Does Beach Grooming Harm Grunion Eggs?

摘要

The grunion (Leuresthes tennis), a charismatic marine fish, makes spectacular midnight spawning runs on some of the most heavily used urban beaches of the California coast. Its eggs incubate buried in the damp sand, fully out of water for 10 to 14 days. Because of numerous human visitors, many urban beaches are groomed regularly to remove trash, kelp, and other debris. Concern over the status of grunion populations prompted a public outcry to the San Diego City Council in 2001, with the accusation that beach grooming had decimated the grunion there by destroying their eggs. San Diego beach maintenance staff countered that they implement a grunion grooming protocol, raking only above a high tide mark set after the highest semilunar tides, during grunion season in an effort to avoid any eggs in the intertidal zone. To assess the status of the grunion population, we initiated a program for volunteers from the community to observe on local beaches during nocturnal spawning runs in 2002. With their help, we identified many areas of San Diego beaches where grunion spawned. We sampled by transects to estimate the density of incubating eggs. Then, to determine whether grooming modifications are necessary during grunion season, we asked the beach maintenance staff to groom directly over the eggs in small experimental areas. Afterwards, we re-sampled the egg population and found that beach grooming directly over spawning sites consistently resulted in a significant loss of eggs. When grooming includes removal of kelp and debris, almost no grunion eggs remain. With collaboration between scientists, civic and environmental organizations and the public, we showed that the vulnerable eggs of this unique fish can be protected in their critical life stage by modifying beach grooming practices, so that our urban beaches can be shared with this natural treasure.