**1. Introduction**

Coral reefs are amongst the most diverse and complex ecosystems in the ocean, despite occupying less than 1% of the ocean floor [1,2]. The combination of soft and hard structures creates an architecturally complex marine habitat that is heavily utilized by numerous organisms and is considered to provide hotspots of diversity and endemism [3–6]. There are both structural and biotic components that make coral reefs multifaceted environments, including rugosity [7–13], algae [8,9,14], hard coral morphology [15,16], and emergent limestone ledges [17,18]; all of these have been described to increase reef fish species diversity.

Caribbean coral reefs have been experiencing severe degradation due to continual disturbances including, but not limited to, disease, sedimentation, and eutrophication, which, together, are eliminating the complex landscapes [19,20]. In many areas, Caribbean coral reef degradation far surpasses that of Indo-Pacific coral reefs, so the Caribbean has become the focal area for studies analyzing the response of reef fishes to this rapid loss of reef structural complexity. Long-term studies following species-specific responses to coral decline found that 43 out of the 72 fish species censused had experienced declines greater than 50% [21–29]. Other studies have found similar trends, with estimated density losses of 2.7–6.0% per year [30], and with predictive models estimating continued losses for particular functional feeding guilds such as invertivores in the years to come [31].

**Citation:** Noonan, K.; Fair, T.; Matthee, K.; Sox, K.; Smith, K.; Childress, M. Reef Fish Associations with Natural and Artificial Structures in the Florida Keys. *Oceans* **2021**, *2*, 634–647. https://doi.org/10.3390/ oceans2030036

Academic Editor: Rupert Ormond

Received: 29 October 2020 Accepted: 18 August 2021 Published: 8 September 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

To combat the loss of complex coral structures, artificial reef structures (ARs) have been used to increase the physical complexity and substrate available to support reef fish communities. There have been many studies that indicated a positive effect of ARs, but these have identified that particular characteristics are necessary for this mitigation strategy to be effective [32–39]. The overall height of ARs has a significant impact on their effectiveness [36,40,41], while the size, surface area available, and complexity of ARs appear to influence the diversity of reef fish across an entire reef [40–42]. In fact, some artificial reefs have been observed to contain species assemblages that are more diverse than those of natural reefs, leading to the conclusion that this method of intervention can be successful [36,37,43–45].

Previous studies have assessed reef fish community responses to coral decline [6,24,27,29] and the use of ARs [32,33,36,37,39], and predicted how reef fish communities may be structured in the future [31], but there is little research investigating how reef fish are utilizing the structures that remain in the Caribbean. The goal of the present study was to investigate how reef fishes utilize both natural and artificial structures, identify structural characteristics that may influence their use, and assess whether reef location and topographic complexity influence the use of structure by different functional feeding guilds. Based on previous literature, we investigated the hypotheses that reef fishes would utilize biotically complex natural structures more often than non-biotic artificial structures, that height and surface area would be the most effective characteristics driving use, and that reef fishes' use of structures would be evident on reefs with higher rugosity but would differ between different feeding guilds. By identifying heavily utilized structures and their associated traits, we can better predict the reef fish community response to structural declines and assess whether artificial structures can mitigate further losses.
