几种幼鱼视觉运动反应研究

对鲻鱼(Mugit cephalus)、鲈鱼(Lateolabrax japonicus)、黄鳍鲷(Sparus latus)、鲤鱼(Cyprinus carpio)和草鱼(Ctenopharyngodon idellus)5种幼鱼进行了视觉运动反应实验。反应率随照度的降低或屏幕转速的提高而下降。高限可跟刺激频率随照度而提高,在10^2 1x下,鲻鱼达37次／秒,鲈鱼26次／秒,黄鳍鲷27次／秒,鲤鱼24次／秒,草鱼29次／秒。明暗视过渡照度值：鲻鱼10^0-10^1 1x,鲤鱼和草鱼10^-1—10^01x,鲈鱼和黄鳍鲷10^-2-10^-1 1x。照度和刺激频率对反应潜伏期有一定影响。跟随角速度普遍落后于屏幕角速度,即跟随率低于100％,并且随屏幕转速提高而下降。照度对跟随率无明显影响。几种幼鱼对运动条纹的反应可分为三阶段：跟随、活泼运动、无反应。     

几种幼鱼视觉运动反应研究

对鲻鱼(Mugil cephalus)、鲈鱼(Lateolabrax japonicus)、黄鳍鲷(Sparus latus)、鲤鱼(Cyprinus carpio)和草鱼(Ctenopharyngodon idellus)5种幼鱼进行了视觉运动反应实验。反应率随照度的降低或屏幕转速的提高而下降。高限可跟刺激频率随照度而提高,在10~2lx下,鲻鱼达37次/秒,鲈鱼26次/秒,黄鳍鲷27次/秒,鲤鱼24次/秒,草鱼29次/秒。明暗视过渡照度值:鲻鱼10~0—10~1lx,鲤鱼和草鱼10~(-1)—10~0lx,鲈鱼和黄鳍鲷10~(-2)—10~(-1)lx。照度和刺激频率对反应潜伏期有一定影响。跟随角速度普遍落后于屏幕角速度,即跟随率低于100％,并且随屏幕转速提高而下降。照度对跟随率无明显影响。几种幼鱼对运动条纹的反应可分为三阶段:跟随、活泼运动、无反应。     

不同屏幕角速度下条纹倾角对鲫鱼视觉运动反应特性的影响

视野结构(屏幕图形结构)对鱼类视觉运动反应的影响极为显著,但至今仍缺乏系统的研究。本文较详细地研究了不同屏幕角速度下条纹倾角对鲫鱼(Carassius auratus)视觉运动反应特性的影响,可为探讨鱼类对各种复杂图形运动的反应机制奠定基础,同时在渔业生产中也有一定参考价值。 选用健康鲫鱼作为试验鱼,体长范围 12.2—16.1cm,平均 14.2cm。试验日期为 1989年4月10日—5月 20日。水温范围 12.5—17.4℃,平均 15℃。水中照度范围 650—800lx,平均 725lx。     

舟山渔场产卵场保护区春季小黄鱼群体结构及资源动态

根据2014—2019年舟山渔场产卵场保护区及邻近海域底拖网调查资料,分析了春季小黄鱼的群体结构、资源密度变化及其与环境因子的关系。结果表明: 小黄鱼的体长-体重关系式为: W=0.44×10^-4×L^2.78,b值<3,说明近年来小黄鱼为负异速生长,肥满度与体长呈负相关,身体趋于细长。2014—2019年小黄鱼的体长和体重均以2014年最高、2019年最低。2014年以来,舟山渔场产卵场保护区及邻近海域小黄鱼的群体规格逐渐减小,说明近年来小黄鱼个体小型化现象并未改善。与产卵场保护区设立前的数据相比,保护区建立后小黄鱼资源密度有所提升,表明保护区管护对小黄鱼的资源恢复起到了一定的保护作用。GAM模型拟合结果显示,与保护区及周边海域小黄鱼资源密度分布关系密切的环境因子主要是水深和底层水温。随着水深的增加,小黄鱼资源量呈波动上升趋势,在水深60 m附近时资源量最高;在12~16 ℃水温范围内,小黄鱼资源量随底层水温升高而增大;当水温>16 ℃时,其资源量随底温的上升而下降。     

硇洲岛海域文昌鱼生长与繁殖特性

2006年3-12月对硇洲岛海域的文昌鱼及其栖息环境进行了调查,为在该地实施文昌鱼保护措施提供参考依据.结果表明:硇洲岛海域的文昌鱼主要集中分布以20°55.35'N,110°31.5'E为中心的周边4.08km2的狭小范围内;平均资源密度约为209ind·m-2,最高达到1053ind·m-2,平均生物量为40.8g·m-2,最高达到424g·m-2;体长组成具明显的季节变动;3-6月为该海域文昌鱼的繁殖期;秋冬季节是文昌鱼的主要补充季节;性腺成熟度和性别比例在不同季节和生长阶段具明显的差异,春季性成熟个体占的比重最大达51%,50%性成熟体长为54mm;在30～55mm时,体长范围内雌雄个体的性比接近1,体长＞55mm雌性个体数量明显多于雄性;文昌鱼全年的体质量与体长关系方程为W=0.0014L3.2898;当体长达到60mm后文昌鱼资源群体尾数减少系数为0.361cm-1.     

水丰水库的池沼公鱼生物学

本文报道了1942年横断鸭绿江而成的水丰水库的池沼公鱼生物学,内容包括性状变异、年龄和生长、食性、繁殖、群体结构、群体消长变化及渔业利用。水丰水库的池沼公鱼一龄体长(到尾叉的体长,后同)74毫米,体重3克;二龄体长99毫米,体重6.5克。体长体重关系式:logW=2.5306logL-1.7590。主要以浮游动物为食,但不同栖息地点食物组成有明显差异。一龄即达性成熟;性比为1♀:1.24♂;个体繁殖力905—19051粒,平均4330粒;个体繁殖力Y(百粒鱼卵)与体长L(厘米)的相关方程式为Y=58.4860L-402.7606;群体增殖速度(Vp)等于1494;4至5月在水库岸边产卵,卵粘性,水温10—15℃,约经12天孵出仔鱼。群体由1—3龄3个龄组组成,1321尾标本的平均年龄为1.15;65.1—80毫米体长组的个体为主要渔获对象,占整个渔获的65.5%。池沼公鱼是目前水丰水库第一位的经济鱼类,1982年鱼产量120万斤,占总鱼产的57.8%。       

水丰水库的池沼公鱼生物学

本文报道了1942年横断鸭绿江而成的水丰水库的池沼公鱼生物学,内容包括性状变异、年龄和生长、食性、繁殖、群体结构、群体消长变化及渔业利用。水丰水库的池沼公鱼一龄体长(到尾叉的体长,后同)74毫米,体重3克;二龄体长99毫米,体重6．5克。体长体重关系式：logW=2．5306logL-1．7590。主要以浮游动物为食,但不同栖息地点食物组成有明显差异。一龄即达性成熟;性比为1♀：1．24σ^n;个体繁殖力905-19051粒,平均4330粒;个体繁殖力Y(百粒鱼卵)与体长L(厘米)的相关方程式为Y=58.4860L-402.7606;群体增殖速度(Vp)等于1494;4至5月在水库岸边产卵,卵粘性,水温10-15℃,约经12天孵出仔鱼。群体由1-3龄3个龄组组成,1321尾标本的平均年龄为1．15;65．1-80毫米体长组的个体为主要渔获对象,占整个渔获的65．5％。池沼公鱼是目前水丰水库第一位的经济鱼类,1982年鱼产量120万斤,占总鱼产的57．8％。     

群体大小对青幼鱼群体特征的影响

为考察群体大小对青幼鱼(Mylopharyngodon piceus)群体特征的影响,在(25±0.5)℃条件下对1、2、4和8尾青幼鱼组成的群体(N=12)进行视频拍摄和轨迹分析,得到游泳速度和空间分布特征数据,实验同时测定了不同群体大小对恐吓刺激的反应差异。结果显示:青幼鱼自发游泳速度多处于0—10 cm/s,不受群体大小的影响,但不同个体速度同步性随群体大小增加而显著下降;青幼鱼最近邻距离多数位于0—10 cm,且随着群体大小增加,最近邻距离和群体排列的极性均随群体数量上升而显著下降;青幼鱼群体中位置分布具有个体差异,且随着群体数量上升差异加剧;青幼鱼对恐吓刺激的反应率不受群体大小的影响,但群体内部不同个体对刺激的反应的一致性随群体大小上升有所下降。结果表明:青幼鱼具备集群性,在2—8个体范围内随群体大小上升凝聚力上升,协调性下降;群体特征的改变对恐吓刺激反应率没有显著影响,但协调性的下降导致反应一致性降低;青幼鱼群体中不同个体在集群活动中可能存在社会分工,且这种分工的作用在大群体大小上升表现更为明显。     

黄海中南部细纹狮子鱼的生物学特征及资源分布的季节变化

根据2009年7—8月、10月和2010年1月、5月黄海中南部渔业生物底拖网调查数据,对该海域细纹狮子鱼的生物学特征及其分布的季节变化作了分析。结果表明,细纹狮子鱼平均体长和平均体重从春季(4.7 cm、3.3 g)到冬季(34.2 cm、764.9 g)呈显著增加,并且雄性个体平均体长显著大于雌性个体(P<0.05,春季除外)。性比(♀∶♂)随体长组和季节变化,体长越大趋向于雄性,反之趋向于雌性;夏季雄鱼居多(0.70∶1,P<0.05),秋季则为雌鱼居多(1.35∶1,P<0.05),而冬季(产卵群体)和春季性比接近于1∶1。细纹狮子鱼各季节摄食等级均在2.5以上,冬季雄性个体摄食等级显著大于雌性个体(P<0.05),但雄性个体肥满度为全年最低(1.52)。细纹狮子鱼相对资源量和贡献率从春季(0.17 kg/h,1.54%)到秋季(15.36 kg/h,33.05%)呈上升状态,而冬季(2.37 kg/h,5.60%)有所下降。相比2000年,夏秋季相对渔获量和贡献率提高明显。全年集中分布于7.8—13.6℃,3.20%—3.38%的水域,平均体重和水深有显著的相关性(秋季除外)。另外,根据性成熟个体分布区和稚幼鱼分布的相关历史资料分析发现,除海州湾外,黄海中部深水区可能是细纹狮子鱼的产卵场。     

个体大小对草鱼耐高温能力及升温过程中群体行为的影响

为考察个体大小对升温过程中草鱼的耐高温能力和群体行为的影响, 研究以草鱼(Ctenopharyngodon idellus)幼鱼为实验对象, 结果显示: 大个体群体的个体游泳速度明显大于小个体群体(P=0.039), 个体间距离和最近邻距离显著高于小个体群体(P<0.001); 大个体鱼群的极性与小个体群体并无差异。虽然大个体群体的致死高温与小个体群体无差异, 但前者的临界高温显著大于小个体群体(P<0.001)。升温过程会使鱼群个体游泳速度变快, 个体间距离和最近邻距离也相应增加, 但游泳速度同步性和群体极性维持不变。研究表明: 个体大小的增加可增强草鱼幼鱼对环境水温的耐高温能力; 升温不会影响不同个体大小草鱼幼鱼群体的协调性, 但会降低草鱼幼鱼的群体凝聚力。     

RHEOTROPISM IN FISHES

(1) The fluid properties of air and water enable animals to orientate to flow and this behaviour in water is termed rheotaxis. Fish, however, have a wide range of responses to currents, extending beyond simple orientation, and the term rheotropism is therefore used as a ‘portmanteau’ word to describe all such reactions. (2) Fish detect currents directly by flow over the body surface or indirectly by other stimuli. Indirect responses are more common and occur in response to visual, tactile and inertial stimuli resulting from displacement of the fish by the current. Reactions to displacement of visual images are called optomotor reactions. The lateral line is not involved except in the detection of small localized jets of water. It has not been demonstrated that any fish can detect the current by electrical stimuli, although it is theoretically possible for some to do so. (3) In the basic form of rhotaxis the fish heads upstream and maintains station by stemming the current. Current detection thresholds fall within the range 0.4 to 10 cm/s for tactile stimuli but may be as low as 0.03 cm/s for visual stimuli. (4) Visual responses have been studied by simulating displacement by the current in optomotor apparatus. Fish respond to a rotating black-and-white-striped background by compensatory movements of the head and eyes - optokinetic nystagmus - or by the optomotor reaction, in which the fish swims with the background. (5) Fish show an orthokinesis in optomotor apparatus, their mean swimming speed increasing with the speed of rotation of the background. The precise form of the relationship varies between species and there is also considerable individual variation in performance. Fish accelerate and decelerate relative to the background, fixating on a particular stripe for short periods. (6) Factors limiting the appearance of the optomotor response are contrast, illuminance, acuity, critical flicker fusion frequency and spectral sensitivity. (7) Fish tolerate retinal image movements equivalent to those received when they are carried forwards by the current but not to those received when they are carried backwards. There are ganglion cells in the optic tectum which are sensitive to the direction of movement of targets across the visual field. In the goldfish there are significantly more units sensitive to movements in the temporo-nasal than in the opposite direction. (8) There are close parallels between the behaviour of fish in schools and in an optomotor apparatus. The optomotor response is apparently innate, occurring in newly hatched fry. (9) Physical and chemical factors can modify rheotaxis. Temperature and olfactory stimuli affect both the sign of the taxis and the kinetic component of the behaviour. (10) Thyroid hormones which are involved in the control of migration have been shown to affect the kinetic component of rheotaxis. (11) Fish show a number of hydrodynamic adaptations to life in currents. Morphological modifications are greatest in fish from torrential streams, which show extreme dorsoventral flattening and have specialized adhesive organs. Other fish select areas of low velocity or decrease their buoyancy with increasing current speed. (12) Rheotropic behaviour plays an important role in the distribution of fish within stream systems, in the maintenance of territory and station and in feeding behaviour. Territory, station and spawning sites in salmonids are all selected in relation to water velocity. (13) Water currents are thought to provide either a transport system or directional clues for fish on migration. The fish either does not respond to the current and is carried passively downstream, or it makes an orientated movement, swimming up- or downstream. (14) Eggs and larvae are known to drift passively downstream from their spawning grounds and some adult fish may also drift passively. In the sea both adult and juvenile fish use a form of modulated drift associated with vertical migration. Fish move up into midwater either by direct tidal selection or in relation to the diel cycle of illuminance. In fresh water the downstream migrations of salmonid fry, and smolts under some conditions, occur by modulated drift. (15) There is no evidence that fish migrating in the sea orientate to the current, but in fresh water the upstream migrations of diadromous fish are clearly orientated movements. (16) Water velocity is a major factor for salmonids migrating upstream. For fry it limits the occurrence of upstream migrations and for adults it can also prevent upstream movement. But migrations are often initiated by freshets, and changing water velocity is thought to be the most important factor associated with a freshet. (17) Both environmental and genetic factors affect the direction of migration in relation to the current. In some sockeye salmon fry direction is determined by temperature, but in others the overall direction of movement is genetically determined and environmental factors only modify the behaviour. (18) Rheotropic behaviour has a number of important practical applications in the capture of fish and in guiding them past dams and power stations. (19) The optomotor response plays a basic role in the capture of roundfish by trawls under conditions when the fish can see the gear. Many fish are caught because they become fatigued after a prolonged period of swimming at the same speed as the trawl. (20) Most success in guiding fish away from hazardous areas and bypassing them round dams has been achieved with mechanical barriers which depend on rheotropic reactions of the fish. (21) Louvre screens are very successful in deflecting juvenile salmonids migrating downstream past small dams but are impracticable at large dams. Instead, the turbine intakes are commonly sited at a considerable depth and fish are bypassed by mechanical screens either at the surface of the forebay or into the gatewells immediately upstream of the turbine intakes. (22) With upstream migrants the basic problem is to attract fish to the lower end of the fishways. An adequate ‘attraction velocity’ is an important feature of fishways, which must be sited so that the fish avoid the high velocity discharges from spillways and turbines.     

Warmer and browner waters decrease fish biomass production

Climate change studies have long focused on effects of increasing temperatures, often without considering other simultaneously occurring environmental changes, such as browning of waters. Resolving how the combination of warming and browning of aquatic ecosystems affects fish biomass production is essential for future ecosystem functioning, fisheries, and food security. In this study, we analyzed individual‐ and population‐level fish data from 52 temperate and boreal lakes in Northern Europe, covering large gradients in water temperature and color (absorbance, 420 nm). We show that fish (Eurasian perch, Perca fluviatilis) biomass production decreased with both high water temperatures and brown water color, being lowest in warm and brown lakes. However, while both high temperature and brown water decreased fish biomass production, the mechanisms behind the decrease differed: temperature affected the fish biomass production mainly through a decrease in population standing stock biomass, and through shifts in size‐ and age‐distributions toward a higher proportion of young and small individuals in warm lakes; brown water color, on the other hand, mainly influenced fish biomass production through negative effects on individual body growth and length‐at‐age. In addition to these findings, we observed that the effects of temperature and brown water color on individual‐level processes varied over ontogeny. Body growth only responded positively to higher temperatures among young perch, and brown water color had a stronger negative effect on body growth of old than on young individuals. Thus, to better understand and predict future fish biomass production, it is necessary to integrate both individual‐ and population‐level responses and to acknowledge within‐species variation. Our results suggest that global climate change, leading to browner and warmer waters, may negatively affect fish biomass production, and this effect may be stronger than caused by increased temperature or water color alone.     

Experimental study of young fish distribution and behaviour under combined influence of baro-, photo- and thermo-gradients

The complex influence of baro-, photo- and thermo-gradients on distribution and behaviour of young physostomous Leuciscus leuciscus L., leaciscus idus L. and physoclistous fish Perca fluviatilis L. was investigated. 40 different combinations consisting of 4 types of photogradients, 3 types of termo gradients and 4 types of baro-gradients were tested. All considered factors influenced the distribution of physostomous and physoclistous fish with high degree of significance. Under a multi-factorial experiment fish behaviour and distribution are determined not only by a separately taken environmental factor but by the presence of other accompanying factors. Under different combinations of imposed factors, the reaction of fish to an individual factor and the character of the response (positive or negative) may change. Hydrostatic pressure is one of the strongest factors which influence fish distribution. The presence of its gradient may change the character of response of perch Perca fluviatilis to light (a changing of a sign of its photoreaction). Water temperature influences fish distribution more than illumination. However, the definite value of water temperature at which most fish prefer to stay may be changed depending on both the intensity of illumination and the value of hydrostatic pressure.     

Diel vertical migration of freshwater fishes – proximate triggers,ultimate causes and research perspectives

1. Diel vertical migrations (DVM) are typical for many cold‐water fish species such as Pacific salmons (Oncorhynchus spp.) and coregonids (Coregonus spp.) inhabiting deep lakes. A comprehensive recent overview of DVM in freshwater fish has not been available, however. 2. The main proximate trigger of DVM in freshwater fish is the diel change in light intensity, with declining illumination at dusk triggering the ascent and the increase at dawn triggering the descent. Additional proximate cues are hydrostatic pressure and water temperature, which may guide fish into particular water layers at night. 3. Ultimate causes of DVM encompass bioenergetics efficiency, feeding opportunities and predator avoidance. None of these factors alone can explain the DVM in all cases. Multi‐factorial hypotheses, such as the ‘antipredation window’ combined with the thermal niche hypothesis, are more likely to explain DVM. It is suggested that planktivorous fish move within a layer sufficiently well illuminated to capture zooplankton, but too dark for predators to feed upon the migrating fish. In complete darkness, fish seek layers with a temperature that optimises bioenergetics efficiency. The strength of each factor may differ from lake to lake, and hence system‐specific individual analyses are needed. 4. Mechanistic details that are still poorly explored are the costs of buoyancy regulation and migration, the critical light thresholds for feeding of planktivorous and piscivorous fish, and predator assessment by (and size‐dependent predation risk of) the prey fish. 5. A comprehensive understanding of the adaptive value of DVM can be attained only if the behaviour of individual fish within migrating populations is explicitly taken into account. Size, condition and reproductive value differ between individuals, suggesting that migrating populations should split into migrants and non‐migrants for whom the balance between mortality risk and growth rate can differ. There is increasing evidence for this type of partial DVM within populations. 6. Whereas patterns of DVM are well documented, the evolution of DVM is still only poorly understood. Because experimental approaches at realistic natural scales remain difficult, a combination of comprehensive data sets with modelling is likely to resolve the relative importance of different proximate and ultimate causes behind DVM in fish.     

Fixation and optomotor response of walking colorado beetles: interaction with spontaneous turning tendencies

Abstract The paths of Colorado beetles ( Leptinotarsa decemlineata Say) in a featureless environment are circular, like those of other species studied. The turning velocity may reach 35^o/s and is due to an internal asymmetry, which may change spontaneously. Normally, all control loops of the insect, like fixation or optomotor responses, must work against this asymmetry to stabilize the insect's path. Stationary vertical patterns damp this turning tendency, but their effect is not strong enough to induce a straight path. Only 70% of the turning tendency can be so eliminated. This reaction is termed optomotor response because it can be adequately described with the parameter turning velocity alone. The insect's path was stabilized more effectively when pattern wavelengths were greater than 60^o. The insects seemed to fixate these wider stripes. This reaction is termed fixation because the correlation between pattern components and insect's course becomes prominent.
A comparison was made between these reactions to stationary patterns and to turning patterns. No differences could be found in the behavioural reactions to the different situations. This suggests that the insect does not use an internal representation of its spontaneous turning tendency to discriminate between the type of turning of the optical environment. These results can be explained by a simple feedback control loop with an additive interaction between the internal turning command and feedback signals from the eyes.     

Intensity-dependent alteration of minnow (Pimephales promelas) behavior by a brain-encysting trematode

We examined the relationship between the numbers of brain-encysting trematodes (Ornithodiplostomum ptychocheilus) and the magnitude of altered behaviors in fathead minnows (Pimephales promelas). Because cysts develop within a brain region that integrates visual stimuli with motor response. we evaluated the standard optomotor response (OMR). Monitoring this task involved recording the time minnows spent following a spinning drum, on which alternating black and white stripes had been painted. Minnows were exposed to 0, 5, 20, 120, and 300 cercariae and then their OMR was evaluated at 2-wk postinfection. Surprisingly, only minnows that had high numbers of parasites (155 +/- 31 worms/fish) or low numbers of parasites (3 +/- 3 worms/ fish) differed significantly in their optomotor performance compared with controls. Reduced OMR of heavily infected minnows was positively correlated with reduction in minnow activity. In contrast, reduced OMR in lightly infected minnows was independent of host activity and was likely associated with the rapid development of parasite larvae within the optic tecta. The nonlinear relationship between parasite intensity and effect on host behavior was consistent with an earlier study, but the underlying mechanisms producing this pattern are unknown.     

Bioenergetic model of planktivorous fish feeding, growth and metabolism: theoretical optimum swimming speed of fish larvae

The feeding activity of an individual fish larva is described by an equation which includes parameters for the area successfully searched, probability of food capture multiplied by the cross-sectional perceptive visual field, larval swimming speed and the time required to consume a unit of food energy. The proportion of ingested food energy used for metabolism increases exponentially with increasing swimming speed. The model predicts that food consumption rate increases asymptotically whereas metabolic rate increases exponentially. This results in a predicted growth rate curve that reaches a maximum at a certain swimming speed and decreases at both higher and lower speeds. The model can be used to predict the influence of type of prey, prey density, water temperature etc. on larval growth. An expression describing how many hours per day fish larvae must forage in order to grow at a certain daily body weight gain allows the limits of environmental conditions for positive, zero and negative growth rate to be set. Results of simulations demonstrated that the optimum swimming speed for maximum growth of coregonid larvae increased with an increase in food density, decrease in water temperature or decrease of prey vulnerability. At optimum ‘theoretical’ swimming speed an increase in water temperature from 5 to 17° C required the food density to be increased from 20 to 80 copepods l^?1 in order to maintain a daily growth increment of 2%. The minimum Artemia density required for maintenance metabolism increased from 10 to 30 items 1¹ over the same temperature increase from 5 to 17° C, and food densities required for 8% growth rates were 26 and 56 Artemia nauplii l^?1 at 5 and 17° C, respectively. Contrary to previous findings, results of the present study suggest that metabolic rates of actively feeding fish larvae may be from 5 to 50 times the standard metabolic rate: earlier studies suggested that a factor of 2–3 may be generally applicable.     

Swimming ability and behaviour of post-larvae of a temperate marine fish re-entrained in the pelagic environment

The degree to which behaviour, vertical movement and horizontal transport, in relation to local hydrodynamics, may facilitate secondary dispersal in the water column was studied in post-larval Sillaginodes punctata in Port Phillip Bay, Australia. S. punctata were captured in shallow seagrass beds and released at the surface in three depth zones (1.5, 3 and 7 m) off-shore at each of two sites to mimic the re-entrainment of fish. The behaviour, depth and position of S. punctata were recorded through time. The direction and speed of local currents were described using an S4 current meter and the movement of drogues. Regardless of site, fish immediately oriented toward the bottom, and into the current after release. In shallow water (1.5 m), 86% of fish swam to the bottom within 2 min of release. At one site, the net horizontal displacement of fish was largely unrelated to the speed and direction of local currents; at a second site, fish could not maintain their position against the current, and the net horizontal displacement was related to the speed and direction of currents. In the intermediate depth zone, wide variability in depths of individual fish through time led to an average depth reached by fish that was between the shallow and deep zones. Based on daily increments in the otoliths, however, this variability was not related significantly to the time since entry of fish into Port Phillip Bay. In the deepest depth zone, 81% of fish remained within 1 m of the surface and their horizontal displacement was significantly related to the direction and speed of currents. Secondary dispersal of post-larval fish in the water column may be facilitated by the behaviour and vertical movements of fish, but only if fish reach deeper water, where their displacement (direction and distance) closely resembles local hydrodynamic regimes. In shallow water, fish behaviour and vertical migration actually reduce the potential for secondary dispersal.     

The Optomotor Response in Weak-electric Mormyrid Fish: Can they See?

The present study has explored the optomotor response in two species of mormyrid fish, Gnathonemus petersii and Brienomyrus niger. Both species tended to follow a black and white striped stimulus pattern under illumination levels of 6, 12, and 60 lx. The optomotor response ceased to occur under 540 lx. The behavioral data support earlier histological findings implicating the mormyrid retina in dim light vision.     

Swimming performance of the freshwater neotropical fish: Pimelodus maculatus Lacepède, 1803

The present study used fixed and increasing velocity tests in an experimental apparatus based on Brett's respirometer to examine prolonged and sustained speeds of the "mandi-amarelo", Pimelodus maculatus. When comparing the curves of critical speed versus total length between the mandi and the sockeye salmon Oncorhynchus nerka, it is observed that for an equal total length, the mandi presents a greater speed, probably due to water temperature differences. The sustained speed for the species was estimated in 5 lengths per second and the percentage of fatigued fish within time in a certain velocity was established. The data raised for the mandi represents an important contribution to the improvement of the handling of the species, providing guidance and criteria for designing several structures, such as fishways, fish screens and guidance systems.