Environmental, spatial, temporal and operational effects on swordfish ( Xiphias gladius) catch rates of eastern Mediterranean Sea longline fisheries

Abstract

Generalized additive models (GAMs) were applied to examine the relative influence of environmental, spatial, temporal and operational factors on swordfish catch rates in the Greek swordfish longline fishery between 1998 and 2004. GAM analysis accounted for 47% of the variance in nominal catch per unit effort (CPUE) expressed in number of fish per 1000 hooks. Stepwise GAM building revealed the relative importance of eight variables ranked by decreasing magnitude: Fishing gear type, Month, Year, Sea surface temperature, Longitude, Latitude, Lunar index and Bottom depth. Longlines having deeper, thicker and more resilient branch lines with illuminated fish attractants yielded significantly higher swordfish catches. CPUE peaked during the last quarter of the fishing season, at sea surface temperatures 16–18 °C and over 26 °C, when the Lunar disc illumination was high and at greater depths. Elevated relative abundance was observed in southern and eastern longitudes, corresponding to the Levantine region. A moderate decline in swordfish abundance was detected from 1998 to 2003 followed by a sharp rise in 2004, while average individual swordfish weight decreased from 30 kg in 1998 to 23 kg in 2004.