, small sizes, high species’ diversity, and complex interrelationships. Most of these attributes are typical of fish communities in Greek waters, which tend to be dominated by thermophile tropical and subtropical fauna (Papaconstantinou, 1988; Stergiou et al., 1997).
The ratio of the discarded to the marketed fraction fluctuated greatly, as has also been reported in similar multispecies fisheries (Machias et al., 2001; Monteiro et al., 2001; D’Onghia et al., 2003; Sanchez et al., 2004). Despite the multispecies nature of a fishery that includes more than 88 commercial and 106 discarded species, the bulk of the catch was just two commercial species (M. merluccius and M. barbatus), which are realistically the targets of the trawl fishery, as defined by Stergiou et al. (2003), and two non-marketed species (L. cavillone and A. sphyraena).
The trawl fishery is also characterized by low values (overall 13.6 cm) of the length at which 50% of the fish in the catch are discarded (L50), again typical for the Mediterranean fisheries. The L50 varied among seasons, being highest in winter, mainly because the duration of sorting tended to be shortened then by the adverse weather conditions (Machias et al., 2004), and lowest in autumn, the recruiting season for most fish species (Stergiou et al., 1997).
There were declining trends for species richness (S), Margalef’s d, and + indices, and an increase in +. The significant trends detected in all different fractions implied deterioration over time which was more readily detected through the indices based on the phylogenetic setup of each fraction (+ and +). Phylogenetic indices have been proposed as more sensitive measures of an ecosystem’s response to disturbance (Warwick and Clarke, 1995). On the other hand, the decline in S and d in the cases of discarded and small fractions, and the absence of such a trend in the case of the marketed fraction, could be attributed to the "transfer" of specimens and species over time from the discarded to the marketed fraction.
The comparisons of community metrics between the discard-related clusters of the trawl catch indicated (i) differences between diversity metrics, (ii) differences between the trophic level of the fractions, and (iii) a consistent seasonal trend in community metrics. Between-fraction differences in terms of diversity metrics reflected human decisions on separation of fractions (i.e. marketed and discarded) rather than ecological processes. The fraction of marketed species showed a higher dominance than that of discards (average similarity higher by a factor of 2), which is a reasonable finding because our analysis showed that trawl landings consisted of mainly two species. On the other hand, the discarded fraction showed high diversity because it included non-marketed species as well as the discarded fraction of the commercially important species. The non-marketed fraction showed lower species richness and higher equitability than the marketed fraction, because most trawled species had a commercial cluster and partially discarded species were not included in the non-marketed fraction. Similar patterns can be seen in the comparisons between small and big fractions, where the big fraction is closely related to the marketed fraction (larger fish are those mainly retained). The variation in taxonomic distinctness (+) was consistently higher in the marketed and the big fractions, which is related to the presence of elasmobranch species, which contribute a lot to the indices based on taxonomic distinctness (Rogers et al., 1999).
The examined fractions also showed differences in mean trophic level. The discarded and non-marketed fractions contained species at a significantly lower trophic level than the marketed fraction, and there were no significant trends in mean trophic level over the timespan analysed. The low trophic level of the discarded and non-marketed fractions was to be expected, because they were mainly small species and individuals. It is worth noting that the use of a single trophic level per species has probably overestimated the trophic level of discards that are small, so the differences between the two fractions are expected to be even more significant that those presented here. Moreover, several fish species (Gadidae, Triglidae, and various elasmobranchs) feed on the discarded catch (Jennings and Kaiser, 1998). Discarded catches create a shortcut in trophic relationships, increase the recycling rate of organic matter, and enhance secondary production (Groenewold and Fonds, 2000). They may therefore support populations of marketable fish species as well as increase the abundance of scavenging species (seagulls, crustaceans, molluscs, etc.). Especially in an oligotrophic marine environment such as the eastern Mediterranean, discarding may influence trophic relationships greatly, so any recorded changes in community structure have to be examined critically.
The emergence of an ecosystem approach to fisheries management has encouraged scientists and managers to give greater consideration to the hidden side of fishing, such as discarding (Rochet and Trenkel, 2005). Here, the composition and/or the trophic level of discards in relation to the marketed catch seem to be indicative of the exploitation status of the demersal community. Most of the indices examined (diversity and/or trophic level) showed a consistent seasonal variation. Specifically, the differences in the values of the indices between the discarding-related community clusters were high at the beginning of the fishing season (autumn), but converged at its end (spring). The marketed fraction showed an increase in diversity and evenness (H, 1–) towards the end of the fishing period, whereas the opposite applied for the discarded and non-marketed fractions. These changes seem to be associated with the exploitation pattern on the demersal community. The beginning of the fishing period in Greek waters (after four months of closure, June–September) coincides with the recruitment period for most species (Stergiou et al., 1997). More juveniles are therefore present, contributing extensively to the discarded and small fractions. Towards the end of the fishing period, in spring, fish abundance decreases because of the fishing mortality through the season (Stergiou and Petrakis, 1993; Stergiou et al., 1997) and more fish are "transferred" to the marketed fraction by a change in sorting strategy (notably affecting species such as Spicara maena, B. boops, and Scorpaena spp.). These changes in sorting strategy have also been mentioned for other fisheries in the area (Tzanatos et al., 2007). In other words, the converging of the indices at the end of the fishing season may well be related to a convergence in the size and the number of individuals and species, as well in the trophic level of the species that constitute the discarded and marketed fractions, and likely reflect a change in discarding habits in response to the heavily exploited status of the community at the end of the fishing season. We therefore propose that indicators such as the ratio of diversity and/or trophic level of the discard-related fractions (e.g. marketed vs. discarded) could be useful in defining an index of community exploitation. Further meta-analysis of existing data from different areas with varying levels (low or high) of exploitation (e.g. Tudela et al., 2005) could help to test the appropriateness and sensitivity of such discard-related environmental indicators in management of the local resources.
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Acknowledgements |
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The study was supported and funded by EU DGXIV, projects 94/065 95/061, 97/044 and the National Fisheries Data Collection Programme (2002–2006). We also thank Niels Daan for his assistance in size spectra analysis and three reviewers for helpful comments on the submitted draft.
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References |
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Allen M., Kilpatrick D., Armstrong M., Briggs R., Perez N., Course G. Evaluation of sampling methods to quantify discarded fish using data collected during discards project EC 95/094 by Northern Ireland, England and Spain. Fisheries Research (2001) 49:241–254.[CrossRef][Web of Science]
Alverson D. L., Freeberg M. H., Murawski S. A., Pope J. G. A global assessment of fisheries bycatch and discards. (1994) 339:233. FAO Fisheries Technical Paper.
Anon. Patterns and propensities in Greek fishing effort and catches. (2001) 162. Institute of Marine Biology of Crete Final Report Contract no: 00/018.
Bianchi G., Gislason H., Graham K., Hill L., Jin X., Koranteng K., Manickchand-Heileman S., et al. Impact of fishing on size composition and diversity of demersal fish communities. ICES Journal of Marine Science (2000) 57:558–571.
Borges L., Zuur A. F., Rogan E., Officer R. Choosing the best sampling unit and auxiliary variable for discards estimations. Fisheries Research (2005) 75:29–39.[CrossRef][Web of Science]
Borges T. C., Erzini K., Bentes L., Costa M. E., Goncalves J. M. S., Lino P. G., Pais C., et al. By-catch and discarding practices in five Algarve (southern Portugal) métiers. Journal of Applied Icthyology (2001) 17:104–114.[CrossRef]
Carbonell A., Martin P., de Ranieri S. Discards of the western Mediterranean trawl fleets. Rapport de Commitee Internationale de Mer Mediteranee (1998) 35:392–393.
Cochran W. G. Sampling Techniques. (1977) 3rd edn. NY: John Wiley and Sons. 428.
Clarke K. R., Warwick R. M. Change in Marine Communities: An Approach to Statistical Analysis and Interpretation. (1994) Plymouth, UK: Natural Environmental Research Council. 144.
Clarke K. R., Warwick R. M. A taxonomic distinctness index and its statistical properties. Journal of Applied Ecology (1998) 35:523–531.[CrossRef][Web of Science]
Clarke K. R., Warwick R. M. A further biodiversity index applicable to species lists: variation in taxonomic distinctness. Marine Ecology Progress Series (2001) 216:265–278.[CrossRef][Web of Science]
Daan N., Gislason H., Pope J. G., Rice J. C. Changes in the North Sea fish community: evidence of indirect effects of fishing? ICES Journal of Marine Science (2005) 62:177–188.
D’Onghia G., Carlucci R., Maiorano P., Panza M. Discards from deep-water bottom trawling in the eastern-central Mediterranean Sea and effects of mesh size changes. Journal of Northwest Atlantic Fisheries Science (2003) 31:245–261.
Erzini K., Costa M. E., Bentes L., Borges T. C. A comparative study of the species composition of discards from five fisheries from the Algarve (southern Portugal). Fisheries Management and Ecology (2002) 9:31–40.[CrossRef][Web of Science]
Froese R., Pauly D. Fishbase. (2004) World wide web electronic publication. http://www.fishbase.org.
Gislason H., Rice J. Modelling the response of size and diversity spectra of fish assemblages to changes in exploitation. ICES Journal of Marine Science (1998) 55:362–370.
Greenstreet S. P., Hall S. J. Fishing and the ground-fish assemblage structure in the North-Western North Sea: an analysis of long-term and spatial trends. Journal of Animal Ecology (1996) 65:577–598.[CrossRef][Web of Science]
Groenewold S., Fonds M. Effects on benthic scavengers of discards and damaged benthos produced by the beam-trawl fishery in the southern North Sea. ICES Journal of Marine Science (2000) 57:1395–1406.
Hall S. J. The Effects of Fishing on Marine Ecosystem and Communities. (1999) London: Blackwell Science. 274.
Jennings S., Kaiser M. J. The effects of fishing on marine ecosystems. Advances in Marine Biology (1998) 34:201–352.[Web of Science]
Jennings S., Greenstreet S. P. R., Hill L., Piet G. J., Pinnegar J. K., Warr K. J. Long-term trends in the trophic structure of the North Sea fish community: evidence from stable-isotope analysis, size-spectra and community metrics. Marine Biology (2002) 141:1085–1097.[CrossRef]
Jensen A. L., Reider R. H., Kovalak W. P. Estimation of production forgone. North American Journal of Fisheries Management (1988) 8:191–198.[CrossRef]
Kelleher K. Discards in the World's Marine Fisheries: An update. FAO Fisheries Technical Paper (2004) 470:131 pp..
Machias A., Maiorano P., Vasilopoulou V., Papaconstantinou C., Tursi A., Tsimenides N. Sizes of discarded commercial species in the eastern-central Mediterranean Sea. Fisheries Research (2004) 66:213–222.[CrossRef][Web of Science]
Machias A., Vasilopoulou V., Vatsos D., Bekas P., Kallianotis A., Papaconstantinou C., Tsimenides N. Bottom trawl discards in the northeastern Mediterranean Sea. Fisheries Research (2001) 53:181–195.[CrossRef][Web of Science]
Monteiro P., Araujo A., Erzini K., Castro M. Discards of the Algarve (southern Portugal) crustacean trawl fishery. Hydrobiologia (2001) 449:267–277.[CrossRef][Web of Science]
Murawski S. A., Idoine J. S. Multispecies size composition: a conservative property of exploited fishery systems? Journal of Northwest Atlantic Fisheries Science (1992) 14:79–85.
Myers R. A., Worm B. Rapid worldwide depletion of predatory fish communities. Nature (2003) 423:280–283.[CrossRef][Medline]
Papaconstantinou C. Check-list of marine fishes of Greece. Fauna Graeciae. National Centre for Marine Research and Hellenic Zoological Society, Athens (1988) 257.
Pauly D., Palomares M. L., Froese R. J., Sa-a P., Vakily M., Preikshot D., Wallace S. Fishing down Canadian aquatic food webs. Canadian Journal of Fisheries and Aquatic Sciences (2001) 58:51–62.
Petrakis G., Stergiou K. I. Size selectivity of diamond and square mesh codends for four commercial Mediterranean fish species. ICES Journal of Marine Science (1997) 54:13–23.
Pope J. G., Knights B. J. Comparison of the length distributions of combined catches of all demersal fishes in surveys in the North Sea and at Faroe Bank. In: Multispecies Approaches to Fisheries Management Advice—Mercer M. C., ed. (1982) 59. Canadian Special Publication in Fisheries and Aquatic Sciences. 116–118. 169 pp.
Rice J., Gislason H. Patterns of change in the size spectra of numbers and diversity of the North Sea fish assemblage, as reflected in surveys and models. ICES Journal of Marine Science (1996) 53:1214–1225.
Rochet M-J., Trenkel V. M. Factors for the variability of discards: assumptions and field evidence. Canadian Journal of Fisheries and Aquatic Sciences (2005) 62:224–235.
Rogers S. I., Clarke K. R., Reynolds J. D. The taxonomic distinctness of coastal bottom-dwelling fish communities of the North-east Atlantic. Journal of Animal Ecology (1999) 68:769–782.[CrossRef][Web of Science]
Sanchez P., Demestre M., Martin P. Characterisation of the discards generated by bottom trawling in the northwestern Mediterranean. Fisheries Research (2004) 67:71–80.[CrossRef][Web of Science]
Stergiou K., Petrakis G. Description, assessment of the state and management of the demersal and inshore fisheries resources in the Hellenic Seas. Fresenius Environmental Bulletin (1993) 2:312–319.
Stergiou K. I., Christou E. D., Georgopoulos D., Zenetos A., Souvermesoglou C. Hellenic Seas: physics, chemistry, biology and fisheries. Oceanography and Marine Biology: An Annual Review (1997) 35:415–538.
Stergiou K. I., Economou A., Papaconstantinou C., Tsimenides N., Kavadas S. Estimates of discards in the Hellenic commercial trawl fishery. Rapport de Commitee Internationale de Mer Mediteranee (1998) 35:490–491.
Stergiou K. I., Karpouzi V. S. Feeding habits and trophic levels of Mediterranean fish. Reviews in Fish Biology and Fisheries (2002) 11:217–254.[CrossRef][Web of Science]
Stergiou K. I., Machias A., Somarakis S., Kapantagakis A. Can we define target species in Mediterranean trawl fisheries? Fisheries Research (2003) 59:431–435.[CrossRef][Web of Science]
Stobberup K. A., Inejih C. A. O., Traore S., Monteiro C., Amorim P., Erzini K. Analysis of size spectra of northwest Africa: a useful indicator in tropical areas? ICES Journal of Marine Science (2005) 62:424–429.
Stratoudakis Y., Fryer R. J., Cook R. M. Discarding practices for commercial gadoids in the North Sea. Canadian Journal of Fisheries and Aquatic Sciences (1998) 55:1632–1644.
Tudela S., Coll M., Palomera I. Developing an operational reference framework for fisheries management on the basis of a two-dimensional index of ecosystem impact. ICES Journal of Marine Science (2005) 62:585–591.
Tzanatos E., Somarakis S., Tserpes G., Koutsikopoulos C. Discarding practices in a Mediterranean small-scale fishing fleet (Patraikos Gulf, Greece). Fisheries Management and Ecology (2007) 14:277–285.[CrossRef][Web of Science]
Warwick R. M., Clarke K. R. New ‘biodiversity’ measures reveal a decrease in taxonomic distinctness with increasing stress. Marine Ecology Progress Series (1995) 129:301–305.[CrossRef][Web of Science]
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