Assessment of Sparus aurata (Gilthead Seabream) stock in Bardawil lagoon, southeast Mediterranean Sea

During present study period from October 2020 to April 2021, gilthead seabream Sparus aurata stock was estimated by using many assessment models to evaluate actual status on Bardawil lagoon. Length at first capture (Lc) was 17.10 and 16.70 cm for females and males respectively, but it was 16.9 cm for all individuals that corresponding age at first capture (tc= 2.49 year). Length at first recruit Lr was 10.70 and 9.70 cm for females and males, respectively, corresponding age tr= 0.77 year. The length at first maturity was Lm= 18.18 cm that corresponding age tm= 2.85 years. Virtual population analysis shows some of intensive capture on S. aurata at its early life’s. Yield per recruit and biomass per recruit tend to collapse as a result of high fishing mortalities. The current study concludes an exact overfishing pressure of S. aurata stock in Bardawil lagoon.

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.

Stock assessment of the Tigertooth croaker, Otolithes ruber (Bloch & Schneider, 1801) from the commercial prawn trawl fishery by-catch in coastal Kenya

Commercial bottom prawn trawling has been reported to generate a higher proportion of by-catch of up to 70% in Kenya. The Tigertooth croaker, Otolithes ruber is one of the species caught in large quantities as commercial by-catch and also by artisanal fishers. This has led to growing concern that the species could be at risk of over-exploitation. The purpose of this study was to carry out a stock assessment of O. ruber. Stock assessment parameters were estimated using ELEFAN with the generic algorithm as included in the R package TropFishR. The length-converted catch curve and the length-based yield per recruit model were employed. The exploitation rate (F/Z = 0.71) indicates that the stock is overfished based on the length-converted catch curve. The current fishing mortality (F = 2.3) based on the catch curve is larger than the reference level ( = 1.1) based on the yield per recruit analysis and also indicates that the stock is overfished (= 2.09). To reverse the current trend of exploitation, improved management of the stock is required, which should include further studies on other by-catch species and the generation of data to capture the whole fishery for a better estimation of stock status.

Using length data to derive biological reference points for kiddi shrimp Parapenaeopsis stylifera (Milne Edwards, 1837) from the south-eastern Arabian Sea, India

Parapenaeopsis stylifera, a major commercial penaeid shrimp fishery resource in the Indian Ocean, has lacked adequate information on life history parameters for nearly two decades. In this study, growth and mortality parameters of P. stylifera from the southwest coast of India were estimated using length data and used to derive biological reference points for the species. The asymptotic length for females was L∞ = 131 mm; k = 1.1 y−1 and for males L∞ = 117 mm; k = 1.25 y−1. Mortality parameter estimates were Z = 4.42, M = 1.24, F = 3.18 y−1 and exploitation rate E = 0.72 for females; Z = 5.76, M = 1.39, F = 4.37 y−1 and E = 0.76 for males. Thomson and Bell yield biomass, Beverton and Holt yield per recruit, and relative yield per recruit models were applied to predict the stock status and length cohort analysis for estimating the stock size. The Beverton and Holt analysis gave Emax = 0.69 in females and 0.75 for males, which is below the Ecurrent values obtained for the sexes. The Thomson and Bell analysis indicated that if Fcurrent at which the yield is 121 460 t in females and in males 128 064 t is further increased, rise in yield will be modest. B/B0 and SB/SB0 at Fcurrent were 24% and 18% for females and 21% and 16% for males, respectively. Target reference point F0.1 and F0.5 at different levels of age at capture tc (0.5, 0.6, 0.7 and 0.8 yrs) was estimated by Beverton and Holt yield per recruit model. The outcome from these models forms integral inputs for multispecies/multigear tropical fisheries management. Parapenaeopsis stylifera is one of the inshore penaeid shrimp identified by the Marine Stewardship Council for certification from the region and, moreover, biological reference points are a prerequisite to assessment and management of tropical multispecies fisheries for ecosystem-based fisheries management.

Growth, Mortality and Yield-per-recruit of Nile Tilapia (Oreochromis niloticusM/em>) 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

Population Dynamics and Management of Invasive Blue Tilapia (Oreochromis aureus) in Garmat Ali River, Basrah, Iraq

The blue tilapia, Oreochromis aureus is an invasive species that has successfully established itself in most of the Iraqi waters. However, there is little information on the population dynamics of the species in these waters. Hence, the growth parameters, mortality rates, probability of capture, recruitment pattern and yield per recruit of blue tilapia in Garmat Ali River, Iraq was assessed using FiSAT II software. A total of 1664 blue tilapia fish were collected by different fishing gears from October 2019 to September 2020 for recording the relevant data. The length-weight relationship obtained was W=0.0147*L3.0748 for fish ranging from 7.5 to 26.3 cm total length suggesting that the species shows positive allometric growth. The asymptotic length (L∞), growth constant (K), theoretical age at zero-length (t0), growth performance index (Ø') and longevity (tmax) were 29.9 cm, 0.205, -1.293, 2.345 and 10.7 years, respectively. The total mortality (Z), natural (M), fishing (F) and exploitation (E) were 1.09, 0.61, 0.48 and 0.43, respectively. Length at first capture (L50) was found to be 13.92 cm. The main recruitment pulse was from March to July with a peak in April, which account for 18.4% of the total recruitment in the year. The relative yield per recruit analysis revealed that the present exploitation rate (Epresent) for blue tilapia was below than the biological target reference points (E0.1 and Emax), which denotes that this stock was not over-exploited. For management purposes, higher yields can be achieved by reducing the mesh sizes of the nets during fishing.

Stock assessment of the peacock bass Cichla temensis (Humboldt, 1821), an important fishing resource from the middle Negro river, Amazonas, Brazil

Peacock bass Cichla temensis is an important species at the Amazon basin, since commercial, subsistence and recreational fisheries simultaneously exploit it. Cichla temensis is the preferred species by recreational fishers and it has been strongly exploited, mainly at the Negro river, the second largest tributary of the Amazon River. It was used data from experimental fisheries, collected at the middle stretch of Negro river, which were coupled with previously published data on its population dynamics, to run a yield per recruit model and build scenarios of sustainable fisheries. The results showed that the age of the first catch is a key variable to successful management of the peacock bass stocks at this region.

Cormorant (Phalacrocorax carbo) predation on a coastal perch (Perca fluviatilis) population: estimated effects based on PIT tag mark-recapture experiment

The number of cormorants has rapidly increased in the northernmost Baltic Sea. In 2018, 50 km × 50 km ICES catch rectangle 55H1 had 3140 breeding pairs. To estimate the predation effect of cormorants on perch populations, we Passive Integrated Tags tagged 1977 perch and 9.9% of tags were found. The median instantaneous cormorant-induced mortality during the breeding time, with consumption by non-breeding individuals, was estimated at 0.23 and at 0.35 during the whole residing period. We estimated with a yeild-per-recruit model that the long-term maximum loss of perch yield of tagged sub-population would be at 80% probability interval 32–67%, and when extended to the entire 55H1, 10–33%, respectively. The cormorants’ share of the >2-year-old perch biomass and production would be 8%, while that of other natural mortality would be 63% and that of fishing 29% in 55H1. The yield-per-recruit-results should be interpreted as an estimate of maximum cormorant effect because the dependence of predation rate on prey density was not accounted for, and density-dependence of growth, mortality, and reproduction of perch could partly compensate the loss. The results indicate that high density of cormorants can reduce the perch stocks and catches locally.