Effects of Density Dependence and Environment on Recruitment of Coastal Invertebrates

Author(s): Lundquist, Carolyn J. | Abstract: The processes influencing recruitment in sea urchins are poorly understood. To examine the depensatory density-dependent reproduction of broadcast spawning invertebrates, I created a simulation model of fertilization dynamics. The model described decreased fertilization success as density decreased, and increased fertilization as the aggregation of adult spawners increased. Highest fertilization success resulted from simulations with wide gamete dispersal distributions with an extended tail, representing low flow conditions. Surprisingly, the model did not predict a deterministic Allee effect or minimum threshold density of spawners for successful fertilization. but instead produced a gradual non-linear decline in larval production at low densities. A stochastic Allee effect was observed, as the variability in larval production increased substantially at low density.To estimate current larval production of the red sea urchin Strongylocentrotus franciscanus, I applied this fertilization model to observed spatial distributions. Adult sea urchins were censused at eleven sites to determine density and local spatial distribution. Reserve sites generally had higher densities than fished sites. All sites were significantly aggregated, and aggregation increased as density decreased. Fertilization success was strongly influenced by both density and aggregation. Estimated current larval production was approximately one-third of estimated pre-fishery larval production, with 75% of the decline attributable to declines in mean density, and the fertilization Allee effect responsible for the remainder of the decline.To evaluate density-independent influences on recruitment, I examined settlement patterns of juvenile sea urchins and crabs in northern California Prior research had determined an intra-annual pattern of settlement during local wind relaxation events. This pattern was present during the 1998 El Nino, resulting in high settlement of cancrid crabs and sea urchins. The 1999 La Nina lacked the typical upwelling/relaxation pattern, and low settlement was observed.These results illustrate several aspects of population dynamics that should be considered in fisheries management and reserve design. Decreased adult density can have long-lasting effects if reproductive potential is affected. Local oceanographic transport of planktonic larvae can result in spatial and temporal variability in juvenile settlement. Researchers will need to consider both reproductive potential and settlement variability when designing reserve networks.