Peak Recruitment Research Articles

Distribution, drivers and population structure of the invasive alien snail Tarebia granifera in the Luvuvhu system, South Africa

AbstractInvasive alien species continue to spread and proliferate in waterways worldwide, but environmental drivers of invasion dynamics lack assessment. Knowledge gaps are pervasive in the Global South, where the frequent heavy human‐modification of rivers provides high opportunity for invasion. In southern Africa, the spatio‐temporal ecology of a widespread and high‐impact invasive alien snail, Tarebia granifera, and its management status is understudied. Here, an ecological assessment was conducted at seven sites around Nandoni Reservoir on the Luvuvhu River in South Africa. The distribution and densities of T. granifera were mapped and the potential drivers of population structure were explored. T. granifera was widespread at sites impacted to varying extents due to anthropogenic activity, with densities exceeding 500 individuals per square meter at the most impacted areas. T. granifera predominantly preferred shallow and sandy environments, being significantly associated with sediment (i.e., chlorophyll‐a, Mn, SOC, SOM) and water (i.e., pH, conductivity, TDS) variables. T. granifera seemed to exhibit two recruitment peaks in November and March, identified via size‐based stock assessment. Sediment parameters (i.e., sediment organic matter, sediment organic carbon, manganese) and water chemistry (i.e., pH, total dissolved solids, conductivity) were found to be important in structuring T. granifera populations, with overall snail densities highest during the summer season. We provide important autecological information and insights on the distribution and extent of the spread of T. granifera. This may help in the development of invasive alien snail management action plans within the region, as well as modelling efforts to predict invasion patterns elsewhere based on environmental characteristics.

Population attributes of Littoraria angulifera (Gastropoda: Littorinidae) in mangroves in Bahia State, northeastern Brazil.

Littoraria angulifera (Lamarck, 1822) is an estuarine gastropod of tropical occurrence, which lives mainly on trunks of mangrove tree species. This study aimed to evaluate the population attributes of this species, such as abundance, space-time distribution, sex ratio and recruitment in mangroves in Bahia State, northeastern Brazil. The specimens were collected monthly throughout 2018, on trunks of the red mangrove Rhizophora mangle L. at two heights and on two horizontal levels. Specimens that were on the marine grass Spartina alterniflora Loisel were also collected. After counting, biometrics and sexing, the specimens were returned alive to the environment. The average size and weight of L. angulifera was higher (p<0.05) in places with taller and less sparse trees and the vertical distribution on the trunks showed a preference for strata close to the soil. Both results are related to shading and protection against desiccation. Females were more abundant than males, at an approximate sex ratio F: M of 1.4: 1. Recruits were observed throughout the period, showing continuous reproduction of the species with a recruitment peak in spring (September to November). The study revealed the importance of keep the mangroves intact to allow the maintenance of the natural stocks of the species.

Pull the trigger: interplay between benthic and pelagic cues driving the early recruitment of a natural bivalve assemblage

AbstractLarval settlement and recruitment are crucial phases in the benthic‐pelagic life cycle of marine benthic invertebrates that controls population dynamic and habitat connectivity. Our study investigated potential triggers driving the settlement of bivalve larvae in a highly dynamic intertidal coarse sand habitat. The early recruitment rate of five dominant bivalve families and abiotic conditions, particulate (&lt;20 µm) organic matter and sediment organic matter, were monitored from May to October 2014. Pelagic particulate organic matter (&lt;20 µm) was dominated by picoplankton throughout the sampling period, with a substantial diatom bloom in spring. Sediment was characterized by fresh organic matter in spring, as suggested by the dominant contribution of polyunsaturated fatty acids, and by a higher proportion of bacterial fatty acid markers during late summer. Different dynamics were also observed in early bivalve recruitment rates, with four different patterns observed over the sampling period. Multiple regression analysis on selected bivalve families showed species‐specific responses to trophic settlement triggers. Indeed, the larva recruitment rate of Mytilidae paralleled pelagic concentration of picoeucaryotes, with the peak early recruitment rate occurring in spring. Surprisingly, the early recruitment rate of Mactridae larvae showed a significant relation to bacterial concentration in the surficial sediment at the end of summer. While the Mytilidae results in such a eutrophic system confirmed those of a previous study in oligotrophic lagoons, therefore supporting the trophic settlement trigger hypothesis, more work is needed to understand the potential role of bacteria in the early recruitment of Mactridae. These results highlight for the first‐time inter‐specific differences in trophic cues that potentially trigger primary settlement in natural bivalve assemblages.

Extremely high but localized pulses of coral recruitment in the southwestern lagoon of New Caledonia and implications for conservation

Recruitment processes largely drive spatial distributions, dynamics, and recovery potential of marine communities. Determining scales of variation in recruitment rates and composition can help in understanding population replenishment mechanisms, while identifying recruitment hotspots is crucial for improving conservation strategies, particularly for threatened marine ecosystems such as coral reefs. We examined the spatial and interannual variability (2012-2014) of coral recruitment at multiple scales within and among reef habitats (14 stations) in the southwestern lagoon of New Caledonia. Recruit assemblages were characterized by high recruitment rates compared to other regions (overall mean of 34.9 recruits per 11 × 11 × 1 cm settlement tile, corresponding to 1220.9 recruits m-2) and strong dominance of Acroporidae. We found a marked spatial heterogeneity among habitats but also exceptionally high interannual variation (100-fold), with extreme recruitment peaks (up to 13572.8 recruits m-2, with a maximum of 811 recruits on a single tile) recorded in 2014 at some fringing and mid-shelf reefs, the highest records ever reported to date. These encouraging results contrast with other reefs where recent declines in coral recruitment rates have been documented with major concerns for their resilience capacities. However, the marked spatio-temporal variability of coral recruitment complicates conservation strategies, as it makes it difficult to identify ‘recruitment hotspots’ as priority sites to protect for their potential capacity to boost the replenishment of local populations.

Episodes of high recruitment buffer against climate-driven mass mortality events in a North Pacific seabird population.

Longitudinal studies of marked animals provide an opportunity to assess the relative contributions of survival and reproductive output to population dynamics and change. Cassin's auklets are a long-lived seabird that maximizes annual reproductive effort in resource-rich years through a behaviour called double brooding, the initiation of a second breeding attempt following the success of the first during the same season. Our objective was to explore whether double brooding influenced population change by contributing a greater number of future recruits. We fit temporal symmetry models to 32 years of mark-recapture data of Cassin's auklets to infer the mechanisms underlying the observed variability in per capita recruitment rates. We found that periodic peaks in recruitment were explained by an increase in available nest sites, the proportion of the population double brooding 4years prior, and spring upwelling conditions. Estimates of population change suggests a relatively stable population throughout the time series, attributable to a 'floating' demographic class of sexually mature individuals excluded from breeding by competition which quickly fill vacant sites following periods of low adult survival. Our results highlight the importance of recruitment in maintaining the population of a long-lived seabird periodically impacted by adverse environmental conditions.

Internal wave intensity and angle of propagation modulate small-scale settlement patterns of intertidal barnacles during peak recruitment

The settlement of intertidal barnacles Chthamalus spp. was measured for an entire recruitment season at 3 sites separated by 100 m in Baja California, Mexico. During a 10 d pulse that accounted for nearly 30% of yearly settlement, coastal wind speed and direction, nearshore water column temperature, and current velocities were also measured. During this period, strong internal tidal forcing was observed, with short episodes (1-1.5 h) of rapid fluctuations in water column temperature, stratification, and currents. Chthamalus spp. settlement was significantly and positively correlated with cumulative high-frequency fluctuations in temperature, thermal stratification, and surface current flows, but not with onshore winds. Furthermore, the spatial pattern in the number of settlers was correlated with the angle of propagation estimated for onshore-moving internal waves, potentially modulated by nearshore bathymetry. This relationship between Chthamalus spp. settlement and high-frequency changes in water column temperature was also shown for a site in La Jolla, California, USA, but not for a similar barnacle, Balanus glandula, suggesting that interspecific differences in larval behavior and nearshore depth distribution may translate into differences in transport. Our results suggest that both the number of settlers at shore and their spatial pattern can be modulated by the intensity and direction of internal wave events, at least during peak settlement periods. Future research should consider both the intensity and geometry of internal-wave-induced variability in the nearshore, as well the small-scale features of coastal geomorphology and bathymetry.

Intrinsic motoneuron excitability is reduced in soleus and tibialis anterior of older adults.

Age-related deterioration within both motoneuron and monoaminergic systems should theoretically reduce neuromodulation by weakening motoneuronal persistent inward current (PIC) amplitude. However, this assumption remains untested. Surface electromyographic signals were collected using two 32-channel electrode matrices placed on soleus and tibialis anterior of 25 older adults (70 ± 4 years) and 17 young adults (29 ± 5 years) to investigate motor unit discharge behaviors. Participants performed triangular-shaped plantar and dorsiflexion contractions to 20% of maximum torque at a rise-decline rate of 2%/s of each participant’s maximal torque. Pairwise and composite paired-motor unit analyses were adopted to calculate delta frequency (ΔF), which has been used to differentiate between the effects of synaptic excitation and intrinsic motoneuronal properties and is assumed to be proportional to PIC amplitude. Soleus and tibialis anterior motor units in older adults had lower ΔFs calculated with either the pairwise [-0.99 and -1.46 pps; -35.4 and -33.5%, respectively] or composite (-1.18 and -2.28 pps; -32.1 and -45.2%, respectively) methods. Their motor units also had lower peak discharge rates (-2.14 and -2.03 pps; -19.7 and -13.9%, respectively) and recruitment thresholds (-1.50 and -2.06% of maximum, respectively) than young adults. These results demonstrate reduced intrinsic motoneuron excitability during low-force contractions in older adults, likely mediated by decreases in the amplitude of persistent inward currents. Our findings might be explained by deterioration in the motoneuron or monoaminergic systems and could contribute to the decline in motor function during aging; these assumptions should be explicitly tested in future investigations.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11357-021-00478-z.

Growth, mortality, exploitation rate and recruitment pattern of Octopus cyanea (Mollusca: Cephalopoda) in the WIO region: A case study from the Mafia Archipelago, Tanzania

Octopus cyanea is a commercially important cephalopod in the Western Indian Ocean (WIO) region, but scientific information to inform management strategies for the species is limited. A study was conducted in 2014, 2015, 2017 and 2018 to investigate biological parameters including growth, mortality, exploitation rates and recruitment patterns in the sea around Mafia Archipelago, Tanzania. Virtual population analysis (VPA) indicated differential mortality between two sampling sites; the lowest and highest fishing mortality of F = 1.5yr-1 and F = 2.7yr-1 were observed in Bwejuu (Dorsal mantle length, DMT = 18-20 cm) and Jibondo (DMT = 8-12 cm) fishing villages, respectively. The maximum exploitation rate (Emax), which gives the maximum relative yield per recruit, was estimated at 0.380 and 0.379 for Jibondo and Bwejuu, respectively. The exploitation rates E 0.5, which corresponded to 50% of the unexploited stock relative biomass per recruit, were estimated at 0.248 for Jibondo and 0.247 for Bwejuu. These values differ greatly from the exploitation rates of 0.53 and 0.41 for Jibondo and Bwejuu, respectively, suggesting that the stock of O. cyanea is probably being overfished both in terms of yield per recruit and biomass per recruit. The stock-recruit pattern was observed to be continuous year-round, with the peak being between May and July. Since the peak in recruitment of both areas coincides with the south-east monsoon (SE Monsoon) and the level of maximum sustainable yield has been overshot, it is recommended that management plans are implemented that will reduce effort while increasing biomass, for example, implementing temporal octopus fishery closures at a village level.

Quantifying the nature and strength of intraspecific density dependence in Arctic mosquitoes.

Processes that change with density are inherent in all populations, yet quantifying density dependence with empirical data remains a challenge. This is especially true for animals recruiting in patchy landscapes because heterogeneity in habitat quality in combination with habitat choice can obscure patterns expected from density dependence. Mosquitoes (Diptera: Culicidae) typically experience strong density dependence when larvae compete for food, however, effects vary across species and contexts. If populations experience intense intraspecific density-dependent mortality then overcompensation can occur, where the number of survivors declines at high densities producing complex endogenous dynamics. To seek generalizations about density dependence in a widespread species of Arctic mosquito, Aedes nigripes, we combined a laboratory experiment, field observations, and modeling approaches. We evaluated alternative formulations of discrete population models and compared best-performing models from our lab study to larval densities from ponds in western Greenland. Survivorship curves from the lab were the best fit by a Hassell model with compensating density dependence (equivalent to a Beverton-Holt model) where peak recruitment ranged from 8 to 80 mosquitoes per liter depending on resource supply. In contrast, our field data did not show a signal of strong density dependence, suggesting that other processes such as predation may lower realized densities in nature, and that expected patterns may be obscured because larval abundance covaries with resources (cryptic density dependence). Our study emphasizes the importance of covariation between the environment, habitat choice, and density dependence in understanding population dynamics across landscapes, and demonstrates the value of pairing lab and field studies.

Year-round high abundances of the world\u2019s smallest marine vertebrate (Schindleria) in the Red Sea and worldwide associations with lunar phases

Very little is known about the ecology and biology of the smallest marine vertebrates, fishes in the genus Schindleria. Even though over half of named Schindleria species have been identified in the Red Sea, the collection of only very few specimens has been documented. Here, we assessed abundance patterns of nearly two thousand Red Sea long dorsal fin (LDF) adults and found evidence for putative seasonal and spatial differences, likely related to differing habitat and environmental conditions. The highest abundances were outside local seasonal temperature extremes and decoupled from peaks of coral reef fish recruitment. We also found evidence for global trends in abundances related to lunar cycles using our Red Sea data and that from a recently published large collection of specimens from the DANA Expedition (1928–1930). The abundance of adult LDF Schindleria in relation to lunar phases differed significantly, with most Schindleria caught outside the full moon, and mostly during the new moon in the Red Sea and the 3rd quarter moon in the DANA collection. We further suggest that the abundances of Schindleria at coral reefs may be related to reproductive cycles and that these cycles may be timed with the moon as back-calculations of hatch dates from otoliths from the Red Sea significantly resulted after the new moon, making Schindleria the fastest-lived coral reef fish with the shortest generation times. Schindleria could be the most numerous coral reef fish in the world, for which we encourage increased research.

Population dynamics of pronghorn spiny lobster (Panulirus penicillatus) in Palabuhanratu Bay, Sukabumi, West Java

Pronghorn spiny lobster (Panulirus penicillatus) is one of the dominant lobsters and has high economic value. The fishing activities are conducted continuously and put aside the preservation of these resources. This study purposed to analyze the population dynamics of pronghorn spiny lobster (Panulirus penicillatus) in Pelabuhanratu Bay, Sukabumi, West Java. The data were collected from January until December 2016 with samples from local fisherman. Lobsters were measured carapace length and total weight. Data analysis included the relationship between length and weight, growth parameters, recruitment patterns, mortality, exploitation rate, length at first maturity or incubating eggs (Lr), and length for first capture (Lc). Panulirus penicillatus from the Palabuhanratu Bay has growth pattern of a negative allometric in both males and females. The asymptotic carapace length (CL∞) of males is higher than females while the growth coefficient (K) of males is lower than females. The peak recruitment for males is expected to be in April and July, while for females in May and August. The total mortality rate of males is higher than the natural mortality rate and vice versa for females. The male lobsters have occurred overexploitation, while females have not occurred overexploitation. The female lobsters have an Lr &lt;Lc value which indicates that the female lobsters are caught after incubating the eggs.

Population dynamics of Arabian yellowfin seabream, Acanthopagrus arabicus Iwatsuki, 2013 from Iraqi marine waters, Arabian Gulf

In this study, the population dynamics of the Arabian yellow fin sea bream, Acanthopagrus arabicus in the Iraqi marine waters, Arabian Gulf were assessed, to contributing towards the development of management plans for its sustainable exploitation. A. arabicus is one of the species caught in large quantities as commercial by artisanal fishers. Fish samples were collected by the Shaheen steel-hulled dhow and from the artisanal fishermen from February 2020 to January 2021. The length-weight relationship was described as W = 0.030L2.867, indicating a negative allometric growth pattern. Of 3484 individuals ranging in total length from 13.0 to 41.0 cm were used to evaluate the growth and mortality parameters. The asymptotic length (L∞), growth rate (K) and growth performance index (O') were 44.6 cm, 0.34 and 2.830, respectively. The total mortality (Z) was calculated as 1.17, while the natural and fishing mortality rates were 0.75 and 0.42, respectively, and the current exploitation rate (Epresent) was 0.36. Length at first capture (L50) was 21.9 cm. The recruitment pattern occurs throughout the year, with two recruitment peaks. The yield per recruit analysis indicates that the current exploitation rate was below the economic yield target and the maximum sustainable yield, indicating that this stock underexploited. Virtual population analysis results showed that the mid-lengths (16-22 cm) experienced the highest fishing mortality. So, for management purposes, more yields could be obtained by increasing the fishing activities on this species for a substantial harvest.

Bio ecology of the white shrimp Litopenaeus schmitti in Babitonga Bay. Do the current regulation of closed seasons is suitable to the lifecycle of this species?

The white shrimp Litopenaeus schimitti is the main way of living for artisanal fishermen who use the gerival for their capture in one of the main estuaries off south Brazil: Babitonga Bay. The abundance of the species is regulated by closed fishing seasons to protect spawning and recruitment, which may not in accordance with species’ life cycle. The present study evaluated the size structure, maturation, and abundance of white shrimp, to verify the effectiveness of these measures for the region. The highest abundances of the L. schmitti occurred between February and May, both in the inner bay and the adjacent marine area. Spawning occurred in the marine environment around 15 m depth, mainly in October and November, with the subsequent entry of post-larvae into the estuary around December, being available for artisanal fishing in January in the inner bay area. In February and March, juveniles migrated to the adjacent marine area, defining the main recruitment peak. Some lagging shrimp remained within the bay until September and October, with a complete disappearance in December. The first size of maturation for the white shrimp females was estimated at 15.16 cm. The results hereby obtained showed that the current closed season partially protects white shrimp spawning and recruitment in Babitonga Bay and the adjacent marine area suggesting that fishing restrictions during the months of February and March may benefit juvenile specimen’s migration from the estuary, however, the protection of the spawning females in October/November could also be taken into account as a way of maintaining stocks of white shrimp in the region.

Role of Non-Coding Regulatory Elements in the Control of GR-Dependent Gene Expression.

The glucocorticoid receptor (GR, also known as NR3C1) coordinates molecular responses to stress. It is a potent transcription activator and repressor that influences hundreds of genes. Enhancers are non-coding DNA regions outside of the core promoters that increase transcriptional activity via long-distance interactions. Active GR binds to pre-existing enhancer sites and recruits further factors, including EP300, a known transcriptional coactivator. However, it is not known how the timing of GR-binding-induced enhancer remodeling relates to transcriptional changes. Here we analyze data from the ENCODE project that provides ChIP-Seq and RNA-Seq data at distinct time points after dexamethasone exposure of human A549 epithelial-like cell line. This study aimed to investigate the temporal interplay between GR binding, enhancer remodeling, and gene expression. By investigating a single distal GR-binding site for each differentially upregulated gene, we show that transcriptional changes follow GR binding, and that the largest enhancer remodeling coincides in time with the highest gene expression changes. A detailed analysis of the time course showed that for upregulated genes, enhancer activation persists after gene expression changes settle. Moreover, genes with the largest change in EP300 binding showed the highest expression dynamics before the peak of EP300 recruitment. Overall, our results show that enhancer remodeling may not directly be driving gene expression dynamics but rather be a consequence of expression activation.

Some population parameters of the common carp (Cyprinus carpio, 1758) in Lake Paniai, Papua

Common carp (Cyprinus carpio) is known as introduced fish on some lakes in Indonesia and included in herbivore fish. The common carp population in Lake is the dominant number, so that it’s become at of fishing target. Estimation of some parameters of common carp population in Lake Paniai was aimed to evaluate the growth parameters, mortality, the fishing rate and recruitment pattern. Collecting and measuring the fish sample was done in the period from February to October 2016 by using the fishing gear of gill nets with mesh sizes of 1.00 - 4.50 inches. The results showed that common carp population in Lake Paniai was dominated by individual lengths between 15-25 cm with a frequency of 55.78%, the growth pattern of male fish was allometric (-) and females were isometric. Asymptotic length (L∞) = 61.43 cm and the growth coefficient (K) = 0.32 per year. The rate of natural mortality (M) = 0.65 per year, the fishing mortality rate (F) = 0.52 per year, the total mortality rate (Z) = 1.17 per year and the exploitation rate (E) = 0.44. The exploitation rate of common carp fish was still below the optimum value, thus an attempt to catch this fish, could be still improved. Common carp populations caught by mostly fishermen have had a chance to spawning thus common carp fish population in the lake was estimated to still be preserved and could be utilized in a sustainably. Peak recruitment occurred only once in a year, namely in June.

Some biological parameters of Glossogobius aureus population from the Mekong Delta

Glossogobius aureus is an important commercial fish for food supply and has been caught increasingly, but data on its population biology is limited. The present study was carried out along coastlines from Soc Trang to Bac Lieu, southern Vietnam to contribute new information on the biological characteristics of G. aureus population. Data analysis of length-frequency distribution of 666 individuals showed that the equation of von Bertalanffy curve of this species was . The species displayed 4.47 y longevity and 2.75 in the growth performance. The length-converted catch curve analysis showed that the fishing, natural and total mortalities were 2.73 yr-1, 1.52 yr-1 and 4.25 yr-1 respectively and fish exploitation rate was 0.64. There was one recruitment peak in July and the analyses of relative yield-per-recruit and biomass-per-recruit gave Emax=0.408, E0.1=0.284 and E0.5=0.404. This goby is potentially an aquaculture species due to its high growth rate. The fish stock has been overfished, so the gill net mesh size should be increased and fish should not be caught during the recruitment period for sustainable management.

Age, growth and mortality of the anchovy Stolephorus commersonnii (Lacepède,1803) (Clupeiformes) caught off the coast of Tanga, Tanzania

The population dynamics of Stolephorus commersonnii (Lacepède, 1803) from a ringnet fishery operating off the northern coast of Tanga Region were evaluated based on monthly length-frequency data collected from August 2016 to August 2017. The total length (TL) and total weight (TW) of 14,410 individuals ranged from 22 to 130 mm and from 0.39 to 14.64 g respectively. S. commersonnii exhibited a negative allometric growth pattern with the length-weight relationship model: W = 0.00001 x L2.886. The von Bertalanffy growth function was Lt = 86.03 x (1 – e–1.19(t – (–0.01))) using ELEFAN I from the FiSAT II software tool package. Growth performance index and longevity were estimated at (ɸ) = 3.9 and Tmax = 2.5 yrs, respectively. The total (Z), fishing (F) and natural (M) mortalities were determined at 1.39, 0.53 and 0.86 yr-1, respectively. The current exploitation rate (Ecur) was estimated at 0.38. S. commersonnii exhibited a year-round breeding pattern, with two recruitment peaks in March and June/July. Length-at-first-capture (Lc50) and length-at-first-sexual maturity (Lm50) were 40.51 and 57.35 mm TL, respectively, suggesting growth overfishing. The stock of anchovy indicates an overfishing scenario requiring management intervention such as reducing fishing effort levels, increasing mesh sizes and introducing seasonal closures during peak spawning periods.

Population parameters of the orange mud crab Scylla olivacea (Herbst, 1796) from the Sundarban mangrove forest in Bangladesh

Population parameters of orange mud crab (Scylla olivacea) were estimated, aiming to determine sex ratios, carapace width-body weight (CW-BW) relationships, asymptotic width (CW∝), growth coefficient (K), mortality (Z, M, and F), recruitment and level of exploitation (E) in the Sundarban mangrove forest, located in the south-western part of Bangladesh. Year-round samples were collected using hook-lines and baited traps, the population parameters were measured from CW frequency data using FiSAT-II analyzer. The study showed that the overall male and female sex ratio was 1:0.66, revealing a male domination in the study area. The CW-BW relationship indicated that the increment rate in the BW of the male crabs (b = 3.06, R2 = 0.98) were higher than that of female (b = 2.62; R2 = 0.98) S. olivacea. The b value differed significantly (P < 0.006) from isometric growth (b = 3) where males exhibited positive and females exhibited negative growth allometry. Estimated CW∝ for male and female were 164 mm and 152 mm along with K values 0.90 yr−1 and 0.76 yr−1, respectively. Total mortality (Z) was 2.67 yr−1 and 1.57 yr−1, natural mortality (M) was 0.98 yr−1 and 0.90 yr−1 and fishing mortality (F) was 1.69 yr−1 and 0.67 yr−1 for male and female, accordingly. Recruitment of both sexes exhibited a bimodal recruitment pattern where young population occurs continuously throughout the year and a major peak of recruitment for males was observed from November to January and for female it was from February to April. The estimated exploitation rate (E) for male (0.63) was higher than the female (0.43) where the E for male exceeded the maximum permissible limit (E = 0.50). A remarkable range of fishing pressure at lower size classes was revealed in this study and thus framing minimum legal size is crucial for effective management of the population.

By-catch-associated demographics of two threatened seahorses from the south-east coast of India

Seahorses comprise a charismatic, high-value and high conservation-concern group of fishes, whose demographics, including the dynamics of populations, are poorly studied. In this paper, we fill this key knowledge gap by determining the population dynamics of two threatened species, Hippocampus kuda and H. trimaculatus, from the south-east coast of India using samples encountered in trawl by-catch. Hippocampus kuda showed a comparatively greater asymptotic length, whereas H. trimaculatus had a greater growth coefficient and growth performance index. Further, H. trimaculatus showed two recruitment peaks (March/April and December) in a year, against a single peak (February) for H. kuda. An assessment of mortality rates (instantaneous total, natural and fishing) indicated a high vulnerability of the local population of both species to fishing pressure. Hippocampus kuda had a greater mean length at first capture than H. trimaculatus. Biomass-per-recruit analysis showed that the exploitation of H. kuda is closer to the exploitation rates under which stock will be reduced to half the unexploited biomass, suggesting their extreme vulnerability to overexploitation, even as ‘by-catch’. Demographic parameters of the two threatened seahorses indicated that even as ‘incidental catch’, these species are vulnerable to overfishing, and species-specific conservation guidelines need to be developed and their on-ground implementation and enforcement ensured.

Growth, Mortality and Yield-per-recruit of Nile Tilapia (&lt;em&gt;Oreochromis niloticusM/em&gt;) in Garmat Ali River, Iraq

The study was conducted to estimate the growth, mortality, recruitment and yield-per-recruit of Oreochromis niloticus from Garmat Ali River, Basrah, Iraq from October 2019 to September 2020. The population parameters were analyzed using the FAO-ICLARM stock assessment tool (FiSAT). A total of 2696 O. niloticus ranged from 7.0 to 25.0 cm and the sizes 13.0-18.0 cm constituted 64.2% of the total catch. The length-weight relationship was W= 0.012L3.109 suggesting that the species shows positive allometric growth. The growth parameters for the species were estimated as L∞= 30.45 cm, K= 0.45, to= -0.313 and Ǿ= 2.622. The coefficients of total mortality (Z), natural mortality (M) and fishing mortality (F) were 3.26, 1.03 and 2.24, respectively. The exploitation rate for the species computed to be 0.69. Fish were recruited to the fishery at a mean size of L50= 14.92 cm. The peak of recruitment was 23.51% in June. The analysis of yield-per-recruit (Y/R’) indicates that stock is not being overfished since the present exploitation rate was below the biological target reference points (E0.1 and Emax). So, more yields could be achieved by reducing the mesh sizes of the nets for fishing the species