MEPS 333:117-128 (2007)  -  doi:10.3354/meps333117

Trophic signatures of small-scale fishing gears: implications for conservation and management

Konstantinos I. Stergiou1,2,*, Dimitrios K. Moutopoulos1,3, Hernando J. A. Casal4, Karim Erzini2

1Aristotle University of Thessaloniki, School of Biology, Department of Zoology, Box 134, Thessaloniki 54124, Greece
2CCMAR, Universidade do Algarve, 8005-139 Faro, Portugal
3University of Patras, Department of Biology, Section of Animal Biology, Rio-Patras 265 00, Greece
4Facultad de Ciencias del Mar, Departamento de Biologia, Grupo Dinamica de Poblaciones de Peces, Universidad de Cádiz, Avda. Republica Saharahui s/n, 11510 Puerto Real, Cádiz, Spain

ABSTRACT: We quantified the ecosystem effects of small-scale gears operating in southern European waters (Portugal, Spain, Greece), based on a widely accepted ecosystem measure and indicator, the trophic level (TL). We used data from experimental fishing trials during 1997 to 2000. We studied a wide range of gear types and sizes: (1) gill nets of 8 mesh sizes, ranging from 44 to 80 mm; (2) trammel nets of 9 inner panel mesh sizes, ranging from 40 to 140 mm; and (3) longlines of 8 hook sizes, ranging from Nos. 15 (small) to 5 (large). We used the number of species caught per TL class for constructing trophic signatures (i.e. cumulative TL distributions), and estimated the TL at 25, 50 and 75% cumulative frequency (TL25, TL50, TL75) and the slopes using the logistic function. We also estimated the mean weighted TL of the catches (TLW). Our analyses showed that the TL characteristics of longlines varied much more than those of gill and trammel nets. The longlines of large hooks (Nos. 10, 9, 7, 5) were very TL selective, and their trophic signatures had very steep slopes, the highest mean TL50 values, very narrow mean TL25 to TL75 ranges and mean TLW > 4. In addition, the mean number of TL classes exploited was smaller and the mean TL50 and TLW were larger for the longlines of small hooks (Nos. 15, 13, 12, 11) in Greek than in Portuguese waters. Trammel and gill nets caught more TL classes, and the mean slopes of their trophic signatures were significantly smaller than those of longlines as a group. In addition, the mean number of TL classes exploited, the mean TL50 and the TLW of gill nets were significantly smaller than those of trammel nets. We attribute the differences between longlines of small hooks to bait type, and the differences between all gear types to their characteristic species and size-selectivity patterns. Finally, we showed how the slope and the TL50 of the trophic signatures can be used to characterise different gears along the ecologically ‘unsustainable–sustainable’ continuum.

KEY WORDS: Trophic level · Trophic signatures · Gill nets · Trammel nets · Longlines · Indicators · Northeast Atlantic Ocean · Mediterranean Sea

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