Bycatch of loggerhead sea turtles: insights from 14 years of stranding data

We present a detailed analysis of sea turtle strandings (n = 619) over a 14 yr period (1993 to 2006) from the Valencian Community (eastern Spain). Loggerhead turtles Caretta caretta accounted for 98.1% of recorded strandings. Although we detected an increasing trend in the annual number of strandings over the study period, we expect that this was driven primarily by the increase in detection effort. Loggerhead turtles were mainly juveniles (mean ± SD curved carapace length from notch to tip [CCL] = 53.6 ± 12.6; range: 16 to 80.2 cm, n = 312) and strandings were far more frequent in summer months (69.6% June to September). We believe that interaction with longline fisheries was by far the main cause of observed stranding (>28% of all cases, 43.5% of 393 turtles with likely mortality cause identified). Turtles showing signs of interaction with longlines were, on average, larger (CCL = 57.5 ± 10.4; range: 29.8 to 80.2 cm, n = 116) than turtles stranded due to other causes (CCL = 51.4 ± 13.3; range: 16 to 79 cm, n = 196; t-test: t = –4.49, p < 0.001) and were more frequent in summer months, when longline fishing effort off the coast was highest. Recent reductions in longline effort may have led to a decrease  in recent years in the proportion of stranded turtles with evidence of longline interaction. Although inferences from stranding data must be subject to a number of caveats, when considered over wide spatio-temporal extents and in conjunction with other data sources, they can offer useful insights into the geographic ranges, seasonal distribution and life history of marine species of conservation concern.