Ontogeny of critical and prolonged swimming performance for the larvae of six Australian freshwater fish species

Critical (<30 min) and prolonged (>60 min) swimming speeds in laboratory chambers were determined for larvae of six species of Australian freshwater fishes: trout cod Maccullochella macquariensis, Murray cod Maccullochella peelii, golden perch Macquaria ambigua, silver perch Bidyanus bidyanus, carp gudgeon Hypseleotris spp. and Murray River rainbowfish Melanotaenia fluviatilis. Developmental stage (preflexion, flexion, postflexion and metalarva) better explained swimming ability than did length, size or age (days after hatch). Critical speed increased with larval development, and metalarvae were the fastest swimmers for all species. Maccullochella macquariensis larvae had the highest critical [maximum absolute 46·4 cm s^?1 and 44·6 relative body lengths (L_B) s^?1] and prolonged (maximum 15·4 cm s^?1, 15·6 L_B s^?1) swimming speeds and B. bidyanus larvae the lowest critical (minimum 0·1 cm s^?1, 0·3 L_B s^?1) and prolonged swimming speeds (minimum 1·1 cm s^?1, 1·0 L_B s^?1). Prolonged swimming trials determined that the larvae of some species could not swim for 60 min at any speed, whereas the larvae of the best swimming species, M. macquariensis, could swim for 60 min at 44% of the critical speed. The swimming performance of species with precocial life‐history strategies, with well‐developed larvae at hatch, was comparatively better and potentially had greater ability to influence their dispersal by actively swimming than species with altricial life‐history strategies, with poorly developed larvae at hatch.     

Settlement behaviour of larvae of the Stripey Snapper, <Emphasis Type="Italic">Lutjanus carponotatus</Emphasis> (Teleostei: Lutjanidae)

Larval behaviour is important to dispersal and settlement, but is seldom quantified. Behavioural capabilities of larval Lutjanus carponotatus in both offshore pelagic and reef environments at Lizard Island, Great Barrier Reef were observed in situ to determine if they were sufficient to influence dispersal. Offshore, larvae swam with higher directional precision and faster on the windward side of the island (28 cm.s⁻¹) than on the leeward side (16 cm s⁻¹). Most larvae swam directionally. Mean swimming directions were southerly in the windward area and northerly in the leeward area. Larvae avoided the surface and remained mostly between 3–15 m. Larvae released near reefs were 2–3 times faster swimming away from reefs (19 cm s⁻¹) than swimming toward or over them (6–8 cm s⁻¹). Speed swimming away was similar to that offshore. Of 41 larvae released near reefs, 73% reached the reef, 59% settled, and 13% of those reaching the reef were eaten. Larvae settled onto hard and soft coral (58%), topographic reef features (29%) and sand and rubble (13%). Settlement depth averaged 5.5 m (2–8 m). Before settling larvae spent up to 800 s over the reef (mean 231 s) and swam up to 53 m (mean 14 m). About half of the larvae interacted with reef residents including predatory attacks and aggressive approaches by residents and aggressive approaches by settling larvae. Settlement behaviour of L. carponotatus was more similar to a serranid than to pomacentrids. Settlement-stage larvae of L. carponotatus are behaviourally capable, and have a complex settlement behaviour.     

Ontogenetic changes in the critical swimming speed of Gadus morhua (Atlantic cod) and Myoxocephalus scorpius (shorthorn sculpin) larvae and the role of temperature

Most studies on behavioural contributions to dispersal and recruitment during early life history stages of fishes have focused on coral reef species. For cold ocean environments, high variation in seasonal temperature and development times suggest that parallel studies on active behaviour are needed for cold-water species. Thus, we examined the critical swimming speed (U_crit) of marine fish larvae from 2 contrasting species: Gadus morhua (Atlantic cod) and Myoxocephalus scorpius (shorthorn sculpin), a pelagic and bottom spawner respectively. Within-species comparisons showed that sculpin reared at 6 °C had lower initial U_crit values, but a faster U_crit increase through development compared with 3 °C conspecifics, ultimately resulting in faster critical swimming speeds at metamorphosis (10.5 vs. 9.1 cm·s^− 1). In contrast, although cod larvae reared at 10 °C were faster swimmers at first feeding than 6 °C fish, temperature differences were absent after the first week. These results show that temperature influences the trajectory of larval critical swimming speed development, but that the relationship is species-specific. Although 6 °C sculpin and cod of similar length had equivalent U_crit values, the smaller size of cod at hatch (5.3 vs. 10.8 mm for sculpin) resulted in much lower age-specific U_crit values for cod. These data have significant implications for how swimming activity of the two species might affect dispersal, particularly in the first few weeks post-hatch. Overall, our data suggest that temperature during larval development influences the swimming capacity of cold-water marine fishes, and has important ramifications for biophysical models of dispersal.     

Swimming speed performance in coral reef fishes: field validations reveal distinct functional groups

Central to our understanding of locomotion in fishes are the performance implications of using different modes of swimming. Employing a unique combination of laboratory performance trials and field observations of swimming speed, this study investigated the comparative performance of pectoral and body-caudal fin swimming within an entire assemblage of coral reef fishes (117 species 10 families). Field observations of swimming behaviour identified three primary modes: labriform (pectoral 70 spp.), subcarangiform (body-caudal 29 spp.) and chaetodontiform (augmented body-caudal 18 spp.). While representative taxa from all three modes were capable of speeds exceeding 50 cm s⁻¹ during laboratory trials, only pectoral-swimmers maintained such high speeds under field conditions. Direct comparisons revealed that pectoral-swimming species maintained field speeds at a remarkable 70% of their maximum (lab-tested) recorded speed; species using body-caudal fin propulsion maintained field speeds at around 50% of maximum. These findings highlight a profound influence of swimming mode on performance, with the relative mechanical and energetic efficiency of each swimming mode being of major importance. Combining attributes of efficiency, maneuverability and speed in one mode of propulsion, pectoral swimming appears to be a particularly versatile form of locomotion, well suited to a demersal lifestyle on coral reefs.     

Avoiding the flow: refuges expand the swimming potential of coral reef fishes

While many coral reef fishes utilise substratum refuges, the direct influence of water flow and swimming ability on such refuging patterns is yet to be established. This study examined the swimming ability and refuging behaviour of a labrid (Halichoeres margaritaceus) and a pomacentrid (Pomacentrus chrysurus) that inhabit high flow, wave-swept coral reef flats. Field observations of refuging patterns were combined with experimental evaluations in a flow tank using a replica of a substratum hole frequently used by these species. Under a range of flow speeds commonly found on the reef flat (0–60 cm s⁻¹), flow within the substratum refuge was reduced to speeds of 0–12 cm s⁻¹, representing a 75–100% flow reduction. Swimming ability of each species was then tested at 60 cm s⁻¹ with and without access to this flow refuge. Both species were able to maintain activity within the 60 cm s⁻¹ flow for considerably longer when provided with a refuge, with increases from approximately 39 min to 36 h for H. margaritaceus and 8 min to 88 h for P. chrysurus. Despite H. margaritaceus having the strongest swimming ability without access to a refuge, P. chrysurus was able to maintain swimming activity more than twice as long as H. margaritaceus when provided with a refuge. These increases in activity are probably due to energetic savings, with this type of refuge providing an estimated 95% energy saving over swimming directly into a unidirectional flow of 60 cm s⁻¹. These results highlight the major advantages provided by refuging behaviour and emphasise the importance of habitat refuges in shaping patterns of habitat use in reef fishes.     

Swimming patterns and behaviour of upriver-migrating adult pink (Oncorhynchus gorbuscha) and sockeye (O. nerka) salmon as assessed by EMG telemetry in the Fraser River,British Columbia,Canada

Little is known about the behaviour patterns and swimming speed strategies of anadromous upriver migrating fish. We used electromyogram telemetry to estimate instantaneous swimming speeds for individual sockeye (Oncorhynchus nerka) and pink salmon (O. gorbuscha) during their spawning migrations through reaches which spanned a gradient in river hydraulic features in the Fraser River, British Columbia. Our main objectives were to describe patterns of individual-specific swim speeds and behaviours, identify swimming speed strategies and contrast these between sexes, species and reaches. Although mean swimming speeds did not differ between pink salmon (2.21 BL s^–1) and sockeye salmon (1.60 BL s^–1), sockeye salmon were over twice as variable (mean CV; 54.78) in swimming speeds as pink salmon (mean CV; 22.54). Using laboratory-derived criteria, we classified swimming speeds as sustained (<2.5 BL s^–1), prolonged (2.5–3.2 BL s^–1), or burst (>3.2 BL s^–1). We found no differences between sexes or species in the proportion of total time swimming in these categories – sustained (0.76), prolonged (0.18), burst (0.06); numbers are based on species and sexes combined. Reaches with relatively complex hydraulics and fast surface currents had migrants with relatively high levels of swimming speed variation (e.g., high swimming speed CV, reduced proportions of sustained speeds, elevated proportions of burst speeds, and high rates of bursts) and high frequency of river crossings. We speculate that complex current patterns generated by river constrictions created confusing migration cues, which impeded a salmon's ability to locate appropriate pathways.     

Relationship between swimming capacities and morphological traits of fish larvae at settlement stage: a study of several coastal Mediterranean species

Experimental measurements were made in the laboratory to determine the swimming capacities of settlement-stage fish larvae of several Mediterranean coastal species collected from the nearshore waters of Corsica, France. Critical swimming speed (U_crit, cm s⁻¹) was measured to provide a realistic laboratory estimate of in situ swimming speed. Morphometric traits were measured to assess potential predictors of a species’ swimming ability and, when possible, daily otolith increments were used to estimate age. Observed swimming speeds were consistent with other temperate species and demonstrated that the tested species are competent swimmers and not passive components of their environment. Morphological traits varied in their correlation with U_crit across groups and species. Direct measurements of morphological traits were better predictors than calculated ratios. Pelagic larval duration had little relationship with swimming speed among species for which daily otolith increments were counted. In addition to expanding the database on swimming capacities of settlement-stage fish larvae in the Mediterranean Sea, this study also developed methods that simplify the assessment of larval fish swimming ability. Swimming speed data are essential for improving larval dispersal models and for predicting recruitment rates in coastal fish populations.     

Mechanisms of Population Structuring in Giant Australian Cuttlefish Sepia apama

While a suite of approaches have been developed to describe the scale, rate and spatial structure of exchange among populations, a lack of mechanistic understanding will invariably compromise predictions of population-level responses to ecosystem modification. In this study, we measured the energetics and sustained swimming capacity of giant Australian cuttlefish Sepia apama and combined these data with information on the life-history strategy, behaviour and circulation patterns experienced by the species to predict scales of connectivity throughout parts of their range. The swimming capacity of adult and juvenile S. apama was poor compared to most other cephalopods, with most individuals incapable of maintaining swimming above 15 cm s⁻¹. Our estimate of optimal swimming speed (6–7 cm s⁻¹) and dispersal potential were consistent with the observed fine-scale population structure of the species. By comparing observed and predicted population connectivity, we identified several mechanisms that are likely to have driven fine-scale population structure in this species, which will assist in the interpretation of future population declines.     

Fish swimming stride by stride: speed limits and endurance

Summary Steadily swimming fish show a species-specific stride length and tail tip amplitude. These are constant over the entire speed range if expressed as a fraction of the body length. The speed of a fish equals the stride length times the tail beat frequency. We describe how maximum tail beat frequencies, and hence maximum swimming speeds, are related to temperature and body length.Maximum sustained swimming speeds, endurance during swimming at higher speeds, and maximum burst velocities of 27 species are compared. The rate of decline of endurance with increasing speed is either gradual or steep, with only a few cases in between Steady swimmers show the steepest decline.The published effects of temperature on endurance are not consistent.The effect of body size on the endurance curve could be investigated for two species. The maximum sustained speed decreases with increasing length, and the slope of the endurance curves steepens with increasing length with the same factor in both species. The maximum burst speed is 10 Ls^-1 on average.     

Swimming Speed of Larval Snail Does Not Correlate with Size and Ciliary Beat Frequency

Many marine invertebrates have planktonic larvae with cilia used for both propulsion and capturing of food particles. Hence, changes in ciliary activity have implications for larval nutrition and ability to navigate the water column, which in turn affect survival and dispersal. Using high-speed high-resolution microvideography, we examined the relationship between swimming speed, velar arrangements, and ciliary beat frequency of freely swimming veliger larvae of the gastropod Crepidula fornicata over the course of larval development. Average swimming speed was greatest 6 days post hatching, suggesting a reduction in swimming speed towards settlement. At a given age, veliger larvae have highly variable speeds (0.8–4 body lengths s⁻¹) that are independent of shell size. Contrary to the hypothesis that an increase in ciliary beat frequency increases work done, and therefore speed, there was no significant correlation between swimming speed and ciliary beat frequency. Instead, there are significant correlations between swimming speed and visible area of the velar lobe, and distance between centroids of velum and larval shell. These observations suggest an alternative hypothesis that, instead of modifying ciliary beat frequency, larval C. fornicata modify swimming through adjustment of velum extension or orientation. The ability to adjust velum position could influence particle capture efficiency and fluid disturbance and help promote survival in the plankton.     

Influence of temperature and ambient oxygen on the swimming energetics of cyprinid larvae and juveniles

Synopsis The relationship between respiration and swimming speed of larvae and juveniles (2–100 mg fresh mass) of Danube bleak, Chalcalburnus chalcoides (Cyprinidae), was measured at 15° and 20° C under hypoxic (50% air saturation), normoxic, and hyperoxic (140% air saturation) conditions. In a flow-tunnel equipped with a flow-through respirometer the animals swam at speeds of up to 8 lengths · s^-1; speeds were sustained for at least two minutes. The mass specific standard, routine, and active respiration rates declined with increasing body mass at both temperatures. Metabolic intensity increased with temperature, but also the critical swimming speed (at which oxygen uptake reached its maximum) was higher at 20° than at 15° C by about 30%. Nevertheless, the oxygen debt incurred by the fish at the highest speeds was about 40%, and the net cost of swimming about 32%, lower at 20° than at 15°C. The standard metabolic rate was more strongly dependent on temperature (Q₁₀ around 2.5) than the maximum active rate (Q₁₀ below 2). Whereas standard and routine respiration rates were well regulated over the pO₂-range investigated (8.5–25.8 kPa), the active rates showed a conformer-like pattern, resulting in factorial scopes for activity between 2 and 4. Under hypoxia, the critical swimming speed was lower than under normoxia by about 1.51 · s^-1, but the net cost of swimming was also lower by about 30%. On the other hand, hyperoxia neither increased the swimming performance nor did it lead to a further increase of the metabolic cost of swimming. The hypoxia experiments suggest that in response to lowered tensions of ambient oxygen maintenance functions of metabolism not directly related to swimming may be temporarily reduced, leading to increased apparent swimming efficiency under these conditions. The responses of the larvae of Danube bleak to low temperature and low ambient oxygen are discussed in terms of the metabolic strategies by which energy-limited animals meet the challenge of environmental deterioration.     

Effect of larval ontogeny, turbulence and light on prey attack rate and swimming activity in herring larvae

The effects of ontogeny (larval size), light and turbulence on the attack rate and swimming activity (proportion of time swimming and duration of swimming bout) of herring larvae (15-28 mm TL) have been investigated. Emphasis was put on the experimental design in order to create a set-up where the turbulence intensity distribution could be accurately measured as well as controlled in the entire experimental tank.Both larval size (ontogeny) and light had a significant positive effect on prey attack rate. Likewise, an intermediate increase in turbulence had a positive effect on prey attack rate, but this effect was dependent of light intensity and larval size.At low light (1.5 μE m² s⁻¹) intermediate turbulence increased the prey attack rate significantly for larger larvae (26 and 28 mm), while at high light (18 μE m² s⁻¹) intermediate turbulence had only a significant positive effect on the attack rate of smaller larvae 20 and 23 mm.In general, our data show a dome-shaped response of turbulence on attack rate and a U-shaped response of turbulence on swimming activity.For herring larvae >20 mm, the maximum (attack rate) and minimum (swimming activity) response of turbulence were found at intermediate turbulence intensities (energy dissipation rates between 7∗10⁻⁸ and 1∗10⁻⁶ W/kg). The highest turbulence level tested (8∗10⁻⁶ W/kg) showed only negative effects, as attack rates where at the lowest and swimming activity at the highest.Swimming activity increased with larval size or light, and decreased at intermediate turbulence. Compared to turbulent intensities under natural conditions this implies that larger herring larvae at 10 m depth have to be exposed to wind speeds of more than 17 m/s before negative effects on attack rate and swimming activity occurs.     

Behaviour controls post-settlement dispersal by the juvenile bivalves Austrovenus stutchburyi and Macomona liliana

Post-settlement dispersal is a key process affecting the population dynamics of many soft sediment benthic invertebrates. Despite its importance, few studies have quantified those factors that influence juvenile dispersal. In a laboratory flume, we examined the effects of three flow velocities and two substrate types on the dispersal of two common intertidal bivalves: the deposit-feeding wedge shell Macomona liliana and the suspension-feeding cockle Austrovenus stutchburyi. Juveniles of three size classes (<2, 2-4 and 4-8 mm shell length) were added to cores of defaunated natural sediment or glass beads. We recorded the number of bivalves remaining in cores versus those recovered downstream either on the flume floor, in bedload traps or in a plankton net at the end of the working section of the flume after 48 h at three freestream velocities (4.8, 11.0 and 16.6 cm s⁻¹). At flow speeds of 4.8 cm s⁻¹, <5% of individuals were recovered outside the cores for both species. At higher flows, the dispersal mode (crawling, bedload or in suspension) and frequency of dispersal differed markedly between species. Austrovenus dispersed primarily by crawling in the low flow treatments. The frequency of dispersal increased substantially (2-6×) between 11.0 and 16.6 cm s⁻¹, and most Austrovenus were found in bedload traps at the highest flow. At the highest flow, twice as many Austrovenus individuals left the glass bead treatment as from the natural sediment. The number of dispersing Macomona also increased with increasing flow from the natural sediment, but numbers dispersing from glass beads were similar for the two higher flows (11.0 and 16.6 cm s⁻¹). Macomona dispersal mode was associated with size; smaller size classes were collected in the bedload traps and the plankton net in approximately equal proportions, while only a small proportion of the largest size class were collected in the plankton net. In contrast to flume dispersal experiments with live bivalves, most dead Austrovenus did not move at the highest flow speed, while most dead Macomona were transported at the highest flow speed. The live-dead comparisons, in conjunction with our primary experimental results, imply that there are active behavioural components to both water column and bedload transport. Our research emphasises that both species actively influence post-settlement transport in response to both substrate type and flow regime, and that bedload transport, often categorised as a passive transport process, is also greatly influenced by active behavioural processes.     

Temperature influences swimming speed, growth and larval duration in coral reef fish larvae

The effects of temperature on growth, pelagic larval duration (PLD) and maximum swimming speed were compared in the tropical fish marine species Amphiprion melanopus, to determine how temperature change affects these three factors critical to survival in larvae. The effects of rearing temperature (25 and 28 °C) on the length of the larval period and growth were examined in conjunction with the effects of swimming temperature (reared at 25 °C, swum at 25 and 28 °C, reared at 28 °C, swum at 25 and 28 °C) on critical swimming speed (U-crit). Larvae reared at 25 °C had a 25% longer pelagic larval duration (PLD) than larvae reared at 28 °C, 12.3 (±0.3) days compared with 9 (±0.6) days at 25 °C. To offset this effect of reduced developmental rate, growth and U-crit were measured in larvae reared at 28 and 25 °C at the same absolute age (7 days after hatching (dah)) and same developmental age (7 dah at 28 °C cf. 11 dah at 25 °C), corresponding to the day before metamorphosis. Larvae reared at 25 °C were smaller than larvae reared at 28 °C at the same absolute age (7 dah at 25 °C cf. 7 dah at 28 °C), yet larger at similar developmental age (11 dah at 25 °C cf. 7 dah at 28 °C) when weight and standard length were compared. This stage-specific size increase did not result in better performance in larvae at the same developmental age, as there was no difference in U-crit in premetamorphic larvae reared at either temperature (7 dah at 28 °C c.f 11 dah at 25 °C). However, U-crit was considerably slower in 7-day-old larvae reared at 25 °C than larvae of the same absolute age (7 dah) reared at 28 °C. Swimming temperature controls demonstrated that a change in temperature immediately prior to swimming tests did not effect swimming performance for larvae reared at either temperature.A decreased in rearing temperature resulted in longer larval durations, reduced growth rates and slower swimming development in larvae. However, the magnitude of the response of each of these traits varied considerably. As such, larvae reared at the lower temperature were a larger size at metamorphosis but had poorer relative swimming capabilities. This study highlights the importance of measuring a range of ecologically relevant traits in developing larvae to properly characterise their relative condition and performance in response to environmental change.     

Direct observation of swimming behaviour in a shallow-water scavenging amphipod Scopelocheirus onagawae in relation to chemoreceptive foraging

Direct observations on foraging behaviour of scavenging lysianassid amphipods have been limited, and no previous study has examined the effect of food odour quantitatively on the behaviour. The present study recorded the swimming behaviour of the amphipod Scopelocheirus onagawae using videographic techniques before and after the introduction of food odour (amino acid solution). S. onagawae showed consistent nocturnal activity swimming at a high speed (16.8 cm s^− 1) with an approximately straight trajectory in various directions before and after the introduction of odour in which the amino acid concentration was below the behavioural threshold concentration for this species (1.0 × 10^− 7 mol l^− 1). High speed multidirectional linear swimming is thought to be advantageous for these amphipods, enabling them to survey across a broad area. After the first encounter with the odour plume above the behavioural threshold concentration, the amphipods slowed down their swimming speed (ca. 9.7 cm s^− 1) with a short time-lag (ca. 0.42 s), and thereafter they frequently turned so that they remained within the odour plume. Once moved out of the odour plume, the amphipods quickly returned to the plume with a shorter response time (ca. 0.1 s) than that in the first detection of the odour plume, suggesting that the sensory adaptation is involved with the tracking of the odour. Our study demonstrated that chemoreception is a major factor causing behavioural change in scavenging amphipods at the edge of the odour plume.     

Ontogeny of behaviour in larvae of marine demersal fishes

The development of behaviours that are relevant to larval dispersal of marine, demersal fishes is poorly understood. This review focuses on recent work that attempts to quantify the development of swimming, orientation, vertical distribution and sensory abilities. These behaviours are developed enough to influence dispersal outcomes during most of the pelagic larval stage. Larvae swim in the ocean at speeds similar to the currents found in many locations and at 3–15 body lengths per second (BL s⁻¹), although, based on laboratory measurements, species from cold environments swim slower than those from warm environments. At least in warm-water species, larvae swim in an inertial hydrodynamic environment for most of their pelagic period. Unfed swimming endurance is >10 km from about 8–10 mm, and reaches more than 50 km before settlement in several species. Larval fishes are efficient swimmers. In most species, a large majority of larvae have orientated swimming in the ocean, but the precision of orientation does not improve with growth. Swimming direction of the larvae frequently changes ontogenetically. Vertical distribution changes ontogenetically in most species, and both ontogenetic ascents and descents are found. Development of schooling is poorly understood, but it may influence speed, orientation and vertical distribution. Sensory abilities (hearing, olfaction, vision) form early, are well developed and are able to detect cues relevant to orientation for most of the pelagic larval stage. All this indicates that the passive portion of the pelagic larval duration will be short, at least in most warm-water species, and that behaviour must be taken into account when considering dispersal, and in particular in dispersal models. Although quantitative information on the ontogeny of some behaviours is available for a relatively small number of species, more research in this field is required, especially on species from colder waters.     

The effect of flow on larval vertical distribution of the sea urchin, Strongylocentrotus droebachiensis

Most meroplanktonic larvae have been considered to behave as passive particles in the water column, and their dispersal determined by advection. However, larvae may influence their horizontal transport by sinking or swimming between overlying water masses. The flow conditions under which larvae influence their vertical distribution through depth regulation are presently unclear. Using an annular flume, we examined the effect of increasing flow, repeated exposure to flow, and acceleration and deceleration on the vertical distribution of 4-arm stage echinoplutei of Strongylocentrotus droebachiensis. Specifically, we generated different levels of vertical velocity and shear strengths by manipulating horizontal velocity (u). We increased and decreased flow speed incrementally from no flow (u = 0 cm s^− 1) to intermediate flow (u = 0.48 cm s^− 1) to high flow (u = 1.02 cm s^− 1) for each of 3 cycles within each of 2 independent trials. We used a high resolution digital camera to record, and image-analysis to quantify, larval distribution. In the absence of flow, larvae swam upwards and aggregated near the surface of the flume. With increasing flow, increasing numbers of larvae were observed in the mid to low water column indicating a negative influence on larval ability to aggregate near the surface. No differences were observed between distributions in acceleration and deceleration phases of the cycles; however, results suggest that increased exposure can decrease the ability of larvae to regulate their vertical position over time. Vertical shear can result in the re-orientation of swimming larvae and likely compromised larval ability for directed swimming in our study. The threshold shear level beyond which larvae cannot regulate their vertical position is > 2 s^− 1, suggesting that echinoid larvae may be more vulnerable to shear than other weak swimmers, most likely because of their shape. However, echinoid larvae can likely influence their vertical distribution within many areas in the ocean, since shears > 2 s^− 1 are present only in highly turbulent regions such as fronts.     

Measurements of aerobic metabolism of a school of horse mackerel at different swimming speeds

Oxygen consumption rates were measured in a school of 56 horse mackerel Trachurus trachurus while at rest and while swimming at steady sustained speeds. Resting values of 38.76 and 42.10mg O₂ kg^?1 h^?1 were measured in a sealed cylindrical tank (535 l) while observing that the fish school remained neutrally buoyant and inactive with only gentle pectoral fin movements and no swimming motion. The same school was trained to swim with projected light patterns within a 10-m diameter annular doughnut respirometer. The oxygen consumption increased from the resting level through 51 mg O₂ kg^?1 h^?1 at the slowest swimming speeds of 0.29 m s^?1 (0.95 L s^?1) to around 259 mg O₂ kg^?1 h^?1 at the higher measured swimming speed of 0.87 m s^?1 (2.82 L s^?1). The data fitted a curve where oxygen consumption rose in proportion to velocity to the power of 2.56 with the intercept at the resting level. The maximum sustained speed (80 min) of 1.12 m s^?1 (3.63 Ls^?1) was not achieved within the respirometer but corresponded to an estimated oxygen consumption of 458.33 mg O₂ kg^?1 h^?1 giving a scope for aerobic activity of 419.02 mg O₂ kg^?1 h^?1. At a speed of 0.87 m s^?1, there was a lower bound on the aerobic efficiency of at least 38% and at 1.12 m s^?1, the highest aerobic speed, of 40%. Sustained speeds swum in a curved path as here should be increased by 5% for a straight path giving a maximum sustained 80 min speed of 1.18 m s^?1.     

In situ settlement behaviour of damselfish (Pomacentridae) larvae

Settlement‐stage damselfish (Pomacentridae) larvae of 13 species in seven genera were obtained from light traps at Lizard Island, Great Barrier Reef, Australia. Behaviour, observed in situ by SCUBA divers, of 245 larvae (6–13 mm, L_S; 5–60 individuals per species) released individually within a few m of reefs during the day differed markedly among species. From 0–28% (range among 13 species) of individuals of each species swam away from the adjacent reefs without swimming to the reefs. Of those that swam to a reef, 0–75% settled. For three species, sufficient data were available to test the hypothesis that these percentages did not differ amongst reefs: the hypothesis was rejected in one species. From 0–75% of larvae that reached the reef were eaten, 0–63% subsequently left the reef and 0–60% were still swimming over the reef at the end of the observation period. Swimming speeds of all but one species were greater when swimming away from the reef than toward it. Most species exceeded average current speeds when swimming away from reefs, but not when swimming toward and over them. Average swimming depths were in the upper half of the water column for most species, and were somewhat greater where the water depths were greater. The time the larvae swam over the reef before settling and the distance swum varied greatly among species from 0 to a mean of 5.5 min and 43 m. Settlement habitats chosen differed amongst species, and in some species, they were very specific. Average settlement depth varied among species from 6–13.5 m. In one species, settlement depth varied between reefs. About half of the 53 observed interactions between larvae and reef resident fishes were predation attempts: fishes of eight species (six families) attacked larvae. The other interactions were aggressive approaches by 11 species of resident fishes, all but one of which were pomacentrids. Many of these aggressive interactions discouraged settlement attempts. Larvae of some species experienced no predatory or aggressive interactions, whereas in other species interactions averaged >0.6 per released larva. Species that swam more‐or‐less directly to settlement sites near the reef edge experienced more interactions. Even within the same family, settlement behaviour differed among species in nearly all measures.