Effects of Macroalgal Dynamics on Recruitment of a Temperate Reef Fish

Abstract

Spatial and temporal variation in the recruitment of dispersive life stages can strongly affect the distribution, dynamics, structure, and gene flow of local populations. I investigated the causes of variable larval recruitment to populations of a temperate reef fish, the kelp bass (Paralabrax clathratus). Spatial and temporal variation in the density of settlement habitat on a reef, the giant kelp (Macrocystis pyrifera), explained much of the variation in recruitment of kelp bass. Although recruitment was positively related to Macrocystis density, the relationship was asymptotic at high plant densities, indicating that kelp bass recruitment saturates at intermediate densities of Macrocystis (°100—130 stipes/30 m2). Field experiments demonstrated causation between Macrocystis density and kelp bass recruitment and indicated that kelp bass recruitment was linearly related to the local abundance of kelp structure (i.e., the number and biomass of overlapping Macrocystis blades). This linear (nonasymptotic) relationship between Macrocystis blade biomass and density of fish recruits indicated that both the quantity and quality of the recruitment habitat limited larval recruitment. Structural complexity per unit length of plant (e.g., blade biomass per unit plant length) was inversely related to plant density. This relationship underlies the asymptotic relationship between structural complexity per unit reef area (the product of per—plant structural complexity and plant density) and recruit density. Thus, density—dependent constraints on blade biomass in the macroalga, rather than larval supply, better explained the general asymptotic nature of the relationship between Macrocystis and kelp bass recruitment. These results indicate that density—dependent effects on plant biomass and architecture (e.g., plant height and structural complexity), common among both macrophytes and terrestrial plants, can greatly influence the relationship between plant density and recruitment of other species. For organisms with dispersive life stages whose local recruitment is influenced by the occurrence of living habitat structure, spatial and temporal variability characteristic of such biogenic habitats can be a major source of variation in their recruitment.