Fish Density Research Articles

Do Two Different Approaches to the Season in Modeling Affect the Predicted Distribution of Fish? A Case Study for Decapterus maruadsi in the Offshore Waters of Southern Zhejiang, China

The relationships between environmental factors and fish density are closely related, and species distribution models (SDMs) have been widely used in exploring these relationships and predicting the spatial distribution of fishery resources. When exploring the prediction of the spatial distribution of species in different seasons, the method of choosing the appropriate approach to the season will help to improve the predictive performance of the model. Based on data collected from 2015 to 2020 during a survey off southern Zhejiang, the Tweedie-GAM was used to establish the relationship between the density of Decapterus maruadsi and environmental factors at different modeling approaches. The results showed that water temperature, salinity and depth were the main factors influencing D. maruadsi, and they operated through different mechanisms and even resulted in opposite trends of density in different seasons. Spatially, the two modeling approaches also differed in predicting the spatial distribution of D. maruadsi, with the seasonal model showing a higher density trend in inshore waters than in offshore waters in spring but showing the opposite trend in summer and autumn, which was more consistent with the actual spatial distribution of the resource. By analyzing the effects of two different approaches on the prediction of fishery resources, this study aims to provide research ideas and references for improving the predictive performance of SDMs.

Seasonal Development of Phytoplankton in South Bohemian Fishponds (Czechia)

Fishponds with a relatively small water volume, high fish abundance, and wide range of nutrient concentrations serve as suitable models for ecological studies. Intensified fish production, together with increased input of nutrients from the watershed, resulted in hypertrophic conditions in the majority of fishponds, the most common type of lentic ecosystems worldwide. In order to understand the processes driving plankton succession, we analyzed eight-year data from nine fishponds in Czechia with differing trophic status. The mean concentration of phosphorus (P) was 200 µg L−1 in hypertrophic ponds, 130 µg L−1 in eutrophic, and 40 µg L−1 in mesotrophic. Correspondingly the mean concentration of phytoplankton was 14.9 mg L−1 in hypertrophic ponds, 7.3 mg L−1 in eutrophic, and 1.96 mg L−1 in mesotrophic. Although the fish stock of 200–900 kg ha−1 eliminated zooplankton in eutrophic and hypertrophic ponds the faster-growing algae did not prevail over cyanobacteria. Zooplankton grazing pressure on algae is thus not relevant in studied food webs. Due to the rapid biological denitrification in hypertrophic and eutrophic fishponds resulting in low concentration of mineral nitrogen (N), these ponds were dominated by N-fixing cyanobacteria throughout the whole season. Similarly, the faster-growing algae prevail over cyanobacteria in mesotrophic ponds until the decrease of available mineral nitrogen. The limitation by mineral N is thus the primary driver of phytoplankton composition reflected in cyanobacterial dominance, independently of the trophic status and fish density in studied fishponds.

Habitat selection of high trophic-level predatory fishes in protected and unprotected areas: The effects of density and maturity stage

Marine Protected Areas (MPAs) are useful tools for the conservation of fishes, providing that they include the essential habitats for the focal species. Habitat selection may depend on external (i.e., environmental conditions, food availability, and shelter) and internal (i.e., density-dependent processes) factors. Additionally, most fishes undergo ontogenetic shifts along their life cycles, thus, understanding the habitat requirements at each stage is of great interest for the correct design and functioning of MPAs. Here, we evaluated habitat preferences of high-level predatory fishes, namely Epinephelus marginatus, Epinephelus costae, Mycteroperca rubra, Dentex dentex, Sciaena umbra and Sphyraena viridensis to assess differences between species, maturity stages and density of fish. To do so, we applied Species Distribution Models (SDMs) in protected and unprotected areas along the coast of Murcia (SE Spain) to determine if those areas are properly located for the conservation of the focal species. We included three areas with different density of fishes. One area with high density in the long-term protected Cabo de Palos-Islas Hormigas MPA, and two with low density in the newly established Cabo Tiñoso MPA and the unprotected area of Cabo Cope. We evaluated the environmental variables that affect the occurrence of the species at two life stages, before and after achieving size at maturity. SDMs showed that bathymetry, habitat complexity and rocky bottoms were the most important drivers determining species distribution in both areas with high and low density of fish, although there were differences in the contribution of the variables at each location. Additionally, we found that habitat requirements were different for the immature and mature individuals, generally being the immatures the most sensitive to changes in habitats. Both MPAs and the unprotected area largely included areas of high suitability for the studied species, indicating that the MPAs are established in proper areas, and suggesting another area for the establishment of a new MPA to enhance a MPA network in the region.

Interactions between demersal fish body condition and density during the regime shift of the Gulf of Lions

Abstract Environmentally driven changes in small pelagic fish condition and size have been observed in the Gulf of Lions (GOL) since 2008, leading to a significant fishery crisis. However, the effect of changes in environment and/or in the small pelagic community on the demersal community remain unknown. For the first time, this study examines the body condition (bc) and population density of 22 demersal species since 1994, using dynamic factor analysis (DFA). Most (but not all) of demersal species have shown a common shift between 2006 and 2009, which is synchronous with that observed in small pelagic species and the environmental conditions in the GOL. It had been concluded that the environmentally driven changes detected in the pelagic fish community also affected the demersal fish community, but with less drastic and lasting consequences. As the DFA revealed that the bc displayed important variations for several species, notably hake (Merlucius merluccius), the interaction between the bc and population density was investigated using the Multivariate Autoregressive (MAR) model on hake population at three life stages (recruits, juveniles, and adults). Results showed that adult bc, while negatively affected by density, had a positive effect on recruitment. So hake bc could have affected population dynamics by promoting higher recruitment at low densities. Further work is needed to ascertain whether such effects exist in other demersal species.

Using the hydroacoustic and mini trawl data for estimating fish density in the eastern part of Banyuasin coastal waters, South Sumatra of Indonesia

The hydroacoustic and mini trawl sampling was conducted to estimate a distribution pattern of fish density in the eastern part of Banyuasin coastal waters, South Sumatra, Indonesia. These surveys were carried out in September 2017 using a SIMRAD EK15 single‐beam echo sounder with a 200 kHz operating frequency. Catch data obtained from the mini bottom trawl were used to examine a dominant species for refining the TS value concerning the fish density estimation along the hydroacoustic track. The ponyfishes (Eubleekeria jonesi) from the Leiognathidae family were found at each station with the highest relative abundance (50.98%). Hence this species was used to refine target strength values for estimating the acoustic volume density. These volume densities of 206 ESDU ranged from 0 to 9048 fish 1000m–3, with an average of 930 fish 1000m–3. The results also described a distribution pattern of fish densities hence this information could be valuable to the fishery manager for improving sustainable management approaches.

Estimating Deep-Sea Fish Population Density From the Odour Extension Area: A Theoretical Basis and Comparison With the Conventional Methods

Accurately estimating the population density of deep-sea fish with a baited camera system has long been a significant challenge. Although several theoretical models have been developed using the first arrival time of an individual fish or time-varying fish abundance at the bait, none of the models allows for the spatio-temporal variability of the odour plume area extending from the bait. This study shows theoretically that the population density can be formulated as the inverse of the sample mean of the odour plume area extended until it reaches a first fish under the condition that fish at rest have a random dispersion. Each area estimate is governed by the homogeneous Poisson process and, hence, its probability density follows an exponential distribution. A large uncertainty can occur for each area estimate (sample), but the uncertainty decreases as the number of samples used to derive the sample mean increases by the law of large numbers. Numerical experiments conducted in the study indicate that the proposed method for inferring population density is also potentially applicable to cases in which the fish have a uniform or large-scale clumped dispersion. The experiments also show that the conventional method based on first arrival time fails to estimate the population density for any of the dispersion cases. This study also indicates that the reliability of the most popular inference method for estimating population density from the time-profile of fish abundance at the bait site was found to depend on the extension of the odour plume area and the dispersion pattern.

Coral reefs recovery post bleaching event in Central Tapanuli, North Sumatra

Temperatures on the west coast of Sumatra, South Java, Bali, and Nusa Tenggara in the Indian Ocean rose dramatically in 2016. The extreme increase in temperature greatly affects sea surface temperature and coral reef ecosystems. Central Tapanuli waters were also the area that could not escape this phenomenon. Coral bleaching has caused damage to coral reefs which is characterized by decreasing live coral’s cover. Observations on coral reef recovery have been carried out at 13 sites in Central Tapanuli waters. Observations were made using the UPT (Underwater Photo Transect) method to determine live coral’s cover. The coral fish density was determined using the UVC (Underwater Visual Census) method. From the observations, coral cover decreased by 57,68% in 2016. Observations were made in 2015, 2016, 2018, and 2021. Observations in 2018 have shown increasing live coral cover by approximately 5%. Meanwhile, in 2021 there was an increase in live coral cover by approximately 8.25% from 2018. Even though coral composition changes and mostly the number of coral decreases, after 5 years, the number of corals starts to increase. The reef health index likewise rises in tandem with these changes. Despite some chronic stress being present at some places, the water quality was favorable for coral recovery.

Mechanistic support for increased primary production around artificial reefs.

Understanding factors controlling primary production is fundamental for the protection, management, and restoration of ecosystems. Tropical seagrass ecosystems are among the most productive ecosystems worldwide, yielding tremendous services for society. Yet they are also among the most impaired from anthropogenic stressors, prompting calls for ecosystem-based restoration approaches. Artificial reefs (ARs) are commonly applied in coastal marine ecosystems to rebuild failing fisheries and have recently gained attention for their potential to promote carbon sequestration. Nutrient hotspots formed via excretion from aggregating fishes have been empirically shown to enhance local primary production around ARs in seagrass systems. Yet, if and how increased local production affects primary production at ecosystem scale remains unclear, and empirical tests are challenging. We used a spatially explicit individual-based simulation model that combined a data-rich single-nutrient primary production model for seagrass and bioenergetics models for fish to test how aggregating fish on ARs affect seagrass primary production at patch and ecosystem scales. Specifically, we tested how the aggregation of fish alters (i) ecosystem seagrass primary production at varying fish densities and levels of ambient nutrient availability and (ii) the spatial distribution of seagrass primary production. Comparing model ecosystems with equivalent nutrient levels, we found that when fish aggregate around ARs, ecosystem-scale primary production is enhanced synergistically. This synergistic increase in production was caused by nonlinear dynamics associated with nutrient uptake and biomass allocation that enhances aboveground primary production more than belowground production. Seagrass production increased near the AR and decreased in areas away from the AR, despite marginal reductions in seagrass biomass at the ecosystem level. Our simulation's findings that ARs can increase ecosystem production provide novel support for ARs in seagrass ecosystems as an effective means to promote (i) fishery restoration (increased primary production can increase energy input to the food web) and (ii) carbon sequestration, via higher rates of primary production. Although our model represents a simplified, closed seagrass system without complex trophic interactions, it nonetheless provides an important first step in quantifying ecosystem-level implications of ARs as a tool for ecological restoration.

Fish stock mass reduction is indicated in standard abundance and biomass estimates from gillnets and hydroacoustics

Gillnetting is a technique commonly used in relative abundance and biomass estimates of fish. However, due to its passive nature, the direct recalculation of the catch to reservoir volume or area is not trivial. This issue is often solved by using hydroacoustics, which provides information about fish density, though without the ability to distinguish species. However, the precision of such density estimates are also questionable. In this study, we estimated the abundance and biomass of dominant fish species before and after a biomanipulation program (fish removal) in 2020 using gillnetting and hydroacoustic surveys in a temperate reservoir. Between the two sampling periods, nearly 27,000 individuals (9000 kg) older than 0 + of bleak (Alburnus alburnus) and bream (Abramis brama) were removed during the biomanipulation program. Decreases in abundance and biomass estimates were expected for both techniques and both species. The gillnet decrease was 68% in CPUE (catch per unit of effort) and 48% in BPUE (biomass per unit of effort) for both species and all gillnets grouped together. Hydroacoustic observations showed a decrease of approximately 79% in abundance and 74% in biomass after fish reduction. Considering the numbers and biomass of fish removed, the absolute hydroacoustic estimates were underestimated for fish abundance but credibly estimated for biomass. The average weight of the fish taken was between the values of the fish caught with gillnets and the weights determined by hydroacoustics. In terms of results, both methods appear to be a suitable tool for estimating fish abundance and biomass in large inland waters, although the spatial and temporal distribution of fish of different sizes should be considered when using different sampling methods for fish monitoring.

Spatiotemporal Response of Fish Aggregations to Hydrological Changes in the Lower Pearl River, China, during the Main Spawning Season

The Pearl River is a global hotspot of fish biodiversity, yet has the most threatened endemic fish species in China. Since the establishment of the Changzhou Dam in the lower reach, changes in hydrological rhythm have negatively impacted fish downstream of the dam, but their spatiotemporal distribution in response to flood alteration has received little attention. In this study, hydroacoustic surveys were undertaken monthly in 2016 to monitor the distribution and behavior of fish. Fish densities were higher during the water discharge rising stage than during the falling stage, indicating that the fish aggregate during flooding (coefficient of variation [CV] > 100%) and depart after flooding (CV < 100%), especially aggregations of large fish. The target strength (TS) was allocated to two groups as per their frequency distributions, defined as small fish (−55 dB < TS < −40 dB) and large fish (TS > −40 dB). The sizes of both groups were significantly larger during the rising stage when compared to those during the falling stage (p < 0.01). Comparatively more fish were present with a greater average TS, and a substantially greater proportion of large fish was detected during rising stages. Hydrological variation importantly influences fish aggregations, including the numbers and sizes present, with the differences being particularly pronounced between the rising and falling stages. Combined with relevant studies, it is suggested that water releases from the Changzhou Dam should be regulated to satisfy fish spawning and migration demands during the main breeding season.

Mesoscale assessment of sedentary coastal fish density using vertical underwater cameras

Accurate and precise monitoring of the absolute density (i.e., number of fish per area or volume unit) of exploited fish stocks would be strongly advisable for deriving stock status and for designing proper management plans. Moreover, monitoring should be achieved at relevant (i.e., sufficiently large) temporal and spatial scales. This objective is particularly challenging for data-poor fisheries, as is often the case for recreational fisheries. Therefore, the feasibility of underwater video monitoring (vertical unbaited cameras) for estimating, as a proof of concept, the absolute density (and its ecological drivers) of a coastal sedentary fish species is demonstrated. The absolute density of a small serranid (Serranus scriba) targeted by recreational fishing was estimated along the southern coast of Mallorca Island (nearly 100 km). The median fish density ranged between 111 ind/km2 (Es Molinar) and 14,110 ind/km2 (Cabrera). Absolute density was correlated with fishing exposure, habitat, and depth. Site specific, seemingly long-term, effects of fishing exposure were negatively correlated with fish density, but short-term effects (assessed by the interaction between fishing exposure and before/after the season when recreational fishing occurred in the study area) were not detected. We suggest that the short-term effects of fishing may remain undetected because highly exploited sites could contain fish that are already not vulnerable to recreational fishing gear, irrespective of the short-term fishing pressure exerted. Such a process may explain some hyper-depletion patterns and should preclude the use of fisheries-dependent data for monitoring fish density. The results reported here indicate that monitoring fish abundance with vertical unbaited cameras at large spatial and temporal scales can be a reliable alternative for many species.

Emerging marine protected areas of eastern Indonesia: Coral reef trends and priorities for management

Indonesian coral reefs are under pressure from illegal, unregulated and unreported fishing, the use of destructive fishing practices, land-based pollution, coastal development and climate change. Marine protected areas (MPAs) are necessary to allow habitat recovery and fish stock replenishment. Through the United States Agency for International Development Sustainable Ecosystems Advanced Project (2016–2021), Indonesia worked to improve fisheries productivity and sustainable livelihoods within the three provinces of Maluku, North Maluku and West Papua, where 13 new coral reef MPAs were designed for multiple use, with a zoning system to support biodiversity conservation and sustainable fisheries. At the time of writing in mid-2021, regulations to prohibit fishing within the no-take areas (NTAs) were not yet implemented. This paper presents baseline and trends in percent coral cover and reef fish density and biomass in the MPAs. In 2020, overall coral cover had increased from 42% to 45% across all MPAs. The average target fish biomass across all NTAs had declined from 1709 ( ± 176 SE) kg per hectare in 2017 to 884 ( ± 76 SE) kg per hectare in 2020, representing a significant decline of about 48%. Large and significant declines in many target fish families across multiple MPAs are reflective of unsustainable levels of exploitation. It is evident that management plans need to be implemented with adequate enforcement and stakeholder engagement to stem the decline of target species and to secure livelihoods for local fishing communities.

Spatial patterns and behaviour of notothenioid fishes off the northern Antarctic Peninsula

A photographic seabed survey conducted off the Antarctic Peninsula region provided the opportunity to study spatial patterns, abundance and behaviour of the notothenioid benthic fish fauna. Overall, a total of 12,715 images taken with the Ocean Floor Observation System (OFOS) along 26 transects in three ecoregions (Joinville Island, Bransfield Strait and Drake Passage) were analysed. The fish fauna consisted of at least 34 species belonging to four families of both low-Antarctic and high-Antarctic origin. Nototheniids showed the highest relative abundance and species richness, followed by channichthyids, bathydraconids and artedidraconids. Direct in-situ observations in OFOS seabed images allowed descriptions of fish behaviour, such as aggregation of individuals (Notothenia coriiceps), specific body postures (Cygnodraco mawsoni and Cryodraco antarcticus) and parental care (Chaenodraco wilsoni, Chionodraco rastrospinosus, Pagetopsis macropterus and Trematomus hansoni). Fish density and species richness was primarily correlated with the occurrence of bryozoans, ascidians, and large cup-shaped sponges, providing a three-dimensional habitat suitable for fish settling, foraging, breeding and refuge from predators. Fish diversity was higher (a) off Joinville Island and in Bransfield Strait than in Drake Passage, where almost exclusively low-Antarctic species were recorded, and (b) between 100 and 600 m than at greater depths. Overall, the benthic fish fauna off the northern Antarctic Peninsula is zoogeographically composite and widespread, with well-structured spatial partitioning.

Posidonia oceanica wrack beds as a fish habitat in the surf zone

Drifting macrophyte detritus cast along the shore, often called wrack, can wash around, accumulate in the surf zone and constitute a habitat where fishes forage and/or hide. We compared fish assemblages associated with Posidonia oceanica wrack beds with those associated with bare sand in the surf zone of Sardinia Island (Italy), NW Mediterranean Sea. In July–August 2020, in situ non-destructive visual censuses were performed to count fishes and record their behaviour at 3 locations, each including 3 shores characterized by both P. oceanica wrack beds and bare sand. The assemblages were mainly composed of small and juvenile/sub-adult individuals and significantly differed between the two habitats. Nine taxa were shared by both habitat types, seven were exclusively associated with P. oceanica wrack and only one species was found exclusively on sand. Higher species richness and fish density (without considering the contribution of gregarious fish in the water column) were observed on P. oceanica wrack than on sand. Assemblages were numerically dominated by Diplodus sargus, D. vulgaris and Mullus surmuletus. These species and other fishes, mostly represented by invertivorous (e.g., labrids) and omnivorous species, were often observed feeding on P. oceanica wrack and using the resuspended dark-brown vegetal fragments to hide. These findings clearly suggest that P. oceanica wrack beds attract fishes from surrounding habitats, some of them of commercial importance, providing food and habitat for juvenile and sub-adult stages.

Fish and invertebrate use of restored vs. natural oyster reefs in a shallow temperate latitude estuary

AbstractCoastal marine habitats continue to be degraded, thereby compelling large‐scale restoration in many parts of the world. Whether restored habitats function similarly to natural habitats and fully recover lost ecosystem services is unclear. In estuaries, oyster reefs have been degraded by multiple anthropogenic activities including destructive fishing practices and reduced water quality, motivating restoration to maintain oyster fisheries and other ecosystem services, often at relatively high cost. We compared fish and invertebrate communities on recently restored (0–1 year post‐restoration), older restored (3–4 years post‐restoration), and natural oyster reefs to determine if and when restored reefs support functionally similar faunal communities. To test the influence of landscape setting on the faunal communities, the restored and natural reefs, as well as a control without reef present, were distributed among three landscapes (on the edge of salt marsh away from seagrass [salt marsh landscape], on mudflats [mudflat landscape], and near to seagrass and salt marsh [seagrass landscape]). Oyster density and biomass were greatest on restored reef habitat, as were those of non‐oyster bivalve species. Total abundance of invertebrates was much greater on oyster reefs than in control plots, regardless of reef or landscape type, yet were frequently highest on older restored reefs. Meanwhile, juvenile fish densities were greatest on natural reefs, at intermediate densities on older restored reefs, and least abundant on controls. When comparing the effects of reef age and landscape setting, juvenile fish densities were greatest on younger reefs within the mudflat landscape. Collectively, these results indicate that oyster reefs harbor higher densities of resident invertebrate prey, which may explain why reef habitat is also important for juvenile fish. Laboratory and field experiments supported the notion that gag grouper (a predatory demersal fish) forage more effectively on oyster reefs than on unstructured mud bottom, whereas our experiments suggest that flounders that utilize oyster reefs likely forage on adjacent mud bottom. Because landscape setting influenced fish and invertebrate communities on restored reefs, the ecological consequences of landscape setting should be incorporated into restoration decision making and site selection to enhance the recovery of ecosystem goods and services.

Strong Behavioral Effects of Omnivorous Fish on Amphibian Oviposition Habitat Selection: Potential Consequences for Ecosystem Shifts

Perceived predation risk to offspring may have similar ultimate community-level impacts to those of consumptive trophic interactions. The present study investigated the behavioral effects of common carpCyprinus carpio–an omnivorous fish capable of triggering an ecosystem shift to an algae-dominated state–on anurans, using a natural experiment conducted in a system of fish-stocked ponds. We compared oviposition patterns and larval densities of anurans and abundance of zooplankton and phytoplankton in ponds where fish were virtually absent and ponds where common carp was dominant. All studied anuran species bred in fish-poor ponds, while in ponds with high fish densities most of them oviposited infrequently or virtually did not breed. Oviposition habitat selection coupled with fish trophic pressure resulted in diametrically different tadpole densities between fish-poor and fish-dominated ponds. The alimentary tract contents of tadpoles of three locally common anurans,Pelobates fuscus,Hyla orientalis, andRana temporaria, contained large numbers of unicellular algae, but also significant numbers of zooplanktonic grazers. According to stable nitrogen isotope analyses, tadpoles occupied a trophic level similar to a primary consumer, snailLymnaea stagnalis, indicating that they fed mainly on algae. While total biomass of crustacean zooplankton did not differ between pond types, chlorophyllaconcentrations were low in fish-poor ponds compared to fish-dominated ponds and negatively related to total tadpole biomass. Our findings indicate that scarcity of anuran larvae, resulting mainly from the behavioral responses of breeding anurans to fish predation risk, may facilitate algal production, although ecosystem shifts in the presence of fish occur along more pathways than a top-down cascade.

Effects of biological and environmental factors on the variability of Paralonchurus brasiliensis (Sciaenidae) density: An GAMLSS application

The hypothesis of this study was based on the premise that complex interactions between biological and environmental factors may explain variability in fish population dynamics. To test this, Paralonchurus brasiliensis (Sciaenidae), a keystone species associated with coastal waters, was used as a model fish species. Generalized Additive Models for Location, Scale and Shape were performed to evaluate relationships among biological and environmental variables. Fish biomass, repletion index (RI), hepatosomatic index (HSI), condition factor (K) and sex ratio was in models as biological factors whereas water temperature, salinity, organic matter, grain size and depth were included as environmental variables. The results revealed significant effects of biological and abiotic factors on population density of P. brasiliensis. The selected model explained 94% of the variability in the explanatory variables. Fish biomass, HSI and K were the most important biological factors, whereas temperature and organic matter were the most important abiotic factors. Variability in fish density suggests that HSI and K both could be used as an additional ecosystem indicator of environmental status (food availability) and fish population health. We also highlight the importance of using robust and flexible statistical methods to model fish population dynamics and the need to incorporate biological and environmental factors into fishery resource management.

Sea temperature and habitat effects on juvenile reef fishes along a tropicalizing coastline

AbstractAimTemperate marine systems globally are warming at accelerating rates, facilitating the poleward movement of warm‐water species, which are tropicalizing higher‐latitude reefs. While temperature plays a key role in structuring species distributions, less is known about how species’ early life stages are responding to warming‐induced changes in preferred nursery habitat availability. We aim to identify key ecological and environmental drivers of juvenile reef fishes’ distributions in the context of ocean warming.LocationSouth‐eastern Australian coastline from 30 to 37°S.MethodsWe used a decade of underwater visual census data to uncover latitudinal distribution patterns of juvenile reef fishes and habitats across 1000 km of coastline, from subtropical to temperate latitudes. We modelled how benthic habitat cover, depth, wave exposure and sea surface temperature influence distributions of warm‐water and cool‐water juvenile reef fishes on temperate rocky reefs.ResultsWe found sea surface temperature was typically the most important factor influencing densities of juvenile fishes, regardless of species’ thermal affinity or latitudinal range extent. Juveniles of tropical and subtropical range‐expanding fishes responded more strongly to warmer temperatures and lower wave exposure, while juveniles of temperate species responded more strongly to benthic habitats. Species’ responses to greater availability of temperate reef habitat‐formers such as kelp and other macroalgae contrasted, being positive for temperate and negative for tropical and subtropical juvenile fishes.Main conclusionsThe availability of both suitable habitat and sea temperatures for species’ early life stages is important considerations when predicting changes in reef fishes’ distributions in the context of ocean warming. Warming‐induced isotherm shifts and feedback loops constraining the persistence of key temperate reef habitat‐formers will favour range‐expanding tropical reef fishes colonizing higher‐latitude reefs, while disadvantaging some macroalgal‐associated resident temperate species. Such varying responses to warming‐induced environmental changes may strongly influence the structure of emerging tropicalized reef assemblages.

Strong positive relationships between eDNA concentrations and biomass in juvenile and adult pike (Esox lucius) under controlled conditions: Implications for monitoring

AbstractReliable abundance information is the foundation for managing aquatic resources. Species with low catchability are, however, often overlooked in monitoring programmes. Thus, governing bodies lack the data necessary to make well‐informed management decisions. Environmental DNA (eDNA) can produce quantitative estimates of fish abundances, but the precision varies greatly depending on the species and system. It is, therefore, necessary to evaluate its performance and investigate how fish biomass and density affects eDNA dynamics on a case‐by‐case basis before eDNA‐based monitoring can be a viable option. Here, we evaluate how biomass and density of an ecologically and socioeconomically important top predator, the Northern pike (Esox lucius), relate to eDNA concentrations in controlled aquarium and mesocosm experiments. We carried out experiments using both juvenile and adult individuals and evaluated eDNA, biomass and density relationships at three different time points using a previously developed TaqMan assay, targeting the cytochrome oxidase I gene. We also evaluated the performance of multiple extraction methods (DNeasy Blood & Tissue kit, DNeasy PowerWater kit, and Chelex 100), and filtering systems (single‐ vs. double‐membrane filters). The results from both pike experiments showed a strong positive linear relationship between eDNA concentration and pike biomass (R2 = 0.74 – 0.87). Levels of eDNA dropped drastically within the initial 24 h of juvenile pike being removed from the aquaria, and low levels were detectable for up to 308 h. Of the extraction methods, Chelex 100 yielded the highest DNA concentration, offering a quick and cost‐effective alternative compared with existing widely used extraction methods. Using double membrane filters of different material showed no increase in DNA yield regardless of the extraction method but it allowed more water to be processed. Although several challenges remain, our results show that eDNA holds promise to become a useful tool for monitoring fish biomass in natural environments.

Differential Responses of Native Fishes in Two Headwater Tributaries of the Gila River Following Severe Wildfires

High-severity wildfires are becoming increasingly common across the American Southwest, and knowledge of how these fires affect native organisms is essential for their conservation. We evaluated changes in fish densities and habitat over 7 and 23 years in 2 tributaries of the Gila River, New Mexico, that experienced large wildfires. The Miller Fire affected Little Creek in 2011 and was followed by moderate monsoonal flooding. The Silver Fire affected Black Canyon in 2013 and was followed by a large monsoonal flood. These 2 headwater streams responded differently to wildfire. Influx of sediments reduced stream depth by 59% and increased fine substrates by 51% following the fire in Black Canyon, while these parameters were relatively unchanged by fire in Little Creek. Native fish densities declined to zero immediately following the wildfire and monsoonal flooding in Black Canyon, and recovery was slow (∼8 years). In contrast, Little Creek fish density declined marginally following wildfires and returned to near prefire levels within one year. The response to wildfires at these 2 locations illustrates how the interaction of wildfire characteristics, catchment features, and post-wildfire precipitation events influence the impact of wildfire disturbance of stream ecosystems.