Journal of Fish Biology

Integrative taxonomy reveals a new troglophilic species of Schistura, from a cave‐dwelling population inhabiting stream draining (Barak–Surma–Meghna drainage) through the Krem (=cave) Mawjymbuin in East Khasi Hills of Meghalaya. Morphologically, the new species is easily distinguished from its congeners in having a combination of characters, including a unique colour pattern in the form of 14–20 greyish black to faint black bars overimposed to a black midlateral stripe on a greyish brown to pale beige (yellowish green in life); pre‐dorsal bars thin, numerous, wider than interspaces, weakly contrasted, sometimes broken and/or incomplete; bars posterior of anal‐fin base numerous and variable, mostly broken or incomplete, located on lower flank, weakly contrasted; a complete lateral line; presence of an axillary pelvic lobe; an uninterrupted thick black stripe along the base of the dorsal fin; an incomplete black basicaudal bar, dissociated, occupying median one‐third of caudal‐fin base; conspicuous black botch on the procurrent rays of the upper lobe of the caudal fin; and a type of sexual dimorphism: males with slender body, bars mostly irregular on flanks and slightly puffy cheek with greater lateral head length; females with regular‐shaped bars and swollen body. Analysis of the mitochondrial cytochrome c oxidase I (COI) gene sequence of the new species shows significant genetic divergence with p distances ranging 4.5%–13.6% from its sister species found in northeast India, supporting its report as new species. Further, this finding is also validated by phylogenetic analysis and the species delimitation approaches, Assemble Species by Automatic Partitioning (ASAP) and Poisson Tree Processes (PTP), which clearly resolved that the Schistura densiclava sp. nov. is distinct from its sister species. Although the species has been sampled from the cave, it retains a fully pigmented body and functional eyes, which are not reduced when compared to other epigean Schistura, tentatively indicating it as a troglophile. Distribution of the new species is limited to the cave, which suggests an endemism in this cave system.

The first documented case of predation on a juvenile critically endangered angelshark Squatina squatina by a greater amberjack Seriola dumerili was observed in 2024 at Las Teresitas Beach, Tenerife, Canary Islands. The sequence shows a roughtail stingray Bathytoshia centroura initially attempting to predate on S. squatina, which briefly escaped before being captured by S. dumerili. This observation provides new insights into predator–prey dynamics of S. squatina, highlighting the critical role of these interactions in shaping conservation strategies for critically endangered species.

Tumour necrosis factor alpha (TNF‐α) is a pivotal cytokine in vertebrate immunity. Considering the increasing scientific interest for TNF‐α in piscine biology, the paper detailed the characteristics of a novel tnf‐α gene from a high‐value mariculture species, Trachinotus blochii. The tbtnf‐α complementary DNA (cDNA) of 1385 bp encoded an open reading frame of 762 bp, 3′ untranslated region (3′ UTR) of 484 bp and 5′ UTR of 139 bp. The deduced Tbtnf‐α1 showed the highest sequence identity to that of Seriola dumerili (∼83%). The comparative phylogenetics identified the protein as the TNF‐α paralog 1. Tbtnf‐α1 displayed all the hallmark features of other teleost TNF‐α, suggesting similar immune‐related functions. However, the Tbtnf‐α1 was predicted to be more acidic and less thermostable. The study generated the three‐dimensional (3‐D) structure model of Tbtnfα‐1 based on the protein sequence that can be applied in future research. The genomic organization of tbtnfα‐1 contained four exons and three introns. Real‐time polymerase chain reaction (PCR) analysis in healthy conditions showed the constitutive expression and wide distribution of tbtnfα‐1 in a tissue‐specific manner, with maximum expression in the kidney. As in silico analysis predicted the biological function of the novel TNF‐α as a regulator in the defence response to bacterial infection, functional validation was done through the expression analysis following exposure to a marine pathogen (Vibrio harveyi) challenge. The results showed that the expression of tbtnf‐α1 in the liver, spleen, heart, gill and kidney was significantly upregulated compared to the control fish after the challenge at differential time points post‐infection. The study also demonstrated the kinetics of tbtnfα‐1 expression in the liver, spleen, heart, gill and kidney at 6 to 48 h post‐infection, and the results showed the peak expression at 24 h in all the tissues followed by a drop in the expression. Briefly, the present study detailed the sequence, structural and functional characteristics of tnf‐α1 in the immune response of silver pompano.

Squalus megalops and Squalus mitsukurii are recognized as the most abundant Squalus species in the southern Brazil region. Currently, S. megalops is listed as ‘least concern’ (LC) and S. mitsukurii as ‘endangered’ (EN). However, at the regional level, both species are classified as ‘data deficient’ – DD, due to the lack of population local data. In this context, the present study aimed to determine the reproductive biology parameters of these species. In addition, insights on their abundance were provided. Research cruises using bottom trawls were conducted on the continental shelf off southern Brazil between 2001 and 2002. A total of 298 specimens of S. megalops (♂143|♀156) and 297 specimens of S. mitsukurii (♂129|♀166) were captured. For S. megalops, the L50 was 38.2 cm for males and 50.22 cm for females, with ovarian fecundity ranging from 1 to 4 vitellogenic follicles and uterine fecundity from 1 to 4 embryos. S. mitsukurii presented an L50 of 54.12 cm for males and 66.4 cm for females, with ovarian fecundity ranging from 1 to 8 vitellogenic follicles and uterine fecundity ranging from 1 to 8 embryos. The highest catch per unit of effort (CPUE) for S. megalops was in zone B (2360.43 ind/mn²) and for S. mitsukurii in zone A (5287.97 ind/mn²). The study observed differences in the reproductive stages and fecundity of both species compared to other studies for the same species. In southern Brazil, the calculated L50 sizes were smaller than those obtained in the northeast, potentially due to higher fishing pressure. Obtaining data on poorly known species reinforces the importance of monitoring fisheries in the region. The maturity data presented are crucial for species classified as DD, such as S. megalops and S. mitsukurii in southern Brazil, potentially generating new information to support local fisheries management.

Migratory animals often transport allochthonous materials, energy or organisms from donor to recipient ecosystems, thereby affecting the dynamics of consumers, communities and ecosystems in the recipient systems. The biomass of migrants is commonly assumed to be equal to that of the allochthonous materials they transport, with the inherent assumption that migrant bodies are produced purely from allochthonous resources. However, the extent to which the body composition of many migratory animals consists of allochthonous materials [e.g., the proportion of total body mass consisting of allochthonous resources, hereafter degree of allochthony (DOA)] may gradually decrease if the animals consume autochthonous resources during their migration. Ignoring this process leads to an overestimation of the magnitude and temporal features of material subsidies. In this study, we quantified the DOA of nine amphidromous fish species that migrate from marine to rivers, using the sulphur stable isotope (δ³⁴S) analysis. The DOA varied substantially among species (11%–82%) and even within species (Sicyopterus japonicus had the highest DOA: 22%–97%). Species with larger body sizes tended to exhibit a lower DOA. This trend was also observed at the species level for six of the nine species, with five species showing an additional pattern of later migrating individuals with lower DOA. These results imply that quantifying DOA is important for accurately estimating material subsidies across ecosystems, owing to the upstream migration of amphidromous fish. Life‐history studies of amphidromous fishes are crucial for elucidating the mechanisms behind inter‐ and intraspecific variation in DOA, which ultimately contributes to a better understanding of marine‐river ecosystem linkages mediated by these fishes.

Blacktip sharks (Carcharhinus limbatus) form large seasonal aggregations numbering in the thousands. These aggregations are typically spotted from the air which makes it impossible to determine the sex and size distribution. We sampled C. limbatus that occur in large aggregations off the coast of southeast Florida. The aggregations were comprised primarily of adult male sharks (95.2%) that were of a similar size (170.8 ± 8.07 SD cm). This provides evidence of both size and sex segregations for this species during the overwintering phase of their annual migration.

Orestias chungarensis Vila & Pinto, 1986 is a small‐bodied (max fork length = 120 mm) cyprinodontiform fish with a very restricted global distribution. The species is limited to a single, small (283 km²), high‐altitude (4520 m) catchment located in the Altiplano of northern Chile. Until the late 20th century, O. chungarensis was the only fish species inhabiting both Lake Chungará and its main afferent river, the River Chungará. The introduction of rainbow trout [Oncorhynchus mykiss (Walbaum, 1792)] at this time raised concerns for the long‐term conservation of Orestias. By 2017, O. chungarensis were no longer present in the River Chungará but remain relatively numerous in Lake Chungará. Although O. chungarensis are of elevated conservation concern, little is known regarding their ecology, and the few studies conducted have relied on individuals captured from shallow littoral habitats. Here, we captured O. chungarensis from different lake habitats and analysed multi‐tissue stable isotopes (δ¹³C, δ¹⁵N, δ³⁴S) and stomach contents to characterise their trophic ecology. We also used geometric morphometrics to analyse any putative habitat‐associated variation in body shape. O. chungarensis showed very wide variation in their stable isotope values (range: δ¹³C = −15.1 to −8.0‰; δ¹⁵N = 8.9–14.1‰; δ³⁴S = −10.5–1.7‰). A k‐means cluster analysis indicated that individuals could be best classified into two groups in stable isotope space. A discriminant function analysis supported the separation of the sampled population into two groups (jack‐knifed classification success = 98%). Individuals belonging to either a putative littoral group (¹³C‐enriched, ¹⁵N‐depleted and ³⁴S‐depleted) or a group associated with pelagic‐derived materials (¹³C‐depleted, ¹⁵N‐enriched, ³⁴S‐enriched), which likely fed offshore or in deeper waters. Stomach contents results showed that O. chungarensis from the two putative groups had consumed similar prey prior to capture, feeding mainly on benthic macroinvertebrates (amphipods, chironomid larvae and pupae and gastropods). Mixing models analysis showed a broadly similar diet between groups, but the scale of contribution to the assimilated diet differed between groups. Comparisons of stable isotope niche size and overlap showed limited niche overlap, providing more evidence for differential foraging patterns. The dichotomy between the results from stable isotope and stomach content analysis suggests that O. chungarensis individuals forage on taxonomically similar diets, but their prey are fuelled from materials derived from different lake habitats (littoral and open‐water). Given the remarkable plasticity found in the genus, our results could reflect the existence of a previously unrecognised ecotype.

Deep‐sea sharks represent a vulnerable group due to their unique life‐history traits, including slow growth and late maturity. This study aims to fill knowledge gaps on key demographic parameters of Etmopterus molleri, a lanternshark species, one of the smallest shark species recorded, classified as ‘data deficient’ in the East China Sea. A total of 280 specimens (165 females and 115 males) were analysed to estimate age, growth and sexual maturity. Post‐cranial vertebral section was treated with cobalt nitrate hexahydrate for improved age determination. Age estimation from band pair ranged from 0 to 13 in females and from 1 to 12 in males [coefficient of variation (CV) = 2.27%, average percent error (APE) = 1.60%]. The von Bertalanffy curve with the Bayesian growth Markov chain Monte‐Carlo (MCMC) models using observed and back‐calculated data provides the best‐fit estimation compared to all models, demonstrating an asymptotic length of L∞ = 391.61 ± 10.06 mm for males and L∞ = 486.28 ± 14.71 mm for females, with a common birth size (L0) of around 100 mm. The growth completion rates were (k) of 0.16/year for the males and 0.11/year for the females, marking sexual dimorphism, with females reaching larger sizes (L50 = 287.78 mm) and maturing later (A50 = 6.57 years) than males (L50 = 260.33 mm, A50 = 4.77 years). This maturity life strategy, typical of deep‐sea elasmobranchs, underscores the vulnerability of E. molleri. A prudent management approach and continuous monitoring are essential for collecting more data to understand their ecology and preserve their fragile future.

The tope (Galeorhinus galeus, Linnaeus, 1758) is a critically endangered shark, which, like numerous elasmobranchs, faces severe global decline. There are, however, substantial disparities between this species' global conservation status and those of some local populations, with the Northeast Atlantic representing a relative stronghold for this species. However, several areas of uncertainty, particularly regarding individual movement patterns, currently hamper region‐specific conservation efforts. Therefore, utilising capture‐mark‐recapture tagging data, collected predominantly by recreational anglers in Ireland – but with recaptures throughout the Northeast Atlantic and Mediterranean – we investigated regional population structure, spatial segregation and individual movement patterns, and estimated sex‐specific individual growth rates. This revealed a marked pattern of sexual segregation, with females residing further south than males overall, and with an Irish‐specific trend towards male‐dominated catches in Atlantic regions versus more even sex ratios in the Irish and Celtic Seas. Recapture timings and locations suggest that female movements are more strongly driven by seasonal water temperatures changes, being broadly in line with the north–south migratory paradigm. Spatiotemporal overlap of mature individuals suggests that the North Channel, Irish Sea and northern Celtic Sea may constitute a key mating area. Female sharks may utilise southerly regions during gestation, with the presence of young‐of‐the‐year indicating that the Irish Sea and neighbouring regions subsequently act as parturition/nursery areas. Our results demonstrate the value of such long‐term programmes, in this case facilitated by citizen science, in identifying broad‐scale movement patterns in wide‐ranging species and specific regions of interest for further study and/or implementation of targeted conservation measures.

The roosterfish (Nematistius pectoralis) is a highly prized organism in the sport fleet for its fight to catch and the showiness of its dorsal fin. However, despite its popularity, studies on its basic biology are limited. This study aims to understand the feeding behaviours of the roosterfish by analysing the stomach content (SCA) and performing bulk stable isotope analysis (BSIA). The study also seeks to identify ontogenetic changes in its feeding habits and habitat preferences. To achieve this, 505 stomachs and 165 muscle samples were analysed in three locations in Baja California Sur (Cabo San Lucas, La Paz and Puerto San Carlos) between 2010 and 2017. The SCA determined that the diet consisted mainly of fish from the families Carangidae, Engraulidae and Clupeidae, which together constituted almost 50% of the total diet. Statistical differences in diet composition were found by size class but not by sex or the interaction between these variables. Concerning BSIA, there were no significant differences between males and females; however, differences were observed in the ‘undifferentiated sex’ grouping, which was comprised largely small juvenile individuals. Concerning size, BSIA values were correlated with roosterfish fork length, so the observed differences for the ‘undifferentiated sex’ category were related to size variation rather than sexual maturity. The findings from this study suggest that roosterfish experience an ontogenetic increase in trophic position, relying less on sardine‐like species as they grow. Furthermore, BSIA values could inform on relative residency of the roosterfish to the Baja California peninsula.

Fewer than 50 of the over 30,000 extant species of fishes have developed anatomical specializations facilitating endothermy in specific body regions. The plankton‐feeding basking shark (Cetorhinus maximus), traditionally classified as an ectotherm, was recently shown to have regionally endothermic traits such as centralized red muscle (RM) along its body trunk and elevated (white) muscle temperatures. However, key anatomical features essential for classification as a regional endotherm, such as the presence of vascular rete mirabile, could not be confirmed in this cold‐water giant. This study compared the morphology of heat‐generating and heat‐retaining tissues – associated with skeletal RM, the cranium and viscera – in the basking shark with those of a confirmed regional endotherm, the porbeagle (Lamna nasus), and a polar ectotherm, the Greenland shark (Somniosus microcephalus). Despite the presence of more medial RM in the basking shark's trunk, the absence of paired lateral vessels and a perfusing rete strongly suggests a lack of RM endothermy in this species. However, the presence of small arterial plexuses in the orbit, along with the discovery of visceral retia associated with the stomach, spleen and valvular intestine, in addition to distinct vascular arrangements in the liver and kidney, indicates potential for cranial and, particularly, visceral endothermy in C. maximus. These specializations, combined with reduced conductive heat loss from partial RM internalization and large body size, may enable C. maximus to maintain regionally elevated body temperatures, facilitating their active lifestyle also in cold‐water environments. Enhanced sensory perception and digestive efficiency may aid prey acquisition and processing in the dimly lit meso‐ and bathypelagic zones, as well as high‐latitude regions. Our findings provide initial insights into the thermal adaptations of these circum‐globally distributed, highly migratory ram filter feeders. Further research is needed to better understand the eco‐physiological implications of these adaptations, especially in the context of rapid ocean warming across their range, including Atlantification in the Arctic, and other anthropogenic pressures in the Anthropocene.

As a member of the bone morphogenetic protein (BMP) family, Bmp4 plays a crucial role in bone formation, early developmental stage and the regulation of gonadal development. This study aims to investigate the expression patterns and potential functions of the Bmp4 in Pampus argenteus during its reproductive cycle. A systematic evolutionary analysis of the Bmp4 was performed, and its expression was quantified and localized using real‐time polymerase chain reaction (PCR), Western blotting and immunohistochemistry. Additionally, the effects of various hormones on hormone‐related genes were analysed in vitro using cultured testes, ovaries and oocytes. The results revealed that Bmp4 is evolutionarily conserved and broadly expressed across tissues. Notably, its expression significantly increased at 4 and 13 days post‐hatching (dph), with no significant differences observed at other developmental stages. Western blotting identified a specific Bmp4 target band in Stage III ovaries, and immunohistochemistry localized Bmp4 expression in oogonia, oocytes and spermatogonia. In vitro hormonal treatments demonstrated that oestradiol (E2) significantly upregulated bmp4 expression in both testes and ovaries, with expression levels being notably higher in ovaries. In immature vitellogenic oocytes cultured in vitro, sex hormone treatments did not significantly affect the expression of bmp4, lhr, fshr and lhβ. However, under LH + hCG treatment (50 ng/mL and 5 IU/mL), the expression of fshβ was upregulated. In summary, our findings reveal that Bmp4 serves as a pivotal factor in the early development of the gonads and germ cells in P. argenteus. The expression of Bmp4 within the ovary is likely regulated by E2. During the early‐stage development of the ovary, E2 and Bmp4 may jointly promote the early‐stage development and differentiation of germ cells. This study offers a valuable theoretical foundation for addressing challenges in the breeding process.

Understanding genetic population structure and connectivity is essential for effective species‐specific management and conservation strategies. The American elephantfish Callorhinchus callorynchus is targeted and retained as incidental catch in commercial and recreational fisheries in Chile and Argentina. Its wide‐ranging distribution across southern South America may require transnational co‐operation to ensure sustainable use, but its current population structure is not known. In this work, we analysed the levels of genetic diversity and differentiation within C. callorynchus in South America using two mitochondrial markers, the control region (CR) and the cytochrome oxidase subunit I gene (CO1). Moreover, we assessed levels of genetic diversity within, and divergence among, the three extant callorhinchids (genus Callorhinchus), a group that exhibits allopatric geographical distributions in the southern hemisphere. Overall, sequence analyses of the mitochondrial CR and the CO1 revealed extremely low levels of sequence variation both within and among Callorhinchus species. Genetic homogeneity was found throughout the range of C. callorynchus coupled to low‐frequency haplotype sharing across spatially distant locations in Chile and Argentina, suggesting gene flow along the South American coast. Moreover, our analyses supported a scenario of recent population expansion of the species in South America. Given the absence of dispersive eggs or juvenile stages in chondrichthyans, gene flow is mainly mediated by actively swimming adults. Based on the available data, gene flow in callorhinchids appears to occur along continuous coastal regions, with deep oceanic waters serving as strong barriers. Findings here provide an important baseline for future research on dispersal and gene flow in holocephalans.

We examined the age and growth of the Pacific goliath grouper (Epinephelus quinquefasciatus) from 210 samples collected from the main fish markets in Panama and Colombia. Ages from otoliths ranged from young‐of‐the‐year to 13 years, predominantly between 1 and 4 years. The von Bertalanffy growth parameters were L∞ = 207 cm, k = 0.13 year⁻¹ and t0 = −1.87 years. Instantaneous mortality rates (Z) were estimated at 36% in Colombia (ages 2–11) and 26% in Panama (ages 2–13).

Early life development in fishes is a period of high phenotypic plasticity. Water temperatures during embryogenesis can lead to alterations in growth and metabolic and morphological phenotypic variations. This study aimed to characterize the effects of temperature on the growth and morphology of yellow perch throughout early development, a species of significant ecological and economic importance in North America. Yellow perch (Perca flavescens) embryos were incubated at either constant temperature (12, 15, or 18°C) or under an ambient seasonal regime, where temperature increased throughout embryogenesis until it reached 18°C. All groups were gradually warmed at hatch until they reached 18°C, and morphology was assessed. Inverse relationships between temperature, length and yolk‐sac volume were found at the end of the embryonic period. Optimal embryonic growth was associated with the ambient seasonal regime. Fish reared at 15 or 18°C were larger and had higher growth rates when compared to those at 12°C following hatch. However, fish reared in cooler temperatures may possess more advantageous body shapes. Surprisingly, the ambient seasonal incubation had the lowest growth rate post‐hatch, even though this most closely mimicked natural incubation for this species. Because suitable larval morphology is related to survival and successful recruitment, these data suggest that yellow perch may be vulnerable to climate change and thermal pollution, although further work is needed to better predict the ecological implications of the phenotypes.

The branchial epithelium of Potamotrygon—a member of the only strictly freshwater elasmobranch family Potamotrygonidae—was observed via immunohistochemistry, and two distinct forms of ionocytes were identified. The acid (A‐type) and base (B‐type) secreting cells with respective basolateral–apical localizations of Na⁺/K⁺‐ATPase–Na⁺/H⁺ exchanger 3 in A‐type and V‐type H⁺‐ATPase–pendrin Cl⁻/HCO3⁻ exchanger in B‐type were detected and morphometric changes in response to acid–base disturbances (3 mmol/kg body mass acid [HCl] or base load [NaHCO3] by intraperitoneal injection) were characterized. A‐type cells were found in distinct follicular clusters in the interlamellar region, while B‐type cells were found individually in both the filament and lamellar epithelia. Lamellar B‐type cells were more abundant and larger, with greater fluorescence than their filament counterparts. The B‐type cells responded to the base load by increasing in number in the filament and showed a more peripheral H⁺‐ATPase distribution, indicative of activation by translocation to the basolateral membrane labyrinth. A‐type cells decreased in size with the base load. In contrast, no changes in A‐type or B‐type cells were observed with the acid load compared to the sham group. Together, these results indicate that the stenohaline Potamotrygon rays have branchial ion uptake cells that allow them to effectively ionoregulate under the challenging ion‐poor water conditions of the Amazon.

Sexual maturation of male Atlantic salmon post‐smolts (‘jacking’) is undesired in aquaculture and seems to occur due to the intensification in modern facilities. Maturation depends on internal and external factors (temperature, photoperiod, feed availability, energy levels, body size, genetic background) that act on the brain‐pituitary‐gonad axis to trigger maturation. This study assessed the influence that the body weight at the time of the photoperiod manipulation performed to induce smoltification can have on early maturation or smoltification. For that, 900 parr (41.3 ± 3.7 g) were assigned to four size‐photoperiod treatments: constant light (LL), a winter signal (5‐week LD12:12) starting when fish was 70.0 ± 8.4 g (70WS), a similar winter signal at 114 ± 9.9 g (110WS) and at 182.9 ± 35.7 g (180WS). The trial took place in fresh water, except for the last 2 weeks in each treatment carried out in full seawater. Body weight and condition, gonadosomatic index and proportion of maturing males, gill Na⁺, K⁺, ATPase activity and blood parameters (chloride and sodium, cortisol, glucose and cholesterol) were measured to assess progression of smoltification or maturation. In addition, pituitary messenger RNA (mRNA) transcription of tshβb (a paralog of the thyrotropin thyroid‐stimulating hormone) was analysed to investigate its potential role in the regulation of any of the two processes. Results evidenced that body size of male salmon was relevant for the developmental response to a winter signal, be it maturation or smoltification. An early winter signal (70WS and 110WS) induced clearer signs of smoltification (reduced condition factor, high energy mobilization and little stress response in seawater among others) and no tendency to mature early. In contrast, a late or no winter signal (180WS and LL) induced a strong stress response in seawater, poor morphological signs of smolting and energy mobilization, and a higher tendency to mature early. It was difficult to directly relate pituitary tshβb transcription with both smoltification or maturation, because this gene seemed to act primarily as a timekeeper displaying large upregulations in response to increases in daylength (irrespective of size) but no changes under constant light. These results evidence that an increase in daylength when salmon is large enough to have developed sufficiently or accrued energy resources may pose high risk that a proportion of them prioritize sexual maturation over smoltification. Producers may take this into consideration when optimizing protocols for post‐smolt production.

Sexual size dimorphism (SSD) and skewed sex ratios are common in the animal kingdom, and marine fishes, such as the Atlantic cod (Gadus morhua), are no exception. This study utilized newly discovered sex‐linked genetic markers to investigate young‐of‐the‐year cod in the Skagerrak system, comprising approximately 2700 individuals from North Sea and fjord ecotypes. Overall, no significant difference in sex ratio was observed overall (51% females, 49% males), with consistent findings across 14 locations and three sheltering degrees. Although slightly more females were found in exposed areas, and slightly more males in sheltered regions, these differences were not significant. Size variation between female and male cod was generally negligible. These results indicate a likely purely genetic sex determination system in Atlantic cod and a lack of prematuration growth differences between sexes. This research enhances our understanding of the Atlantic cod, providing more insight into the Atlantic cod population in the Skagerrak and emphasizing the importance of genetic factors in sex determination.

Stellifer naso is known from the State of Bahia (Brazil), but there is a lack of an accurate definition of its type locality. Thayer's expedition (around 1865–1866), led by Louis Agassiz and staff from the Museum of Comparative Zoology (MCZ, Harvard University), intended to explore Brazilian fauna and flora, with most of its material housed in the MCZ and used to describe several species, including S. naso. Examination of specimens from the original description of that species, housed in the MCZ and the Smithsonian National Museum of Natural History, helped review the accurate point of origin for such specimens. A combination of data from reproductive biology and the history of the region further support the actual type locality of S. naso as Iguape Bay, with an error radius of 2.5 km. Damming in the Paraguaçu River in the early 1980s might have had an impact on the morphological variation over time recorded to this species.

Hatchery practices are a critical tool of fisheries management to supplement diminished fish populations and restore extirpated species. However, stocking programs that successfully restore self‐sustaining populations are rare. Unintentional artificial selection and domestication of hatchery‐reared fish are potential limitations to the success of stocked individuals by selecting for behaviours that are poorly suited for natural conditions. Here, we compared seasonal movement behaviours between an established population of hatchery origin (stocked) and naturally produced (wild) lake trout (Salvelinus namaycush) in Lake Champlain at a range of sizes, including immature and mature fish. Fifty‐six stocked and 34 wild lake trout were implanted with acoustic transmitters, including 45 transmitters with temperature and pressure (depth) sensors, to evaluate three‐dimensional movement. Movement behaviours were assessed based on the number of distinct lake regions used, proportional time spent among lake regions, average daily distance travelled and depth distribution. Overall, horizontal and vertical movements were similar between stocked and wild lake trout across sizes, although individuals tended to occupy shallower depths at larger sizes. Seasonal differences in movement behaviours were observed and in some cases were dependent on origin. For all lake trout, average daily movement was greatest during fall and least during summer. Depth occupied, however, had an opposite trend, with the deepest average depths during summer and shallowest during fall and winter. The proportion of time spent among lake regions and variability in depth occupied also varied seasonally but only for wild fish, and included less time spent in individual regions and greater depth variability during fall compared to other seasons. While origin had insignificant effects in most models we evaluated, model predictions consistently suggested stocked lake trout had slightly smaller movements than wild fish. These results suggest that hatchery practices may have long‐term, unintended effects on fish behaviour yet overall differences are likely subtle.

We present a random pattern of body size ratios between adjacent ranked group members in the humbug damselfish Dascyllus aruanus. This random distribution of size ratios differs from the other well‐known group‐living coral reef fishes which exhibit non‐random, well‐defined and closely regulated size differences between ranks. Our results suggest a relaxation of social constraints in D. aruanus which may be due to its social system, mating system, capacity for movement or a combination thereof. Investigating how these traits co‐vary within and among populations is an interesting avenue of future research.

Spawning behaviour, ontogenetic development and sexual dimorphism of the Taranetz's lumpsucker Eumicrotremus taranetzi are described based on aquarium observations. Reproductive behaviour was observed from ca. 1000 days after hatching [dah, female ca. 45.0 mm body length (BL), male ca. 37.0 mm BL], including territorial males remaining on the artificial substrate after spawning and protecting eggs until hatching. Some of the eggs were moved to a smaller tank for hatching. Eggs hatched approximately 110 days after spawning at an average water temperature of 2.8°C. Pelvic‐fin sucker formation begins in the egg at 83 days (completed at 90 days) after fertilization (daf). Newly hatched larvae have a smooth body (without spines or bony tubercles), fully developed fins (except for the caudal fin) and adhesive suckers. Larvae attach themselves to the bottom or side of the tank immediately after hatching. Bony tubercles on the body appear first on the head at 50 dah. Males and females exhibited distinct sexual dimorphism in 15 body measurements and the number of bony tubercles at 200 days post hatching (dph), probably related to the egg‐guarding behaviour of males.

Southern Africa is a region denoted by both high levels of fish diversity, some of it cryptic and unrecognised by current taxonomy, and severely threatened freshwater ecosystems. The Waterberg, a key aquatic ecoregion of the greater Limpopo River basin in South Africa, represents an area with high terrestrial conservation value but is lacking in aquatic biodiversity information. This study characterised this unique aquatic ecoregion's fish diversity, their biogeographic patterns and threats to this biodiversity. A total of 29 fish species (11 families, 19 genera) were identified, with many distinct upland fish communities occurring within the high‐altitude headwaters of the ecoregion, whereas lowland fish communities tended to be more homogeneous. Mitochondrial CO1 barcoding revealed genetically distinct lineages in four presumed‐widespread southern African species: the shortfin barb, Enteromius brevipinnis (Jubb, 1966); hyphen barb, Enteromius bifrenatus (Fowler, 1935); straightfin barb, Enteromius paludinosus (Peters, 1852) and snake catfish, Clarias theodorae Weber, 1897, that were restricted to the Waterberg aquatic ecoregion. The level of genetic divergence suggests that these four Waterberg‐restricted lineages are likely new candidate species. These findings indicate the Waterberg to be a biogeographic island within the greater Zambezian ichthyofaunal region of southern Africa, which should be prioritised for aquatic ecosystem conservation. Current terrestrial conservation structures in the region, encapsulated within the Waterberg Biosphere Reserve, appear to protect this distinct ichthyofauna from human land‐use‐derived impacts. Nonetheless, the presence of the invasive predatory largemouth bass (Micropterus nigricans) inside the biosphere represents a credible conservation threat. Engagement with biosphere stakeholders will be critical for managing this threat to the Waterberg's unique ichthyofauna going forward.

Fishes undergo dramatic physiological changes upon consumption of a meal, including an increase in oxygen consumption to support the metabolic cost of digestion [specific dynamic action (SDA)] and an increase in the excretion of ammonia. Channel catfish (Ictalurus punctatus) and blue catfish (I. furcatus) are two species commonly used for commercial aquaculture production in the United States. Postprandial ammonia excretion and oxygen consumption of both channel and blue catfish were measured at 25 and 32°C. Rates of both ammonia excretion and oxygen consumption increased quickly after feeding and were significantly higher within 2 h postfeeding. Ammonia excretion of channel catfish peaked 6 h postfeeding at both 25 and 32°C, with peak ammonia excretion rates increasing 8.3‐ and 4.7‐fold higher than fasting rates at 25 and 32°C, respectively. Ammonia excretion of blue catfish at 25°C peaked 6.6‐fold higher than fasting rates at 12 h postfeeding and 5.5‐fold higher than fasting rates 6 h postfeeding at 32°C. Relative to fasting levels, postprandial oxygen consumption of channel catfish peaked 1.8‐ and 2.0‐fold higher at 25 and 32°C, respectively. Blue catfish oxygen consumption peaked 1.9‐ and 1.8‐fold higher at 25 and 32°C, respectively. Both channel and blue catfish rapidly increase ammonia excretion and oxygen consumption in response to feeding, with temperature mostly affecting changes in peak and minimum rates.