Trammel nets’ ghost fishing off the Portuguese central coast

The loss of fishing gear has negative consequences to marine communities if the gear preserves its catching abilities for a significant period, a phenomenon called “ghost fishing”. The present study assessed the impact of lost trammel nets in both sandy and rocky bottoms in the central area of the Portuguese coast. Ten trammel nets, each 50 m long and corresponding to the most common type used by the local commercial fleet, were allowed to fish continually for 285 days. During this time, changes in the structure and catching ability of each net were monitored by scuba divers in regular time intervals (1, 5, 10, 15, 20, 30, 40, 60, 80, 120, 250 and 285 days). Three control nets were also set in each bottom type the day before each monitoring dive. Irrespective of bottom type, nets’ fishing area decreased to about 40% during the first 30 days, and then gradually (rocky bottoms) or sharply (sandy bottoms). It was estimated that during the experiment 541 and 257 individuals were caught per 100 m of net in rocky and sandy bottoms, respectively. Catching efficiency decreased in a negative exponential manner in parallel with the nets deterioration. The nets’ effective fishing lifetime, when catching efficiency became lower than 1%, was 10–11 months in the rocky bottom and 8 months in the sandy bottom.