Changes in fish abundance do not cascade to sea urchins and erect algae in one of the most oligotrophic parts of the Mediterranean

The abundance of fish, sea urchins, and erect algae over carbonated and non-carbonated rocks was measured in two areas differing in the level of fishing intensity off northern Minorca (Balearic Islands, western Mediterranean) to test the hypothesis that changes in the abundance of invertebrate-feeding fish cascade to erect algae through changes in the density of sea urchins in oligotrophic areas, as has been reported previously for eutrophic areas in the northwestern Mediterranean. Although the biomass of invertebrate-feeding fish and the cover of erect algae increased where fishing intensity decreased, independently of rock type, the biomass and the horizontal test size of sea urchins, represented almost exclusively by Paracentrotus lividus, also increased. Furthermore, the total cover of erect algae and that of the two dominant algae species were not correlated with sea urchin density or sea urchin biomass. Rock type did not affect the total cover of erect algae, but Cystoseira brachycarpa J. Agardh preferred non-carbonated rocks and Dictyota dichotoma v. intricata (C. Agardh) Greville favoured carbonated ones. The only herbivorous fish (Sarpa salpa) and the omnivorous breams of the family Sparidae were more abundant around carbonated rocks, although only inside the marine protected areas. The wrasses of the family Labridae were insensitive to rock type and the level of fishing intensity. The overall evidence indicates that trophic cascades do not operate in the area studied, as changes in the abundance of invertebrate-feeding fish did not cascade to sea urchins and changes in the abundance of sea urchins did not affect erect algae. The limited recruitment of fish and sea urchins due to extreme oligotrophy is invoked as an explanation, in agreement with the ecosystem exploitation paradigm.