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

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, TL_50, TL₇₅) and the slopes using the logistic function. We also estimated the mean weighted TL of the catches (TL_W). 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 TL_W > 4. In addition, the mean number of TL classes exploited was smaller and the mean TL₅₀ and TL_W 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 TL_W 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