The degree and result of gillnet fishery interactions with juvenile white sharks in southern California assessed by fishery-independent and -dependent methods

摘要

Previous reports have documented juvenile white shark interactions with gillnet fisheries in southern California; however, there has been no quantification of the degree of this interaction using fishery-independent methods. We compared geopositioning data from juvenile white sharks fitted with acoustic (n = 11) or satellite transmitters (n = 13) to fisheries data to determine the degree and effect of white shark interactions with the gillnet fisheries in southern California. Between 2006 and 2008, set gillnet effort comprised a majority of the total gillnet effort (88%) and both set gillnet and inshore drift gillnet effort were significantly and positively correlated with incidence of white shark capture (p < 0.0001, tau = 0.34 and 0.32) and number of satellite detections (p < 0.0001, tau = 0.34 and 0.33). However, spatial and temporal overlap of white sharks with gillnet fisheries was limited. Approximately 18% of CDFW fishing blocks where white sharks were detected overlapped with blocks that were also heavily utilized by gillnet fisheries. Total gillnet effort tended to peak in the month of July before declining substantially whereas SPOT detections of tagged sharks were the most numerous in fall months. Although juvenile white sharks were shown to overlap with gillnet fisheries in their vertical, horizontal and temporal distributions, post-release survival of sharks retrieved live in gillnets was high (92.9%). Sharks were more often found live in gillnets when net soak times were low. Therefore, continued research is needed to further evaluate the potential benefit of reducing soak times to improve incidental capture survival of white sharks at this age class