Nekton Assemblages Research Articles

Intertidal Creek Pool Nekton Assemblages: Long-term Patterns in Diversity and Abundance in a Warm-Temperate Estuary

Intertidal Creek Pool Nekton Assemblages: Long-term Patterns in Diversity and Abundance in a Warm-Temperate Estuary

Seines and baited remote underwater video document consistent spatiotemporal patterns in nekton communities of subtropical coastal habitats

Monitoring nekton using multiple types of gear over a range of estuarine habitats is necessary for assessing changes in ecosystems, the effects of natural and anthropogenic disturbances, and management related to habitat restoration and protection. To assess the utility of 2 types of gear (seines and baited remote underwater videos [BRUVs]) used in many monitoring programs, nekton community metrics were compared across 3 adjacent, hydrologically distinct coastal basins in Florida Bay (Florida, USA) across seasons (wet vs. dry) and 3 water-years (2016-2019). In these basins, projects are underway to restore freshwater inflow. Spatiotemporal patterns in catch per unit effort and species richness were consistent among types of gear, and nekton assemblages differed significantly among basins. Although similar in size and spatial area, the 3 study basins were notably different in habitat (hydrology, water depth, substrate, submerged aquatic vegetation), and 1 basin exhibited distinct basin-specific trends in nekton community structure and lower nekton abundance and species richness across types of gear. Temporal differences in nekton assemblages reflected seasonal shifts and were strongly affected by the passage of Hurricane Irma in 2017. Although seines and BRUVs revealed similar patterns, seine surveys captured more overall species diversity and prey species, while BRUVs could be used in a greater variety of habitats and documented more predators. These results demonstrate the effectiveness of complementary types of gear in tracking variation in estuarine communities and highlight the importance of a sampling design that successfully monitors species throughout the food web and estuarine seascape.

Nekton use of co-occurring aquaculture and seagrass structure on tidal flats

On the extensive tidal flats of Willapa Bay (Pacific coast, USA), oyster culture, seagrass, and mudflats create a mosaic of intertidal habitats. Structured habitats are generally considered to increase abundance and diversity of associated species, but less attention has been paid to roles of different kinds of structure (seagrass meadows, reefs, farm infrastructure) or co-occurring structure in shaping nekton assemblage structure. Here, we investigated the effects of different oyster culture methods (suspended culture and bottom culture) on nekton communities and abundance across a gradient of seagrass habitats, during multiple seasons, and using both seine and video sampling methods. Of 23 major estuarine taxa, 2 were generally associated with vertical structure (eelgrass or suspended culture), 3 were seagrass specialists, and 3 primarily used habitats lacking vertical structure (mudflats and bottom culture). Where oyster culture was present, 5 taxa associated with on-bottom and 2 taxa associated with suspended culture. Assemblage structure responded to co-occurring structure as expected from responses to each structure type independently (i.e. additive effects of seagrass and oyster culture). In contrast to much empirical evidence in structured habitats, seagrass density was a poor predictor of overall fish abundance. These findings together suggest that maintaining a mosaic of available habitats is favorable for promoting diversity in Willapa Bay.

Trophodynamics of Nekton Assemblages and Relationships with Estuarine Habitat Structure Across a Subtropical Estuary

Understanding the spatiotemporal dynamics of trophic structures, or trophodynamics, is important for assessing the overall condition and function of estuarine ecosystems. We examined spatial and seasonal trophodynamics of nekton assemblages across the seascape of a subtropical estuary (Charlotte Harbor, FL, USA) using historical nekton data from an ongoing monitoring program. We also examined relationships between trophodynamics and the spatial distribution of vegetated habitats, such as salt marshes, mangroves, and seagrass beds, as well as their absence along hardened shorelines (seawalls or riprap) using seascape metrics (area and edge density of habitats within several search radii of sample locations). We found that nekton assemblages in shallow waters (from the shoreline to a depth of ≤ 1.5 m) transitioned from mainly planktivores in the lower Peace and Myakka Rivers to mainly benthivores (those feeding on benthic infauna, epifauna, or seagrass) in the middle and lower portions of Charlotte Harbor. Overall nekton assemblages throughout the estuary appeared to be largely sustained through benthic pathways in areas with abundant seagrass and nearby mangrove habitat, particularly during the cool-dry winter when planktivore densities were seasonally low. The dramatic seasonal decline in planktivore densities we found in the lower rivers may be related to low freshwater inflow and increased predation resulting from seasonal recruitment pulses of several omnivorous species. Trophodynamics linked with seascape metrics of estuarine habitat structure described here may be used as a spatially explicit baseline of the estuarine seascape to monitor ecosystem health as well as parameterize spatial ecosystem models used in fisheries management in Charlotte Harbor and similar subtropical estuaries.

Seasonal succession and spatial heterogeneity of the nekton community associated with environmental factors in Hangzhou Bay, China

Spatiotemporal variations of nekton communities in estuaries are closely associated with habitat quality and environmental shifts. However, the importance of environmental conditions in shaping the spatial heterogeneity and seasonal succession of nekton in Chinese estuaries has rarely been reported. We collected nekton seasonally between 2018 and 2019 from 10 sites in the semi-enclosed Hangzhou Bay (HZB) and investigated the spatiotemporal dynamics of the nekton assemblage structure. A wide taxonomic diversity was identified, including 57 taxa from 23 families (36 fish, 15 shrimp, and six crabs). The nekton was mainly composed of small coastal, nearshore, and estuarine species, dominated by Miichthys miiuy, Collichthys lucidus, Exopalaemon annandalei, and Palaemon gravieri. The nekton biomass and abundance distribution were higher in the upper and middle bay than in the lower bay. The year-round northeast–southwest front zone was high related to the high-value dense area of nekton. Diversity and evenness indices of nekton biomass were higher during July and October than January and April. The unstable and high variability of the nekton community composition was reflected in the species diversity indices, seasonal alternate index, and seasonal migration index. Significant differences were found between fauna clusters among seasons, revealing the relationship between the habitat adaptability of species and seasonal variation of water masses. The nekton assemblage structure and distribution pattern were predominantly correlated with seasonal variations in natural and anthropogenic factors. In this study, the observed effects of anthropogenic and natural stressors on the seasonal variation of the nekton community and density distribution in the estuarine-bay ecosystems provide new insight into fishery ecosystems and ecosystem functioning.

Mercury concentrations and stable isotope ratios (δ13C and δ15N) in pelagic nekton assemblages of the south-western Indian Ocean

Mercury (Hg) concentrations and stable isotope values (δ13C and δ15N) were investigated in micronekton collected from La Pérouse and MAD-Ridge seamounts, Reunion Island and the southern Mozambique Channel. Organisms occupying epipelagic habitats showed lower Hg concentrations relative to deeper dwelling benthopelagic ones. Increasing Hg concentrations with increasing body size were recorded in the Mozambique Channel and Reunion Island. Positive relationships were observed between Hg levels and δ15N values in pelagic nekton assemblages collected at MAD-Ridge seamount and the southern Mozambique Channel, suggesting biomagnification of Hg. Concentrations of Hg in organisms across the south-western Indian Ocean were within the same range of values. Total Hg concentrations depend on a range of factors linked to habitat range, body size and trophic position of the individuals. To our knowledge, this is the first study investigating the patterns of Hg concentrations in pelagic nekton assemblages from the south-western Indian Ocean.

Effects of sample gear on estuarine nekton assemblage assessments and food web model simulations

Long-term fisheries-independent sampling data inform population status and trends of species-specific biomass and are often used to drive biomass-based food web models such as the Comprehensive Aquatic Systems Model (CASM). Indicators such as total biomass and mean trophic level derived from these data and from CASM outputs inform management and facilitate assessments of on-going and predicted coastal change and restoration activities on fisheries, but rely on consistent sampling to enable comparisons across space and time. Changes in coastal estuarine gradients, combined with the availability of new sampling technologies, highlight a need to assess the potential consequences of changing sampling technologies on fisheries data and the cascading impact on model outputs. In Louisiana, USA, CASM models are used to inform coastal restoration projects, relying on 40 years of fisheries-independent data derived from 50′ seine sampling. However, alternative use of electrofishers as a sampling method has been proposed to replace the seine sampling. In this study, we examine data from concurrent seine and electrofisher sampling in Barataria Basin, Louisiana, and compare biomass, assemblage data and CASM outputs related to species biomass, food web structure and energy cycling. In a paired comparison of data in 2018–2019, the electrofisher captured higher total catch and diversity compared to the seine. The electrofisher samples were dominated by shrimp (grass, white, brown) and larger bodied fish, while seine samples were dominated by smaller-bodied fish (i.e., anchovy, menhaden). Ecosystem indicators derived from running the CASM using biomass data from seine and electrofisher sampling separately in two different simulation exercises provide contrasting results. In Simulation Exercise 1, the use of different datasets (long-term CASM calibration, 2018–2019 seine, 2018–2019 electrofisher) to initialize the CASM biomasses did not result in large or long-running changes in the simulated biomasses over time. In contrast, in Simulation Exercise 2, CASM model outputs using adjusted gear ratios indicated changes in biomass structure when using electrofisher data, with a doubling of total food web biomass due to the increased shrimp count, and a 13% increase in total energy flow through the food web. Conversions based on area and gear efficiency for overall catch may be useful in maintaining the continuity of historical data. However, differences in species-specific catch due to gear selectivity could have large consequences for constructing and calibrating fish and ecosystem models and remain difficult to reconcile. These differences in assemblages, and estimated biomasses for key food web species, suggest careful consideration in changing gears.

Tidal influence on fish faunal occurrence and distribution in an estuarine mangrove system in Ghana

This study assessed the effects of tidal stages and habitat conditions on nekton assemblage and distribution in the Kakum River estuarine mangrove system in the Central Region of Ghana. Teleosts and crustaceans were sampled using pole seine and cast net from a tidal mangrove pond and a channel, and characterised using morphometry and ecological guilds concept. In total, 1 146 specimens were collected, with 35 species from 19 families represented. The most abundant species encountered were Liza spp. (Mugilidae; 36%), Sarotherodon melanotheron (Cichlidae; 16%), Elops lacerta (Elopidae; 6%) and Sardinella aurita (Clupeidae; 5%), which exhibited pronounced spatial distribution. Sarotherodon melanotheron showed preference for ponds with minimal water flow and depth, Sardinella aurita occurred only in deeper sections of the mangrove channel with significant flow velocity, and Elops lacerta in the pond and all channel stations. Mugilids were found exclusively in the channel, whereas Palaemonidae (Macrobrachium macrobrachion and M. vollenhoveni) occurred in specific locations in the channel, with species- specific preferences for tidal stages. Most species encountered were identified to be predominantly marine migrants with bentophagous feeding habit. Site selection and tidal stages were found to affect the occurrence and distribution of fishes over a tidal cycle and interactively influence species diversity.

Evaluating the Sampling Design of a Long-Term Community-Based Estuary Monitoring Program

Community-based monitoring programs (CBMPs) are a cost-effective option to collect the long-term data required to effectively monitor estuaries. Data quality concerns have caused some CBMP datasets, which could fill knowledge gaps for aquatic ecosystems, to go unused. The Community Aquatic Monitoring Program (CAMP) is a CBMP that has collected littoral nekton assemblage data from estuaries in the southern Gulf of St. Lawrence since 2003. Concerns with the CAMP sampling design (station placement and numbers) have prevented decision-makers from using the data to inform estuary health assessments. This study tested if CAMP’s sampling design that accommodates volunteer participation provides similar information as a scientific sampling approach. Six CAMP stations and six stations selected using a stratified random design were sampled at ten estuaries. A permutational-MANOVA revealed nekton assemblages were generally not significantly different between the two sampling designs. The current six CAMP stations are sufficient to detect the larger differences in species abundances that may indicate differences in estuary condition. The predicted increase in precision (2%) with twelve stations is not substantive enough to warrant an increased sampling effort. CAMP’s scientific utility is not limited by station selection bias or numbers. Furthermore, well-designed CBMPs can produce comparable data to scientific studies.

Preference, avoidance or coincidence? How fish and crustaceans use intertidal salt-marsh creeks in the German Wadden Sea

Worldwide, salt marshes have proven important or even essential for a wide variety of marine nekton. But in the world's largest intertidal wetland, the Wadden Sea, their role for nekton is surprisingly unexplored. Building on extensive field sampling, this study presents an inventory of fish and crustacean species using German Wadden Sea salt marshes and investigates their habitat use and affinity. Ten species dominated the nekton assemblage of the salt-marsh creeks both in frequency of occurrence and in relative abundance. Repetitive samplings resulted in highly variable catches, probably caused by the abundance in the subtidal. However, common goby (Pomatoschistus microps) and brown shrimp (Crangon crangon) were depleted with the 9 consecutive hauls when occurring in high numbers, indicating a high habitat affinity. Seasonal and diurnal quantitative sampling revealed seasonal shifts within the nekton community and significant higher densities during night than day for seven out of the nine dominant salt-marsh nekton species. Nekton densities tended to be lower than elsewhere in the Wadden Sea and thus animals using the salt-marsh creeks may benefit from lower resource competition. Compared to the subtidal not only the species composition differed significantly but also the size composition. The majority of fish in the marsh creeks were small (97% < 4.5 cm), consisting of larvae, juveniles and adults of small species. Thus, the use of salt-marsh creeks seems predominantly be triggered by size than by life stage. The absence of aquatic and avian piscivores indicates that salt-marsh creeks function for small nekton as refuge from predators. However, the habitat value depends also on the connected low tide refuges and the access pathways in between. The difference of the nekton community of the marsh creeks from the subtidal raises the demand for habitat-specific monitoring programmes and further research on the connectivity and interactions between different habitat types, both considering seasonal and diurnal abundance variabilities.

Estuarine Nekton Assemblages along a Marsh-Mangrove Ecotone

The marsh-mangrove ecotone along the southeastern US Atlantic coast occurs in northeast Florida within the Guana-Tolomato-Matanzas (GTM) estuary, where emergent vegetation transitions from marsh-dominated in the north to mangrove-dominated in the south. Dominant vegetation type has been shown to influence creek bank slope, nekton access to refuge, predation risk, and access to food. The northward distribution of mangroves in the estuarine mosaic is in flux in northeast Florida, and the effect on subtidal nekton, including commercially important species, is not known. To determine if estuarine nekton assemblages differ along the marsh-mangrove ecotone, a 60-km transition zone within GTM estuary was divided into 20 sub-zones where nearshore subtidal nekton communities were sampled monthly with trawls for 1 year. A total of 15,750 individuals consisting of 100 species were collected during the study period; 13 species made up 90% of the total catch. Subtidal nekton assemblages in marsh sites were dominated by typical salt marsh species (i.e., Leiostomus xanthurus, Anchoa spp., Bairdiella chrysoura) and had little overlap with assemblages in mixed and mangrove sites, which were dominated by structure-oriented species (i.e., Lagodon rhomboides and Eucinostomus spp.). Despite similar environmental conditions among the zones, there were clear differences in the subtidal nekton community along the marsh-mangrove ecotone, largely driven by fish species. This change in nekton community along the ecotone suggests that ecological processes such as food availability or predator/prey dynamics affected by changes in marsh surface habitats may result in differences in nekton species distribution and abundance across interconnected habitats such as in subtidal nekton that we observed in the GTM estuary.

Nekton community dynamics within active and inactive deltas in a major river estuary: potential implications for altered hydrology regimes

High fisheries production within estuaries is associated with coastal upwelling, tidal mixing, and land-based runoff facing increasing impacts from climate and human activities. Active river deltas receive large riverine inflows compared to inactive river deltas, providing contrasting estuaries to compare impacts of river inflow on estuarine nekton. We quantified nekton assemblages and stable isotopes (δ13C, δ15N) of commercially important blue crab Callinectes sapidus Rathbun, 1896 within an active and inactive delta in coastal Louisiana to explore the impacts of differing riverine inflow. Crustaceans dominated estuarine assemblages, differing only by season and not delta type, with summer and fall supporting highest densities. Fish density and assemblages differed by the interaction of season and delta due to differences during the 2019 record high spring river inflow. During this period, the active delta supported reduced fish densities and richness compared to the inactive delta. Nekton densities across deltas and seasons reflect a combination of species life history characteristics and habitat conditions. The high spring river discharge in 2019 impacted habitat availability (reduced presence of submerged aquatic vegetation), water conditions (decreased temperature and salinity), and potentially displaced nekton to unsampled habitat areas (i.e. interior marsh surface) within the active delta. While differences in nekton density and assemblages were only evident during the high spring river discharge, δ15N values of blue crabs were approximately 1.5 times higher in the active delta, potentially indicating more terrestrial influence. Understanding how altered inflow impacts environmental variables supporting estuarine nekton production remains critical for supporting management within these hydrologically managed regions.

Genetic analysis of the diet of red‐footed boobies (Sula sula) provisioning chicks at Ulupa'u Crater, O'ahu

Abstract The diet of red‐footed boobies (Sula sula) provisioning chicks was quantified using 106 regurgitations collected from 81 adults over two study years with contrasting oceanographic conditions: 2014 and 2015. A total of 1,049 prey items were sorted into three broad categories (fish, squid, and other, consisting of highly‐digested ‘mush’ and parasitic isopods) and assigned a categorical freshness value of 1 (perfect condition), 2 (superficial digestion), or 3 (highly digested and incomplete). A total of 492 undigested prey items (freshness 1 and 2) were measured and sampled for genetic identification with 401 (82%) successfully identified. Rarefaction analyses suggested robust diet sampling, with 32 documented species: 29 fishes and three squids. While the prey‐specific index of relative importance highlighted that fish dominated the diet in 2014 (32.3% fish, 15.1% squid) and in 2015 (29.9% fish, 22.3% squid), diet composition varied significantly between the two study years. The purpleback flying squid (Stenoteuthis oulaniensis) accounted for 98.7% (232 of 236) of the identified squid specimens. Mantle lengths were significantly longer in 2015 (7.2 ± 1.1 SD cm) than in 2014 (6.3 ± 1.7 SD cm). These results indicate a higher relative abundance and size of the epipelagic squids available to foraging red‐footed boobies in 2015, during warm‐water conditions associated with the positive phase of the El Niño Southern Oscillation. Due to their diverse diet, red‐footed boobies are useful bioindicators of epipelagic nekton assemblages during changing oceanographic conditions. Moreover, their reliance on the purpleback flying squid makes these seabirds ideal samplers of the juveniles of this poorly studied species. Understanding how interannual and longer‐term oceanographic variability affects central‐place foraging seabirds and the recruitment and abundance of their squid prey is critical for precautionary ecosystem‐based fisheries management of oceanic islands and the surrounding marine environment.

The Differential Importance of Deep and Shallow Seagrass to Nekton Assemblages of the Great Barrier Reef

Seagrass meadows are an important habitat for a variety of animals, including ecologically and socioeconomically important species. Seagrass meadows are recognised as providing species with nursery grounds, and as a migratory pathway to adjacent habitats. Despite their recognised importance, little is known about the species assemblages that occupy seagrass meadows of different depths in the coastal zone. Understanding differences in the distribution of species in seagrass at different depths, and differences in species diversity, abundance, biomass, and size spectra, is important to fully appreciate both the ecological significance and economic importance of these seagrass meadows. Here, we assess differences in the assemblage characteristics of fish, crustacea, and cephalopods (collectively, nekton) between deep (&gt;9 m; Halophila spinulosa dominant) and shallow water (&lt;2 m; Halodule uninervis and/or Zostera muelleri dominant) seagrass meadows of the central Great Barrier Reef coast of Queensland, Australia. Nekton assemblage structure differed between deep and shallow seagrass. Deeper meadows were typified by juvenile emperors (e.g., Lethrinus genivittatus), hairfinned leatherjacket (Paramonacanthus japonicus) and rabbitfish (e.g., Siganus fuscescens) in both biomass per unit effort (BPUE) and catch per unit effort (CPUE), whereas shallow meadows were typified by the green tiger prawn (Penaeus semisulcatus) and pugnose ponyfish (Secutor insidiator) in both BPUE and CPUE. Both meadow depths were distinct in their nekton assemblage, particularly for socioeconomically important species, with 11 species unique to both shallow and deep meadows. However, both meadow depths also included juveniles of socioeconomically important species found in adjacent habitats as adults. The total nekton CPUE was not different between deep and shallow seagrass, but the BPUE and body mass of individual animals were greater in deep than shallow seagrass. Size spectra analysis indicated that in both deep and shallow meadows, smaller animals predominated, even more so than theoretically expected for size spectra. Our findings highlight the unique attributes of both shallow and deeper water seagrass meadows, and identify the distinct and critically important role of deep seagrass meadows within the Great Barrier Reef World Heritage Area (GBRWHA) as a habitat for small and juvenile species, including those of local fisheries value.

Inferences on the role of coral bleaching or seasonality on cross-habitat movement of nekton assemblages in adjacent coral reef, seagrass, and mangrove habitats

We investigated shifts in nekton assemblages in coral reef, seagrass, and mangrove habitats that coincided with a coral bleaching event in June 2016. The study was conducted in May 2016 (prebleaching), July 2016 (bleaching), October 2016 (4-mo postbleaching), and February 2017 (7-mo postbleaching) in relatively undisturbed coastal areas in Busuanga, Palawan, western Philippines. We used triangular trap nets to capture nekton samples from each habitat. In coral reef and mangrove habitats, there were increases in nekton abundance and biomass from the prebleaching to the bleaching period. After the bleaching event, however, there were reductions in nekton abundance and biomass at the coral reefs until 7-mo postbleaching. Species composition changed at all sites where shifts in dominant species, habitat affinity, and trophic category were observed. The postbleaching increase in nekton abundance in mangroves coincided with the decreased nekton abundance in the coral reef, suggesting a cross-habitat movement, likely due to the reduced suitability and/or food in the bleached coral reef. The changes in the nekton assemblage may also have been due to seasonal fluctuations in environmental parameters, especially salinity. Our study presents evidence of the possible role of coral bleaching or seasonal changes on cross-habitat movements of nekton assemblages, which can be inferred as an indicator of disturbance. The presence of adjacent vegetated habitats may provide refuge for the affected nekton assemblage of the coral reef.

Fish and tadpole composition as an indication of differences between micro-estuaries and micro-outlets in the Eastern Cape Province, South Africa

The fish and tadpole assemblages inhabiting micro-estuaries and micro-outlets along a warm-temperate section of the Eastern Cape coast were investigated over four seasons between 2015 and 2016. The specific aim of this study was to elucidate whether nekton assemblages in coastal microsystems are similar to those found in larger estuarine systems on the same coastline, and whether the presence or absence of tadpoles can indicate a difference between the two microsystem types. Oligohaline and/or freshwater conditions prevailed in micro-outlets for much of the sampling period, while micro-estuaries were mostly mesohaline. Nekton communities in micro-outlets were dominated by tadpoles (Xenopus laevis), with juvenile Pseudomyxus capensis sometimes present, while Oreochromis mossambicus, Gilchristella aestuaria, P. capensis and Mugil cephalus were the most common fish species in micro-estuaries which had an almost complete absence of tadpoles. The micro-estuaries had a low estuarine function when compared to temporarily open/closed estuaries, while the micro-outlets had very limited estuarine characteristics and an almost complete absence of fishes. Due to their restricted periods of stability and small size, micro-estuaries served as only intermediate nursery areas for small numbers of 0+ marine fish which, after less than a year, need to migrate to nearby larger estuarine systems that can provide the full range of nursery habitats for subsequent growth and development. The limited numbers of estuarine resident fishes (e.g. G. aestuaria) and freshwater fish species (e.g. O. mossambicus) confirm the marginal habitat and food availability in these microsystems for fishes that are normally abundant in other types of Eastern Cape estuaries and rivers. The results of this study confirm that there are clear distinctions between the nekton of the two microsystem types, thus making fish and tadpole assemblages a viable tool for differentiating between micro-estuaries and micro-outlets in this province.

Decadal-Scale Changes in Subtidal Nekton Assemblages in a Warm-Temperate Estuary

Coincidental long-term changes in estuarine nekton assemblages and environmental conditions are widely reported. In this study, from a warm-temperate, high salinity, salt marsh-dominated estuary in the southeastern USA, decreases in overall abundance, shifts in species and life stage composition, and changes in seasonal patterns of occurrence coincided with increased water temperature. Biweekly trawl collections in a subtidal creek were made during two 4-year periods separated by more than 30 years. Of the total 111 taxa in the North Inlet estuary, South Carolina, 64% (71) occurred during both the historic (1981–84) and recent (2013–16) periods. The top five species and their proportions of the total annual catches changed between periods. In the recent study period, near-bottom species (Lagodon rhomboides, Bairdiella chrysoura, Litopenaeus setiferus) increased, and pelagic species (Anchoa spp. and Lolliguncula brevis) decreased. The mean abundance of total nekton in the recent period was approximately 50% of historic abundance. Large, but temporary increases in nekton abundance occurred when salinity decreased after major storms. In the recent study period, shifts in the timing of peak abundances from spring to fall, the occurrence of juveniles during winter, and increased diversity suggested responses to significantly warmer winters and summers. Over the 30 year period, the subtidal nekton assemblage transitioned to a state of lower abundance and different composition. Future increases in water temperature, incidences of major storms, and modifications of estuarine habitats due to rising sea level could lead to additional changes in the fauna of warm-temperate estuaries.

Oyster Reefs in Northern Gulf of Mexico Estuaries Harbor Diverse Fish and Decapod Crustacean Assemblages: A Meta-Synthesis

Oyster reefs provide habitat for numerous fish and decapod crustacean species that mediate ecosystem functioning and support vibrant fisheries. Recent focus on the restoration of eastern oyster (Crassostrea virginica) reefs stems from this role as a critical ecosystem engineer. Within the shallow estuaries of the northern Gulf of Mexico (nGoM), the eastern oyster is the dominant reef building organism. This study synthesizes data on fish and decapod crustacean occupancy of oyster reefs across nGoM with the goal of providing management and restoration benchmarks, something that is currently lacking for the region. Relevant data from 23 studies were identified, representing data from all five U.S. nGoM states over the last 28 years. Cumulatively, these studies documented over 120,000 individuals from 115 fish and 41 decapod crustacean species. Densities as high as 2800 ind m-2 were reported, with individual reef assemblages composed of as many as 52 species. Small, cryptic organisms that occupy interstitial spaces within the reefs, and sampled using trays, were found at an average density of 647 and 20 ind m-2 for decapod crustaceans (range: 18 - 2843) and fishes (range: 2 - 56), respectively. Both groups of organisms were comprised, on average, of 8 species (range: 3 - 15). Larger-bodied fishes captured adjacent to the reef using gill nets were found at an average density of 6 ind m-2 (range: 1- 68) which came from 23 species (range: 9 - 46). Decapod crustaceans sampled with gill nets had a much lower average density, < 1 ind m-2, and only contained 2 species. On average, seines captured the greatest number of fish species (n = 33), which were made up of both facultative residents and transients. These data provide general gear-specific benchmarks, based on values currently found in the region, to assist managers in assessing nekton occupancy of oyster reefs, and assessing trends or changes in status of oyster reef associated nekton support. More explicit reef descriptions (e.g., rugosity, height, area, adjacent habitat) would allow for more precise benchmarks as these factors are important in determining nekton assemblages, and sampling efficiency.

Habitat structure influences the seasonality of nekton in seagrass

The presence of a seasonally variable biogenic habitat (eelgrass, Zostera marina) increased the spring–summer variability of associated nekton relative to unvegetated bare tidal flat. This spatio-temporal pattern emerged because most eelgrass-associated taxa tracked the decline in eelgrass biomass from summer to spring, but in one case reached greater density in spring when predation intensity was low. Among 21 taxa (26,884 individuals) captured, a strong correlation arose between structure association and summer dominance, and certain functional traits, in particular morphology and on- vs. off-bottom position, were strong predictors of eelgrass association. Structure-associated taxa were slender-bodied and pelagic schooling fishes, while habitat generalists or bare-associated taxa were more consistent seasonally, primarily benthic, and cryptically colored with sand. Estuarine use (transient, reproducing, or estuarine resident) was not a strong predictor of structure association or seasonality. Because an identical sampling design was used in five regions of Washington State, USA, coarse-scale (> 100 km) differentiation in nekton assemblages was identified, representing less of the total variation than across seasons but more than across different habitats. While regional nekton differences were attributable in part to geographic distance and eelgrass morphotypes, the most seasonally variable nekton were at sites with morphotypes adding the least vertical structure but highest density. These results support two mechanisms that increase seasonal variability of taxa using structured habitats, including both bottom-up provision of habitat and resources, and subsequent possibilities for negative interspecific interactions and top-down control.

Predicting the response of nekton assemblages to seagrass transplantations in the Venice lagoon: An approach to assess ecological restoration

Abstract One of the major challenges to ensure effective restoration of estuarine habitats is to establish success criteria to determine whether the goals of restoration are met. The aim of this work is to propose and test an approach to identify reference conditions and assess the recovery of nekton (fish, decapods and cephalopods) assemblages at seagrass restoration sites. Nekton sampling took place from 2014 to 2017 in the northern Venice lagoon (northern Adriatic Sea, Italy) during spring at eight sites subjected to seagrass (Zostera marina and Zostera noltei) transplantation. In spring 2016, five natural seagrass sites in the same area were additionally sampled, and physico‐chemical water parameters and habitat structure were also recorded. A multivariate generalized linear model approach was adopted in order to disentangle the relative effect of water quality and seagrass habitat structure on nekton assemblages of natural habitats. Models were subsequently employed to predict species composition of nekton fauna at each transplantation site, to identify the reference assemblage expected under site‐specific abiotic and habitat characteristics. The average distance of the observed assemblage from reference conditions was used to track temporal trajectories of nekton colonization at transplantation sites, and to ultimately evaluate the recovery rate towards restoration goals. This study highlights how a predictive approach could serve management purposes in ecological restoration, providing a concise tool to assess the functionality of restored habitats for associated fauna.