Surveying while fishing in the slope areas off Brazil: direct assessment of fish stock abundance from data recorded during commercial trawl fishing operations
Alvarez Perez,José Angel
Estimating the total biomass of fish stocks available for fishing at any one time can highly improve the process of deciding upon sustainable fishing management measures. However, countries where fishing resources are not abundant or valuable can hardly justify a regular investment in costly direct stock assessment programs. One way of overcoming such difficulties is to use catch rate data recorded from monitored commercial fishing operations, usually made available through the implementation of less expensive Observers programs. However, this data is prone to overestimate biomass of targeted stocks, because fishermen tend to operate in high-density areas. This study addresses this issue by exploring the use of geostatistical models to estimate biomass of three slope fish stocks of the Brazilian EEZ: codling (Urophycis mystacea), hake (Merluccius hubbsi) and monkfish (Lophius gastrophysus). The analysis included 25,824 commercial trawls conducted off Brazil between 2000 and 2009. Global biomass tonnage estimations were: codling 16,402 ton in 2001 and 23,816 ton in 2002; hake 267,712 ton in 2001 and 233,107 ton in 2002; monkfish 51,947 ton in 2001 and 16,998 ton in 2002. Values exceeded those obtained by scientific surveys conducted in the same period and area, but an in-depth analysis of methods employed demonstrated that such overestimation was only significant in the case of hake, the main target of the trawl fleet during the study period. The study showed that geostatistical models are most effective in reducing bias introduced by fishermen behavior when some catches away from high density areas are available. Those tend to be rare in the case of major target-species.