Abstract
Kelp forests are declining in many parts of the globe, which can lead to the spreading of barren grounds. Increased abundances of grazers, mainly due to reduction of their predators, are among the causes of this development. Here, we compared the species richness (SR), frequency of occurrence (FO), and maximum abundance (MaxN) of predatory fish and their predation pressure between kelp forest and barren ground habitats of northern-central Chile. Sampling was done using baited underwater cameras with vertical and horizontal orientation. Two prey organisms were used as tethered baits, the black sea urchin Tetrapygus niger and the porcelanid crab Petrolisthes laevigatus. SR did not show major differences between habitats, while FO and MaxN were higher on barren grounds in vertical videos, with no major differences between habitats in horizontal videos. Predation pressure did not differ between habitats, but after 24 h consumption of porcelanid crabs was significantly higher than that of sea urchins. Scartichthys viridis/gigas was the main predator, accounting for 82% of the observed predation events on Petrolisthes laevigatus. Most of these attacks occurred on barren grounds. Scartichthys viridis/gigas was the only fish observed attacking (but not consuming) tethered sea urchins. High abundances of opportunistic predators (Scartichthys viridis/gigas) are probably related to low abundances of large predatory fishes. These results suggest that intense fishing activity on large predators, and their resulting low abundances, could result in low predation pressure on sea urchins, thereby contributing to the increase of T. niger abundances in subtidal rocky habitats.
Highlights
Kelp forests are communities formed by large brown seaweeds, which can reach high densities and several meters in height (Wernberg et al, 2019)
Frequency of occurrence and maximum abundance We observed a total of 11 fish species analysing the entire video duration, 10 species in kelp forests and eight on barren grounds (Fig. 3)
A species that was absent in kelp forests was Paralabrax humeralis, which was only observed at BG-Punta Choros (PC), while the following species were not recorded on barren grounds: G. nigra, Semicossyphus darwini, and Acanthistius pictus
Summary
Kelp forests are communities formed by large brown seaweeds, which can reach high densities and several meters in height (Wernberg et al, 2019). These ecosystems are vital to a wide diversity of invertebrate and fish species In the absence of these kelp forests, there are often extensive patches of rock devoid of vegetation called ‘barren grounds’, which have low biodiversity and high abundances of grazers, such as sea urchins and snails (Norderhaug & Christie, 2009; Wernberg et al, 2019)
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