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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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© 2008 International Council for the Exploration of the Sea. Published by Oxford Journals. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Seasonal and temporal trends in metrics of fish community for otter-trawl discards in a Mediterranean ecosystem

K. Tsagarakis1,2, A. Machias1, M. Giannoulaki1, S. Somarakis1 and I. Karakassis2

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 ({Delta}+), whereas the variation in taxonomic distinctness ({Lambda}+) 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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