Environmental Biology of Fishes

Stoplight parrotfish, Sparisoma viride, are hermaphroditic fish that exhibit a complex social structure in which terminal phase (TP) males control a territory of initial phase (IP) individuals (mostly female, rarely male) called a harem. These fish are prolific herbivores that maintain the health of coral reefs by consuming or removing algae that competes with coral for space. In this study, we estimate the influence of food availability, structural complexity, and conspecific density on territory size in S. viride TP males on reef sites in the middle Florida Keys. Divers estimated the territory sizes of both TP and IP individuals by following fish and dropping markers. Divers also estimated the substrate composition, conspecific density, and physical complexity around these territories. TP territories were roughly circular and only overlapped with IP individuals on the outer edges of the TP territory, resulting in TP territories being significantly larger. While TP territory size was positively influenced by the body size of the TP male and negatively influenced by the number of IP individuals, the average ratio of IP:TP individuals was 2.99:1 (± 0.31 SE, range 1–12) for each TP territory regardless of body size. These results suggest that TP males adjust their territory sizes to maintain an apparent optima for the number of females within harems and that the density of conspecifics can potentially influence the timing of sexual transitions in these fish. These results also suggest that S. viride have a polygynous mating structure that is driven by female defense rather than resource defense.

Bonefish are members of one of the oldest extant teleost superorders, Elopomorpha, which also includes eels, tarpons, and ladyfish. All share a common larval stage called the leptocephalus or “glass eel” stage. Though bonefish serve an important role in Caribbean ecosystems and fisheries, little is known about their life history and reproductive process. In order to observe and better describe bonefish spawning characteristics, wild bonefish (Albula goreensis) were collected off of Stuart, FL, USA. The group was transported to Harbor Branch Oceanographic Institute and acclimated to captive habitats where they underwent monthly hormonal manipulation to induce spawning. Spawning events occurred on October 4 and 6, 2020. For both spawns, eggs were collected in an egg collector and transferred to Kreisel tanks to hatch. Samples of eggs were collected prior to hatching to determine biometrics such as size, fertilization success, and hatching success. Leptocephalus larvae were collected daily for up to 3 days post-hatch, and biometric data was recorded for a subsample of larvae. Total fatty acids were extracted from egg and larvae samples and analyzed using GC–MS detection methods. Larvae samples showed a general decrease in polyunsaturated fatty acids (PUFAs) coinciding with growth after hatching, supporting the notion that PUFAs are utilized for growth and metabolic processes. This project is part of a multi-year effort to successfully spawn bonefish in captivity and can be utilized as a baseline in future studies intended to inform conservation methods for wild populations.

Annual killifish live in temporary pools which completely lose water during the dry season. They bury their eggs in the muddy bottom where they stay through the entire dry season. The development of killifish embryos involves three strictly defined stages, where the embryo may go dormant and stop developmental progression. The system of the three facultative diapauses — diapause I, II, and III — was described in the 1970s and remained supported until now. Here, we report on the potential of the embryos of an African killifish species Nothobranchius furzeri Jubb to enter an additional developmental stasis occurring in between the stages defined for the diapause II and III. The stasis manifests as a full developmental halt in a stage previously regarded as non-diapausing or a markedly slowed down developmental rate. It occurs under usual laboratory incubation conditions, and the embryos that entered the stasis are capable of normal hatching after they exit the dormancy and complete their development. The aim of the study is to inform on the increased, cryptic complexity of the killifish diapause system while we discuss the possible reasons why the stasis, arguably occurring with some regularity, remained unnoticed for so long.

Oxydoras kneri and Hemisorubim platy-rhynchos are catfish species of high ecological and commercial relevance from the Paraná River (La Plata Basin, South America) whose life history remains unknown. The aim of this study was to reveal the nursery areas and describe the habitat use throughout the ontogeny of both catfish species from the Paraná River using the oto-lith 87 Sr/ 86 Sr ratio. Otolith core-to-edge 87 Sr/ 86 Sr analyses were measured using a LA-MC-ICPMS system. A Quadratic Discriminant Analysis (Wilks' Lambda = 0.029, p < 0.0001) based on the water 87 Sr/ 86 Sr baseline of the La Plata Basin was run to infer the natal origin using the core 87 Sr/ 86 Sr ratio as an independent variable. Oxydoras kneri was classified as originating from Para-guay (35.7%), middle Paraná (57.1%), and upper Paraná (7.1%) rivers, while H. platyrhynchos was classified as originating from the middle (56.3%) and upper (43.7%) Paraná River. Otolith 87 Sr/ 86 Sr time series suggested a high complexity in the stock structure of both species from upper Paraná River, with a significant contribution of specimens originating in other areas. Both species showed isotopic variations in their otoliths compatible with short-and medium-scale movements between different reaches of the Paraná and Paraguay rivers that could exceed 200 km.

The shovelnose sturgeon ( Scaphirhynchus platorynchus ) and endangered pallid sturgeon ( S. albus ) deposit demersal and adhesive eggs in swift currents, near or over coarse substrate. Hydrographic surveys have demonstrated the dynamic nature of spawning habitats and that coarse substrates may episodically be buried (partially or completely) by fine sediments. To evaluate embryo survival of both species in various substrate conditions, laboratory trials were conducted with substrates of clean glass, gravel, medium-coarse sand (MCS), and fine sand-silt (FSS). Embryos in MCS and FSS were tested three ways: unburied, partially buried, and fully buried (1–2-mm depth). Embryos were exposed to trial conditions for 10 days from the day of fertilization (5 days beyond expected hatching). For both species, mean hatch of normally developed free embryos was highest in unburied treatments where embryos were incubated on substrates and not covered with sediments and ranged from 81.0 to 87.1% for shovelnose sturgeon and 55.2–80.0% for pallid sturgeon. Mean hatch of normal free embryos was lowest where incubating embryos were fully buried by MCS or FSS and ranged from 2.4 to 11.6% for shovelnose sturgeon and 4.8–15.2% for pallid sturgeon. We observed free embryos with physical abnormalities in all treatments; however, the occurrence was most variable in treatments fully and partially buried by MCS. Hatch of both species was also delayed in treatments where embryos were incubated fully and partially buried by MCS. Our results may be useful to estimate the relative suitability of spawning substrates in relevant river reaches.

The impacts of mine contaminants on ecological connectivity in rivers and streams are poorly documented globally. We used acoustic telemetry to evaluate and refine conceptual models of fish movement in Magela Creek, a stream in the wet-dry tropics of Australia. This creek receives wastewater discharge from a nearby uranium mine, and a secondary objective was to describe behavioural responses of fish to one such discharge event. Of 55 fish (black bream Hephaestus fuliginosus, saratoga Scleropages jardinii, sharp-nose grunter Syncomistes butleri) tagged in dry season refuge pools 18 km upstream of the mine lease area (RPA [Ranger Project Area]), 16 (29%) moved downstream after the first wet season flows, using the RPA as habitat for 3–5 months before moving upstream to their previous locations as flows receded. Of 39 fish (spangled perch Leiopotherapon unicolor, barred grunter Amniataba percoides, black catfish Neosilurus ater) tagged ~ 8–12 km downstream of the RPA in the late wet season, only two were subsequently detected in the RPA. Direct and camera-based observations of 12 species of upstream-migrating fish during mine-water discharge in the late wet season showed no evidence of mine-water avoidance. Our results demonstrate that Magela Creek provides wet season habitat for fish within the RPA and acts as a migration pathway that connects lowland reaches and floodplains to upstream dry season refuges. Use by fish of waterbodies within the RPA highlights the need to manage the site to ensure that future contaminant egress and water quality do not adversely affect fish migration and habitat suitability.

Coastal marine fisheries and the habitats that support them are under extensive and increasing pressures from numerous anthropogenic stressors that occur at multiple spatial and temporal scales and often intersect in unexpected ways. Frequently, the scales at which these fisheries are managed do not match the scales of the stressors, much less the geographic scale of species biology. In general, fishery management is ill prepared to address these stressors, as underscored by the continuing lack of integration of fisheries and habitat management. However, research of these fisheries is increasingly being conducted at spatial and temporal scales that incorporate biology and ecological connectivity of target species, with growing attention to the foundational role of habitat. These efforts are also increasingly engaging stakeholders and rights holders in research, education, and conservation. This multi-method approach is essential for addressing pressing conservation challenges that are common to flats ecosystems. Flats fisheries occur in the shallow, coastal habitat mosaic that supports fish species that are accessible to and desirable to target by recreational fishers. Because these species rely upon coastal habitats, the anthropogenic stressors can be especially intense—habitat alteration (loss and degradation) and water quality declines are being exacerbated by climate change and increasing direct human impacts (e.g., fishing effort, boat traffic, depredation, pollution). The connections necessary for effective flats conservation are of many modes and include ontogenetic habitat connectivity; connections between stressors and impacts to fishes; connections between research and management, such as research informing spawning area protections; and engagement of stakeholders and rights holders in research, education, and management. The articles included in this Special Issue build upon a growing literature that is filling knowledge gaps for flats fishes and their habitats and increasingly providing the evidence to inform resource management. Indeed, numerous articles in this issue propose or summarize direct application of research findings to management with a focus on current and future conservation challenges. As with many other fisheries, a revised approach to management and conservation is needed in the Anthropocene.

Understanding the effects of species invasions on vulnerable freshwater habitats is crucial to protecting those ecosystems. The vermiculated suckermouth sailfin armored catfish, an abundant, invasive grazer in Florida springs and around the world, may interact with native communities in complex ways, including alteration of nutrient availability, which in turn may influence primary production, with cascading effects through the food web. Armored catfish may differ from native fishes in their body nutrient content because of their high proportion of bony material; therefore, increasing densities of armored catfish may shift nutrient availability to primary producers due to their consumption, excretion, and egestion of nutrients. To evaluate this, we measured N:P ratio of catfish tissues compared to the N:P ratios of two abundant native species and found that catfish had a significantly lower body N:P than native species. We also measured the N and P in catfish gut contents and excreta and found that posterior gut (egesta) N:P was significantly higher than anterior gut (ingesta) N:P. Finally, we measured excreted N and P, finding that excreted N:P was high. These data demonstrate that, because high proportions of P are retained in the bodies of invasive armored catfish, they are a P sink and are, therefore, more likely to limit P availability than native fishes in freshwater spring ecosystems both in Florida and worldwide.

Greater amberjack are active swimmers that occur over a range of depths around reefs and artificial structures. In this study, we describe overall diel patterns of depth use and swimming activity for greater amberjack. We tested for impacts of fishing-related stress on diel patterns, and we describe the potential influence of cyclonic storms on diel behavior and site attachment. Fifty-five fish were tagged with depth and accelerometer tags off coastal Alabama, USA, in three field efforts (FEs): FE1, summer–fall 2018; FE2, spring 2019; and FE3, summer–fall 2020. Fish displayed diel depth patterns that varied among individuals: fish that occupied a narrow depth range midway within the water column, fish that spent time both at mid-water and near bottom, and fish with more generalized patterns. Diel depth use was best predicted by both site depth and field effort, while swimming patterns were best predicted by field effort among tested factors: legal or sublegal sizes, field effort, site, and use of a descender device vs. surface release. Swimming activity was elevated on post-release days 1–5, consistent with predicted impacts of fishing stress. For most fish, however, depth use did not vary appreciably following release. Release with a descender did not prevent altered diel patterns, and thus, barotrauma was not likely a factor. Fish occupied shallower depths when Hurricane Laura (2020) passed south of the study area, and swimming activity was elevated during Hurricane Sally (2020). Our study highlights that depth use and swimming activity of greater amberjack are influenced by multiple factors and recreational fishing discards may impart sublethal stress that results in elevated swimming activity.

A telemetry study was conducted on lake sturgeon to determine the seasonal use and environmental factors that stimulate movement within a tributary of a larger system. Over a 10-year period, 52 lake sturgeon implanted with acoustic transmitters were monitored through an array of Vemco receivers with the Blanche River, Ontario. An average of 68% of the tagged fish entered the tributary annually whereas only 19.2% of those were detected on known spawning areas approximately 54 km upstream. Water temperature, lunar phase, lake water level, presumed spawning fish, and water discharge were all considered significant variables associated with the onset of upstream migration. Water temperature (mean 12.9 °C) was the only variable that explained the timing and the arrival at the spawning grounds. Sturgeon remained at the spawning area for a mean duration 7.9 days; however, none of the variables examined explained the duration of stay. Water temperature was the only variable that explained departure from the spawning areas (mean 15.8 °C). Mean time spent in the river after spawning was 30.5 days. There was not a significant difference in sex or total length in relationship to the duration tagged fish stayed in the river after spawning. The entire Blanche River up to the insuperable rapids was used by the sturgeon with the exception of minor tributaries. Water temperature, river discharge, and fish size were all considered significant factors initiating out-migration back into the lake. This study demonstrated the importance and seasonal use of a tributary by lake sturgeon. Correlations from abiotic variable and seasonal response could be used to establish guidelines for water management purposes when the goal is to conserve or restore sturgeon populations within a river system. This study also illustrates the importance of tributaries in association to a larger waterbody for lake sturgeon.

Both ecological and anthropogenic changes constantly shape the form, function, and longevity of tropical marine ecosystems. With emerging technologies in remote sensing and Geographic Information Systems (GIS) analysis, these changes can be accurately and efficiently monitored to provide actionable information to help shape the future of marine protected areas (MPAs). The South Water Caye Marine Reserve (SWCMR), an MPA in southern Belize, contains the largest unbroken stretch of the Belize Barrier Reef as well as many small mangrove islands, patch reefs, and seagrass beds that sustain myriad threatened and economically important fisheries. While this is an important marine reserve, no study has documented change within it. This study fills this knowledge gap by applying land use and land cover (LULC) change analysis methods to 3- and 5-m resolution satellite images from Planet taken in 2010 and 2019 to measure changes in open water, seagrass, terrestrial vegetation, and developed and barren land cover within the SWCMR. The results of this study have an accuracy of at least 80%. They reveal decreases in seagrass cover and increases in the developed and barren areas in and around the SWCMR. While greater amounts of change are found outside the marine reserve boundaries, within the SWCMR, the total area of developed and barren land cover increased by 203.77% and 70.97%, respectively, from 2010 to 2019. Documenting these changes in the SWCMR establishes the reserve as a model for understanding the tropical marine environments throughout Belize and provides valuable data for management decision-making.

Sneaking is a parasitic reproductive behavioral tactic for stealing fertilization opportunities from other males that monopolize females; therefore, sneaker males always face sperm competition. The timing of sneaking is an important trait that contributes to their fertilization success. In the dusky frillgoby Bathygobius fuscus, an intertidal gobiid fish, spawning between a nest-holding male and a paired female continues over several hours. We investigated the behavior of sneaker males of this species in the wild by examining the number of sneaker males around the nest, aggressive interactions between sneaker males, sneaking attempts, and sneak intrusions as indices of their sneaking motivation. Pair spawning lasted for 23.7-192.8 min, during which a maximum of 0-23 sneaker males were observed around the nests. All indices of sneaking motivation, except the number of sneak intrusions, significantly decreased with time elapsed since the pair spawning began. This suggests that sneaking motivation was high at the early stage of pair spawning. Early sneaking may contribute to high fertilization success of sneaker males, likely owing to the long-surviving effect of their long-lived sperm.

Microspathodon dorsalis is a conspicuous territorial species found in rocky reef systems along the Eastern Pacific whose mating system is not fully understood regarding aspects such as nest distribution, egg predation patterns, and overall reproductive-related behavior. Spatially explicit models indicate that individuals and their nests correlated with substrate structure, principally rocks followed by live Pocillopora spp. corals. Nest size was not related to fish size in the center and along the periphery of the nesting colonies, suggesting that female did not choose males based on size. Up to 11,792 interactions (536 ± 2256.97 interactions/nest) at a rate of 5.95 (± 0.47) interactions/minute were recorded, most of which resulted in unsuccessful predation attempts (63%), while the remaining percent (37%) resulted in successful predation attempts. Thalassoma lucasanum fed heavily on M. dorsalis eggs, followed by Stegastes acapulcoensis and Johnrandallia nigrirostris. The frequency, interaction type, and interacting species varied across space and time, probably due to the spatial location of the nests and the distribution patterns of the interacting species, as well as a consequence of the seasonal environmental variation of the area.

Although many shark species display predictable and seasonal aggregations related to breeding activities, few studies have reported direct evidence of reproductive interactions of large sharks. In this context, the present study reports the first evidence of mating scars in female tiger sharks Galeocerdo cuvier at the Fernando de Noronha Archipelago (FEN), an oceanic remote insular system in the South Atlantic Ocean. Results from the records of seven wounded females suggest that mating activity may occur between February and August. These females measured between 267 and 372 cm in total length, which is aligned with previous studies reporting size at sexual maturity for Atlantic tiger sharks. Although some females exhibited healed wounds, three of them had fresh, open wounds. One of the females also had a swollen and red cloaca. We also report the first presumably gravid female tiger shark for the region. Although sporadic, the observations herein reported indicate the importance of FEN for tiger shark reproduction in western equatorial Atlantic waters. This finding is especially relevant as FEN has been identified as an important genetic hotspot for tiger sharks, with the largest global genetic diversity for the species.

Economic impact assessments (EIAs) have long been used to estimate the economic benefits of human activities. More recently, environmental economists have applied EIAs to demonstrate the benefits of conservation, including assessments of recreational fisheries. Here, we summarize findings of EIAs of the recreational catch-and-release flats fishery in The Bahamas, Belize, and the Florida Keys (USA). Results show high economic impact, often twice that of other tourism activities, for a mostly catch-and-release fishery with high post-release fish survival rates. The EIAs have already provided leverage for improved management, including spatial protections and catch-and-release designations. The EIAs, in combination with data on fish ecology, are useful for formulating effective conservation strategies and should be applied more widely. Here we report these EIAs as case studies and propose applicability for recreational fisheries in other regions.

On 11 March 2011, the Pacific coast in northeastern Japan was affected by the tsunami and land subsidence caused by the great east Japan earthquake. The disaster brought about various major environmental changes to the region. In our sampling site, examined over the period 2007–2015, the Zostera seagrass bed covered an area of approximately 3000 m2, but almost all of the shoots of the seagrasses were washed away by the tsunami. However, the seagrasses started to grow again in the summer of 2011, and the seagrass bed area expanded to approximately 3400 m2 by August 2014. Underwater observations revealed that the process of expansion resulted from regrowth from survived rhizomes and from seeds which were produced after 2012. A total of 28,969 fish larvae and juveniles, representing at least 65 species, were collected from 2012 to 2015, which showed a change in the composition of the dominant species. However, based on Jaccard’s coefficient of community, the species composition observed in 2015 returned to almost the same composition as observed before the tsunami. This implies that the seagrass formed a key habitat for the fish faunal recovery. Furthermore, it is implied that the occurrence of some transient species and casual (intermittent occurrence) species in the seagrass bed were associated with increase of these fishes in areas near to the seagrass bed. On the other hand, some species which occurred intermittently before the tsunami were not sampled or decreased after the tsunami.

Within-clutch variation in the size or the amount of yolk in the egg has been widely studied because of its importance for both maternal and ofspring ftness. In particular, egg size has been used as a proxy for survival in first-feeding salmonids alevins. We tested the hypothesis that egg size has implications for the phenotypes (development, physiology, and behavior) of juvenile steelhead trout (Oncorhynchus mykiss) up to 1 year post fertilization in both hatchery and wild fish. Egg diameter was measured for 38 families; the smallest and largest eggs were selected and reared individually to study embryonic development, and then collectively as “small” and “large” egg groups. The development and growth of individual embryos were followed up to first feeding. We showed that hatchery fish have smaller eggs than wild fish, offspring from small eggs grow better during their first year than those from larger eggs, offspring from large eggs hatch later and at a more advanced stage than those from small eggs, and the degree of smolting and saltwater preference differed between hatchery and wild offspring. By looking at not only mean egg size but also individual mediated– egg size phenotypic traits, we show the significant contribution of egg size in promoting phenotypic variation within a population and its potential for among population variation.

The Hawaiian sandburrowers Crystallodytes cookei and Limnichthys nitidus were found to be protandrous hermaphrodites based on a histological examination of gonad morphology and development. The majority of individuals of both species (71% and 75%, respectively) had delimited ovotestes in which ovarian and testicular tissue were divided by a connective tissue barrier. In juveniles and functional males, the ovarian and testicular regions were similar in cross-sectional area, whereas in functional females, the testicular portion was absent or greatly reduced. Reproductive females were significantly larger in body size than functional males but did not differ significantly in size with transitionals (individuals which contained both developing ova and visible spermatozoa). The complete absence of functional females at the smallest size ranges suggests that both species are monogynic; all females are derived from previously mature males. The ovotestis morphology and sequence of gonad development present in C. cookei and L. nitidus are most similar to protandrous porgies (family Sparidae). When compared to sex changing species of the closely related genus Trichonotus (family Trichonotidae), the distinct ovotestis morphology (delimited in creediids vs. mixed in Trichonotus) and differing direction of sex change (protandry vs. protogyny) suggest that sex change evolved independently in these taxa.

Pollution is one of the most relevant issues for the conservation of freshwater environments. Fishes are the main group of organisms studied in aquatic toxicology, and their behaviours, including feeding, courtship, parental care, and territoriality, are essential to their health. Cichlids are one of the most abundant fish families and are both economically and ecologically relevant. In this review, we found that metals, pesticides, pharmaceutical products, and endocrine disruptor chemicals are the main groups of pollutants that have been studied in the context of cichlid behaviour. African tilapias are the most evaluated cichlids on this topic, and Nile tilapia was the most frequently studied species. Exposure to these pollutants has been shown to affect cichlid fish behaviours in aspects including erratic swimming, activity versus inactivity, preference for the bottom and/or the surface of the tank, aggressiveness, feeding, predation, and reproduction. Oxidative stress, failures in neurotransmission, energetic imbalances, and histopathological damage are among the most common mechanisms underlying the behavioural changes. It is necessary to establish a set of integrative criteria for evaluating the ecotoxicological behaviour of cichlid fish, given the current lack of standardised methods. It is also important to consider more pollutants—both individually and in combination—such as polyaromatic hydrocarbons, flame retardants such as polybrominated diphenyl ethers, perfluoroalkyl substances, and microplastics, due the increasing relevance of these pollutants of emerging concern. In line with this, cichlid fish species with reduced native distributions are suitable study species to provide information for the conservation of freshwater environments.

Climate models suggest that by 2100, maximum temperatures where many larval Pacific lamprey, Entosphenus tridentatus , rear now may approach 27–31 °C. Little information exists on whether larval Pacific lamprey can tolerate these temperatures. We used acclimated chronic exposure (ACE) and direct acute exposure (DAE) experiments to determine the water temperature that is lethal to larval Pacific lamprey and whether sublethal water temperatures influence larval burrowing behavior. After 30 days in ACE experiments, all larvae survived in temperatures averaging ≤ 27.7 °C, no larvae survived in temperatures averaging ≥ 30.7 °C and the ultimate upper incipient lethal temperature (UILT) was estimated to be 29.2 °C. After seven days in DAE experiments, all larvae survived in 27.1 °C, only larvae acclimated to 23.3 °C survived in 29.1 °C, and no larvae survived in temperatures ≥ 30.6 °C. Using a time to death analysis, estimates of the UILTs ranged from 27.5 to 30.2 °C and the ultimate UILT was estimated to be > 28.3 °C. Using a percent mortality analysis, estimates of the ultimate UILT were not definitive but indicated it may be > 30.2 °C. To burrow, larvae reared for 30 days at temperatures averaging 26.9–27.7 °C exhibited more total time, active time, and stops than those reared for 30 days at temperatures averaging 22.3–23.9 °C. Our findings suggest that larval Pacific lamprey may be resilient to the most likely temperature increase scenarios predicted by climate models. However, they may be vulnerable to the high-temperature increase scenarios and, relatively high but sublethal temperatures may impact the behavior, and ultimately survival, of larval Pacific lamprey.

Acute stress response in the European bitterling, Rhodeus amarus, to glochidia parasitism by the invasive unionid mussel, Sinanodonta woodiana, was quantified by analysing cortisol plasma levels using liquid chromatography coupled to mass spectrometer. We tested a novel method which required as little as 4 µL of plasma by increasing the volume using charcoal-treated plasma. Bitterling were observed to increase cortisol levels significantly in response to glochidia infestation. Overall, this technique allows the precise measurement of steroid hormone plasma concentrations in small fish.

Arctic charr ( Salvelinus alpinus [L.] complex) has been widely used as a model system for studies in evolutionary ecology because of its diversity in feeding ecology, habitat use, life history forms, and associated morphologies observed in matured individuals. However, we still know relatively little about traits exhibited early in life of the species, although the trait diversity of matured individuals may largely be shaped during development. Egg size is a key determinant of various traits exhibited early in life. Therefore, describing egg size variation within- and between-individuals as well as the link between egg size and adult traits will be a useful step in understanding the early life trait diversity of Arctic charr. Here, using Greenlandic Arctic charr, which includes alternative life history forms (i.e. anadromous and resident) and spawning habitat use (i.e. lake and river spawner), we described egg size variation (i.e. clutch-mean egg diameter and within-clutch variation) and explored the link between egg size variation and female body length, life history form, and spawning habitats. As in many other fishes, clutch-mean egg diameter increased with female body length. No significant effect of other female traits on clutch mean-egg diameter was detected, suggesting that female body size variation could be a direct cause of early life history trait variation. On the other hand, we found that the degree of within-clutch variation of the anadromous life history form was higher than that of the resident life history form. The pattern could be interpreted in an adaptive context. For instance, given that the anadromous life history form tends to be semelparous, anadromous females could decrease the likelihood of complete reproductive failure by producing variable-sized offspring within a clutch since at least some offspring are expected to be matched to the prevailing environment.

Much new information on the movement ecology of fishes is present in this Special Issue of Environmental Biology of Fishes. Authors submitted manuscripts in response to now-deceased Editor-in-Chief David Noakes’ call for papers under the theme of “fish movement: molecules, models, and migration.” In a little over a year and a half, 30 papers were submitted on the molecular biology, behavior, physiology, and ecology of bony and cartilaginous fishes (and even a jawless fish) that inhabit both fresh water and salt water. Briefly summarized in this introductory article are the findings of the authors published in this Special Issue, organized under six main themes—movement patterns, innovative techniques, anthropogenic disturbances, behavior and environmental relationships, predictive models, and management and conservation biology, and suggestions are offered for future research. We are pleased to see David’s goal of integrating information at every scale of research, from genomics to theoretical models and landscape ecology, realized in this multi-disciplinary compilation. We dedicate this Special Issue to our fond remembrance of his friendship and his long-term leadership in ichthyological science.

There are millions of built structures existing today in thousands of rivers. While these structures provide important services to society, e.g., power, transportation, and water for drinking and irrigation, the structures are not without consequences for provisioning the whole of a rivers’ goods and services. A major issue for these structures is their creation of barriers for fish passage. While most provide some form of fish passage, the solutions are restricted to economically important species and barriers in isolation. We are slowly accepting that there are broader ecological consequences of barriers and more holistic approaches are emerging for the planning and managing created barriers in river ecosystems. We develop a holistic and adaptive, fish passage decision-making framework that uses key science questions to inform and support the development of successful fish passage management plans for a barrier and the river ecosystem. The framework builds from the biological needs of fish for functional passage, which can then support the complex social and economic considerations that are entwined in a comprehensive management plan. The framework uses a multi-species, ecosystem focus, embraces uncertainty, and embraces an adaptive approach. We recognize this approach advocates for a paradigm shift in fish passage decision making and management, but cracks in the old paradigm are emerging, and it is imperative that operators, regulators, rightsholders, stakeholders, and science keep working together to build this new paradigm that embraces a whole ecosystem approach.

Realized thermal niche and habitat use are two conceptualizations of fish habitat based on organismal performance or lake-specific ecology, respectively. Both habitat types were compared for lake trout (Salvelinus namaycush) and brook trout (S. fontinalis) co-occurring in four large (> 500 ha) oligotrophic lakes. Lakes were partitioned into two morphological categories based on possession of a central or non-central deep basin with corresponding differences in adjoining shelf areas. Lake asymmetry in basin location has been shown to strongly influence food web connections based on isolation of basins from shelf areas. Generally, overlap between both habitat types occurred in several comparisons with lake trout, suggesting that thermal habitat is a reasonable proxy for habitat use boundaries though not a full replacement for insights gained from habitat use models. For brook trout, overlap was not as consistent especially for lakes with non-central basins. In central basin lakes, there was closer proximity between the two species and overlap in both thermal niche and habitat use models. There was very limited overlap of either habitat type in lakes with non-central basins. Further, there was no shared areas of interspecific overlap between thermal niche and habitat use in non-central basins pointing to additional complexity governing habitat partitioning between lake trout and brook trout in these types of lakes. The shelf area effect on spatial structure of habitat, and likely food web connections, can occur in lakes regardless of basin centrality so long as shelf areas are large. In this lake set, lakes were sufficiently large to observe this phenomenon.

The genus Bothus includes small flatfishes classified as left-eyed flounders. Within the Gulf of Mexico (GoM), B. ocellatus and B. robinsi are the most widely distributed species, whose differentiation using morphological characters has been a challenge limiting their accurate identification. Larvae were sampled during two summer seasons (2015 and 2017) in the GoM. Morphological characters allowing identification of both larval species were limited, so they were classified as Bothus spp. Two morphotypes were observed based on the melanophore’s presence on the tip of the caudal fin. Species identification was made using a portion of the cytochrome oxidase I (COI) gene. COI sequences for 126 larvae were identified using Basic Local Alignment Search Tool (BLAST) searches of the National Center for Biotechnology Information (NCBI) GenBank and the Barcode of Life Data (BOLD) System repositories. COI sequences for larvae and reference sequences from databases were used to build phylogenetic trees and perform genetic distance analyses. COI analyses identified 21 larvae as B. ocellatus and 105 as B. robinsi. Both morphotypes were detected in both species and no diagnostic characteristics were observed. The geographic distribution of the species was similar, but B. robinsi abundance was higher than B. ocellatus in contrast to findings of previous studies. These differences between the results in this study and those of previous ones could have been due to errors in identifications based solely on morphological characters. Therefore, the results obtained demonstrate the power of molecular approaches to improve accuracy of the information generated for Bothus genus larvae.

Nearshore fish communities in marine flats ecosystems are recognised as being key for early life stages of socio-economically valued fish species, as well as small-bodied forage fishes, yet little is known about the spatial ecology of these fishes. Recent advances in acoustic telemetry have allowed for the tagging of small fish. Here, we used the smallest commercially available acoustic transmitter to tag and track several juvenile or small-bodied species of a flats fish assemblage in Rock Sound, Eleuthera, The Bahamas. Fish species tagged included juvenile bonefish (Albula vulpes; n = 2), juvenile great barracuda (Sphyraena barracuda; n = 22), juvenile redfin needlefish (Strongylura notata; n = 21), and yellowfin mojarra (Gerres cinereus; n = 20), and their movements were recorded by twenty hydrophone receivers deployed in nearshore flats habitats extending ~ 3 km along the shoreline. Yellowfin mojarra had the highest site fidelity and were detected most commonly during diurnal periods. Juvenile bonefish had the lowest site fidelity and travelled throughout the array area, primarily detected at night (albeit sample size was low). Juvenile barracuda and juvenile redfin needlefish were mobile but tended to spend the majority of their time near several receivers. Juvenile barracuda were least present during the morning but were detected during all other times of the day. Similarly, juvenile redfin needlefish had the lowest residency during the morning and were more resident during the other period. Some of the space use patterns observed appeared to be correlated with water temperature (e.g. for barracuda there were more detections at warmer water temperatures). This preliminary study reveals that it is possible to tag and track small flats fishes which opens the door for longer-term and more fine-scale (e.g. with 2-day positioning) studies to understand habitat associations and environmental drivers of behaviour although receiver detection range was somewhat limited in these shallow and dynamic habitats.

Elasmobranch fishes (sharks, skates and rays) are some of the most morphologically and behaviourally diverse vertebrate species on the planet, demonstrating a wide repertoire of feeding strategies. The nurse shark (Ginglymostoma cirratum) is a large, widely distributed shark species which is commonly observed on tropical reefs worldwide; yet it remains vastly understudied relative to other large species. To advance our understanding of nurse shark behaviour and ecology, we used opportunistic video observations gathered throughout the islands of Turks and Caicos from September 2020 – April 2021. We made 233 observations from 78 camera deployments and identified five behaviours, four of which were attributed to foraging. Stationary feeding behaviour was most observed and was strongly influenced by habitat type with a greater number of observations occurring on sand banks relative to reefs. Unique to this study was the first empirical description of novel pectoral positioning behaviour, by which individuals can position their body relative to a food-source through mobilization of the pectoral fins. We discuss these findings in relation to mechanical processes and kinematics, and the implications of expanding our knowledge of the functional role nurse sharks play in the transfer of energy across tropical seascapes.

Many freshwater ecosystems worldwide are threatened by increasing water turbidity and extensive submerged aquatic vegetation (SAV) loss, with potential consequences on aquatic communities. Such changes in water quality and habitat features affect the relative abundance of species in the community and/or the composition of ecological guilds altering the trophic network. Here, we estimated the relationship between fish species and both SAV abundance and water quality in Lake St. Pierre (Québec, Canada), a shallow fluvial lake of the St. Lawrence River. To explain the association between the fish community and environmental variables, we performed multiple linear regressions on fish abundance, species richness, and Shannon diversity calculated at 133 stations, along a gradient of turbidity, temperature, and SAV abundance. In addition, we estimated the relationship between dominant fish species abundance and environmental variables by using canonical correspondence analysis (CCA). Species richness and abundance were positively related to SAV. Turbidity was negatively related to fish abundance, but had an unexpected positive effect on diversity (Shannon and species richness). By quantifying the association between fish species and habitat features, this study contributes to a better understanding of mechanisms structuring fish communities in changing environments.

The threespine stickleback (Gasterosteus aculeatus) is a small, mesopredatory fish that is widespread in coastal regions of the northern hemisphere. Although studied extensively as a model organism in evolutionary biology, behavioral ecology, genomics, and numerous related subfields, this research relies heavily on populations from the Pacific coastal regions of North America and Asia, and those of northern Europe. However, based on the morphology of some western Atlantic populations, the different ecological context, and the evolutionary history of the species, not all of the knowledge gained from Pacific and European lineages is likely to be fully transferrable to the populations of North America’s East Coast. Nevertheless, work in eastern Canada does suggest high levels of intraspecific phenotypic diversity and local adaptation, though much of this diversity may be under threat from climate change, altered land use patterns, and introduced species. These factors warrant a research program focused on broad sampling of previously identified populations, identifying previously undocumented populations, determining whether there are unique genetic mechanisms underlying the unusual trait combinations present in the region, and exploring novel community interactions. Such a research program would facilitate future studies in ecology, conservation, and evolution by documenting phenotypic changes and establish baselines for future work. Because the work on nearshore marine threespine stickleback populations is sparse in the western Atlantic, I focus here on freshwater populations—with the exception of a brief discussion of the “white” stickleback populations of Nova Scotia—but this is not to suggest that nearshore marine populations are not phenotypically diverse.

Phenotypic plasticity, the ability of an organism to express multiple phenotypes in response to the prevailing environmental conditions without genetic change, may result in a response to anthropogenic environmental change. Given that increasing climate variability is predicted to pose a greater risk than directional climate change, we tested the effect of a water temperature differential of 4 °C on the Arctic charr phenotypic within a single generation. We demonstrate that Arctic charr phenotype can respond rapidly and markedly to an environmental temperature cue. The plastic response to different temperature regimes comprised a shift in the mean expressed phenotype but also coupled with a reduction in the between-individual phenotypic variation in the expressed head shape. The magnitude of shape difference between temperature conditions was cumulative over time but the rate of divergence diminished as fish became larger. Overall, individuals raised in the elevated temperature treatment expressed a phenotype analogous to a benthivorous ecotype of this species, rather than that of the parental pelagic feeding form. The response of cold-water freshwater species to temperature change is likely to be an interaction between the capacity of the organism for phenotypic plasticity, the mean speed of change in the environment, and the degree of short interval variation in the environment.

Many marine fisheries rely on production and energy flow in the pelagic zone; thus, sustainable management of exploited pelagic fishes benefits from insight into temporal, spatial, and ontogenetic variability in the trophic ecology of these species. Here, we analyze stomach contents and stable isotopes to reveal spatial variability (focusing on two fishing grounds, north and south of an oceanographic barrier in the Pacific Ocean) and ontogenetic changes (contrasting immature and mature) in Southern Ray’s Bream (Brama australis) diet composition in Chilean waters. Stomach contents analysis indicated that euphausiids were predominant components of the diet in both fishing grounds and ontogenetic stages. Patterns of prey long-term assimilation, revealed in Bayesian mixing models of predator and prey isotopic values of δ¹⁵N and δ¹³C, differed from diet as indicated in stomach contents. Shrimps and crustacean larvae were more important than euphausiids in the northern and southern fishing ground, respectively. In both fishing grounds, diet shifted after maturity towards increased use of shrimps. Combining methods of stomach contents analysis and stable isotopes analysis is a powerful approach to determining predator–prey relationships and energy flow in pelagic fishes.

Salmonine fishes are generally thought not to exhibit male parental care, although brief post-spawning mate-guarding by males is known for a number of species, often for populations where eggs are subjected to intense predation (egg cannibalism). Here, detailed in situ data on the spawning behaviour of male brook charr, Salvelinus fontinalis, from a naturally reproducing lake population is used to test whether post-spawning mate guarding (i.e. paternal care) is related to perceived paternity. From video records of spawning events (n = 20), we documented a novel behaviour whereby the dominant male curtails spawning ahead of the female (by ~ 3 s) and immediately inspects the nest. Afterwards, the dominant male significantly reduces courtship behaviour (quivers) but aggressively defends the female with the same intensity as prior to spawning. The duration of mate guarding (range: 3–48 min), however, declined with increasing numbers of peripheral males that had rushed into the nest at spawning. Neither the number of active females present on the spawning grounds nor male body size influenced the amount of time dominant males spent mate guarding. Extensive revisitation (80% of visits) by males to spawning sites and to females during focal sampling bouts highlights the importance of a male’s ability to predict female readiness to spawn as a significant component of reproductive success. Thus, dominant males appear to guard females at a cost of acquiring other mating opportunities and adjust the time spent guarding depending upon their perceived paternity.

Neotropical cichlids are a group of fish among which there are species with reversed sexual dichromatism, where females have carotenoid-based coloration that males lack. Carotenoid coloration can vary depending on the environment and the distribution of other types of chromatophores in the pigment pattern. The role of thyroid hormones (THs), one of the main signaling molecules regulating fish ontogeny, in the diversity of carotenoid coloration in Neotropical cichlid females remained poorly understood. In this work, using the species Amatitlania nigrofasciata, the effect of elevated TH level on the development of reversed sexual dichromatism was shown. Increases in TH signaling in experimental fish led to heterochrony in the development of the pigment pattern and phenotypic variability with the appearance of variants of carotenoid pigmentation similar to other Neotropical cichlid species. The extent of carotenoid-based ventral coloration of females depended on TH-induced changes in the melanistic pattern. The results obtained testify in favor of the important role of the TH signaling pathway in the diversification of the carotenoid coloration of Neotropical cichlids and provide new perspectives for studying the role of plasticity in the evolutionary adaptation of cichlid fishes.

The bonnethead Sphyrna tiburo is a wide-spread, small species of coastal hammerhead shark with a long history of taxonomic uncertainty, including, according to some authors, disjoint subspecies in the western Atlantic and eastern Pacific oceans. The present study investigates the level of morphometric and genetic variation within the Sphyrna tiburo complex throughout most of its distributional range including the western Atlantic, the Gulf of Mexico, the Caribbean, and the eastern Pacific. A morphometric study, including a comparison of ampullary densities, and a molecular analysis of two mitochondrial genes, was performed. We analyzed variation in cephalofoil shape between and within lineages, based on landmarks and Fourier analysis of the silhouettes. All tests delineated discrete morphological groups, including differences of ampullae density for those lineages in which this information could be unambiguously assessed. Both morphometric and molecular analysis support the existence of three different evolutionarily significant units (two in the Atlantic: one northern and one south-central, and a third in the eastern Pacific) that deserve specific taxonomic status. We urge a deep taxonomic review of this complex to gather biological data on each lineage and develop adequate independent conservation strategies and actions to protect the various evolutionary units. Graphical abstract

Animals that congregate in large numbers to reproduce in spatially and temporally distinct locations are particularly susceptible to overexploitation. Many fishes form spawning aggregations that are intentionally targeted given ease of capture. Bonefish (Albula spp.) species aggregate to spawn and are culturally and economically important, but generally lack management such as spawning area protections to ensure that fisheries are sustainable. Here, we use Cuba as a case study to inform the development and refinement of management strategies for bonefish. Recommendations for the management of bonefish pre-spawning aggregations were based on international experiences, which have been adapted to the Cuban context from results of surveys and interviews with Cuban fisheries professionals and fishing guides. The achievability and feasibility of recommendations were further reviewed by additional experts in the field of fisheries, management and Cuban policy. The process revealed extensive data-limitations for bonefish fisheries and underscored the importance of including fishing guides, local ecological knowledge and the context of marine protected areas in Cuba for bonefish management. Recommendations include (1) initiating information exchange between Cuban management agencies and third-party institutions related to bonefish management; (2) utilizing local ecological knowledge to gather information, formulate management strategies and enforce regulations; (3) implementing spatial and temporal management measures for bonefish spawning sites; (4) using what is already in place, by protecting spawning sites in the context of existing marine protected areas; (5) collaborating with all stakeholders to manage bonefish spawning sites; and (6) reducing the commercial harvest of the species. Supplementary information: The online version contains supplementary material available at 10.1007/s10641-022-01355-0.

This account serves as the introduction to a Special Issue of the journal Environmental Biology of Fishes. It includes 18 contributions, 12 of which deal predominantly with warming, four explicitly with deoxygenation, one providing the framework, and one addressing how warming and deoxygenation—which all contributions view, implicitly or explicitly, as leading to “oxythermic” stress—are used as a pretext to cover up overfishing. The “Mean Temperature of the Catch” (MTC) concept of Cheung et al. (2013, Nature 497:365–368) inspired five of the studies involving mainly temperature, including its first applications to fresh water and to the past, i.e., to the past 130, 7–8 thousand, and 2 million years. Four contributions, jointly representing 4200 + populations and 1100 + species, deal with the effect of temperature on the maximum and/or the asymptotic length of fish behaving as predicted by the Gill-Oxygen Limitation Theory (GOLT). This theory is also evoked in one of these studies to explain how cold denaturation causes fish to grow to a smaller size when temperatures decline below 4 °C. These contributions, which are here summarized and whose conceptual affinities are also presented in graphic form as a tree-like structure, provide a basis for understanding the changes in fish community composition and size structure resulting from marine or freshwater warming. Jointly, they explain some of the changes in fish behavior and position in the water column resulting from the deoxygenation of their habitats.

David Lloyd George Noakes (1942–2020) is best known for his insatiable curiosity, his quick wit and dry sense of humor, his scientific contributions to the field of animal behaviour, and his ability to form and maintain long-lasting connections. His research interests were vast but remained grounded in early life history, behaviour, social behaviour, the evolution of behaviour, behavioural genetics, and evolutionary ecology. David had a remarkable ability to establish and maintain strong connections within the international academic community. David was also internationally recognized for his numerous contributions as a scientific editor, promoting accessibility to the international community that he served. We memorialize David’s legacy in this tribute article, ensuring that his accomplishments and the momentous impact he had on the scientific community are not soon forgotten.

Maintaining natural thermal regimes in montane stream networks is critical for many species, but as climate warms, thermal regimes will undoubtedly change. Mitigating impacts of changing thermal regimes on freshwater biodiversity requires knowledge of which elements of the thermal regime are limiting factors for aquatic biota. We used full-year stream temperature records sampled across a broad latitudinal gradient to describe the diversity of the thermal landscapes that bull trout (Salvelinus confluentus) occupy and identify potential divergences from thermal regimes where this species has been studied previously. Populations of bull trout occupied stenothermic, cold thermal niches in streams that exhibited low to moderate thermal sensitivity throughout the species' range. However, winter thermal regimes in the central and northernmost streams were colder and more stable than in the southernmost streams, reflecting differences in sensitivity to air temperature variation and contributions of perennial groundwater to baseflow. In the southernmost streams, bull trout distributions appeared to be regulated by warm summer temperatures, whereas in northern streams, unsuitably cold temperatures may be more limiting. Our results also suggest that local differences in the extent of complete freezing during winter among northern streams may further limit the distributions of suitable habitats. Contrasts in limiting factors at bull trout range extents would suggest differential responses to climate warming wherein northern populations extend their range while southern populations contract, and an overall change in species status that is less dire than previously anticipated.

Differences in environmental conditions experienced throughout development are known to produce differences in phenotypic traits. Hatchery and wild fish often express different phenotypic traits driven, at least in part, by differences in the early rearing environment. These differences can influence their ability to adapt and survive in the wild. In this study, we compared juvenile Chinook salmon (Oncorhynchus tshawytscha) that were reared at high and low density, with and without complex structure, and used a psychological test to evaluate impacts on behavior, including motivation, exploration, and what we consider to be an antipredation response. Juvenile Chinook salmon are generally motivated to be near conspecifics. In the test, fish were presented with one of three stimuli (conspecifics, structure, or nothing) across an aversive area from their starting location (start box). Fish reared in low density spent more time in the start box and had a longer latency to leave the start box (for those that left without bolting) compared to those reared in high density, regardless of the stimulus presented. However, there was no impact of structure on these behaviors, although those reared with structure were more likely to either bolt out of or stay in the start box. Rearing treatment also influenced caudal fin condition such that fish reared in low density had larger and more pointed fins than fish in high density and fish reared with structure had larger and more symmetrical fins than those reared without structure. We speculate that reduced density and rearing with structure led to fish with enhanced antipredation responses which may confer adaptive advantages if released in a novel environment and possibly improve their survival upon release in the wild.

Migration patterns and habitat use of adult barbel ( Barbus barbus ) and nase ( Chondrostoma nasus ) were monitored by radio telemetry over a period of 13 months along a 58-km-long section of the Austrian part of the Danube River. The study site is confined upstream and downstream by two hydropower plants, and contains a larger tributary, the Pielach River. Telemetry transmitters were implanted into fish caught in this tributary after spawning in June (25 individuals per species). The results show that both species use the entire available width and depth spectrum of the Danube along the full migratable river length. Nase had an average home range of 22.4 km, while that of barbel was 34.4 km. The habitat use of the two species differs significantly. While the nase was primarily encountered in the free-flowing section, barbel mainly used deep areas of the impoundment during the year. Nase showed a distinct site fidelity to certain areas in the free-flowing reach which were periodically revisited. During the spawning season, distinct homing behavior was observed in both species. All seven nase that could still be detected during the spawning season returned to the tributary (homing rate 100%). Six homing nase migrated up to the first migration barrier in the tributary but did not pass the existing fish passage facility. In contrast, only nine barbel returned to spawn in the tributary (homing rate 50%), while nine barbel were most likely using a spawning location in the head of impoundment section. Homing fish entered the tributary during darkness.

Life history traits of three commercial and recreational important black seabreams were characterized from the Ryukyu Archipelago of the Northwest Pacific. The active spawning periods were similar among three species and were determined to be between January and March, coinciding with a lower water temperature. The Okinawan yellow-fin seabream Acanthopagrus chinshira is a large-bodied species which exhibits a moderate initial growth, late female maturation, and an estimated intermediate lifespan of 15 years. The Pacific seabream Acanthopagrus pacificus is a medium-bodied species with a slow initial growth and an estimated intermediate lifespan of 20 years. The Okinawa seabream Acanthopagrus sivicolus is a large-bodied species with a rapid initial growth, early maturation, and an estimated intermediate lifespan of 15 years. The present study strongly suggested that all three species of the Okinawan populations showed protandrous hermaphroditism based on the available evidence (size and age frequencies and gonadal histology) and contributed toward the understanding of sexual systems in closely related black seabreams. Detailed age-based demography, which relates directly to productivity and recovery potential, also provided a necessary component for future management and conservation decisions: A. chinshira has a particularly high extinction risk and key approaches for the species preservation are urgently needed.

Identifying critical habitats is challenging for a relatively rare species like green sturgeon (Acipenser medirostris), which spends most of its life at sea. We used a fixed array and an autonomous underwater vehicle (Slocum glider) as monitoring platforms to detect acoustically tagged green sturgeon in the coastal ocean. For 1 month in 2018 during spring (May) and again in fall (September–October), both methods were used simultaneously to detect sturgeon bearing uniquely coded transmitters. For the fixed array, nine acoustic receivers were interspersed with three sentinel transmitters along a single line of latitude off Winchester Bay, OR. For the glider, two 3-week deployments were completed along the coast of Oregon from nearshore to the 200-m depth contour. For both deployments, the glider flew a zig-zag course southward as it profiled the water column and collected water quality information. Tagged green sturgeon and sentinel transmitters were successfully detected by both the fixed array and the glider. The fixed array provided indications of onshore and offshore movement, while the glider indicated along-shore movement. Although more green sturgeon were detected by the fixed array, the glider provided information on potential sturgeon aggregation areas. In addition, this application of the underwater glider may provide a unique opportunity for public engagement, teaching, and outreach.

Garden eels are known to form large single-species colonies on the sandy bottom of tropical nearshore areas throughout the world’s oceans. Passing divers are met by areas full of almost identical snake-like creatures, with their upper bodies protruding out of their burrows, in which they retract quickly as intruders approach. However, during a 15-day expedition to the Gorgona Island, in the Pacific Ocean of Colombia, it was discovered that the garden eels might not be so identical after all in that area. Between August and September 2019, individuals of both Heteroconger klausewitzi and Heteroconger pellegrini were observed co-existing in the same colony, which was previously believed to be exclusively comprised of the species H. klausewitzi. Photo and video evidence of this co-existence was obtained, and three individuals of H. pellegrini were collected. A ratio of 95.9% of H. pellegrini and 4.1% of H. klausewitzi was obtained from counting individuals appearing in the obtained videos. Heteroconger klausewitzi was found at depths greater than 12 m, while H. pellegrini was present from 5 to 12 m depth. This appears to be the first report of the existence of multi-species colonies in garden eels, suggesting it is a new case of inter-species colony association in fishes, which opens a new window for ecological and social behavior studies. In addition to multi-species coexistence, this work also is the first report of H. pellegrini in South America (i.e., Colombia), extending its distribution range southward by approximately 550 km.

Artificial islands and reefs have been used in the Laurentian Great Lakes for over 40 years as a means of improving aquatic habitat; but research on their efficacy has primarily focused on their ability to increase the abundance of specific sportfish, top predators, or other keystone species. To understand the importance of islands in structuring the whole fish community, we took a holistic approach and analysed the effect of islands, location, and the interaction effect between the two in structuring fish communities in Hamilton Harbour, Lake Ontario using a 30-year electrofishing dataset. The effect of islands varied by location within the harbour, with some species showing a preference for islands in some locations while avoiding them in others. Island communities also tended to have significantly different species compositions, with higher index of biotic integrity scores and species richness, greater numbers of pollution intolerant fishes, centrarchids, and fewer generalist species. However, these results paled in comparison to the level of inter-annual variation in the fish community of the harbour, which has changed markedly over the 30-year time span. Taken together, our results highlight that while island creation can influence the fish community, the type and magnitude of effect will vary based on their distance to other suitable habitats (i.e., location within the system) and the design of the island itself. Further, the noted inter-annual variability emphasizes the importance of considering long time scales (> 10–20 years) when exploring fish community responses to habitat creation. Collectively, these results will help the design of more effective management strategies for restoring fish communities.

Information on fish movements is essential for effective spatial management, including marine-protected areas. Unfortunately, this information is missing for many species of management interest. This lack of information is true for fish species that support the economically important, catch and release, recreational flats fishery in Cuba. Here, we present findings of the first assessment of movements of flats fishes in and around marine-protected areas on the south coast of Cuba, based on mark-recapture and local knowledge. Data were sufficient for analysis for bonefish, which showed high levels of fidelity to the protected areas in which they were tagged. Tarpon showed larger-scale movements and were less fidelic to the protected areas in which they were tagged. Findings suggest that these marine-protected areas are sufficient in size to protect a large portion of the areas used by bonefish, with the exception of likely spawning migrations. The life history and movement patterns of tarpon mean that protected areas provide only partial protection, necessitating additional management measures. These findings will inform marine-protected area management in Cuba and can be used to guide spatial management in other locations.

Assessment of distribution pattern of species with small populations is a prerequisite for conservation efforts. In our investigation, we evaluated the use of environmental DNA (eDNA) in assessing the presence and distribution pattern of a vulnerable fish species in Pranmati hill stream, a tributary of Pinder river in Chamoli, Uttarakhand. Fish specimens caught at one of the sites were classified as Schizothorax richardsonii after morphometric and meristic analysis. DNA isolated from these individuals was subjected to PCR for barcoding. DNA barcode confirmed that the sampled individuals belonged to S. richardsonii. We developed a real-time quantitative polymerase chain reaction (qPCR) assay to target 112 bp sequence of S. richardsonii COI gene. The primers on in silico analysis showed specificity for COI gene of Schizothorax spp. qPCR was employed to quantify the Schizothorax spp. DNA in eDNA samples collected from the four sampling sites on the Pranmati stream. Schizothorax spp. DNA was found in three out of four sites. Detection of Schizothorax spp. DNA at sites where fish specimens were neither visible nor caught, reinforces the importance of use of eDNA in sampling for rare and endangered species. This study demonstrates the efficacy of eDNA detection method for determining the distribution pattern of thinly populated fish species and can be employed to detect their presence at sites where it is difficult to locate or catch them, and can be made use of, while devising conservation strategies.

Few published studies have measured the oxygen consumption rates of elasmobranchs larger than 1.5 m, with only one measured at temperatures below 10 °C. This study provides initial measurements of the metabolic rate of three juvenile Pacific sleeper sharks, SP1904, SP1908, and SP2005 (199 cm, 162 cm, 144 cm), which were caught and temporarily housed at the Alaska SeaLife Center in Seward, Alaska. While at the Center, multiple respirometry trials were performed utilizing an intermittent-closed system respirometer to measure oxygen consumption. The average routine metabolic rates for each individual were calculated at 17.8 ± 1.3 mgO2kg⁻¹ h⁻¹, 20.5 ± 1.6 mgO2kg⁻¹ h⁻¹, and 4.0 ± NA mgO2kg⁻¹ h⁻¹, respectively, while the resting metabolic rate of SP1908 was calculated at 14.3 ± 2.4 mgO2kg⁻¹ h⁻¹. Oxygen consumption rates were measured between 6.0 and 8.8 °C. Both the active and resting metabolic rates for the Pacific sleeper shark were like those of the previously measured and closely related Greenland shark (Somniosus microcephalus) and the temperate zebra shark (Stegostoma fasciatum) when scaled to the temperature in this study. Therefore, Pacific sleeper shark metabolic rate is not unusually low compared to other shark species. These initial measurements of the metabolic rate of Pacific sleeper sharks enhance deep-sea and polar physiology knowledge by filling gaps in respirometry research for large, Arctic elasmobranchs.

Climate-driven declines in oxythermal habitat in freshwater lakes can impose prolonged constraints on cold-water fishes sensitive to hypoxia. How fish cope with severe habitat limitations is not well understood, yet has implications for their persistence. Here, we use acoustic-positioning telemetry to assess seasonal habitat occupancy and activity patterns of lake whitefish (Coregonus clupeaformis), a cold-water benthivore, in a small boreal lake that regularly faces severe oxythermal constraints during summer stratification. During this stratified period, they rarely (< 15% of detections) occupied depths with water temperatures > 10 °C (interquartile range = 5.3-7.9 °C), which resulted in extensive use (> 90% of detections) of water with < 4 mg L-1 dissolved oxygen (DO; interquartile range = 0.3-5.3 mg L-1). Lake whitefish were least active in winter and spring, but much more active in summer, when only a small portion of the lake (1-10%) contained optimal oxythermal habitat (< 10 °C and > 4 mg L-1 DO), showing frequent vertical forays into low DO environments concurrent with extensive lateral movement (7649 m d-1). High rates of lateral movement (8392 m d-1) persisted in the complete absence of optimal oxythermal habitat, but without high rates of vertical forays. We found evidence that lake whitefish are more tolerant of hypoxia (< 2 mg L-1) than previously understood, with some individuals routinely occupying hypoxic habitat in winter (up to 93% of detections) despite the availability of higher DO habitat. The changes in movement patterns across the gradient of habitat availability indicate that the behavioural responses of lake whitefish to unfavourable conditions may lead to changes in foraging efficiency and exposure to physiological stress, with detrimental effects on their persistence. Supplementary information: The online version contains supplementary material available at 10.1007/s10641-022-01335-4.