不同温度条件下异色瓢虫四龄幼虫亲缘关系对自残行为的影响

在14、18、22、24℃及30℃下对异色瓢虫Harmonia axyridis(Pallas)的不同同血缘的四龄幼虫自残行为及其对生长发育的影响进行研究。结果表明,异色瓢虫四龄幼虫自残发生概率受环境温度条件影响显著。在22-24℃处理范围内同血缘个体间自残发生概率均显著小于非同血缘个体。自残滞后时间同样受环境温度条件影响显著,且在同一温度条件下同血缘个体间均显著小于非同血缘处理。个体间相互攻击率不受环境温度变化影响,而在高于22℃下受血缘关系影响显著。此外,总受攻击次数在不同温度处理间差异显著。同血缘个体间自残后死亡率随温度增加而减少,均在30%以上。而非同血缘个体间自残后死亡率不受温度变化影响,均在70%以上。自残后残存个体的化蛹率及成虫羽化率均不受环境温度变化及个体间血缘关系的影响,化蛹率除30℃处理外均在90%以上,而在各温度下两血缘处理的羽化率均大于70%。     

延迟初次投喂对太门哲罗鱼仔鱼生长和存活的影响

在水温为10.4～14.9℃条件下,研究了延迟初次投喂对太门哲罗鱼仔鱼的生长、存活和个体大小差异的影响.该试验共分5组:S0(饥饿0d,对照)、S1(开口9d后投喂)、S2(开口12 d后投喂)、S3(开口15 d后投喂)、S4(开口18d后投喂).结果表明:经过36 d试验,S1组生长率和初次摄食率高于S0组,试验结束时总体的死亡率低于S0组,两组个体大小和体质量均无显著性差异;S2组生长率和初次摄食率高于S0组,两组个体大小差异不显著,但S2组体质量明显低于S0组,且总死亡率和自残死亡率均高于S0组;S3组初次摄食率高于S0组,总死亡率、自残死亡率和个体大小均高于S0组,虽然生长率接近S0组,但体质量明显低于S0组;S4组生长率和体质量均小于S0组,总死亡率和自残死亡率高于S0组.相同条件下延迟9d投喂太门哲罗鱼仔鱼出现了“完全补偿生长”,可应用于太门哲罗鱼初次摄食期仔鱼的培育.     

在不同营养状态下东方田鼠对陌生食物选择学习的研究

通过不同饥饿处理 ,研究了东方田鼠在不同营养状态下 ,对陌生食物尝试和选择的行为模式。结果表明 ,初次遇到陌生食物时 ,各处理组对陌生食物的采食量分别为饱食 6.9± 1 .8g,饥饿组为 4.8± 1 .6g,饥饿 食物短缺组为 3.4± 1 .4g。各处理组动物对陌生食物的摄入量间差异极为显著 ( P=0 .0 0 4 3)。饱食组、饥饿组及饥饿 食物短缺组动物初次遇到陌生食物时 ,所摄入的陌生食物量占总摄入量的百分比分别为72 %、38%与 1 2 %。说明 ,在实验室条件下饱食动物较饥饿和饥饿 食物短缺的动物学习选择摄食陌生食物的倾向较强。     

哲罗鱼稚鱼最佳投喂策略

为探讨哲罗鱼稚鱼的最佳投喂策略,设置了饥饿再投喂试验、饥饿再投喂恢复试验以及日投喂频率试验.结果表明: 饥饿再投喂试验中,各饥饿组未表现出补偿生长现象.但在饥饿再投喂恢复试验中,各饥饿组表现出不同程度的补偿生长,其中S_1/2组(饥饿1/2 d投喂1/2 d)体质量的增加量与对照组接近,表现出完全补偿生长.表明在哲罗鱼早期稚鱼阶段(体质量0～2 g,水温9～15.3 ℃),S_1/2是可以考虑使用的投喂方法.日投喂频率试验中,T₃组(日投喂3次)体长、体质量的增加量以及特定生长率均最高,饵料转化率也相对较高.表明在哲罗鱼后期稚鱼阶段(体质量2～21 g,水温8.8～15.5 ℃),以日投喂3次为宜.     

投喂策略对黄颡鱼生长和繁殖的影响

实验设置饱食投喂(AS)、80%饱食(80AS)、50%饱食(50AS)及先饥饿34d后再饱食投喂31d (HAS)四个处理组,饲养初重(86.6±0.41) g的雌性黄颡鱼(Pelteobagrus fulvidraco) 65d,于实验中期(34d)和实验结束时取样,以探究投喂策略对黄颡鱼母本生长性能和繁殖能力的影响。实验结果显示:80AS组、50AS组和HAS组的总摄食量基本符合实验设计的比例且显著低于AS组。在实验结束时, 80AS组的增重率和体重与AS组无显著差异, HAS组体重显著低于AS组并与80AS组无显著差异,生长未能达到同期饱食水平。各组在实验结束时的肌肉粗脂肪和粗蛋白含量均随投喂水平降低而降低,性腺粗脂肪和粗蛋白则无显著差异,据此可以推测母本将营养优先供给性腺而非肌肉。实验末期AS组和80AS组卵直径无显著差异,均显著高于50AS组并低于HAS组,表明饥饿后恢复摄食对卵的发育有补偿效应。实验结束时各处理组的绝对繁殖力和相对繁殖力未受到投喂策略的显著影响。80AS、50AS及HAS组的star、sf-1、3β-hsd和cyp19a1基因表达由中期的显著低于AS组上调...     

饥饿对转基因鲤与野生鲤生长竞争和性腺发育的影响

为研究转大麻哈鱼(Oncorhynchus keta)生长激素基因鲤(转基因鲤)在不同食物含量条件下的生长竞争(主要是生长、生殖),实验设定两个实验组,分别为正常投饲组和半饥饿实验组,每组的饲养池大小相同、混养的转基因鲤和对照鲤初始总体重相等。连续3个月定期采样后,处理数据并拟合体重体长方程,从形态学和组织学入手对转基因鲤和对照鲤(Cyprinus carpio)的生长状况、饲料占有情况和性腺发育状况开展研究。结果表明,正常投饲组的转基因鲤和对照鲤体重相对增长率分别为127.9%和70.6%,而半饥饿实验组的转基因鲤、对照鲤的体重相对增长率分别是72.3%和52.2%,表明无论在饱食还是在饲料短缺情况下,转基因鲤显示出快速生长的优势;通过比较两组转基因鲤和对照鲤的肠道充塞度,发现正常投饲组的转基因鲤和对照鲤的肠道食物充塞度差异不显著(P0.05),半饥饿组中的两种鱼的肠道食物充塞度差异不显著(P0.05),但是转基因鲤的肠长/体长比大于对照鲤,差异显著(P0.05),表明转基因鲤可能对饲料具有较强的消化、吸收能力;总体来看,两组实验鱼的性腺发育进程无明显差异(P0.05),但在部分月份转基因鲤和对照雌、雄鲤的相对性腺重差异显著(P0.05),无论在饱食还是在半饥饿情况下,转基因鲤的性腺发育情况略好于对照鲤。     

饥饿对高体革鯻消化道黏液细胞分布的影响

利用石蜡组织切片与AB-PAS染色技术研究饥饿0天(S0)、10天(S10)、20天(S20)对高体革消化道黏液细胞形态及分布的影响。结果发现,高体革饥饿20天后存活率为100%,食道含有Ⅱ、Ⅲ、Ⅳ型黏液细胞, S0组黏液细胞总密度为(2 839.13±261.52)个/mm2,饥饿不影响食道黏液细胞的种类,但S10组黏液细胞总密度减少[(1 546.00±70.02)个/mm2], S20组则表现为增加[(3 095.00±162.04)个/mm2]。S0、S10、S20三组胃部黏液细胞分布差异不大。S0和S10组幽门盲囊含Ⅰ和Ⅱ型黏液细胞, S20组则只有Ⅱ型黏液细胞,总密度先增加后减少,分别是(215.22±21.52)、(466.31±63.05)、(369.46±43.49)个/mm2。肠道只含Ⅱ型黏液细胞,饥饿使其密度逐渐增多,尤其是后肠,S0、S10、S20组密度分别是(1 683.50±219.64)、(2 068.71±65.38)、(4 622.50±85.60)个/mm2。不同部位黏液细胞对饥饿的响应存在差异,反映消化道不同部位结构功能的差异,饥饿显著影响消化道中Ⅱ型黏液细胞的分布,推测Ⅱ型黏液细胞分泌的变化有可能是高体革响应饥饿应激的敏感指标之一。     

气体过饱和对长江鲟早期生活史的影响

为研究总溶解气体(Total Dissolved Gas, TDG)过饱和对长江鲟(Acipenser dabryanus Dumeril)早期生活史的影响, 以长江鲟受精卵和仔稚鱼为实验对象, 进行不同TDG饱和度的暴露试验。结果显示: 在不同饱和度的TDG过饱和水体中, 发育至神经胚阶段的长江鲟受精卵的孵化率存在显著差异。其中, 对照组中的受精卵孵化率为(79±3)%, 140%组的孵化率((78±2)%)与对照组无显著差异(P>0.05), 110%组、120%组130%组的孵化率分别为(84±0)%、(92±2)%和(91±1)%, 显著高于对照组(P<0.05)。将出膜1—2d的仔鱼置于不同梯度的TDG饱和度水体中暴露96h, 除对照组外, 各实验组均有仔鱼死亡, 但死亡率均未达到7%。其中130%组的死亡率最高, 为(5.59±1.86)%; 110%和120%组死亡率次之, 分别为(3.45±0.12)%和(3.45±0.00)%; 140%组的死亡率最低, 为(1.15±1.99)%。130%组仔鱼死亡率显著高于140%组(P<0.05), 其他各组之间无显著差异(P>0.05)。将50日龄的长江鲟稚鱼置于不同梯度的TDG饱和水体中暴露96h, 除对照组外, 各实验组均有稚鱼死亡, 且各组死亡率均在50%以上, 140%组死亡率达100%。除110%组和120%组差异不显著外(P>0.05), 其他各组之间差异显著(P<0.05), 且随着TDG饱和度增加, 死亡率增大。各实验组之间的半致死时间存在显著差异(P<0.05), 且呈现出TDG饱和度增加, 半致死时间呈减少趋势。总体来看, TDG饱和度为110%、120%和130%的水体对长江鲟受精卵的孵化率有显著促进作用, 140%的水体对其孵化率无影响。不同TDG饱和度的过饱和水体均可导致长江鲟仔稚鱼死亡, 但对仔鱼影响程度较小, 且死亡率与TDG饱和度之间无显著关联; 对稚鱼影响程度较大, 在研究范围内, 影响程度随饱和度的增加呈增大趋势。     

饥饿对牙鲆幼鱼补偿生长、生化组成及能量收支的影响

在水温(23.0±1.0)℃条件下,以鲜杂鱼(x)和配合饲料(P)为饵料,采用室内循环流水的试验方法研究饥饿后再投喂对牙鲆幼鱼补偿生长、生化组成及能量收支模式的影响.结果表明:试验组S1F5、S5F25均表现出完全补偿生长现象,且S1F5组主要通过提高摄食率(FR)来实现补偿生长,而S5F25组主要通过提高食物转化率(FCE)来实现补偿生长;经补偿生长后,各试验组鱼体的生化组成与对照组均无显著差异(P>0.05);经饥饿处理后牙鲆幼鱼生长能占摄食能的比例增加,用于排泄和代谢的比重有所下降.     

饥饿和再投喂对虎斑乌贼幼体存活、生长和消化酶活力的影响

本文探究了饥饿胁迫与饥饿后再投喂对虎斑乌贼幼体存活率、生长、行为、肝体比、摄食率以及消化酶活力的影响.在室内控制条件下开展了幼体(初始体质量为4.95±0.48 g)的饥饿(0、1、2、3、4、5、6 d)和再投喂(15 d)试验.结果表明: 不同饥饿时间对虎斑乌贼的幼体存活率、体质量降低率、肝体比和消化酶活力影响显著.随着饥饿胁迫时间的增加,其存活率、肝体比呈下降趋势,其中饥饿3 d后,存活率开始明显下降,体质量降低率明显增大,幼体出现喷墨、互相残杀等异常行为;4种消化酶活力呈先下降后上升的趋势,淀粉酶活力以饥饿4 d组最低 (0.07±0.02 U·mg^-1·prot^-1);脂肪酶活力以饥饿2 d组最低(18.47±2.07 U·g^-1·prot^-1),饥饿6 d组最高(57.60±3.98 U·g^-1·prot^-1),胃蛋白酶活力和胰蛋白酶活力以饥饿5 d组(1.98±0.59 U·mg^-1·prot^-1)和饥饿4 d(186.68±20.72 U·mg^-1·prot^-1) 最低.饥饿处理结束后,经15 d再投喂,各试验组存活率、特定生长率、肝体比和摄食率差异显著,幼体的存活率、特定生长率、肝体比和摄食率均与饥饿处理时间呈负相关;饥饿1和2 d组与对照组的存活率、特定生长率和肝肝体比无显著差异,而饥饿3～6 d组显著低于对照组;饥饿1～2 d组的摄食率明显高于对照组,而饥饿6 d组的摄食率明显小于对照组;各组淀粉酶与脂肪酶活力无显著差异,胃蛋白酶与胰蛋白酶活力差异显著,均以对照组最高(胃蛋白酶活力7.06±0.64 U·mg^-1·prot^-1,胰蛋白酶活力914.67±26.54 U·mg^-1·prot^-1),饥饿6 d组最低(胃蛋白酶活力3.21±0.57 U·mg^-1·prot^-1,胰蛋白酶活力660.04±37.92 U·mg^-1·prot^-1).说明虎斑乌贼的幼体饥饿不可逆点(PNR)为第6天,且不能补偿生长.     

Analysis of sibling cannibalism among pike,Esox lucius,juveniles reared under semi-natural conditions

Synopsis Sibling cannibalism in pike, Esox lucius, larvae and juveniles living in outdoor rearing ponds was studied using stomach contents analysis. For the two initial densities tested (6 and 18 larvae m^–2, equivalent to 12 and 36 larvae m^–3), cannibalism was non-existent during the larval period (13 to 35 mm total length) and was observed only during the juvenile stages. Initial density of larvae influenced both the date of first detection of cannibalistic individuals and the rate of development of cannibalism in the population. At initial stocking densities of 18 larvae m^–2 (36 larvae m^–3), cannibalism was observed from 21 days after the start of exogenous feeding (mean total length: 60 mm) onwards. At a mean total length of 100 mm and for initial stocking densities of 6 and 18 larvae m^–2, (12 and 36 larvae m^–3), the average proportions of cannibals in the populations of juveniles were 7.8% and 41.3% and the cannibals accounted for 15.5% and 65.9% of the total pike biomass, respectively. In stomachs of cannibals, young pike were the dominant prey in terms of weight. Dry weights of invertebrate-prey were lower in cannibals than in non-cannibals of similar size. Cannibalism among pike juveniles was characterized by the prey being swallowed whole and head first in the vast majority of cases. There was a strong positive correlation between predator and prey size and the mouth size of a cannibal was found to be an important constraint determining maximum victim size. The overall mean ratio of pike prey length to pike cannibal length was 66.2% and the average ratio of prey head depth to predator mouth width amounted to 87.6%. Prey size selection could be demonstrated for several length-groups of cannibals. These results are compared with the characteristics of early cannibalism in other fish species.     

Sibling cannibalism in the ladybird beetleHarmonia axyridis: Fitness consequences for mother and offspring

The adaptive significance of sibling cannibalism was analyzed in the ladybird beetle Harmonia axyridis at two prey densities. Possible costs and benefits were considered from three points of view: the mother, the cannibal and the victim. Cannibals ate both infertile and fertile eggs, thereby increasing in body length and survival rate with the intensity of sibling cannibalism. The cannibalistic trait was clearly beneficial to the cannibal when aphid density was low. However, it was not always beneficial when aphid density was high and the victims were full siblings. The altruistic behavior of being a victim was beneficial only when the victim was cannibalized by full siblings at low aphid density. The mother attained almost equal fitness at low aphid density, regardless of the intensity of sibling cannibalism. This suggests that sibling cannibalism is not maladaptive for the mother. At high aphid density, however, mother's fitness decreased with the intensity of sibling cannibalism, indicating that sibling cannibalism is maladaptive for the mother when larval food availability is high.     

Modelling Size-Dependent Cannibalism in Barramundi Lates calcarifer: Cannibalistic Polyphenism and Its Implication to Aquaculture

This study quantified size-dependent cannibalism in barramundi Lates calcarifer through coupling a range of prey-predator pairs in a different range of fish sizes. Predictive models were developed using morphological traits with the alterative assumption of cannibalistic polyphenism. Predictive models were validated with the data from trials where cannibals were challenged with progressing increments of prey sizes. The experimental observations showed that cannibals of 25–131 mm total length could ingest the conspecific prey of 78–72% cannibal length. In the validation test, all predictive models underestimate the maximum ingestible prey size for cannibals of a similar size range. However, the model based on the maximal mouth width at opening closely matched the empirical observations, suggesting a certain degree of phenotypic plasticity of mouth size among cannibalistic individuals. Mouth size showed allometric growth comparing with body depth, resulting in a decreasing trend on the maximum size of ingestible prey as cannibals grow larger, which in parts explains why cannibalism in barramundi is frequently observed in the early developmental stage. Any barramundi has the potential to become a cannibal when the initial prey size was <50% of the cannibal body length, but fish could never become a cannibal when prey were >58% of their size, suggesting that 50% of size difference can be the threshold to initiate intracohort cannibalism in a barramundi population. Cannibalistic polyphenism was likely to occur in barramundi that had a cannibalistic history. An experienced cannibal would have a greater ability to stretch its mouth size to capture a much larger prey than the models predict. The awareness of cannibalistic polyphenism has important application in fish farming management to reduce cannibalism.     

Drastic growth effect may explain sympatric cannibalistic polymorphism

Cannibalistic polyphenism is observed in many fishes and amphibians. In the case of amphibian larvae, cannibal morph and typical morph coexist. Benefits and costs of the cannibal morph have been studied empirically but the mechanism of the maintenance of polymorphism is not well known. Here, we construct a game model of typical and cannibal morph strategies to obtain the condition of stable coexistence. Generally, once an individual succeeds in cannibalism, it grows very quickly, which facilitates the next cannibalism. In a model without this 'drastic growth effect', stable coexistence cannot occur. To represent drastic growth effect, it is assumed that cannibal/typical morph stage is followed by giant/normal stage. A cannibal morph that performs cannibalism in the first stage can become a 'giant' in the next stage. This model allows stable coexistence of cannibal and typical morphs. The condition for coexistence is that payoff of a giant is two times larger than normal individuals. As long as direct consumption of victim's body is considered as reward for successful cannibalism, coexistence cannot be explained. When the reward is considered as social standing of being outstanding size in a population, sympatric cannibalistic polymorphism is possible, without regard to the initial size variation or resource shortage.     

An interaction‐driven cannibalistic reaction norm

Cannibalism is induced in larval‐stage populations of the Hokkaido salamander, Hynobius retardatus, under the control of a cannibalism reaction norm. Here, I examined phenotypic expression under the cannibalism reaction norm, and how the induction of a cannibalistic morph under the norm leads to populational morphological diversification. I conducted a set of experiments in which density was manipulated to be either low or high. In the high‐density treatment, the populations become dimorphic with some individuals developing into the cannibal morph type. I performed an exploratory analysis based on geometric morphometrics and showed that shape characteristics differed between not only cannibal and noncannibal morph types in the high‐density treatment but also between those morph types and the solitary morph type in the low‐density treatment. Size and shape of cannibal and noncannibal individuals were found to be located at either end of a continuum of expression following a unique size–shape integration rule that was different from the rule governing the size and shape variations of the solitary morph type. This result implies that the high‐density‐driven inducible morphology of an individual is governed by a common integration rule during the development of dimorphism under the control of the cannibalism reaction norm. Phenotypic expression under the cannibalism reaction norm is driven not only by population density but also by social interactions among the members of a population: variation in the populational expression of dimorphism is associated with contingent social interaction events among population members. The induced cannibalistic morph thus reflects not only by contest‐type exploitative competition but also interference competition.     

Temporal shift of diet in alternative cannibalistic morphs of the tiger salamander

Evolutionary theory predicts that alternative trophic morphologies are adaptive because they allow a broad use of resources in heterogeneous environments. The development of a cannibal morphology is expected to result in cannibalism and high individual fitness, but conflicting results show that the situation is more complex. The goal of the present study was to increase our understanding of the ultimate benefits of a cannibalistic polyphenism by determining temporal changes in the feeding habits and biomass intake in a population of tiger salamanders ( Ambystoma tigrinum nebulosum ). Cannibals in this species develop a larger head than typicals and have prominent teeth, both useful for consuming large prey. Although cannibalism was only detected in cannibal morphs, large temporal variation in resource partitioning was found between morphs. The two morphs always differed in their foraging habits, but cannibalism mainly occurred immediately after the ontogenetic divergence between morphs. Cannibals shifted their foraging later to a more planktivorous diet (i.e. the primarily prey of the typical morph). Cannibals also obtained more prey biomass than typicals. These results indicate that the cannibalistic morph is advantageous over the typical development, but that these advantages vary ontogenetically. Although the results obtained are consistent with models predicting the maintenance of cannibalism polyphenism in natural populations, they show that the foraging tactics utilized by cannibal morphs, and the fitness consequences accrued by such tactics, are likely to be more complex and dynamic than previous studies have suggested.  © 2006 The Linnean Society of London, Biological Journal of the Linnean Society , 2006, 89 , 373–382.     

Cannibalism in Arctic charr: do all individuals have the same propensity to be cannibals?

By comparing large Arctic charr Salvelinus alpinus , which had shown a persistent cannibalistic response varying from zero to very high in succeeding laboratory trials, with their individual cannibalistic behaviour after release into a natural lake inhabited by small Arctic charr, it was found that all Arctic charr had the potential to become cannibalistic, irrespective of their laboratory behaviour. More specifically, Arctic charr that never fed on prey fishes when offered them in the tank experiments turned to cannibalism when released in the lake, highlighting the potential difficulties in extrapolating laboratory results to natural settings. This was also true for naive fish that had no prior experience of eating live food. Since no significant increase in the number of prey consumed during each of the succeeding laboratory trials was found, and naive fish showed a response under natural conditions similar to that of their counterparts, the training of the Arctic charr (or experience or learning) probably had no effect upon the piscivorous or cannibalistic response after stocking. Thus, the study appeared to demonstrate that most variations in cannibalism in Arctic charr was simply a function of environmental conditions, depending on the density of conspecifics v . alternative prey, and the relative size difference between predator and prey, rather than any genetic influence.     

Non-Random Sibling Cannibalism in the Marine Gastropod Crepidula coquimbensis

Sibling cannibalism is commonly observed in marine species. For instance, intrabrood cannibalism has been documented in marine gastropods with direct development, suggesting a relationship between embryo behavior and the evolution of life history strategies. However, there has been little effort to document the factors driving sibling cannibalism in marine species. The kin selection theory suggests that the level of relatedness plays an important role in cannibalism patterns. We examined Crepidula coquimbensis, a marine gastropod that broods and encloses its brooded offspring in capsules. Encapsulated embryos show sibling cannibalism and high levels of intracapsular multiple paternity. Given these features, cannibalistic behavior may be driven by kin-relatedness. To test this hypothesis, we constructed artificial aggregations of embryos to mimic three levels of relatedness: high, medium and low. For each category of aggregation, the cannibalism rate and benefits (i.e. size at hatching of surviving offspring) were estimated. In addition, at the end of embryo development, we performed parentage analyses to determine if cannibalism was associated with the relatedness between cannibal and victim embryos. Our results show that the intensity of sibling cannibalism increased in aggregations characterized by the lowest level of relatedness. There were important benefits of cannibalism in terms of hatching cannibal size. In addition, cannibalism between embryos was not random: the variation in reproductive success between males increased over the course of the experiment and the effective number of fathers decreased. Altogether, these results suggest that polyandry may play an important role in the evolution of sibling cannibalism in C. coquimbensis and that kin selection may operate during early embryonic stages in this species.     

Spring cannibalism on 1 year walleye pollock in the Doto area,northern Japan: is it density dependent?

Cannibalism in walleye pollock off the eastern coast of the Hokkaido Island, Japan was important only during spring (April to June), and its importance increased from 0% in dry mass for <200 mm L _S fish to 48·9% for >400 mm L _S fish. Most of the prey was represented by age 1 year fish, showing a unimodal body size distribution with a mode at 121–130 mm. Although cannibal body size was larger in deeper (>150 m) water, there was no difference in prey size by depth, suggesting impingement of the predators inhabiting deeper water into the shallow areas to cannibalize 1 year fish. The minimum ratio cannibal: prey size was 1·74. There was a positive but non-significant correlation between the contribution of cannibalism to a potential predator's (>300 mm) diet and an estimate of the previous year's recruitment. This was due to an extremely high contribution of cannibalism during 1992, when a distinctly larger size of predators seemed to bias the contribution. When the 1992 data were removed from the analysis, a significant correlation was obtained ( r ²=0·77, P <0·01), showing that Pollock cannibalism is rather density dependent. Based on the results, it is hypothesized that the'overflow' of 1 year fish from the shelf waters due to their high abundance and the weak stratification in the spring water column results in increased co-occurrence with adult fish and consequent cannibalism.     

Sibling cannibalism among juvenile vundu under controlled conditions. II. Effect of body weight and environmental variables on the periodicity and intensity of type II cannibalism

Cannibalism among starved groups of juvenile (19–48 days old) vundu catfish Heterobranchus longifilis was 66·5% nocturnal, and its impact under modified day length was proportional to the duration of the dark phase. Shallow depth and high population density decreased the intensity of cannibalism, whereas low density and deeper environments had an opposite effect. The presence of refuges had no significant effect on cannibalism. The maintenance ( R _maint) and maximum ( R _max) daily food rations (% day⁻¹) of cannibals feeding on live prey were modelled as R _maint=3·899 W _C^0·327 ( r ²=0·684; d.f.=31), and R _max=49.545 W _C^0·321 ( r ²=0·999; d.f.=5), where W _C was the body weight of the cannibal (g). The latter model indicated that the impact of a cannibal on a population decreased by a 20% margin each time the cannibal doubled its body weight, and suggested that cannibalism among vundu would become insignificant for cannibals heavier than 30 g. The significance of these findings is discussed within the contexts of vundu aquaculture and of general, conceptual models of the dynamics of cannibalism among fishes.