Ghost-fishing ability decreasing over time for lost bottom-gillnet and estimation of total number of mortality

The ghost-fishing ability descending over time and ghost-fishing mortality were quantitatively evaluated for a lost bottom-gillnet. Three experiments were conducted for a maximum period of 1,689 days since deployment of a net. The number of ghost-fishing mortalities per day per net was determined by underwater observation of enmeshed fish on two consecutive days. Mortalities were recorded continuously around the start of experiments and intermittently during the following period. The consistent faunal character around the net was confirmed and the number of mortalities was assumed to reflect the change in the ghost-fishing ability of the net. The decline in ghost-fishing ability over time was represented by a sum of two formulae, which reflect two ghost-fishing factors effective respectively for short and long periods of time. From this analysis, the effective ghost-fishing duration which was defined as the time elapsed to the day when the value of the equation declines to 5 percent of the original fishing ability of the net was 142 days during which the total number of ghost-fishing mortalities was 455 fish. Ghost fishing for red sea bream, Pagrus major and jack, Decapterus sp. occurred in the first short period and for filefish Stephanolepis cirrhifer, over a longer period.