Reproductive resilience: a paradigm shift in understanding spawner‐recruit systems in exploited marine fish

A close relationship between adult abundance and stock productivity may not exist for many marine fish stocks, resulting in concern that the management goal of maximum sustainable yield is either inefficient or risky. Although reproductive success is tightly coupled with adult abundance and fecundity in many terrestrial animals, in exploited marine fish where and when fish spawn and consequent dispersal dynamics may have a greater impact. Here, we propose an eco‐evolutionary perspective, reproductive resilience, to understand connectivity and productivity in marine fish. Reproductive resilience is the capacity of a population to maintain the reproductive success needed to result in long‐term population stability despite disturbances. A stock's reproductive resilience is driven by the underlying traits in its spawner‐recruit system, selected for over evolutionary timescales, and the ecological context within which it is operating. Spawner‐recruit systems are species specific, have both density‐dependent and fitness feedback loops and are made up of fixed, behavioural and ecologically variable traits. They operate over multiple temporal, spatial and biological scales, with trait diversity affecting reproductive resilience at both the population and individual (i.e. portfolio) scales. Models of spawner‐recruit systems fall within three categories: (i) two‐dimensional models (i.e. spawner and recruit); (ii) process‐based biophysical dispersal models which integrate physical and environmental processes into understanding recruitment; and (iii) complex spatially explicit integrated life cycle models. We review these models and their underlying assumptions about reproductive success vs. our emerging mechanistic understanding. We conclude with practical guidelines for integrating reproductive resilience into assessments of population connectivity and stock productivity.     

Exogenous xylanase improves growth,protein digestibility and digestive enzymes activities in Nile tilapia,Oreochromis niloticus,fed different ratios of fish meal to sunflower meal

A 3 × 2 factorial experiment was designed to evaluate the effect of different ratios of fish meal (FM): sunflower meal (SFM) with or without exogenous xylanase supplementation on growth, feed utilization, digestive enzymes activities, apparent digestibility, intestinal and liver morphology and chemical composition of Nile tilapia, Oreochromis niloticus. Three isonitrogenous (329.80 g/kg of crude protein) and isoenergetic (18.46 MJ/kg gross energy) experimental diets were formulated as SFM₁ (FM:SFM = 2:1), SFM₂ (FM:SFM = 1:1) and SFM₃ (FM:SFM = 1:2) based on protein content. Each diet was supplemented with 0 or 0.5 g/kg of exogenous xylanase and was fed to triplicate groups of twelve fish (with initial weight, 1.31 ± 0.02 g) for 84 days. After 84 days of feeding period, the highest weight gain, specific growth rate, protein efficiency, protein productive value and the best feed conversion ratio were recorded in fish fed either SFM₁ or SFM₂ supplemented with exogenous xylanase. Whereas lowest growth performance was recorded in fish fed SFM₂ and SFM₃ un‐supplemented with xylanase. The highest activities of chymotrypsin, trypsin, lipase, amylase, alkaline phosphatase and cholecystokinin were observed in fish fed SFM₁ and SFM₂ diets supplemented with xylanase. The highest ADCs of dry matter, protein, lipid and digestible energy were recorded in fish fed SFM₁ and SFM₂ diets supplemented with exogenous xylanase. Supplementation of exogenous xylanase improved muscularis mucosa thickness, height of mucosal folds and enterocytes of intestinal fish. Addition of exogenous xylanase increased the calcium and phosphorus retention. Results of this study indicated that the addition of exogenous xylanase to diet containing high inclusion level of sunflower meal improved growth, digestive enzymes, nutrient digestibility, histological morphometric of liver and intestine and nutrient retention.     

Paraphoma root rot of alfalfa (Medicago sativa) in Inner Mongolia,China

Root rot symptoms were observed in fields of alfalfa in Chifeng city, Inner Mongolia, China in 2016. Disease incidences of seven alfalfa varieties planted in 2014 ranged from 56% to 95%, while incidence of Gongnong No. 1 planted in 2016 was 8%, 31% and 76% in 2016, 2017 and 2018, respectively. Paraphoma isolates were consistently recovered from black necrotic root tissues of diseased plants with a frequency of 77.1%. Based on morphological characters and phylogenetic analysis of rDNA internal transcribed spacer (ITS), elongation factor 1-α (EF1-α) and β-tubulin (TUB), this fungus was identified as Paraphoma radicina. Glasshouse pathogenicity experiments showed that P. radicina significantly reduced above- and below-ground biomass of alfalfa plants 2 months after inoculation. Paraphoma radicina infected 70% of the plants inoculated with a root dip in conidia, and these symptoms were consistent with the symptoms in the field. Paraphoma radicina was successfully reisolated from disease roots of the inoculated alfalfa plants. This is the first report of P. radicina as the causal agent of alfalfa root rot in China.     

Comparison of growth,survivorship, seed production and shedding of eight weed species in a wheat crop in Western Australia

Development of integrated weed management strategies is dependent on a thorough knowledge of the demography of individual species. The current research established eight winter or summer weed species in a winter annual wheat cropping system at Wongan Hills, Western Australia, and investigated emergence of the first cohort of each species, survivorship, plant size, seed production and seed shedding over three years (2016–2019). The winter weeds Bromus diandrus and Lolium rigidum emerged at the same time as the wheat crop, and the initial cohort of marked plants had 100% survival to seed production in each year. By comparison, other winter weed species like Hordeum leporinum, Rumex hypogaeus, Sonchus oleraceus and Polygonum aviculare frequently emerged later than the crop and had a lower percentage of plants surviving to seed production. However, individual S. oleraceus and P. aviculare plants had the greatest seed production compared to other species. All winter weeds had variable patterns of seed shedding between years, with the exception of L. rigidum. Summer weed species emerged at the same time, but plants in the initial cohort of each species did not always survive to produce seed. The early emergence and high survivorship of B. diandrus indicates high competitive ability, but shedding commenced at a similar time to L. rigidum and harvest weed seed control may be a viable control method for this species.     

Dietary Photoprotective Compounds Ameliorate UV Tolerance in Shrimp (Pleoticus muelleri) through Induction of Antioxidant Activity

The aim of this study was to determine the effects of ultraviolet radiation (UVR) on bioaccumulation of UV‐absorbing compounds acquired through the diet, in larvae and postlarvae of Pleoticus muelleri, and to assess tissue antioxidant activity, survival, and development. Mysis stage I were exposed to two artificial radiation treatments: M‐PAR (photosynthetically active radiation, range = 400–700 nm) and M‐PAR + UVR (280–700 nm). The experimental larvae received a mixed dietary treatment of Artemia persimilis and the microalga Pavlova lutheri, reared under two radiation regimes: PAR (D‐PAR) and PAR + UVR (D‐PAR + UVR). Shrimp from all treatments reached 8 d postlarval stage (PL8), except those under M‐PAR + UVR treatment fed the D‐PAR‐cultured algae, which had 0% survival. Larvae in M‐PAR + UVR and M‐PAR treatments fed with D‐PAR + UVR diet presented the highest survival rates (70 and 75%, respectively), with 37 and 41% increase in PL size. UV‐absorbing compounds were detected in microalgae and PL subject to PAR + UVR treatments. Antioxidant activity, quantified by measuring the free‐radical 1,1‐diphenyl‐2‐picrylhydrazyl in homogenates of PL8, decayed drastically under radiation treatment M‐PAR + UVR fed with algae of the D‐PAR + UVR treatment. It is concluded that the bioaccumulation of UV‐absorbing compounds and the highest antioxidant activity in PL could improve the biochemical and photophysiological responses of shrimp under UVR stress.