This study provides a comprehensive examination of juvenile sportfish responses to oyster reef restoration and living shoreline stabilization, incorporating 17 abiotic and biotic factors into this assessment. Many studies have examined the response of resident fishes and invertebrates to restoration, while relatively few have targeted more transient and higher trophic species. Sportfish observations in former studies were generally in low abundances, e.g., [
25,
26,
28], while a recent targeted sportfish study allowed for direct comparisons, which found them to have comparable densities between natural and restored coastal wetlands [
14]. Furthermore, restoration benefits impact species to varying degrees and may take time to accrue, as the majority of increased gross fish production does not occur for up to two years at oyster reefs [
17] and three years at living shorelines [
22], similar to the findings of this study.
Overall, results suggest juvenile sportfish abundance and diversity responded positively to restored habitats, with the biotic factors of the site (e.g., oyster density and percent vegetative cover) and prey abundance representing important predictors of sportfish abundance. Oyster reefs and coastal shorelines are well known for their provision of habitat for macrofauna (e.g., crustaceans and demersal fishes) [
53,
54], as evident by their effect in this study. In addition, abiotic factors such as DO and proximity to the nearest ocean inlet were influential predictors, suggesting restoration location is crucial to the success of a restored habitat to enhance sportfish populations. This sentiment towards site selection has been echoed by others [
28], along with a call to incorporate more biotic interactions in restoration studies [
7], such as those examined here, to fill an important knowledge gap in coastal restoration studies.
4.1. Oyster Reefs
Previous studies of sportfish at restored reefs tend to examine subtidal reefs [
25,
26,
28], while the impact of intertidal reefs, where fish can only access the site during high tide, is less studied. These oyster reefs can increase in height until they reach a maximum threshold set by the tide range [
39]. In the intertidal Mosquito Lagoon system, sportfish abundance was predicted to be highest at live oyster reefs, lowest at dead reefs and intermediate at restored reefs. However, restored sites outperformed other site types, as sportfish abundance was approximately equal at live and 2017 restored reefs and highest at 2018 restored reefs. This relatively higher abundance could be due to: (1) the creation of a new habitat that was previously degraded or inaccessible to foraging, (2) the initial disturbance caused by the act of restoration could stir up sediments and nutrients, thereby attracting prey and ultimately sportfish, and (3) the restored sites provide an intermediate level of habitat heterogeneity and complexity that sportfish can better utilize.
Restored reefs that supported higher sportfish abundances increased in both height and oyster density to that of live reefs by ~20 months and 12 months at 2017 and 2018 restored reefs, respectively [
39], thus lending support to hypothesis 1. Restored 2018 sites experienced an initial pulse of sportfish abundance; however, this peaked at 4–6 weeks post restoration then decreased, which possibly indicates a settlement pulse. Alternatively, sites restored in 2017 had similar abundance to live reefs from the onset. As such, it is unlikely that hypothesis 2 is driving the sportfish response in this system, and the creation of a newly accessible habitat of intermediate complexity is driving observed patterns of abundance.
If hypothesis 3 drove the trend in sportfish abundance, then the highest abundances would occur at sites with intermediate oyster density and reef height. There is some support for this, as sportfish were collectively more abundant at restored reefs where these mean reef metrics were indeed intermediate to controls, with intermediate levels observed during the first year of monitoring (
Table 3,
Table A1). However, their distribution was less correlative to these metrics and demonstrated multimodal abundance peaks relative to oyster density and reef height (
Figure 2C,D). Alternatively, sportfish have been detected in higher abundances at unstructured bottom compared to oyster reef habitat [
25,
26], which could be attributed to the larger body size of those fishes and their transient nature. Sampling gear in the current study targeted juveniles and detected lower sportfish abundances at dead reefs (i.e., reefs with lowest oyster density and reef heights). The size range of juvenile sportfish can presumably allow them to access heterogenous habitat structures, which may subsequently lend to the range of reef metrics in which they were found, but this relationship requires further exploration. Furthermore, habitat complexity can be challenging to quantify and can be measured by various methods. Additional techniques (e.g., rugosity, bathymetry) could contribute to a better indication of habitat heterogeneity and its influence on sportfish abundance and diversity [
55,
56,
57].
Dissolved oxygen (DO) also influenced sportfish abundance, although only in conjunction with the other variables and was largely consistent at ~6 mg/L across sites. Dead 1 and Live 1 were slightly below average, while the restored 2018 sites were lower, which is likely due to a dip in DO for several months post restoration before increasing to that of the other sites. Changes in dissolved oxygen are of global concern [
58], which can negatively impact recreationally and commercially important fishes [
59]. These data suggest that in coastal estuary systems such as Mosquito Lagoon, restoration sites should be selected for initially suitable DO, or a location where there is an opportunity to improve DO. This could occur through vegetative plantings, oyster enhancement for the water filtration of excess nutrients and phytoplankton, or where land based conservation actions can limit nutrient runoff that may result in eutrophication and ensuing algal blooms, and potentially fish kills [
60].
Restored reefs in this study are located near or along boating channels, as boat wakes are a significant source of oyster reef decline in Mosquito Lagoon [
38,
39]. Oyster reefs with higher abundances of sportfish were located closer to Ponce de Leon Inlet, or experienced greater tidal currents. When inlet distance was considered alone, it was not a strong predictor of sportfish abundance (
Table A2). However, when combined with other metrics of the sites (e.g., oyster density and prey abundance), it proved to be influential, presumably acting in conjunction with these other variables. Proximity to the inlet and its subsequent tidal flow could facilitate the movement of adult fishes along migratory pathways [
61] and larval settlement of juvenile fishes. Similarly, this greater tidal exchange created by inlet proximity may facilitate bivalve and macrofauna larval dispersal and ultimately settlement at a site, which can also supply food for these sessile and resident species, thus encouraging the recovery of the habitat and benthic community.
Macrofauna have previously shown improvements following restoration [
19,
28,
62,
63], similar to that in this system [
43,
44,
52]. Macroinvertebrates here were found in highest abundances at ~75–100 mm reef height and 75–150 oysters/m
2 [
43], and were an important predictor of sportfish abundance. Sportfish occupied a similar habitat range; therefore, the correlation between these metrics and macroinvertebrates could be a mechanism to explain sportfish habitat use and, therefore, abundance at restored sites, thus supporting hypotheses 1 and 3. In addition, proximity to natural habitat that acts as a source population for oyster spat and macrofauna can facilitate the establishment of a restored site [
64]. Proximity of restored oyster reefs to other coastal benthic habitats (namely, seagrass) has shown potential to be functionally equivalent for transient piscivorous fishes, while oyster reefs placed near mudflats augmented juvenile fish abundances [
28]. Likewise, results here suggest that, along with the recovery of habitat features at the site, restoration location is essential in maximizing restoration success to enhance sportfish populations. Therefore, it would be prudent to continue to employ restoration methods in areas with high current, which often experience high vessel traffic that can lead to oyster decline, to both mitigate oyster loss and support sportfish and their prey. These efforts may also experience greater success if presented with complementary boater education to inform stakeholders that recreationally important fish populations could benefit from more mindful boating behavior [
39].
Restored and control oyster reefs in this study did not differ in their ability to support a diverse sportfish assemblage, as community composition overlapped between controls and restored sites. This is unsurprising given the relatively few species of sportfish, which is further reduced when looking at diversity within habitats rather than between. Like abundance, species richness experienced temporal variation, which could be due to the influence of seasonal environmental variables and sportfish life histories. Live reefs supported the highest overall species richness, while diversity (i.e., effective number of species) was higher at restored reefs than at controls and was likely driven by higher abundances at those sites. The species in this system showed some surprising preferences. For example,
C. nebulosus are known to forage near oyster reefs and have been shown to benefit from restored reefs [
27], but were found in very low abundances there. Similarly,
P. cromis that are known to associate with both seagrass and oyster reefs and forage on bivalves were not observed at oyster reefs. Alternatively,
L. griseus were more abundant at oyster reefs and
L. synagris were found exclusively at oyster reefs, with a preference for restored 2018 sites (57.9% of their total catch). These are important findings to consider if using habitat restoration as part of a species specific management or recovery plan. For example, management for
L. synagris could emphasize sites with sufficient current, since they were found predominantly at 2018 restored sites which experienced greater current and were located closer to the inlet.
4.2. Living Shorelines
Few studies have examined the impacts of restored shorelines on fish populations, particularly for sportfish. Recently, fish abundance, richness and diversity were found to be similar between natural and restored marsh shorelines along the Florida west coast, and higher relative to impacted shorelines [
57]. Specifically, sportfish densities at those sites were comparable between natural and restored shorelines and higher than impacted shorelines, which included five of the same species highlighted in the current study (
C. nebulosus, C. undecimalis, L. griseus, P. cromis, Sciaenops ocellatus) [
14]. Higher abundances and species richness of fishes have also been observed three years post construction of sill living shorelines (i.e., a hybrid technique which incorporates offshore rock material with shoreline marsh plantings), compared to unvegetated shoreline and natural controls, including some juvenile species observed in our study (
C. nebulosus, L. griseus, S. ocellatus) [
22]. Similarly, two shared sportfish species (
C. nebulosus, S. ocellatus) were enhanced at living shorelines with oyster reef breakwaters, relative to natural shorelines [
21].
Here, living shorelines supported a greater abundance of sportfish compared to oyster reefs. This was likely driven by benthic vegetation and an influx, putatively a settlement pulse of
C. nebulosus in July 2017, which decreased in abundance after three months post stabilization. Mean sportfish abundance was lower at stabilized sites prior to stabilization but converged with control (i.e., natural) shorelines around three months and remained similar throughout the study. The total abundance and CPUE of sportfish were higher at control shorelines compared to stabilized, but would have been comparable if it were not for the settlement pulse of
C. nebulosus. This suggests stabilized shorelines have the ability to support sportfish populations in ways that are comparable to natural shoreline habitats, and in a relatively shorter timeframe than previously reported. Previous research found that restored shorelines may take longer to accrue sportfish, which may be due to the rate of recovery and establishment of submerged aquatic vegetation at stabilized sites [
21,
22,
29]. For example, in a west Florida study, sportfish densities were comparable between restored and natural marsh shorelines six to nineteen years post restoration, although a restoration timeline was not examined directly [
14].
It is not uncommon to observe single species dominance at some sites [
14,
29], or fluctuations such as those observed here for
C. nebulosus, which likely reflect fish life histories [
49]. These episodic fluctuations are an important consideration for assessing the success of restored sites to enhance fisheries, as these “boom and bust” years can influence the presence and absence of species over the course of a study. For example,
C. nebulosus were in highest abundances in living shorelines during the first year of the study (95% of their catch) and less common the following year. Similarly,
L. synagris were uncommon at oyster reefs in 2017 (~8% of their catch) compared to 2018 (92%). The current study ran consecutively for three years and captured these population dynamics, but caution should be exerted when assessing species population trends over relatively short study durations.
Living shoreline sportfish abundance was influenced by biotic variables reflective of the site types. Living shoreline sites are located farther south in the lagoon, where tidal fluctuations and currents are relatively minimal. However, some of these sites contain patches of
H. wrightii seagrass seaward of the oyster breakwaters, which provides an important nursery habitat for many macrofauna [
54,
65]. The coverage of this seagrass contributed to the higher vegetation cover at control sites and better correlated with sportfish abundance (R
2 = 0.14) than at stabilized sites (R
2 = 0.03). Perhaps this reflects a difference in seagrass quality or vegetation density at control sites that better supported sportfish and their prey. Other shoreline vegetative cover (e.g., mangroves) can also provide structure for juvenile sportfish [
54] and data here indicate highest sportfish abundances were present at 25–50% cover at stabilized and control sites, respectively. Vegetation that is too dense likely precludes sportfish from accessing and utilizing a site, as evident in the lack of sportfish catch at vegetative cover >75%, or perhaps our gear was limited in sampling higher vegetation densities. Therefore, results in the current study suggest restored sites can support more sportfish as they continue to develop vegetation to ~50% coverage to resemble that of control sites.
Interestingly, juvenile sportfish also associated with bare substrate at control and stabilized sites, while they are more prevalent at the latter. Common snook (
C. undecimalis) utilized a mosaic of restored coastal wetland habitats in west Florida but were more abundant in restored creeks relative to marshes and ponds [
29], suggesting sportfish also benefit from open subtidal habitat. These two substrates were important predictors of sportfish abundance at living shorelines here, which offer habitat heterogeneity through a mosaic of vegetation, seagrass, and bare substrate that fish appear to utilize [
22,
29,
57]. Juveniles captured in this study may prefer the protection of vegetated and seagrass substrate at living shorelines, while bare substrate may allow for movement of subadult fishes related to ontogenetic habitat shifts e.g., [
29,
66].
Macrofaunal abundance (both fish and macroinvertebrates) was also an important predictor of sportfish abundance in living shorelines (
Table 3), suggesting this habitat provides a healthy prey population to support foraging sportfish. This has also been demonstrated with an increase in crustacean abundance at sill living shorelines relative to controls [
21,
22]. Highest prey fish abundances were found at ~25% and 75% vegetation cover, and macroinvertebrates were in highest abundances at ~30% and 40% vegetation cover for stabilized and control shorelines, respectively [
44,
52]. The abiotic variable that ranked highest with biotic variables in additive shoreline models was dissolved oxygen (DO), which may be related to the reduced tidal flow in this area (
Table A2). DO was largely consistent across sites, with Stabilized 2 and 3 sites slightly below average, but falling within adequate levels (5.8 ± 0.3 mg/L). These data further suggest that restoration site location is important when a management goal is to support relatively mobile sportfish and should aim to mimic conditions at natural shorelines.
Sportfish community composition, species richness and diversity (i.e., effective number of species) were similar between control and stabilized living shoreline sites, with seven species being found at each. Similarly, these metrics did not differ between natural and restored coastal wetland shorelines among large and small bodied fishes in Tampa Bay, Florida [
57]. As described above and demonstrated by the indicator species analysis, some species (e.g.,
P. cromis) showed a preference toward living shoreline habitat, and within this habitat
C. nebulosus were more abundant at control sites (62.5%) and
C. undecimalis were fairly evenly distributed between site types. There was an
a priori expectation of catching more Red Drum
S. ocellatus (
n = 6), as they are a popular recreational sportfish in the region, but perhaps the juvenile size class does not utilize these habitats as much as expected, or a larger gear type (e.g., beach seine, gill net) would better quantify their relative abundance [
14]. Sportfish have shown species preferences for shorelines of different quality [
14,
29]; therefore, the habitat diversity of living shorelines offers valuable options for sportfish species to use and, thus, enhance their populations.
4.3. Across Habitats
Living shorelines and oyster reefs provide different habitat opportunities within the Mosquito Lagoon system; shorelines receive little tidal exchange and are characterized by a soft bottom interspersed with intertidal and submerged aquatic vegetation, whereas oyster reef sites are characterized by hard substrate and some mangroves with significant tidal exchange. Despite these differences, these habitats supported similar sportfish abundance (when excluding the shoreline settlement pulse in the first year) and richness/diversity. However, they differed slightly in community composition, which may be related to species’ nursery or juvenile habitat preferences. This is an important consideration for fisheries managers who are interested in using habitat restoration to meet species specific sportfish enhancement goals, whereas more generalist species (e.g., L. griseus) may broadly benefit from restoration.
Juvenile sportfish abundance at both habitats was poorly predicted by biotic and abiotic variables alone, but rather, were best predicted by additive models of habitat metrics (e.g., oyster density and substrate cover), macrofauna prey, inlet distance and DO. Therefore, consideration should be given to oyster site locations that are 16 km or less from the inlet and can support a reef height of ~55–90 mm and oyster density of 60+/m
2, and shoreline locations that can provide ~50% vegetation cover, with both habitats providing DO around 6 mg/L. These criteria also support a healthy prey population, which contributes to sportfish populations and communities. Dissolved oxygen is particularly noteworthy, as water quality across the Indian River Lagoon system has been under scrutiny due to declines in recent decades [
35,
36]. In addition, proximate source populations should be examined to encourage the restoration success of benthic species and, ultimately, sportfish. It is particularly important to identify characteristics that can support fisheries in light of documented shifts in the occurrence of prey fish communities in this system [
32,
33]. Future studies could manipulate habitat complexity, e.g., [
55,
57], to determine optimal habitat heterogeneity to benefit sportfish. In addition, studies could employ acoustic telemetry to determine the habitat use and movement of fishes related to control and restored sites at different coastal habitats, similar to [
27], before and after restoration to better quantify changes.