1. The longstanding debate in conservation biology on the importance of single large or several small (SLOSS) habitats for preserving biodiversity remains highly relevant, given the ongoing degradation and loss of natural habitats worldwide. Restoration efforts are often constrained by limited resources, and insights from SLOSS studies therefore have important implications if restoration efforts can be optimized by manipulating the spatial configuration of restored habitats. Yet, the relevance of SLOSS for habitat restoration remains largely unexplored.

2. Here, we report the effects of spatial reef configuration on early colonization of marine organisms after restoring boulder reef habitats. Reefs were restored in single large (SL) and several small (SS) designs in the western Baltic Sea, where century-long boulder extraction has severely degraded large reef areas and likely exacerbated regional declines in commercially important gadoids (Gadidae spp.). We sampled the field sites using remote underwater video systems in a before-after control-impact (BACI) design and obtained probabilistic inferences on restoration and SLOSS effects from Bayesian hierarchical models.

3. Probabilities of a positive restoration effect were high (>95%) for gadoids, labrids and demersal gobies, moderate (60-75%) for species richness and sand gobies, and low (<5%) for flatfish abundance. Notably, gadoid abundance increased 60-fold and 129-fold on average at SL and SS, respectively. The species composition at restored reefs deviated from control sites, mainly driven by large-bodied piscivores.

4. Spatial reef configuration had the strongest effect on small-bodied mesopredators, including gobies, which were more abundant and driving a distinct species assemblage at SS. In addition to providing suitable conditions for reef species, results suggest that SS can also benefit soft-bottom taxa, possibly through a dispersed predator-mediated effect relative to SL.    

5. Synthesis and applications. This study demonstrates that boulder reef restoration can strongly promote the abundance of exploited gadoids and is therefore a promising management tool to support top-down controls by predatory fishes in degraded marine systems. The higher abundance of mesopredators at SS reefs suggests that SLOSS could have long-term implications for trophic structure and resilience of restored habitats, and should therefore become an important facet within restoration strategies.        

The data used in this study were collected with use of remote underwater video systems (RUVS). Specifically, these systems included a mono-camera setup (using GoPro Hero 3, 3+ and 4) attached to a timer (Time Lapse Intervalometer or BlinkX; Cam Do Solutions; https://cam-do.com) to trigger the camera to record two-minute hourly video clips. Video clips were analyzed in VLC Media Player (https://videolan.org) with adjusted video filters to attain natural coloration and increased brightness if necessary (mostly for twilight recordings). No artificial light sources were used with the video systems and all night sequences were hence discarded.  

The Bayesian hierarchical models used in the univariate part of this study were run in parallel on high-performance computing (HPC) servers, with an average model convergence time of 2-3 days per model. Due to the high computational demands associated with running MCMC using the Hamilton Monte Carlo (HMC) algorithm, we strongly recommend that users of our script and dataset similarly make use of HPC servers if available.