Migration, aggregation, and philopatry of two nearshore elasmobranch species in the Southern California Bight

Abstract

Over one-third of elasmobranch fishes (sharks, rays, and skates) are threatened with extinction, mostly due to overfishing, habitat loss, and habitat degradation. Understanding the daily and seasonal movement patterns of these species can inform when and where populations are most susceptible to these threats, but these data are often lacking for nearshore species that are not actively managed. Two such species are the shovelnose guitarfish Pseudobatos productus and California bat ray Myliobatis californica; this study quantified the broad- and fine-scale movement patterns of these species using passive acoustic telemetry. Twelve guitarfish (10 female, 2 male) were surgically implanted with coded acoustic transmitters at an aggregation site off La Jolla (San Diego County), California, USA, and tracked for 849.5 ± 548.9 d (mean ± SD). Six bat rays (all female) were also implanted with transmitters and tracked for 1143.8 ± 830.9 d. These animals were detected at 187 acoustic receiver stations between Point Conception, California, and San Quintín, Baja California, Mexico. Both species exhibited annual philopatry to La Jolla, especially in July, after traveling as far north as Santa Barbara (221 km away; guitarfish) and San Miguel Island (259 km away; bat rays), California. Based on their movement patterns and known reproductive phenology, we hypothesize that both species utilize the La Jolla aggregation site as a gestating ground and possibly also a mating, pupping, and nursery ground. This site is within a no-take reserve, and we recommend that similar sites also be protected, given the increased susceptibility to anthropogenic stressors when aggregating.