Spatio-temporal dynamics in the discards of trawl fisheries in the eastern Mediterranean

Discarded catches in the main fishing areas of the Turkish coasts of the Aegean Sea were studied using data from commercial trawlers. The study area spans the Turkish coast of the Aegean Sea from Saros Bay (north) to Gulluk Bay (south), and data were collected from trawl fishing grounds between April 2010 and 2012. Particular attention is paid to discard per unit effort (DPUE) and discard ratio. Generalized additive modelling (GAM) techniques with Tweedie family and log-link function were used to examine various predictor variables (duration, season, depth, longitude and latitude) on the DPUE of total catch. Discard ratio varied from 0.01 in winter at canakkale to 0.90 in summer at Gulluk. Total discard ratio was calculated to be 0.33 for pooled data. The DPUE values ranged between 0.2 kg h-1 in autumn at Sigacik and 45.5 kg h-1 in spring at Gulluk, with a mean value of 7.6 +/- 5.7 kg h-1. Modelling DPUE values in relation to season vs coordinates revealed spatio-temporal differences. The relationship between the dependent variable (DPUE) and the independent variable (depth) showed fluctuations, but haul duration displayed a decreasing trend in the modelling. It was determined that a considerable amount of trawl discards was obtained in the Aegean Sea. The results of this paper should be considered to create a regional discard policy and management plan in the eastern Mediterranean.