Large Fish Research Articles

Sperm Collection and Computer-Assisted Sperm Analysis in the Teleost Model Japanese Medaka (Oryzias latipes).

Japanese medaka (Oryzias latipes) is a teleost fish andan emerging vertebrate model for ecotoxicology, developmental, genetics, and physiology research. Medaka is also used extensively to investigate vertebrate reproduction, which is an essential biological function as it allows a species to perpetuate. Sperm quality is an important indicator of male fertility and, thus, reproduction success. Techniques for extracting sperm and sperm analysis are well documented for many species, including teleost fish. Collecting sperm is relatively simple in larger fish but can be more complicated in small model fish as they produce less sperm and are more delicate. This article, therefore, describes two methods of sperm collection in the small model fish, Japanese medaka: testes dissection and abdominal massage. This paper demonstrates that both approaches are feasible for medaka and shows that abdominal massage can be performed a repeated number of times as the fish quickly recover from the procedure. This article also describes a protocol for computer-assisted sperm analysis in medaka to objectively assess several important indicators of medaka sperm quality (motility, progressivity, duration of motility, relative concentration). These procedures, specified for this useful small teleost model, will greatly enhance understanding of the environmental, physiological, and genetic factors influencing fertility in vertebrate males.

The role of food stereotype in hedonic judgment of a delicacy food: A case study of Korean consumers’ liking for sliced raw fish (sashimi)

For a delicacy food that it is not routinely consumed, the criteria by which consumers determine whether they like it may not be clear. For such foods, it is highly likely that external cues, such as information, play an important role in sensory perception and hedonic judgment. However, such information may have different effects depending on the consumers’ preconceived stereotype. Sashimi is thin slices of fresh raw fish, considered a delicacy food in many countries. This study was conducted to investigate whether information about the size of a fish affected expectations and acceptance of sashimi and whether the effect of such information differed depending on the type of stereotype about the relationship between fish size and the sensory quality of sashimi. Sashimi was prepared from large- (2.5 kg; LS) and small-sized (1.5 kg; SS) olive flounders. The overall sensory difference between LS and SS was determined using a triangle test (n = 58) and liking of the sashimi samples was rated by 48 consumers in blind and informed settings. There was no significant difference in sensory characteristics between LS and SS. Different types of stereotypes were associated with different interpretations of the same extrinsic cues, and affected expectations and acceptance. The stereotype that sashimi from large fish tastes better than that from small fish induced higher expected liking and actual liking for LS than for SS. This suggests that hedonic judgment of sashimi, a delicacy food, is influenced more strongly by external cues and stereotype than by intrinsic sensory traits.

Acoustic researches of arctic cod <i>Boreogadus saida</i> (Gadidae) in the southwestern Chukchi Sea in 2003–2020

Spatial distribution patterns and abundance of arctic cod Boreogadus saida are considered for the southwestern Chukchi Sea on the data of acoustic surveys and trawl samples obtained by TINRO research vessels in 2003–2020. The highest abundance and biomass of this species indicated by the echosounder–measured backscattering coefficient sA (m2 per sq.n.mile) were observed in August 2003, decreased significantly in 2007–2008 and later, with the minimum in 2010, increased in 2014 due to the high portion of juveniles, but continued to decrease in recent years and reached the lowest value in 2020. This negative tendency for arctic cod contradicts to the walleye pollock abundance increasing in the southwestern Chukchi Sea on the background of warming in the Arctic. The arctic cod were distributed mainly in the northeastern and southwestern parts of the surveyed area, usually as vast fields of small schools or sparse layers. To the east from 174°W, the aggregations were formed by large-sized fish distributed mostly at the bottom. In the western area, the aggregations were formed within thermocline at the depth of 10–20 m or had two–layer structure with the juveniles dominated in the upper layer and larger fish at the bottom. Typical echograms showed dense schools of arctic cod with the height up to 30 m and horizontal size of 120–400 m, presumably formed by migrating fish. Diurnal vertical migrations were not expressed for this species, though the aggregations of lower density were observed at night, in general, with some local aggregations of permanently dense schools, both in the daytime and at night.

Onchorynchus mykiss alter nutrient dynamics in high-altitude headwater streams in Boyacá, Colombia through displacement of the native fish community

A primary global threat to aquatic ecosystem conservation and biodiversity is the introduction of fish species, both deliberate and unintentional. Much of the research conducted on large predatory fish introductions has focused primarily on top-down controls that regulate the abundance and composition of native species assemblages, yet exotic species that alter ecosystem structure and function may pose an even greater threat. The objective of this study was to examine whether an introduced piscivorous fish alters nitrogen cycles in high elevation freshwater streams in the tropics by displacing the native species community through predation. To test this, we measured biomass and ammonium (NH4+-N) remineralization rates for introduced Onchorynchus mykiss and the two native species found in the area; Trichomycterus bogotensis and Grundulus bogotensis in six study sites along an elevation gradient in the Pómeca River Basin in Boyacá, Colombia. Nonnative O. mykiss populations remineralized ammonium at a rate that was 20 times higher than native G. bogotensis and 47 times higher than T. bogotensis. Our results indicate that high biomass of an introduced predatory fish species can have important implications in nutrient dynamics in freshwater streams through reduction in native species biomass when combined with differential nutrient remineralization rates.

Size-selective mortality fosters ontogenetic changes in collective risk-taking behaviour in zebrafish, Danio rerio

Size-selective mortality is common in fish populations and can operate either in a positive size-selective fashion by harvesting larger-than-average fish or be negatively size-selective by harvesting smaller-than-average fish. Through various mechanisms (like genetic correlations among behaviour and life-history traits or direct selection on behaviour co-varying with growth rate or size-at-maturation), size-selection can result in evolutionary changes in behavioural traits. Theory suggests that both positive and negative size-selection without additional selection on behaviour favours boldness, while evolution of shyness is possible if the largest fish are harvested. Here we examined the impact of size-selective mortality on collective boldness across ontogeny using three experimental lines of zebrafish (Daniorerio) generated through positive (large-harvested), negative (small-harvested) and random (control line) size-selective mortality for five generations and then relaxed selection for 10 generations to examine evolutionarily fixed outcomes. We measured collective risk-taking during feeding (boldness) under simulated aerial predation threat, and across four contexts in presence/absence of a cichlid. Boldness decreased across ontogeny under aerial predation threat, and the small-harvested line was consistently bolder than controls. The large and small-harvested lines showed higher behavioural plasticity as larvae and developed personality earlier compared to the controls. The large-harvested line showed increased variability and plasticity in boldness throughout ontogeny. In the presence of a live predator, fish did not differ in boldness in three contexts compared to the controls, but the large-harvested line showed reduced behavioural plasticity across contexts than controls. Our results confirmed theory by demonstrating that size-selective harvesting evolutionarily alters collective boldness and its variability and plasticity.

Indicators of Tuna Gigi Anjing (Gymnosarda unicolor) Growth Pattern on the Implementation of Sustainable Development Goals (SDGs)

Tuna is a type of large pelagic fish migrating and spreading in tropical and subtropical waters. In general, Indonesian waters are one of the tuna migration routes. The waters of the western part of Indonesia, especially the Simeulue Waters in the Fisheries Management Area (WPP) 572 Indonesia. One type of tuna was found there, namely Gymnosarda unicolor species, Family: Scombridae with the general name dogtooth tuna while the local name is tuna sisik bergigi, and or tuna karang. The sustainability of tuna fish resources can be threatened if the size of the fish stock is not known, one of which is the suitable size for catching. This study examined and identified the length and growth pattern of tuna gigi anjing that were landed in fish auction area of East Simeulue. The research was conducted in May-July 2021. The data collected was in the form of fork length data. The frequency of fish length to determine the class interval was analyzed using the frequency distribution formula. Fish growth parameters were analyzed using ELEFAN 1 contained in the FISAT II program. The number of tuna gigi anjing (Gymnosarda unicolor (Ruppell)) that was successfully measured was 190 individuals. The size of the fork length ranged in the interval of 44.1 to 101.3 cm. The length of first caught tuna gigi anjing (Lc) was at 59.2 cm, the asymptotic length (L) was 98.7 cm, the growth rate (K) was 0.8/year and the zero fish growth rate (t0) was of 0.09/year.

Length-weight relationships and relative condition factor in razor surgeonfish, Prionurus laticlavius (Teleostei: Acanthuridae) from the southwestern coast of the Gulf of California, Mexico

The razor surgeonfish, Prionurus laticlavius, listed on the IUCN Red List of Threatened Species under the Least Concern category, plays an important ecological role as energy regulator in reef communities, but it is sought by the aquarium and aquaculture industry, and also is consumed by people. This study estimated the length-weight relationship (LWR) and relative condition factor (Krel) in P. laticlavius. The equation for length-weight relationship was TW= 0.02 TL3.01 for the Overall category, and the relative condition factor was 1.28. A total of 379 fish (3-45.6 cm total length) were collected in the southwestern coast of the Gulf of California using harpoon (for large fishes) and hand net (for small fishes), of which 181 were females (20.2-45.6 cm), 137 males (20.6-43.4 cm), and 61 juveniles (3-19.5 cm). Allometric coefficient b of LWR for the Overall category suggested isometric growth (b = 3.01), positive allometric growth for juveniles (b = 3.14), and negative allometric growth for adults (2.75). Results of Krel showed higher values for females during the warm season. The population showed a good nutritional condition, mainly in the warm season. This work is the first to estimate LWR and Krel for P. laticlavius, which will inform stock assessment and the management of this species in the Gulf of California.

Incidental fishing of Epinephelus itajara (Lichtenstein, 1822): the vulnerability of a critically endangered species of the Brazilian coast

Epinephelus itajara, popularly known as grouper, has a wide distribution and is considered the largest fish in the Atlantic Ocean. Behavioral characteristics allied to overfishing and degradation of marine habitats caused declines in E. itajara populations throughout the entire geographic distribution area. The goliath grouper is currently ranked on the IUCN RedList as Vulnerable in Global Status and Critically Endangered in the Gulf of Mexico. In Brazil, even with a fishing moratorium that has protected it since 2002, the species is categorized as critically endangered in the Red Book of the Instituto Chico Mendes de Conservação da Biodiversidade. Taking into account these factors and the information gap on the goliath grouper, this study addresses an account of an occasional “post mortem” encounter with a specimen on the edge of Pajuçara beach, Maceió - Alagoas. It was an adult individual, whose size was estimated at 1.6 m and 70 kg of body mass. This study also presents discussions about the conservation status of the species and the efficiency in the management of marine protected areas, revealing that even with many marine protection areas, incidental captures and port records are still recurrent on the Brazilian coast. This record highlights the need to reinforce inspections, in addition to suggesting that the management of these areas be more efficient, ensuring the reestablishment of grouper populations in the region.

Pollution Evaluation and Health Risk Assessment of Trace Metals in Eleven Tissues of Mylopharyngodon piceus Collected from an Aquaculture Pond in Huzhou, near Southern Taihu Lake

Pollution evaluation and health risk assessment are critical procedures for residents consuming black carp (Mylopharyngodon piceus) contaminated with non-essential hazardous trace elements in an artificial wetland also known as an aquaculture pond. Samples were collected, dissected and digested to analyze the pollution status and health risk associated with four heavy metals present in eleven tissues of black carp fish based on the metal pollution index (MPI) and target hazard quotient (THQ). The results indicated that the concentrations of Cd, Cr, Cu and Pb were 1.36 ± 0.04 mg/kg, 13.70 ± 0.50 mg/kg, 2.85 ± 0.10 mg/kg, and 4.98 ± 0.18 mg/kg in large black carp, while the concentrations of Cd, Cr, Cu and Pb were 4.27 ± 0.08 mg/kg, 50.84 ± 0.82 mg/kg, 9.33 ± 0.18 mg/kg, and 22.49 ± 0.42 mg/kg in small specimens. The MPI values showed that the heavy metal pollution detected in small fish was much more significant than in large fish. Notably, the polluted metals were more likely to accumulate in the viscera (e.g., brain and heart) rather than in the external tissues (e.g., muscle and epidermis). In addition, the estimated THQ and HI values for three edible tissues demonstrated that the health risk associated with muscle tissue intake of both small and large black carp was within the acceptable range, while the contaminants were likely to pose health risks associated with the consumption of fish head tissue. Small black carp are generally not fit for human consumption, thus both the epidermis and muscle of the fish are often cooked in China. However, the ingestion of large black carp is relatively safe. The contributions to THQ values of these four heavy metals decreased in the following sequence: Cr > Pb > Cd > Cu. Cr is the critical pollutant with its contribution to HI values measuring over 50%. We suggest that in artificial wetlands (e.g., aquaculture ponds) more attention should be paid to heavy metal pollution, the conservation of the aquaculture ecosystem, and effects on human health.

Temporal changes of the food web structure and function driven by changes in hydrological regimes and water quality in a large shallow lake

Assessing food web structural properties and energy fluxes under changing hydrological regimes and water quality reveals the temporal patterns of ecosystem dynamics in shallow lakes. Here, we studied northern China’s largest shallow lake (Lake Baiyangdian) using quantitative food web models for five representative years (1958, 1980, 1993, 2009, and 2019). We analyzed the temporal patterns of food web structure and function by combining a Bayesian isotope mixing model with a food web energetics model. We further examined the temporal changes of unweighted and weighted food web topological attributes. Lake Baiyangdian changed from a detritus-based into a phytoplankton-based food web based on the relative contributions of basal food sources and energy flux distributions. The trophic position of fingerlings, large omnivorous fish, and carnivorous fish decreased with increasing eutrophication. The highest energy fluxes were from detritus to zooplankton and mollusks in 1958, from detritus and phytoplankton to zooplankton in 1980, 1993, and 2009, and from phytoplankton to zooplankton and detritus to mollusks in 2019. The highest total energy flux was in 1993, followed by 2019, with the lowest value in 1958. Unweighted food web metrics showed similar patterns. We observed more pronounced temporal variability in the node- and link-weighted food web metrics than in the unweighted metrics. In addition, hydrological factors (threshold, duration, reversals between high, and low water levels), eutrophication, and some water quality factors (chemical oxygen demand, dissolved oxygen, and pH) played important roles in the temporal changes of food web dynamics in Lake Baiyangdian. Our findings demonstrate the importance of integrating unweighted and weighted indicators to holistically comprehend how highly aggregated food webs respond to changing hydrological regimes and water quality, thereby improving management and restoration of shallow lake ecosystems.

The influence of macrophyte ecological groups on food web components of temperate freshwater lakes

Aquatic macrophyte taxonomic composition, species abundance and cover determine the physical structure, complexity and heterogeneity of aquatic habitats – the structuring role of macrophytes. These traits influence richness, distribution, feeding and strength of the relationships between food web communities in lakes. The aim of this study was to determine how lakes with different dominating macrophyte ecological groups affect planktonic food web components, emphasising the influence on young of year (YOY) fish and large (≥1 +) fish community. We hypothesised that different dominating macrophyte ecological groups have different structural effects on food web components and YOY fish growth, abundance and feeding. Studied lakes categorised into three different macrophyte ecological groups – lakes dominated by emergent, floating+floating-leaved or submerged vegetation. We found that all dominating ecological groups had a strong influence on plankton communities (except heterotrophic bacterioplankton and nanoflagellates), YOY fish and large fish. Floating-leaved plant dominance was positively related to planktonic food web structure and YOY fish weight, length, abundance and the consumption of zooplankton as a prey of all major species of YOY fishes. Larger fish tended to favour the presence of emergent vegetation. This conclusion has important implications for local managers and conservationists in respect to the maintenance and protection of littoral habitats and fish resources.

Functional diversity in herbivorous fishes on Caribbean reefs: The role of macroalgal traits in driving interspecific differences in feeding behavior

Understanding how functional traits drive plant-herbivore interactions often illuminates the mechanisms driving niche partitioning and functional diversity in this important guild of consumers. On coral reefs, a diverse suite of herbivorous fishes is key for controlling algal abundance and impacting benthic community dynamics. Here, we focus on how algal traits shape complementarity in feeding for a diverse suite of large herbivorous fishes on a coral reef in the wider Caribbean. We used feeding assays in the field to document feeding patterns of the different herbivorous fishes across 50 species of macroalgae. Then, we used a suite of macroalgal traits such as dry mass, nutrient content, calcification, and chemical defenses to assess how these traits influenced patterns of consumption. Algal defenses were the major traits determining the overall vulnerability of algae to herbivores, with uncalcified algae from undefended genera being consumed 40% more rapidly than algae that were either calcified or chemically defended. Nutrient content of algae explained little in patterns of overall herbivory. We found significant niche diversification in feeding both among and within genera with the three major genera having unique feeding patterns on different suites of algae (Acanthurus spp. - red macroalgae, Sparisoma spp. - brown and green macroalgae, and Scarus spp. - benthic filamentous algae). With respect to how algal traits influenced patterns of feeding in individual species, we found that: (1) some herbivore species were almost exclusively influenced by avoiding algal defenses, (2) some species showed strong interactions between algal defenses and nutrient content that determined their feeding, and (3) some species (Kyphosus spp., Pomacanthus arcuatus) targeted chemically-defended algae with high nutrient content. Our work highlights the role that algal traits play in influencing patterns in the diversity and redundancy of feeding in herbivorous fishes on coral reefs.

How catch uncertainty and harvest regulations drive anglers’ choice for pike (Esox lucius) fishing in the Baltic Sea

In the brackish lagoons located in coastal inshore areas around the German island of Rügen, pike Esox lucius is a highly sought after target species for recreational fishers. Yet, the pike stock is currently in decline, and conflicts with other user groups (specifically commercial fishers) have emerged. To inform future fisheries management, an improved understanding of the human dimensions of recreational fishers, including their preferences for catch outcomes and regulations, is relevant. Our objective was to better understand how German recreational pike fishers react to different types of harvest regulations in trade-off with risky or ambiguous catch outcomes for pike and other predators in the lagoon fishery. We conducted an online survey including choice experiments for angling experiences that differed in seven attributes. Data were analyzed using a latent class model. We found four distinct angler types differentiating resident and tourists and dependency on pike. Specifically, we identified a highly involved pike tourist (42 % average probability), a consumptive, generalist tourist angler type (18 %), a highly specialized local trophy pike angler type that readily engaged in catch-and-release (12 %), and a local generalist angler that also benefitted from the catch of other predators than pike (18 %). Despite heterogeneity in preferences, all four angler types generally accepted more restrictive regulations for pike removals than currently in place, and dispreferred low catch probabilities compared to catch ambiguity. Most angler types also revealed diminishing marginal utility returns for catch rates, where the first three pike in a day provide utility, while the marginal benefits of catch rate decrease to zero for more than three pike per day. Angler types varied more in their preferences for trophy pike, the general attractiveness of the pike as a target and the utility derived from catching fish other than pike. Particularly the more specialized angler types focused on size rather than catch rate. Despite preference heterogeneity, focusing management attention on maintaining a sufficiently high catch rate and increasing the number of trophy pike in the stock seems to satisfy the expectations of most angler types. To achieve such outcomes, the anglers of all types would overwhelmingly support implementing harvest slots of 50–85 cm that allow both small and large fish to be released (rather than the current minimum-length limit of 50 cm), and accept reductions in the daily bag limit to 1 pike per day (relative to the current of 3 pike per day). Implementing these policy measures would unlikely generate substantial conflict with anglers represented in our survey.

Retinal ganglion cell topography and spatial resolution in the smelt Hypomesus japonicus (Brevoort, 1856)

AbstractThe present study deals with the topography of retinal ganglion cells (GCs) and spatial resolution in the smelt Hypomesus japonicus. The eyes and retinae were examined by light microscopy and computerized tomography. DAPI labelling was used to visualize cell nuclei in the ganglion cell and inner plexiform layers. Two zones of increased GC density in the nasal and temporal retina were bridged by a horizontal streak with the GC density ranging from 5600 to 8000 cells/mm2. The maximum cell density (area retinae temporalis) ranged from 9492 to 14,112 cells/mm2, and the total number of GCs varied from 286 x 103 to 326 x 103 cells in three individuals. The theoretical anatomical spatial resolution (the anatomical estimate of the upper limit of visual acuity) was minimum in the ventral periphery (smaller fish, 1.43 cpd; larger fish, 1.37 cpd) and maximum in area retinae temporalis (smaller fish, 2.83 cpd; larger fish, 2.41 cpd). The relatively high density of GCs and presence of the horizontal streak and area retinae temporalis in the H. japonicus are consistent with its highly visual behaviour. The present findings contribute to better understanding of the factors affecting the topography of retinal ganglion cells and mechanisms of visual adaptation in fish.

Brown Trout Upstream Passage Performance for a Fishway with Water Drops between Pools beyond Fish Passage Design Recommendations

This work aims to assess brown trout (Salmo trutta) passage through a free-flow pool-weir-type fishway with hydrodynamic notches and extreme water drops between pools. It consists of an old-school fishway design, commonly constructed in salmon rivers of Spain during the period of 1950–1980. To assess their performance, a field test was designed with confined trial conditions during the spawning migratory season. The mean water drop between pools was 0.65 m and the total water height considered for the trial was 11.8 m. The monitoring was carried out using PIT telemetry. The initial hypothesis, considering the fishway design and assessment guidelines, classified this structure as hardly insurmountable. Results showed an ascent success of 19% with a median transit time of 29.1 min/m of ascended height. Larger fish and fishway sections with lower values of volumetric power dissipation were related to a better performance in the passage. The results suggest that in certain circumstances, such as limited construction areas where other design or management options are difficult to implement (e.g., canyons), this type of fishway may be an alternative for the upstream passage of at least a small proportion of the brown trout population, although a selection effect is expected.

Availability of Marine Fishes in Cox’s Bazar, Bangladesh: A Case Study on the BFDC Landing Center

Abstract Fish availability in the coastal landing center highlights the assumption of stocks in the marine fishing zone of the ocean. This study, therefore, aimed to analyze the availability of marine fishes in the Bangladesh Fisheries Development Corporation (BFDC) landing center, Cox’s Bazar, Bangladesh between January 2021 and May 2021. A total of 54 species were recorded, of which 42 were marine fishes, 7 were shellfishes and 5 were large fishes. The dominant orders were Perciformes (56%), Scombriformes (17%) and Clupeiformes (10%). More than 56% of the total marine fishes were classified as Least Concern, nearly 10% were categorized as Near Threatened and 2% were marked Vulnerable. The dominant orders of shellfish were Portunidae (43%), followed by Penaidae (29%), Loligonidae (14%) and Octopopidae (14%). Shrimp Penaeous monodon had the highest consumer demand, whereas consumer demand for non-conventional shellfish was comparatively low. Most of the shellfish were categorized as Least Concern. Among large fishes, the wider availability of sharks (five species) and rays (two species) was observed in the winter and monsoon season, although the consumer demand for those large fishes was low. The Vulnerable sharks and rays were Sphyma zygaena and Mobula birostris. This study elucidates the present scenario of marine fishes in the BFDC fish landing center, Cox’s Bazar, Bangladesh.

Factors contributing to anglers’ satisfaction and their requests concerning recreational salmonid fisheries management: Insights from a questionnaire survey in strictly regulated, catch-and-release stream fishing areas

Regional fisheries managers need to consider anglers’ satisfaction and requests and balance them with resource conservation when making decisions to promote the sustainability of recreational fisheries. However, there is a marked paucity of information on inland recreational fisheries, particularly in non-Western countries. This study was designed to gain insight on the factors contributing to the anglers' satisfaction and their requests concerning salmonid recreational fisheries management. The approach used was a questionnaire survey of salmonid anglers in two strictly regulated, reservation-only, catch-and-release stream fishing areas in Japan. We evaluated the relationships among anglers’ characteristics and the fishing experience with their satisfaction degree. We also investigated the anglers’ requests concerning the use of fishing license proceeds. The catch numbers of the larger masu salmon available in the area (e.g., 20–40 cm standard length) were positively related to the satisfaction degree of anglers. This study confirms the findings on the determinants of anglers’ satisfaction in Western countries, especially concerning avid anglers. The highest priority for using the fishing license proceeds was for enhancing the number of fish and conserving the fish habitats. Surprisingly, enhancing the number of large fish received the lowest priority among the requests. These results suggest that anglers’ requests for recreational fisheries management do not necessarily reflect their preference to catch more and larger fish. Thus, reflecting only the anglers’ requests and discontinuing stocking the special areas with large fish could paradoxically reduce the anglers’ satisfaction. These results strongly point out that understanding the factors contributing to anglers' satisfaction is crucial to avoid misunderstandings and make sounder and more sustainable recreational fisheries management decisions.

KARAKTERISTIK BIOLOGIS IKAN TONGKOL (Euthynnus affnis) HASIL TANGKAPAN PURSE SEINE DI PERAIRAN HERLANG TELUK BONE, INDONESIA

Purse seine fishing gear is designed to catch both large and small pelagic fish, and commonly used in tuna fisheries. However, this gear is not size-selective, and can pose a threat to fish stocks. The aim of this study was to obtain information on the biological characteristics of little tuna (Euthynnus affinis) caught by purse seiners operating in the waters of Herlang District in Bulukumba Regency, South Sulawesi, Indonesia. Data collected from March to May 2019 included little tuna fork length (FL), weight, sex, and gonad maturity stage (GMS). Fish characteristics analysed in this study were length-weight relationship, size distribution, life phase, and gonadal maturity. Analyses were performed in MS Excel 2010 and Graphpad Prism v.5.03. Male and female little tuna had isometric growth patterns. Modal length class was 24.0-24.4 cm for males and 23.0-23.4 cm for females. All fish caught were below 30 cm TL, while the mean size at first maturity for this species is 39.8 cm. Although the catch was dominated by immature fish (GMS I and GMS II), all GMS stages were present for both sexes, with around a third of females in GMS III-V.
 Keywords: Scombridae, gonad maturity stage, length-weight relationship, early maturation, fishing pressure.

Spatio-temporal dynamics of bluefin tuna (Thunnus thynnus) in US waters of the northwest Atlantic

Atlantic bluefin tuna (Thunnus thynnus) are large, highly migratory fish that support important fisheries. Oceanic conditions influence Atlantic bluefin tuna distribution and it has been hypothesized that stock distributions have shifted in recent years. Distributional shifts can affect regional availability and fleet catchability, introducing a potential bias in fisheries dependent data used for indexing population trends. We developed a vector auto-regressive spatio-temporal model (VAST) to estimate changes in bluefin tuna spatial distribution in US waters and created standardized indices of abundance for large (>177 cm) and small size classes (≤ 177 cm) of fish. Local-scale environmental factors (sea surface temperature (SST), ocean depth) and regional-scale drivers (e.g., Atlantic Multidecadal Oscillation (AMO) and prey biomass) of spatial distribution were explored. Results indicated that from 1993 to 2020, spatial distribution of the larger size class was highly variable, but on average, the total estimated area occupied increased by 96 km2/year and the center of gravity shifted 2 km/year north and 3 km/year east. Results were similar for the smaller size class fish with an average increase in area occupied of 71 km2/year. The center of gravity shifted an average of 1 km/year north and 2 km/year east. The primary factor driving the spatial shifts for both large and small fish was local-scale SST. Standardized indices of abundance were produced and incorporated SST as a covariate of local density. In comparison to prior standardization results, spatio-temporal indices demonstrated less inter-annual variability and produced similar overall trends. This study advanced our understanding of bluefin tuna spatial distributions and generated indices of relative abundance in US waters of the Northwest Atlantic that are more robust to spatio-temporal changes in tuna distributions for consideration in future stock assessments.

Larger Nile tilapia struggle in hot water

When your metabolism depends on the temperature of your surroundings, climate change represents a unique and alarming challenge. As temperatures increase, your metabolic rate rises too, requiring that you consume more oxygen to maintain your body at a faster pace. ‘At warmer temperatures, larger individuals may be prone to limitations in their capacity for oxygen uptake’, says David McKenzie, at the CNRS's Centre for Marine Biodiversity Exploitation and Conservation, Montpellier, France, with the consequence that larger fish may struggle to survive at higher temperatures. ‘If true, this would be important in projecting how global warming will affect fish populations’, says McKenzie; the size distribution of fish populations could become smaller as temperatures rise. ‘But evidence is lacking’, McKenzie admits. So, he and colleagues from the Federal University of São Carlos, Brazil, decided to find out how well Nile tilapia (Oreochromis niloticus), ranging in size from 21 to 313 g, managed to keep swimming at high speed as the temperature rose. First, the team tested how resilient the resting fish were as the water temperature rose. Initially, the fish began behaving erratically but eventually they toppled over when the temperature was too high. However, the largest fish were as resilient as the smallest; getting enough oxygen was not an issue when the fish were resting. ‘They may lose their balance for another reason when they are warmed up while resting in a tank’, McKenzie says.He and his colleagues then set the fish a challenge, encouraging them to swim at 85% of their top speed while the researchers gently increased the water temperature, and they found that the largest animals struggled most as the temperature rose. They gave up swimming at lower temperatures (around 37°C), while the smaller fish managed to keep going at temperatures up to 39.5°C. And when the team investigated the fish's oxygen consumption as the temperature increased, they found the largest fish got out of breath at far lower oxygen consumption rates (∼18 mmol O2 kg−1 h−1) than their smaller counterparts (41 mmol O2 kg−1 h−1).So, larger tilapia struggle to keep swimming as the temperature rises and they seem to find it difficult to get enough oxygen, which could have implications for the size of fish in future populations. McKenzie suspects that ‘Global warming may favour the small’, as smaller individuals may be able to take the heat while larger fish may have more trouble surviving in hot water.