Optimal harvesting strategies: Practice versus theory

Abstract

Over the last decade or so, a number of bioeconomic models have been developed with the expressed objective of identifying the optimal time to harvest farmed fish. The models explicitly incorporate growth rates, prices and costs, but have generally excluded risk except implicitly through the discount rate. Implicit in this is the assumption that risk is continuous, such that the probability of significant changes in key parameters (e.g. prices, mortality rates etc) is independent of time. In reality, these factors can vary over time. In this paper, the optimal harvesting strategies for two farmed species were analysed using a bioeconomic model. The model was based on previous models applied in the literature, with a new feed‐use component applied. The theoretical ‘optimal’ harvest strategy was compared with actual behaviour, and the factors that may affect divergences in the results examined.     

Optimum krill phospholipids content in microdiets for gilthead seabream (Sparus aurata) larvae

The aim of the present study was to determine the optimum dietary levels of krill phospholipids (KPL) for sea bream (Sparus aurata) larvae, and its influence on larval development and digestive enzymes activity. Larvae were fed five formulated microdiets with five different levels of KPL. Complete replacement of live preys with the experimental microdiets for seabream larvae produced high survival and growth rates, particularly in fish fed the highest levels of KPL. In the present study, increase in dietary KPL up to 120 g kg^?1 (100 g kg^?1 total PL) significantly improved larval survival and growth, whereas further increase did not improve those parameters. An increase in alkaline phosphatase, trypsin and lipase activity with the elevation of KPL up to 120 g kg^?1 was also found denoting a better functioning of digestive system. Besides, there was a linear substrate stimulatory effect of dietary KPL on phospholipase A2 activity. Finally, increasing dietary KPL lead to better assimilation of n‐3 HUFA especially eicosapentaenoic acid, reflected in the higher content of these fatty acids in both neutral and polar lipids of the larvae. In summary, KPL were found to be an excellent source of lipids for seabream larvae. Optimum inclusion levels of this ingredient in microdiets to completely substitute live preys at this larval age were found to be 120 g kg^?1 KPL.     

插穗母树年龄和粗度对薄壳山核桃硬枝扦插的影响

探讨薄壳山核桃不同母树年龄和不同粗度对插穗生根能力的影响,采用1年生硬枝进行扦插试验.母树选择2、5、16年生和25年生,采用IBA-0.2 g· L-1处理插穗.插穗粗度设为≤0.5、0.5～0.8、≥0.8 cm3个径级,插穗用不同浓度的IBA、NAA进行处理.结果表明,2年生母树穗条的愈伤组织产生率和生根率分别为87.7％、14.4％,明显高于其他年龄母树.插穗粗度≥0.8cm穗条的效果最好,其平均愈伤组织产生率和生根率分别达33.1％和9.7％.≥0.8 cm穗条的IBA1.0 g·L-1+NAA0.25 g·L-1混合液处理效果最佳,2个指标分别达43.0％和14.6％.相同母树年龄的插穗其愈伤组织产生率和生根率与粗度成正相关.     

三种中国淡水鱼肌肉脂质的研究

测定了鲢、大口黑鲈和团头鲂三种养殖淡水鱼肌肉脂质的含量、分布、成分及脂肪酸组成,调查了鲢肌肉脂质含量与体重的关系。结果表明：（1）肌肉脂质的主要成分是甘油三酯和极性脂,中性脂含量影响决定了肌肉的总脂含量。（2）腹部肌肉的脂质含量是背部的2－5倍。（3）鲢肌肉的脂质含量随体重的增加而增加。（4）主要的脂肪酸有16：0、16：1n-9,18:2n-6,18:3n-3,20:5n-3和22：6n-3,多烯酸的含量是15．8－32．9％,∑n-3与∑n-6的比率是2．5（鲢）、2．7（大口黑鲈）和0．3（团头鲂）。     

不同倍性团头鲂群体遗传变异的初步分析

从鲤的微卫星DNA标记中筛选4对有效引物，进行不同倍性团头鲂遗传变异的微卫星DNA分析。结果表明：（1）4对引物中有2对能在不同倍性团头鲂五群体中探测到多态性，其等位基因数为4～6个，大小在131～307bp之间。发现两个等位基因（MFW19—175和MFW19—256）可作为异源3n特异的微卫星DNA标记。（2）不同倍性团头鲂五群体内的Shannon多样性指数为0．0562～0．2887，Nei’s基因多样性指数为0．0315～0．1969，平均遗传距离为0．0395～0．1542，不同倍性团头鲂群体存在较大的遗传变异。（3）反交3n、异源3n、正交3n和同源4n—F1群体的Shannon多样性指数和Nei’s基因多样性指数均显著地高于2n群体（P〈0．05）。（4）分子方差分析和聚类分析的结果表明：五群体分子系统树分成明显的两支，同源4n—F1、正交3n、反交3n和异源3n为一支，2n为另一支。     

Feeding requirements of sea bream (Sparus aurata) larvae under intensive rearing conditions: Development of a fuzzy logic controller for feeding

Applied methods for larval rearing suffer from the difficulty of accurately predicting larvae quantitative feeding requirements for optimum growth. Even when known, requirements change according to the behavior of each population reared. Computer based feeding systems require tools for adapting the delivered amounts of food to the needs of the reared population. In this work the daily feeding requirements of sea bream (Sparus aurata) larvae were estimated, and based on these estimations a fuzzy logic controller (FLC) for feeding was designed and developed. Sea bream larvae were reared, in triplicate, under intensive conditions in a pilot scale rearing system for 20 days with the pseudo-green water method. Larvae reached 6.7±0.6 mm (mean value and S.D.) at the end of the experimental period while the mean survival of the populations was 48.0±12.6%. Consumption was 0.017±0.009 mg of carbon per individual at first feeding and increased to 0.198±0.032 mg at day 20 post-hatching. The findings were used for the development of a FLC for feeding that adapts the amount of food delivered to the larvae according to changes in the reared population. The FLC utilizes five linguistic variables describing the state of the population and a rule base composed of 316 rules. The developed FLC allows the use of computer based feeding systems during rearing of early larval stages.     

鳊鱼鱼糜保藏特性的研究

对在不同温度（-80％,-20℃,0℃,5℃）下保藏了一个月的鳊鱼鱼糜品质进行比较和研究,检测了凝胶强度、挥发性盐基氮、肌球蛋白含量,保藏效果为：一80℃效果最好,-20℃其次,0℃一般,5℃变质严重,即温度越低,保藏效果越好;通过化学作用力、凝胶溶解率、巯基含量、SDS-聚丙烯酰胺凝胶电泳（SDS-PAGE）分析品质差异原因,结果发现：在鳊鱼鱼糜蛋白质构象稳定时,离子键作用力强、总巯基含量高。在品质较差的鱼糜中,疏水键和非二硫键较强。     

Growth performance and metabolic utilization of diets with native and waxy maize starch by gilthead sea bream (Sparus aurata) juveniles

The effect of dietary starch source and level on growth performance, feed utilization, apparent digestibility coefficients and liver enzyme activities involved in intermediary metabolism of gilthead sea bream juveniles was studied. Five isonitrogenous (47% crude protein) and isolipidic (15% crude lipids) diets were formulated to contain 10% native (diet NS10) or waxy (diet WS10) maize starch; 20% native (diet NS20) or waxy (diet WS20) maize starch or no starch (control). Diets were adjusted with α-cellulose. Another diet was formulated without carbohydrates, and contained 70% crude protein and 15% crude lipids (diet HP). Each diet was fed to triplicate groups of 30 fish (initial weight: 20 g) for 12 weeks. The HP group was fed to near satiation and the other 5 groups were fed on a pair-feeding scheme according to the group that ingested less feed (control diet group). The reduction of dietary protein level from 70% to 47% by the incorporation of 20% starch did not significantly affect gilthead sea bream growth performance or feed efficiency. Compared to the control diet, neither the level nor the nature of starch had any measurable effect on growth performance and feed efficiency. Digestibility of starch was unaffected by source or dietary inclusion level. Diet had no effect on plasma glucose levels, but liver glycogen was higher in diet groups NS20, WS20 and HP. Dietary carbohydrates increased GK and G6PD enzyme activities and decreased ALAT and GDH enzyme activities while had only a minor effect on FBPase activity. The nature of dietary starch tested (native or waxy) had little influence on performance criteria.