Interactions among red abalones and sea urchins in fished and reserve sites of northern California: implications of competition of management

摘要

Red abalones (Haliotis rufescens), red sea urchins (Strongylocentrotus franciscanus), and purple sea urchins (S. purpuratus) share similar food and habitat requirements in northern California. Red abalones and red sea urchins also support importantfisheries. Here we explore spatial interactions and apparent competitive effects among these species at an area where fishing has large impacts on both taxa, and at unfished reserve sites in which invertebrate density and food availability differ. Therewas an inverse correlation between adult red abalone and red sea urchin abundance at the scale of our transects when density of either or both species was high. In the poorest habitat for macroalgae, red abalones seldom occurred on the same transects with red urchins. The results suggest that differences in density, depth, and food availability play an important role in the observed spatial patterns of red abalones and red sea urchins. Purple sell urchins were not correlated to either of the other two species' distributions. An intense fishery for red sea urchins appears to have had a positive effect on kelp availability, and abalone growth and abundance. Aerial photographs during the period of intense urchin fishing (from 1982 to 1989), showed a dramatic increase in the surface canopy. Similarly, during this period, size frequency distributions of fished red abalones Show an increase in the number of individuals in larger size classes. Modal progression in abalone size frequency distributions suggests a faster growth rate during this period when compared with a growth study, at the same location, conducted during the pre-urchin fishery years. Ultimately, red sea urchin removal apparently led to an increase in red abalone abundance even at a site that was heavily fished by recreational abalone fishers. Meanwhile, at a nearby reserve site where kelp populations are lower, red abalones have declined in abundance as red sea urchins increased. Our results suggest the need for multi-species ecosystem-based approaches to management of these valuable resources.