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Reisewitz SE, Estes JA, and Simenstad SA. Indirect food web interactions: sea otters and kelp forest fishes in the Aleutian archipelago. Oecologia

Institute of Marine Sciences, Long Marine Laboratory, 100 Shaffer Road, Santa Cruz, CA 95060, USA.
Oecologia (Impact Factor: 3.09). 02/2006; 146(4):623-31. DOI: 10.1007/s00442-005-0230-1
Source: PubMed

ABSTRACT

Although trophic cascades-the effect of apex predators on progressively lower trophic level species through top-down forcing-have been demonstrated in diverse ecosystems, the broader potential influences of trophic cascades on other species and ecosystem processes are not well studied. We used the overexploitation, recovery and subsequent collapse of sea otter (Enhydra lutris) populations in the Aleutian archipelago to explore if and how the abundance and diet of kelp forest fishes are influenced by a trophic cascade linking sea otters with sea urchins and fleshy macroalgae. We measured the abundance of sea urchins (biomass density), kelp (numerical density) and fish (Catch per unit effort) at four islands in the mid-1980s (when otters were abundant at two of the islands and rare at the two others) and in 2000 (after otters had become rare at all four islands). Our fish studies focused on rock greenling (Hexagrammos lagocephalus), the numerically dominant species in this region. In the mid-1980s, the two islands with high-density otter populations supported dense kelp forests, relatively few urchins, and abundant rock greenling whereas the opposite pattern (abundant urchins, sparse kelp forests, and relatively few rock greenling) occurred at islands where otters were rare. In the 2000, the abundances of urchins, kelp and greenling were grossly unchanged at islands where otters were initially rare but had shifted to the characteristic pattern of otter-free systems at islands where otters were initially abundant. Significant changes in greenling diet occurred between the mid-1980s and the 2000 although the reasons for these changes were difficult to assess because of strong island-specific effects. Whereas urchin-dominated communities supported more diverse fish assemblages than kelp-dominated communities, this was not a simple effect of the otter-induced trophic cascade because all islands supported more diverse fish assemblages in 2000 than in the mid-1980s.

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    • "Consequently, most of the nearshore rocky reefs of the Aleutian Islands, specifically in the central and western regions of the archipelago, are currently dominated by urchin barrens, although some kelp forests persist in isolated locations that are scattered among the islands (Konar et al. 2014). The fish assemblages in these habitats have previously been described (Simenstad et al. 1977), with the dominant rock greenling, Hexagrammos lagocephalus, generally being substantially more abundant in kelp forests than in urchin barrens (Reisewitz et al. 2006). Likewise, fish assemblages in other areas of Alaska differ between the interiors versus margins of the kelp forests (Efird and Konar 2013), suggesting that they may similarly vary in the kelp forests of the Aleutian Archipelago. "
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    ABSTRACT: Fish distributions can be influenced by changes in their local habitat features and regional oceanographic conditions, both of which can occur at different spatial scales. Currently, the coastal waters throughout the Aleutian Archipelago are dominated by two discrete habitats types, kelp forests and urchins barrens, both of which span known U.S. biogeographic breaks near Buldir and Samalga Passes. These passes roughly divide the western Aleutians from the central Aleutians, and the central Aleutians from the eastern Aleutians, respectively, and have been suggested as important breaks for demersal fishes and deep marine fauna due to corresponding changes in ocean conditions. This study assessed how local habitat features compare to regional oceanographic conditions in how they influence nearshore fish assemblages throughout the Aleutian Archipelago. We hypothesized that these assemblages would differ markedly between the kelp forests and urchin barrens, and that local habitat features such as forest size and the abundance of understory kelps and turf algae are more important to assemblage structure than large-scale oceanographic conditions. To explore these hypotheses, 32 kelp forests and 24 non-forested urchin barren areas occurring on 15 Aleutian islands were surveyed for fish assemblage structure and habitat characters using scuba. Within the kelp forests, both the interiors (middles) and the margins (edges) of the forests were sampled for comparison. In general, fish assemblages did not significantly differ between kelp forests and urchin barrens in the western and central Aleutian Islands. Across all three regions, we found that of 26 habitat characters considered, five (density of the stipitate kelp Agarum, the percent bottom covers of understory foliose algae and encrusting coralline algae, and the availability of gravel and bedrock) together provided the best correlation with the observed variation in the fish assemblages, but this appeared to be due to the greater influence of these in the eastern Aleutians where the abundance of Agarum is greatest. However, the relationship between these habitat characters and fish assemblage structure was weak across the archipelago and not statistically significant in any of the regions. Implications of these analyses suggest that a further reduction in kelp forests may not impact fish assemblages in the western and central Aleutians. However, this is likely an artifact of these forests already being in a diminished state, with most of the kelp forests having been replaced by urchin barrens, and thus the impacts to the fish assemblages have presumably already occurred. However, this may be different in the eastern Aleutians where kelp forests remain more abundant and where the influence of Agarum appears strongest. In addition, while nearshore fish assemblages were not significantly different on the opposing sides of Buldir Pass, they were significantly different on the opposing sides Samalga Pass, suggesting that variation in large-scale oceanographic conditions around this pass were important drivers of differences in fish assemblages. However, if ocean properties, such as temperature and salinity, change around Buldir Pass in the future, then corresponding differences in the fish assemblages may develop.
    No preview · Article · Aug 2015 · Environmental Biology of Fishes
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    • "Several species, in particular, depend on kelp forests for finding suitable feeding and nursery areas and protection from predators (e.g. Norderhaug et al. 2005; Reisewitz, Estes & Simenstad 2006; Rosenfeld et al. 2014), leading to the hypothesis that their abundances would be drastically affected by changes in patterns of distribution and density of habitat-forming kelps (e.g. O'Connor & Anderson 2010). "
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    ABSTRACT: 1.Kelp species are ecosystem engineers in temperate coasts, where they provide valuable services to humans. Evidence of the declines of kelp forests exists from several regions, but their effects on fisheries still need to be elucidated. More effective management strategies for sustainable fisheries require a synthesis of research findings and an assessment of how research could be improved to fill current gaps.2.This review aims to: (i) summarize the available evidence on the influence of changes in kelp density and/or area on the abundance and diversity of associated fisheries; and (ii) examine how research on kelp–fisheries interactions could better support effective management.3.Most studies (67%) reported data ascribable, directly or indirectly, to a positive relationship between kelp and fishery-relevant variables, 11% provided evidence of a negative relationship, 15% indicated species-specific findings and the remaining found unclear or ‘neutral’ relationships.4.Important shortcomings were identified, including the paucity of experimental studies suitable to test for unequivocal cause–effect relationships, the disproportion between North America, which is well-studied, and other regions and between the large number of fish-based investigations and the small number of those focusing on other commercially important organisms, and the general lack of studies carried out over spatial and temporal scales comparable to those of global processes driving patterns of distribution of both kelps and fisheries.5.Synthesis and applications. The consistency of most studies in showing a positive kelp–fishery relationship supports the protection of kelp habitats stated by current environmental directives. However, achieving their goals requires that the limitations we detect are addressed through better connections between research, management practice and policy. This would require: (i) researchers to combine multiple approaches (large-scale experimental studies and modelling) for the analysis of kelp–fisheries relationships; (ii) funding agencies to provide resources needed to fill the existing gaps; and (iii) researchers and institutions from less studied regions to strengthen collaborations with those from regions where there have been more investigations into kelp–fishery systems. This is essential under present and predicted environmental changes, with the ultimate aim of conserving and allowing the sustainable use of critically important habitats and of fishery resources relying on these.This article is protected by copyright. All rights reserved.
    Full-text · Article · Jun 2015 · Journal of Applied Ecology
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    • "elp forest habitats and the quantity and quality of the prey resources they provide . Alternatively , kelps may provide habitat for forage fishes and planktonic invertebrates that do not consume kelps directly or indirectly , but are prey for larger predatory fishes . Although fish abundance is higher in the presence of abundant kelp populations ( Reisewitz et al . 2006 ) , the mechanisms by which kelp forests support fish communities are poorly understood . This paper addresses several questions concerning the effects of sea otter recovery on the trophic ecology of two species of rockfish . First , we examined kelp forest sizes and depth distributions in regions with and without sea otters along the w"
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    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.
    Full-text · Article · May 2015 · Ecology
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