Natural and Fishing Mortality

Extensive coastal longlining operations by small scale fishers developed in the eastern tropical Pacific since the mid-1990’s. Presently, most of the coastal areas are incorporated in the fishing operations targeting Mahi-Mahi (Dorado) but are catching significant numbers of billfish and sharks as bycatch in the longline operations. This has resulted in large billfish landings by the mostly transformed artisanal and semi-industrial longliners such as those observed in the figure below.

 

 

Eastern Pacific billfish fishing mortality rates are much larger than rational levels for their exploitation.

Data on size composition of Sailfish caught incidentally in tuna purse seine sets in the Eastern Pacific between 1991 and 2007 were made available to University of Miami scientists by the Inter-American Tropical Tuna Commission (IATTC). These data were measured by observers working for the IATTC aboard the tuna purse seiners and analyzed at the University of Miami Billfish laboratories.

On average, Pacific sailfish fishing mortality rate is at least 2 times as large as the value of the estimated natural mortality rate (M=0.38). Maximum acceptable exploitation is when on average fishing mortality is equal to natural mortality.

 

Fishing mortality rates estimates for the Striped marlin in the Eastern Pacific by the Inter-American Tropical Tuna Commission correspond well with the equally high fishing mortality rates estimated for the Sailfish. Despite great differences in data and methods used to estimate these rates, the trends are indicative of very high levels of exploitation exerted by the fishing fleets operating in the region.