Indirect effects of sea otters on rockfish (Sebastes spp.) in giant kelp forests

Abstract

Abstract. Sea otters are a classic example of a predator controlling ecosystem productivity through cascading effects on basal, habitat-forming kelp species. However, their indirect effects on other kelp-associated taxa like fishes are poorly understood. We examined the effects of sea otter (Enhydra lutris) reintroduction along the west coast of Vancouver Island, Canada on giant kelp (Macrocystis pyrifera) distributions and the trophic niches and growth of two common kelp forest fishes, black (Sebastes melanops) and copper (S. caurinus) rockfishes. We sampled 47 kelp forests, and found that red sea urchins (Strongylocentrotus franciscanus) were eliminated in the presence of otters, and that kelp forests were 3.7 times deeper and 18.8 times larger. Despite order-of-magnitude differences in kelp forest size, adult black and copper rockfishes contained less kelp-derived carbon in their tissues (as measured by stable isotopes of C and N) in regions with otters. Adults of both species had higher mean trophic positions in the presence of otters, indicating more frequent consumption of higher trophic level prey such as fishes. Smaller trophic niche space of rockfishes in the presence of otters indicated a higher degree of trophic specialization. Juvenile black rockfishes rapidly shifted to higher kelp-carbon contents, trophic positions, and body condition factors after settling in kelp forests. The relationships of growth to length, percentage of kelp carbon, and trophic position varied between the two regions, indicating that potential effects of kelp forest size on trophic ontogeny may also affect individual performance. Our results provide evidence that the indirect effects of otters on rockfishes arise largely through the creation of habitat for fishes and other prey rather than a direct trophic connection through invertebrates or other consumers of kelp productivity.

Indirect effects and prey behavior mediate interactions between an endangered prey and recovering predator

“…B, C, ; Estes and Palmisano , Breen et al. , Estes and Duggins , Watson and Estes , Markel and Shurin ). In addition to providing food in the form of algal drift, kelp forests provide shelter for abalone by attenuating wave energy and reducing water flow through understory kelps (Duggins , Eckman and Duggins ).…”

“…), sea otters also transformed two‐dimensional urchin barrens into structurally complex three‐dimensional kelp forest habitat (Fig. C; Estes and Palmisano , Watson and Estes , Markel and Shurin ). This indirect magnification of kelp forest habitat and associated drift kelp increases the availability of food and shelter for abalone, promoting the persistence of abalone as sea otters recovery.…”

“…5,6), likely due to habitat change from urchin barrens to deeper and larger kelp forests created via the otter-urchin-kelp trophic cascade (Figs. 9B, C, 10;Estes and Palmisano 1974, Breen et al 1982, Estes and Duggins 1995, Watson and Estes 2011, Markel and Shurin 2015. In addition to providing food in the form of algal drift, kelp forests provide shelter for abalone by attenuating wave energy and reducing water flow through understory kelps (Duggins 1987, Eckman andDuggins 1991).…”

Megafaunal Impacts on Structure and Function of Ocean Ecosystems

“…Growth rates of suspension-feeding invertebrates are two-to threefold greater in forested compared with deforested systems (52); fish population densities are significantly enhanced where systems are in the forested state (53,54); and the diets and foraging behaviors of other high trophic-level consumers are strongly influenced by the sea otter-sea urchin-kelp trophic cascade. For example, glaucous-winged gulls (Larus glaucescens) switch from piscivory to invertebrivory where sea otters are lost from coastal ecosystems (55); bald eagle (Haliaeetus leucocephalus) diets shift from a roughly even mix of marine mammals, fish, and seabirds where otters are abundant to one that is more strongly comprised of seabirds where otters are absent (56).…”

Acanthocephalan parasites in sea otters: Why we need to look beyond associated mortality…

“…Sea otters have been extensively studied focusing on their trophic level (Estes & Palmisano, ; Estes et al, ; Markel & Shurin, ; Reisewitz, Estes, & Simenstad, ; Watson & Estes, ), behavior, population dynamics (Estes, Riedman, Staedler, Tinker, & Lyon, ; Tinker, Mangel, & Estes, ; Tinker et al, ), and diseases (Kreuder et al, ; Miller et al, , ; Thomas & Cole, ). In disease research, particular emphasis has been placed on the significance of parasitism on sea otter mortality, especially the protozoan parasites Toxoplasma gondii (Burgess et al, ; Kreuder et al, ), Sarcocystis neurona (Miller et al, ; Tinker et al, ), and peritonitis induced by the acanthocephalan Profilicollis sp.…”