Abstract
Marine management interventions are increasingly being implemented with the explicit goal of rebuilding ocean ecosystems, but early responses may begin with alterations in ecological interactions preceding detectable changes in population-level characteristics. To establish a baseline from which to monitor the effects of spatial protection on reef fish trophic ecology and track future ecosystem-level changes, we quantified temperate reef fish densities, size, biomass, diets and isotopic signatures at nine sites nested within two fished and one five-year old marine protected area (MPA) on the northwest coast of Canada. We calculated rockfish (Sebastes spp.) community and species-specific niche breadth for fished and protected areas based on δ13C and δ15N values. We found that rockfish community niche width was greater inside the MPA relative to adjacent fished reefs due to an expanded nitrogen range, possibly reflecting early changes in trophic interactions following five years of spatial protection. Our data also demonstrated that the MPA had a positive effect on the δ15N signature of rockfish (i.e., trophic position), but the effect of rockfish length on its own was not well-supported. In addition, we found a positive interaction between rockfish length and δ15N signature, such that δ15N signatures of rockfish caught within the MPA increased more rapidly with body size than those caught in fished areas. Differences in rockfish size structure and biomass among fished and unfished areas were not clearly evident. Species of rockfish and lingcod varied in trophic and size responses, indicating that life-history traits play an important role in predicting MPA effects. These results may suggest early changes in trophic behavior of slow-growing rockfish due to predation risk by faster growing higher trophic level predators such as lingcod inside MPAs established on temperate reefs. Consequently, spatial protection may restore both the trophic and behavioral roles of previously fished consumers earlier and in measurable ways sooner than observable changes in abundance and size.
Highlights
To establish a baseline for reef fish trophic ecology and assess the initial ecosystem-level effects of an marine protected area (MPA) located off the east coast of Haida Gwaii, an island archipelago located in British Columbia, Canada (Fig 2), we surveyed reef fish and habitat characteristics at nine replicate rocky reef sites nested within two fished areas (Kunghit Island, ‘Fished North’, n = 3 sites; Louise Island, ‘Fished South’, n = 3 sites) and an MPA (Lyell Island, n = 3 sites) in July and August 2009
Niche expansion in the MPA was largely due to an increase in trophic level diversity indicated by a wider nitrogen range (NR extent)
We found very low probabilities (17–34%) that china and copper rockfish niche widths differed between the MPA
Summary
Expanded consumer niche widths may signal an early response to spatial protection conservation science tends to focus on changes in the population abundance or biomass of key species, rather than changes in the trophic and behavioural interactions that underpin ecosystem-level processes. This tendency is borne out of necessity, because it is much harder to observe and measure species interactions than it is to count and estimate individuals within a population. Because species and community isotopic niches are an emergent outcome of the species interactions and energy flows that underlie ecosystem structure and function, alteration to these niches may signal ecosystem-wide effects of management interventions
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