Contrasting Patterns of Fish Density and Abundance at an Artificial Rock Reef and a Cobble-Bottom Kelp Forest

We describe the results of an intensive, short-term (November 1986–January 1987) characterization of numerical and biomass abundances of fishes at Pendleton Artificial Reef (PAR), a quarry rock reef off southern California. We emphasize area and density effects on fish abundance and discuss their implications for monitoring and resource management. PAR is compared with concurrent estimates of fish density and abundance within San Onofre Kelp bed (SOK), a cobble-bottom giant kelp (Macrocystis) forest, 5.5-km upcoast of PAR. An estimated 14,950 resident juvenile-adult fishes weighing 490 kg were present atop the 1.3 ha of rock modules and fringing sand-rock ecotones at PAR; biomass density thus was about 380 kg/ha. The biomass density of resident fishes in the kelp forest was about 325 kg/ha. Numerical density at SOK, however, was less than one-third (3,600 fish/ha) of the density of resident fishes at PAR (11,500 fish/ha). Since the areal extent of the kelp-cobble habitat (∼88 ha) was nearly two orders of magnitude greater than the artificial reef, its standing stock of resident fishes (about 29 MT) was almost 60 times greater than that at PAR. Our results have important implications for the design of reef mitigation studies. Evaluations of fish populations at reefs can be misleading when they are based on density data alone. It is obvious that any assessment of existing or potential biological mitigation provided by an introduced habitat should base its evaluation on abundance, not the density of organisms independent of the quantity of habitat. Another important topic for future research is the relation between fish density and the areal extent of introduced habitat.