Predator-mediated recruitment and size of kelp-associated fishes (funded by National Undersea Research Program; National Science Foundation; SDSU Grant-in-Aid)

Predation is an important process in the post-settlement mortality of reef fishes. Todd Anderson, Andy Davenport, and Carey Galst investigated experimentally the role of a piscivore, the kelp bass, Paralabrax clathratus, in the recruitment success of fishes that settle to giant kelp, Macrocystis pyrifera. For two successive years (2002-03) recruitment of young-of-year kelp bass (left photo; ~ 2 cm), giant kelpfish (Heterostichus rostratus; right photo; ~ 3.5 cm), and senorita (Oxyjulis californica) to plots of giant kelp were examined in three treatments: predator access, predator exclusion, and a cage control.

The species composition of recruits differed between years, but recruitment was significantly higher in each year within plots of kelp which excluded predators than in plots that allowed access to predators. Cage controls indicated that exclusion pens neither enhanced nor inhibited settlement of recruits. In addition, the size distribution of recruits of kelp bass differed among treatments; larger recruits occurred on plots in which predators were excluded than on plots open to predators, which could be caused by differences in consumption rates, growth rates, or size-selective predation. In a third year, recruit kelp bass were marked and placed on kelp plots at different densities and then exposed to older kelp bass predators. Mortality of marked kelp bass was density-dependent despite large influxes of settlers and immigrants, suggesting that predator-induced density-dependent mortality of marked individuals occurred quickly after onset of the experiment. These data collectively show the importance of predators in causing variation in recruitment success of kelp-associated fishes.

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