单选题 .  ①The waters east of Cape Hangklip were once the center of a lucrative wild-caught abalone fishery, but illegal fishing in the mid-1990s escalated to such levels that the recreational fishery was closed in 2003. ②When abalones did not rebound, commercial fishing was also banned. ③Continue declines in abalone were attributed to poaching, but an invasion by rock lobsters during the early 1990s probably intensified the trend. ④Rock lobsters prey on sea urchins, and increased rock lobster densities coincided with significant decreases in urchins. ⑤In that area, urchins feed largely by trapping drift kelp, and in doing so provide juvenile abalone with both protective shelter and nourishment. ⑥Without urchins' presence, juvenile abalones are less likely to survive to adulthood.35.  According to the passage, since the early 1990s, sea urchins in the waters east of Cape Hangklip have ______