Mature Fish Research Articles

COVID-19's effects and adaptation strategies in fisheries and aquaculture sector: An empirical evidence from Bangladesh

The COVID-19 pandemic has wreaked havoc on the aquaculture and fisheries sector all around the world, with the impact being exacerbated in developing countries. This study is an endeavor to identify consequences of the COVID-19 on fisheries and aquaculture sectors based on primary data collected from Bangladesh as an empirical case study. The data were collected through face-to-face interviews with different supply chain actors while analyzed using descriptive statistics and a problem confrontation index. As results depicted, income and employment across fish farmers, fishers, and traders were severely hurt, with a drastic fall in the market demand, coupled with a severe drop in their fish consumption. As market demand declined, fish farmers must be stocked mature fish for an extra period, and feed costs raised, eventually increasing the overall production cost. Besides, inaccessibility to inputs also made fish production and catch more troublesome. The price of all the major cultured and captured species plunged, leading to a depressing return to farmers, while inputs price underwent a significant increase except for labor and fingerling. However, traders seemed to be the worst sufferers amid striking disruption in fish value chain, which ostracized the preponderance of the traders from the chain. Some of the prime obstacles that constrained the production and trading process were but not limited to higher transportation costs, labor shortage, inability to pay for the wage, and reduced consumer demand across fish farmers, fishers, and traders. Nevertheless, our article further identified a myriad of strategies that the fish farmers, fishers, and traders followed to heal the scar of the fisheries and aquaculture sector with hands-on actions.

Morphoregression and Reproduction Aspect of Bonylip Barb (<I>Osteochilus vitattus</I> Valenciennes, 1842) in Tamblingan Lake, Bali Island

Highlight Research Morphoregression and reproduction aspect of bonylip barb was analyzed Fork length more accurately to estimate the body weight Lm50 of bonylip barb in Tamblingan Lake was 116 mmTL Growth pattern is isometric Good reproduction aspect Abstract Bonylip barb (Osteochilus vitattus Valenciennes, 1842) is one of many native fish inhabiting Tamblingan Lake. The information about this species in Tamblingan Lake is rare which requires fulfilment in information gaps for the management of bonylip barb. The aims of this research were to find out the length-length relationship, length-weight relationship, length at first maturity, sex ratio, gonadal maturity stage, gonadosomatic index (GSI), fecundity, as well as spawning period and location in Tamblingan Lake. The fish sample was captured with experimental gillnet that was set in the afternoon and hauled in the next morning. Length and weight of every sample were taken. All samples were dissected to observe the sex and gonad. Fork length was the type of length with higher accuracy to estimate the body weight than the other length character. Growth pattern of this species was isometric. Lm50 of bonylip barb in Tamblingan Lake was 116 mmTL. Sex ratio of bonylip barb was imbalanced (1.00:0.82). Gonad maturity stage I-V was found during the research with GSI between 0.16-15.50 for male fish and 0.43-32.82 for female fish. The highest GSI was found in March, 15.50 for male fish and 32.73 for female fish. Mature fish were discovered in all stations in every month of sampling. The fecundity of bonylip barb ranged between 2,792-279,326 eggs. The length-length and the length-weight showed a strong relationship. The fish was isometric. Based on the growth pattern and the reproductive aspects, the population of bonylip barb in Tamblingan Lake was in good condition.

Patterns of fish community structure in protected and non-protected marine areas of mainland Tanzania

Information on the benefits of Marine Protected Areas (MPAs) for the condition of fish stocks is not well documented in Tanzania. Fish landing sites located in Tanga and Mtwara regions were surveyed to assess patterns of fish community structure; particularly fish abundance, species diversity, growth patterns, and maturity stages, based on catches landed at sites with different protection status. Fish abundance in the catch from protected areas was significantly lower than in non-protected areas (p=0.002). Species diversity was relatively higher in catches from non-protected (H=2.742) compared to protected areas (H=2.232). A high percentage of species (63.24 %) exhibiting negative allometric growth was observed in catches from non-protected areas. Further, a large number of mature fish was observed in catches from protected areas compared to non-protected areas (p<0.01). These indices are useful indicators of the performance of MPAs. The observed negative allometric growth and reduced number of mature fishes in the non-protected areas suggest that extractive pressure and disturbances from fishing gears have negative impacts on the fish stock. Continued high extraction may induce a decline in general fish size due to the constant selection for large-trait fish specimens, potentially causing evolutionally change in morphological traits. In contrast, the lower abundance and species diversity from the protected areas reflected low catch effort as a result of regulated fishing pressure in MPAs, rather than indicating the actual diversity in the fish stocks in these protected waters. Based on these findings it is recommended that more regulatory strategies are implemented in non-protected waters to allow more time for fish to attain appropriate harvest sizes and to ensure the effective protection of marine resources.

Removal of malignant melanophoroma with intramuscular invasion in a bowfin fish (Amia calva)

AbstractA mature male bowfin fish (Amia calva) presented with a left‐sided lateral caudoventral coelomic swelling. Diagnostic imaging (computed tomography and ultrasound) revealed a focal, expansile muscular mass measuring 6.4 × 3.8 cm in diameter and contiguous with the musculature of the left body wall at the level of the pelvic fin. The mass was surgically excised. Histopathology categorized the mass as a malignant melanophoroma. The patient made a full recovery and lived 4 additional years. The patient died from unrelated causes, and necropsy revealed no evidence of recurrence of neoplasia. This is the first documentation of surgical treatment of a melanophoroma in A calva.

Characteristics of protein, lipid, and carbohydrate metabolism of fish of the Kremenchuk Reservoir in the prespawning period

The paper presents the results of scientific research aimed at studying the peculiarities of metabolism in the body of seven species of mature fish in the Kremenchuk reservoir in the pre-spawning period under ecological conditions that differ from existing ones according to the Dnipro Reservoir Rules of Operation. Somewhat increased levels of total protein accumulation were found during this period in the muscles of zander, perch, and gibel carp. More statistically significant differences between the content of total protein in the liver and muscles were found in other fish species, in particular in roach it was 51.2%, in bream – 57.8%, in European flounder, and zope – 40.6%. Slightly elevated total lipids were found in the muscles of these fish. Thus, in the muscles of silver bream, it was 12.07 mg/g of raw weight, and in the muscles of gibel carp – 18.5 mg/g, while in the muscles of all other studied species of fish, this figure was in the range of 6.7 to 8.71 mg/g of raw weight. The glycogen content in the muscles of different objects of the Kremenchuk reservoir in the pre-spawning period was different. Its highest content was found in the gibel carp muscle, which reached 74 mg/g of raw weight. Significantly lower (2.7 times) was the level of glycogen accumulation in zander muscles and 3.2 times – in roach muscles. In the muscles of bream, European perch and silver bream found close, relatively low levels of glycogen, which was in the range of 10 – 13 mg/g of raw mass, and the lowest level of its accumulation was recorded in the muscles of the zope (only 4.9 mg/g). The glycogen content of the liver of all studied fish species significantly exceeded that recorded in their muscles.

Broad Whitefish (Coregonus nasus) isotopic niches: Stable isotopes reveal diverse foraging strategies and habitat use in Arctic Alaska.

Understanding the ecological niche of some fishes is complicated by their frequent use of a broad range of food resources and habitats across space and time. Little is known about Broad Whitefish (Coregonus nasus) ecological niches in Arctic landscapes even though they are an important subsistence species for Alaska's Indigenous communities. We investigated the foraging ecology and habitat use of Broad Whitefish via stable isotope analyses of muscle and liver tissue and otoliths from mature fish migrating in the Colville River within Arctic Alaska. The range of δ13C (-31.8- -21.9‰) and δ15N (6.6-13.1‰) across tissue types and among individuals overlapped with isotope values previously observed in Arctic lakes and rivers, estuaries, and nearshore marine habitat. The large range of δ18O (4.5-10.9‰) and δD (-237.6- -158.9‰) suggests fish utilized a broad spectrum of habitats across elevational and latitudinal gradients. Cluster analysis of muscle δ13C', δ15N, δ18O, and δD indicated that Broad Whitefish occupied four different foraging niches that relied on marine and land-based (i.e., freshwater and terrestrial) food sources to varying degrees. Most individuals had isotopic signatures representative of coastal freshwater habitat (Group 3; 25%) or coastal lagoon and delta habitat (Group 1; 57%), while individuals that mainly utilized inland freshwater (Group 4; 4%) and nearshore marine habitats (Group 2; 14%) represented smaller proportions. Otolith microchemistry confirmed that individuals with more enriched muscle tissue δ13C', δD, and δ18O tended to use marine habitats, while individuals that mainly used freshwater habitats had values that were less enriched. The isotopic niches identified here represent important foraging habitats utilized by Broad Whitefish. To preserve access to these diverse habitats it will be important to limit barriers along nearshore areas and reduce impacts like roads and climate change on natural flow regimes. Maintaining these diverse connected habitats will facilitate long-term population stability, buffering populations from future environmental and anthropogenic perturbations.

Kisspeptin Exhibits Stimulatory Effects on Expression of the Genes for Kisspeptin Receptor, GnRH1 and GTH Subunits in a Gonadal Stage-Dependent Manner in the Grass Puffer, a Semilunar-Synchronized Spawner.

Kisspeptin has an important role in the regulation of reproduction by directly stimulating the secretion of gonadotropin-releasing hormone (GnRH) in mammals. In non-mammalian vertebrates, there are multiple kisspeptins (Kiss1 and Kiss2) and kisspeptin receptor types, and the two kisspeptins in teleosts have different effects depending on fish species and reproductive stages, serving reproductive and non-reproductive functions. In the grass puffer, Takifugu alboplumbeus, which has only a single pair of kiss2 and kissr2, both genes display seasonal, diurnal, and circadian oscillations in expression in association with the periodic changes in reproductive functions. To elucidate the role of kisspeptin in this species, homologous kisspeptin peptide (gpKiss2) was administered at different reproductive stages (immature, mature and regressed) and the expression levels of the genes that constitute hypothalamo-pituitary-gonadal axis were examined in male grass puffer. gpKiss2 significantly elevated the expression levels of kissr2 and gnrh1 in the brain and kissr2, fshb and lhb in the pituitary of the immature and mature fish. No noticeable effect was observed for kiss2, gnih, gnihr, gnrh2 and gnrh3 in the brain and gpa in the pituitary. In the regressed fish, gpKiss2 was ineffective in stimulating the expression of the gnrh1 and GTH subunit genes, while it stimulated and downregulated the kissr2 expression in the brain and pituitary, respectively. The present results indicate that Kiss2 has a stimulatory role in the expression of GnRH1/GTH subunit genes by upregulating the kissr2 expression in the brain and pituitary at both immature and mature stages, but this role is mostly ineffective at regressed stage in the grass puffer.

Temporal variation in length-weight relationship, growth and condition factor of Acentrogobius viridipunctatus in the Mekong Delta, Viet Nam

The fish resource assessment has strongly related to their growth pattern estimated from fish length-weight relationship and condition factor, but these data on Acentrogobius viridipunctatus—a traded gobies in the Mekong Delta, Viet Nam have been limited. Consequently, the present study was performed for a year-round 2020 in order to provide knowledge on the length-weight relationship (LWR), growth and condition factor (CF) of this fish species. A total of 960 fish (432 females and 528 males) were caught at four coastal sites from Long Huu, Tra Vinh to Tan Thuan, Ca Mau. Due to high determination values of LWRs (r2 > 0.8), the fish weight could be estimated from a given fish length. Since the slope value (b= 2.64 ± 0.03) of LWR was <3 (p < 0.05), the goby exhibited negative allometry, implicating that practically fish was caught at a young age. The fish growth pattern did not vary with sex and size since males, females, immature fish, and mature fish displayed negative allometry. Likewise, this fish exhibited a negative allometric growth pattern in dry and wet seasons. The fish growth pattern did not vary with the studied site but changed with month, raging from negative allometry to isometry. The CF of males (1.04 ± 0.01) was significantly higher than that of females (0.99 ± 0.01) (t = 4.07, p < 0.001). Likewise, the CF in the wet season (0.96 ± 0.01) outnumbered in the dry season (1.09 ± 0.01) and was regulated by the season × site interaction (F = 53.60, p < 0.001). However, as the CF was close to 1⎯the well-being threshold, this fish adapted well to their habitat. The findings suggested that the size of fish caught should be increased for sustainable exploitation of this resource.

Condition Factor, Length-Weight Relationship and Sexual Maturity Stages of Labeobarbus tanensis in River Kathita of the Upper Tana River Basin of MT. Kenya

Information on health of fish populations and sexual maturity is useful in formulating fisheries management regulations. There is little such information on fish species of upper Tana River basin of Mt. Kenya. Therefore, a study was conducted to estimate condition factor, length-weight relationship, sexual maturity and length at 50% maturity of Labeobarbus tanensis during March - August 2020. Fish were caught in River Kathita and in adjacent four dams on upper Tana River using electrofishing equipment and gillnets respectively. Specimen of L. tanensis were sorted from the catch and lengths measured to the nearest 0.1 cm TL using a measuring board and weights to the nearest gram using a weighing balance. Specimens were dissected to reveal gonads which were further dissected to evaluate the sexual maturity stages of eggs on a 1 to 6 scale (Witte &amp; Vandensen, 1995). Estimated condition factor was greater than 1 indicating that the fish populations were healthy. Smallest mature male and female were 12.0 and 12.89 cm TL respectively. Length at 50% maturity for males and females were 13.9 and 14.9 cm TL respectively. Over 90% of mature fish were caught in Kathita River and few in dams. Length-weight relationship for both sexes are as follows: Females: Log W = 3.154 log TL - 2.104; R2 = 0.958. Males: log W = 3.184 log TL - 2.125; R2=0.960. Information obtained will be useful in formulating mesh-size regulations for sustainable exploitation of the fish.

Ecological Traits and Trophic Plasticity in The Greater Pipefish Syngnathus acus in the NW Iberian Peninsula.

Simple SummaryThe population of the pipefish Syngnathus acus inhabiting Cíes Archipelago (NW Spain) was monitored in 2017–2018 for spatial and temporal changes in abundances, reproduction traits, trophic niche occupancy, and dietary regimes across reproduction states, through an isotopic (δ13C and δ15N) approach. The population consisted almost exclusively of large adults, whose abundances decreased significantly from mid-autumn after the breeding season. S. acus is a secondary consumer that prefers amphipods, but mature specimens were less selective than immature fish. The present study highlights the outstanding size of the fish and the exceptional occurrence of breeders on the studied area.The great pipefish Syngnathus acus is one of the most representative European syngnathids, being highly associated with seagrass and macroalgal beds. Surprisingly, the ecology of this large ovoviviparous marine fish has received scanty attention. The population inhabiting three sites on Cíes Archipelago (Atlantic Islands National Park, NW Spain) was monitored in 2017–2018 for spatial and temporal changes in abundances, reproduction traits, trophic niche occupancy, and dietary regimes across reproduction states, through an isotopic (δ13C and δ15N) approach. Abundances were highly variable across seasons and sites, decreasing significantly from mid-autumn. The population consisted almost exclusively of large adults that migrate by the end of the breeding season, which extended from mid-spring to summer. Operational sex ratios suggest that the species is sex-role reversed. S. acus is a secondary consumer (Trophic position = 3.36 ± 0.05), preferring amphipods but displaying annual and seasonal dietary plasticity. Mature fish were less selective than immatures (especially females), with a higher preference for amphipods (36–68%) in the former. The second most-preferred prey were carideans, copepods, or isopods, depending on the year and the reproduction state. Overall, the wider trophic niches in females and immature specimens compared to males and mature fish would indicate a higher variability in both the use of prey resources and/or their origin. The present study highlights the trophic plasticity and unique features of S. acus population in the Cíes Archipelago, especially regarding the outstanding size of the fish and the exceptional occurrence of breeders.

Effects of laboratory salmon louse infection on mortality, growth, and sexual maturation in Atlantic salmon

Abstract Elevated salmon louse infection pressure generated by salmon farming represents a major threat for wild Atlantic salmon (Salmo salar). This study explored the effect of salmon lice on mortality, and body and gonad growth in F1 wild Atlantic salmon. Mature males (jacks) and immature fish were either infected with two different louse infection intensities (LIIs, 0.08 or 0.35 lice g−1) or kept as uninfected controls. Fish maturation was thereafter environmentally stimulated in seawater, followed by transfer to freshwater for 38 d to simulate river ascendance. No females matured, while 99% of the initially immature males started puberty. Jacks had high, and immature and maturing fish low, seawater mortality, independent of lice. The parasites had an LII-dependent negative effect on growth in length, weight, and condition factor in seawater. In freshwater, after the lice had detached, fish that were previously infected in seawater had reduced growth in length but not weight when compared to the uninfected control. The parasites did not affect relative gonad size in any fish phenotypes. The present results show that Atlantic salmon has a complex, and unexplored, regulation of growth when recovering from lice infection under laboratory settings. Further, the results suggest that possible negative effects of salmon louse on reproductive success in Atlantic salmon is most likely governed by the reduced body size and condition. However, further work on possible effects of salmon louse on semen quality is encouraged.

Evidence for overfishing of tigerfish Hydrocynus vittatus (Castelnau 1861) in the Kavango River, Namibia

The fishery for the African tigerfish Hydrocynus vittatus in northern Namibia makes a substantial contribution to surrounding food security and stimulates the local economy through subsistence and recreational fishing. However, local fishers suggest that catch rates of H. vittatus have declined and the fish are now smaller. The Namibian Ministry of Fisheries and Marine Resources conducted annual gill net surveys in the Kavango River from 1994 to 2018. These standardised surveys were used to compare catch per unit effort data and length frequencies of H. vittatus between intensively fished areas and a freshwater protected area (FPA). The catch per unit effort (by numbers and mass) and proportion (56% vs 10–20%) of mature fish (males >180 mm fork length and females >280 mm fork length) were higher inside the FPA, compared with those outside. The H. vittatus populations, both outside and inside the FPA, are subject to overfishing and management actions should be taken to secure this important fishery resource for subsistence and recreational fishers.

Modeling for Scenarios for Stock Recovery of Russian Sturgeon Acipenser gueldenstaedtii in the Sea of Azov in the Absence of Natural Reproduction

—A new model describing the dynamics of sturgeon (Acipenseridae) population unable to recover or ranked as depleted is proposed as a depleted artificially-stocking population (DAP) model. The model composed of two submodels is implemented in the R language. One of the submodels provides the opportunity to adjust the appropriate parameters used in a global model (the rate of natural mortality of a mature fish stock in a population and the artificial reproduction efficiency) with the optimization-based methods, while the second submodel deals with various scenarios for fish stock recovery over the forecast period. Scenarios are characterized by two parameters: the quantity of juvenils released by sturgeon hatcheries and the commercial fish catch decline caused by the illegal fishing practices and the fish catch for science-based targets. The estimates for the pattern of the Russian sturgeon Acipenser gueldenstaedtii in the Sea of Azov indicate that its stock may be recovered to the target level of 10 thousand tons with the baseline and optimistic scenarios by 2037—2048 relative to the artificial reproduction efficiency and the extent of illegal, unreported, and unregulated fishing. With respect to pessimistic scenarios, the stocks are not expected to recover.

Evaluation of the skill of length-based indicators to identify stock status and trends

Abstract In data-poor situations, length-based indicators (LBIs) and reference points based on life history parameters have been proposed to classify stocks according to conservation status and yield optimization. Given the variety of potential LBIs, life history traits, and fisheries, it is necessary to evaluate the robustness of length-based advice to ensure that despite uncertainty that management objectives will still be met. Therefore, a simulation procedure was employed where an Operating Model conditioned on life history parameters was used to generate pseudo data. Receiver operator characteristics and the true skill score were then used to screen LBIs based on their ability to identify overfishing and recovery. It was found that LBIs performed better for long-lived species with low individual growth rates, those aimed at ensuring the conservation of mature fish performed better than those aimed at the conservation of immature fish, are better at indicating trends than at quantifying exploitation level, and in general were robust to uncertainty about dynamic processes.

Liposome-Encapsulated Rec8 and Dmrt1 Plasmids Induce Red-Spotted Grouper (Epinephelus akaara) Testis Maturation.

In fish, the maturity of gonads plays an important role in the development and reproduction of the population, and it also dictates the success of captive breeding. Therefore, finding ways to promote gonadal maturation is an important goal in aquaculture. In this study, we injected recombinant dmrt1 and rec8 overexpression plasmids packaged in liposomes into the immature testis of red-spotted grouper (Epinephelus akaara) and measured the expression of Dmrt1 and Rec8 protein in vivo. Gonadosomatic index (GSI) and gonadal histology analyses showed that the testis developed from the immature to the mature state within 7days after plasmid injection. Additionally, the spermatozoa concentration and motility in plasmid-injected fish was the same as that of naturally mature fish. These results provided evidence that delivery of dmrt1 and rec8 expression plasmids into the testis via injection induced testis maturation in vivo.

Horizontal and Vertical Migration of Anglerfish Lophius piscatorius in Relation to Hydrography in Faroese Waters

Adult anglerfish conduct annual migrations between spawning areas and feeding areas; for Faroese waters this migration has so far not been described. Therefore, anglerfish migration and distribution in Faroese waters was investigated by mark-recapture studies, including data storage tags, as well as data from scientific trawl surveys, commercial trawlers and gillnetters. The fish distribution was compared to hydrographical conditions such as sea surface- and bottom temperature. A clear seasonal offshore-onshore migration was observed. Anglerfish occupied shallower waters (&amp;lt;200 m) during summer and deeper waters, especially on the western side of the Faroe shelf, during winter. This seasonal movement was most evident for sexually mature fish longer than 70 cm indicating spawning in deep waters during winter. Further, during winter anglerfish experienced cold water, which indicated that they were distributed close to the main interface of cold water masses surrounding the shelf. The fish were mostly located in warm water (6.5–11°C) and seldom occurred in colder waters than 4°C. Anglerfish appeared generally inactive, only around 5% of the total data storage tag recordings showed more than 5 m vertical movement between two subsequent hourly recordings. This vertical activity varied seasonally by being higher during winter than summer and diurnally by being higher during night time than day time, indicating that sunrise and day length played an important role in vertical movements.

Induction of spawning under artificial moonlight in the honeycomb grouper Epinephelus merra, a lunar-synchronized spawner

ABSTRACT In the reproductive season, lunar-synchronized fish spawn during species-specific moon phases. It remains unknown how they perceive cues from the moon for initiating gonadal development and subsequent spawning. We hypothesized that fish use moonlight to synchronize reproductive events. Using the honeycomb grouper Epinephelus merra, a tropical Serranidae, spawns around the time of the full moon, we report the successful induction of spawning in this species by controlling light condition at night. Following 3 months of acclimatization under long days (14:10) at 28 ± 1.0°C, mature fish were reared under an artificial full (experimental group) or new (controls) moon during scotophase. Rearing fish under these conditions for 1 month resulted in successful spawning in the experimental group between 21h00 and 01h30. Similar results were obtained when control fish were subdivided into artificial full and new moon groups and reared for an additional month. Fish in the experimental group had a high gonadosomatic index and abundant yolk-laden oocytes in the ovary. Therefore, moonlight triggers gonadal development and subsequent spawning in lunar-synchronized fish with a full-moon preference.

Aspects of the biology and population structure of Oreochromis niloticus, Coptodon zillii and Oreochromis leucostictus tilapia in Lake Naivasha, Kenya

AbstractThe Nile tilapia (Oreochromis niloticus), Redbelly tilapia (Coptodon zillii) and Blue spotted tilapia (Oreochromis leucostictus) were introduced into Lake Naivasha in the 1950s to diversify and boost the lake fisheries. These species have since been exploited in the commercial gillnet fishery. The fish stocks, however, are currently facing problems of pollution, wetland degradation and intense fishing pressures. Accordingly, the present study investigated some aspects of the biology and population structure of the three tilapiine species in light of changing lake conditions. Fish samples were collected from January to December 2019 using multifilament gillnets of 2.0‐to‐6.0‐inch mesh sizes. Immediately after their capture, the fish specimens were transported to the lab for morphometric measurements and analysis of maturity. The highest and lowest mean (±SD) total length (TL) were exhibited by O. niloticus (22.9 ± 5.2 cm) and C. zillii (14.5 ± 2.1 cm), respectively. Similarly, O. niloticus exhibited a higher mean weight (268.4 ± 18.0 g) and body depth (7.6 ± 2.5 cm), compared to the other species. O. niloticus exhibited an isometric growth pattern, while C. zillii and O. leucostictus exhibited negative allometric growth. The fish species were in good condition, with mean condition factors of 1.59 ± 0.07, 1.51 ± 0.06 and 1.23 ± 0.03 for C. zillii, O. leucostictus and O. niloticus, respectively. Furthermore, the condition factors for O. leucostictus and O. niloticus varied by size class, exhibiting a decreasing trend with increased fish size. The male:female sex ratio revealed significantly more males than females for O. niloticus (2.30:1.0) and O. leucostictus (2.36:1.0), although not for C. zillii (1.15:1.0). O. niloticus (28.0–29.0 cm TL) matured at a larger size, followed by O. leucostictus (21.0–26.0 cm TL), and C. zillii (15.0–17.0 cm TL). In comparison, values of length‐weight parameters, condition factor, sex ratio and size at first maturity of the studied fishes varied in Lake Naivasha, Victoria, Albert and George. The gillnet mesh ≤3.0″ mainly targeted small‐sized immature fish, while mesh ≥4″ targeted mature fish, particularly O. niloticus. It was concluded that gillnets with ≥4.0″ mesh could safely be applied for this fishery in Lake Naivasha, whereas the use of smaller mesh should be controlled to protect the juveniles as a means of maintaining sustainable fisheries. Furthermore, the present study also provided useful information, including size at maturity that is vital for setting mesh size for managing the lake fishery.

In vitro effect of cholinergic agonists and antagonists, activators and blockers of voltage-sensitive Ca++-channels and Na+-channels on arylalkylamine N-acetyltransferase 2 mRNA levels in pineal organ of the catfish, Clarias gariepinus

The fish pineal organ is innervated by cholinergic nerve fibres. But there is no information on cholinergic regulation of aanat2 gene in the fish pineal organ. Therefore, we investigated in vitro effect of specific agonists and antagonists of cholinergic receptors on aanat2 mRNA levels in the pineal organ of catfish Clarias gariepinus. Acclimatized mature male fish was decapitated and the pineal organ was rapidly removed and placed in culture medium and pre-incubated for 1h. After pre-incubation, the culture medium was replaced with the culture medium containing desired concentrations of specific ago-nists and antagonists of cholinergic receptors, blockers and activators of volt-age-sensitive Ca++ channels and Na+ channels for 1h, after which the pineal organs were collected and processed for measuring relative expression of aanat2 gene by measuring AANAT2 mRNA with the help of Real Time RT-PCR. Acetylcholine and nicotine produced dose-dependent inhibitory and stimulatory effect on aanat2 transcript levels, while carbachol increased aa-nat2 transcript levels in a dose-dependent manner. In vitro treatment of the pineal organ with atropine, hexamethonium, the blockers of voltage-sensitive Ca++ channels and Na+ channels significantly blocked the stimulato-ry effect of carbachol and nicotine on aanat2 transcript levels, respectively. Based on these findings, we suggest that cholinergic mechanism plays an important and direct role in regulation of aanat2 gene expression in the fish pineal organ, and stimulates the gene transcription via both muscarinic and nicotinic receptors. Further, muscarinic and nicotinic receptors seem to in-crease aanat2 transcript levels by affecting voltage-sensitive Ca++ channels and Na+ channels, respectively.

Reproductive dynamics of Pacific bluefin tuna (Thunnus orientalis) off the Nansei Islands, southern Japan

To describe seasonal changes in the reproductive traits of Pacific bluefin tuna (Thunnus orientalis), we examined the spawning season, spawning fraction (SF), spawning interval, batch fecundity (BF) and monthly changes in the horizontal distribution of spawning fish (SP) in the Nansei Islands and the waters off southwestern Honshu Island, Japan. SP were observed at sea surface temperatures (SSTs) between 19 and 29 °C, and the frequency of SP tended to increase with SST. SP were observed from early May to mid-July and most frequently in late June. The SP distribution in the Nansei Islands tended to expand northward with time. Mature female fish were widely distributed between the Nansei Islands and the waters off southwestern Honshu Island throughout the study period. The total SF during the spawning season was 0.24, which was equal to the 4.17-day spawning interval. The SF tended to increase with time. The number of days since the start of the fishing season (Days) positively affected the probability of the occurrence of specimens with postovulatory follicles among mature fish. The mean and relative BFs during the spawning season were 9.74 ± 6.72 (million oocytes) and 64.8 ± 38.4 (oocytes/g), respectively. The BF was positively affected by increases in the fork length (FL) and Days. These results suggest that the reproductive traits of Pacific bluefin tuna in the Nansei Islands are influenced by environmental and maternal conditions.