Monitoring post-spawning movement, habitat use, and survival of adult anadromous rainbow smelt using acoustic telemetry in a New Hampshire estuary.

Anadromous rainbow smelt (Osmerus mordax, [Mitchill 1814]) are found along the northeast Atlantic coastline of North America, with their range now limited to north of Cape Cod, Massachusetts, USA. Although their anadromous life cycles are described broadly, gaps remain regarding how adult rainbow smelt use estuaries post-spawning, including movement behaviors, habitats used, and specific timing of emigration to coastal waters. In spring 2021, we used acoustic telemetry to characterize movements during and after the spawning season of rainbow smelt captured in tributaries to Great Bay, New Hampshire, USA, a large estuarine system near the southern edge of their range. Forty-four adult rainbow smelt (n = 35 male, n = 9 female) were tagged with Innovasea V5 180-kHz transmitters and an array of 22,180 kHz VR2W receivers were deployed throughout Great Bay to detect movements of tagged fish from March to October 2021. Rainbow smelt were detected 14,186 times on acoustic telemetry receivers, with 41 (93%) of the tagged individuals being detected at least once post-tagging. Individuals were detected moving between tributaries, revealing that rainbow smelt can use multiple rivers during the spawning season (March-April). Mark-recapture Cormack-Jolly-Seber models estimated 83% (95% confidence interval 66%-92%) of rainbow smelt survived to the mainstem Piscataqua River, and a minimum of 50% (22 of 44) reached the seaward-most receivers and were presumed to have survived emigration. Most individuals that survived remained in the estuary for multiple weeks (average = 19.47 ± 1.99 standard error days), displaying extended use of estuarine environments. Downstream movements occurred more frequently during ebb tides and upstream movements with flood tides, possibly a mechanism to reduce energy expenditures. Fish emigrated from the estuary by mid-May to the coastal Gulf of Maine. Our results underscore that rainbow smelt need access to a variety of habitats, including multiple tributaries and high-quality estuarine habitat, to complete their life cycle.

Quantifying euryhaline histories in red drum Sciaenops ocellatus: Scale isotope ratios.

Stable isotope analysis of fish muscle tissue has been used to quantify fish migratory behavior in many systems, and these chemical tracers are especially useful for euryhaline fishes that traverse significant salinities and gradients in baseline signatures. However, the removal of muscle tissues often requires lethal sampling. Fish scales may be removed non-lethally and offer potential alternatives to stable isotope ratios from muscle to indicate recent feeding histories. For this study, red drum Sciaenops ocellatus were collected in subtropical estuaries in the northwestern Gulf of Mexico and analyzed for stable isotope (𝛿13 C and 𝛿15 N) compositions of scale exteriors. Decalcification of scale samples was deemed unnecessary given the preservation of linearity and minimal offsets between paired decalcified and untreated scale samples. Stable isotope signatures of muscle tissue and scales were closely matched and indicated high degrees of residence within bays at fine spatial scales. This study also surveyed the existing body of literature comparing scale and muscle isotope values from the same individuals and found that the majority reports linear relationships with slopes close to unity, although species-specific offsets and intercepts vary. This work expands the body of literature indicating that scales are a viable non-lethal alternative for stable isotope assessments of dietary and habitat use histories for mobile fishes.

Quantifying euryhaline histories in red drum Sciaenops ocellatus: otolith chemistry and muscle isotope ratios.

The combined use of otolith chemistry and tissue isotopes has the potential to reveal movements, habitat associations and food web interactions at a variety of spatial and temporal scales. Here, we used a combination of otolith Ba:Ca life history transects with muscle tissue δ13 C and δ15 N values to assess habitat use and oligohaline residence in red drum Sciaenops ocellatus in subtropical estuaries in the northwestern Gulf of Mexico. Tissue isotopes were distinct among capture locations, particularly between bays with differing proximity to freshwater inflow sources. Otolith edge Ba:Ca values and tissue δ13 C values were not correlated. These results indicated that fish were not residing in nor feeding in oligohaline waters for significant periods of time within the tissue turnover window of several months prior to capture. However, spatial differences in tissue isotope values indicated limited mixing among bays and relatively high site fidelity during estuarine occupancy. Lifetime otolith Ba:Ca transects revealed individual variability in the magnitude of residence in oligohaline waters. Using a medium oligohaline occupancy threshold, an estimated 82% of individuals used oligohaline waters at some point in their life. However, 66% of individuals spent less than 20% of their life histories in oligohaline waters, suggesting intermittent and infrequent excursions into low salinity waters. Finally, a literature survey identified 56 peer-reviewed publications using combinations of otolith chemistry and tissue stable isotope ratios with a wide range of marker pairings and study aims. The diversity of ecological questions that can be asked with the combined use of these two approaches will provide valuable insight into fish ecology. This article is protected by copyright. All rights reserved.

Distributive stress: individually variable responses to hypoxia expand trophic niches in fish.

Environmental stress can reshape trophic interactions by excluding predators or rendering prey vulnerable, depending on the relative sensitivity of species to the stressor. Classical models of food web responses to stress predict either complete predator exclusion from stressed areas or complete prey vulnerability if predators are stress tolerant. However, if the consumer response to the stress is individually variable, the result may be a distributive stress model (DSM) whereby predators distribute consumption pressure across a range of prey guilds and their trophic niche is expanded. We test these models in one of the largest hypoxic "Dead Zones" in the world, the northern Gulf of Mexico, by combining geochemical tracers of hypoxia exposure and isotope ratios to assess individual-level trophic responses. Hypoxia-exposed fish occupied niche widths that were 14.8% and 400% larger than their normoxic counterparts in two different years, consistent with variable displacement from benthic to pelagic food webs. The degree of isotopic displacement depended on the magnitude of hypoxia exposure. These results are consistent with the DSM and highlight the need to account for individually variable sublethal effects when predicting community responses to environmental stress.

Environmental Records from Great Barrier Reef Corals: inshore versus offshore drivers.

The biogenic structures of stationary organisms can be effective recorders of environmental fluctuations. These proxy records of environmental change are preserved as geochemical signals in the carbonate skeletons of scleractinian corals and are useful for reconstructions of temporal and spatial fluctuations in the physical and chemical environments of coral reef ecosystems, including The Great Barrier Reef (GBR). We compared multi-year monitoring of water temperature and dissolved elements with analyses of chemical proxies recorded in Porites coral skeletons to identify the divergent mechanisms driving environmental variation at inshore versus offshore reefs. At inshore reefs, water Ba/Ca increased with the onset of monsoonal rains each year, indicating a dominant control of flooding on inshore ambient chemistry. Inshore multi-decadal records of coral Ba/Ca were also highly periodic in response to flood-driven pulses of terrigenous material. In contrast, an offshore reef at the edge of the continental shelf was subject to annual upwelling of waters that were presumed to be richer in Ba during summer months. Regular pulses of deep cold water were delivered to the reef as indicated by in situ temperature loggers and coral Ba/Ca. Our results indicate that although much of the GBR is subject to periodic environmental fluctuations, the mechanisms driving variation depend on proximity to the coast. Inshore reefs are primarily influenced by variable freshwater delivery and terrigenous erosion of catchments, while offshore reefs are dominated by seasonal and inter-annual variations in oceanographic conditions that influence the propensity for upwelling. The careful choice of sites can help distinguish between the various factors that promote Ba uptake in corals and therefore increase the utility of corals as monitors of spatial and temporal variation in environmental conditions.

Movements of diadromous fish in large unregulated tropical rivers inferred from geochemical tracers.

Patterns of migration and habitat use in diadromous fishes can be highly variable among individuals. Most investigations into diadromous movement patterns have been restricted to populations in regulated rivers, and little information exists for those in unregulated catchments. We quantified movements of migratory barramundi Lates calcarifer (Bloch) in two large unregulated rivers in northern Australia using both elemental (Sr/Ba) and isotope ((87)Sr/(86)Sr) ratios in aragonitic ear stones, or otoliths. Chemical life history profiles indicated significant individual variation in habitat use, particularly among chemically distinct freshwater habitats within a catchment. A global zoning algorithm was used to quantify distinct changes in chemical signatures across profiles. This algorithm identified between 2 and 6 distinct chemical habitats in individual profiles, indicating variable movement among habitats. Profiles of (87)Sr/(86)Sr ratios were notably distinct among individuals, with highly radiogenic values recorded in some otoliths. This variation suggested that fish made full use of habitats across the entire catchment basin. Our results show that unrestricted movement among freshwater habitats is an important component of diadromous life histories for populations in unregulated systems.