A direct test of the effects of protective management on abundance and yield of tropical marine resources

Abstract

Despite a vast body of traditional knowledge on management of marine resources in the tropics there are few direct tests, utilizing manipulative or natural experiments, of the effect of protective management on the abundance of marine resources and, more importantly, their yield. This paper reports on a natural experiment which makes such a direct test. Approximately 25% of the sub-tidal coral reef of Sumilon Island (total reef area to 40 m isobath = 0.5 km ² ) in the central Philippines was protected from all forms of exploitation from 1974 until May 1984. Cessation of protective management led to fishing of the entire coral reef. This resulted in asignificant reduction in abundance within the previously protected area of the fishes which constituted the majority of the yield from the reef. Catch per unit effort (c.p.u.e.) measured over a one-year period eighteen months after protective management ceased (1985/1986, $\overline{\text{x}}$ = 0.99 kg man ⁻¹ trip ⁻¹ , 95% CL = 0.42) was significantly less than that measured over a one-year period before protective management ceased (1983/1984, $\overline{\text{x}}$ = 1.98 kg man ⁻¹ trip ⁻¹ , 95% CL = 0.31). There was a decline of 54% in the total yield of reef fishes from Sumilon Island reef between 1983/1984 and 1985/1986 (from 36.9 to 19.87 metric t/km ² ) despite the fact that only 75% of the reef was fished in 1983/1984. The yield from traps and gill nets (approximately 64% of the total yield) from the whole reef in 1985/1986 was significantly less than the average yield of the non-reserve (75% of the reef) measured during three of the years of protection. Similarly, the yield from traps (approximately 45% of the total yield) in 1985/1986 was significantly less than the average yield of the non-reserve (75% of the reef) measured during six of the years of protection. Protective management maintained high abundances of fishes in the reserve and significantly higher yields to fishermen from areas adjacent to the reserve. Migration of adult fish from the reserve to the non-reserve area during protection is the simplest explanation of these results.