Gradients of abundance of fish across no-take marine reserve boundaries: evidence from Philippine coral reefs

Abstract

1. An abundance gradient from high inside to low outside a no-take marine reserve may indicate net emigration of adult fish from the reserve (spillover). 2. We examined spatial patterns of abundance of fish across two 900 m long sections of coral reef slope at each of two small Philippine islands (Apo and Balicasag). One section sampled the entire length of a no-take reserve and extended 200-400 m outside the two lateral reserve boundaries. The other section, without a reserve, was a control. The reserves had had 20 (Apo) and 15 (Balicasag) years of protection when sampled in 2002. 3. Significant spatial gradients of decreasing abundance of target fish occurred across only one (Apo Reserve northern boundary = ARNB) of four real reserve boundaries, and across none of the control boundaries. Abundance of non-target fish did not decline significantly across reserve boundaries. 4. Abundance of target fish declined sharply 50 m outside the ARNB, but enhanced abundance extended 100-350 m beyond this boundary, depending on fish mobility. 5. Density of sedentary target fish declined 2-6 times faster than density of highly vagile and vagile target fish across the ARNB. 6. Habitat factors could not account for these ARNB results for target fish, but did influence abundance patterns of non-target fish. 7. The lack of abundance gradients of target fish at Balicasag may reflect reduced fishing outside the reserve since it was established. 8. Apo Reserve had a gradient of abundance of target fish across at least one boundary, a result consistent with spillover.