Foraging behaviour of larval cod (Gadus morhua) influenced by prey density and hunger

Fish larvae meet diverse environmental conditions at sea, and larval growth and chance of survival depend on a flexible response to environmental variability. The present study focuses on the flexibility of the foraging behaviour of larval cod in a series of laboratory experiments on larval search activity, prey selectivity, and hunger in a variable prey environment. Gadus morhua eggs were collected in March 1992 and 1993 from the Kattegat area, Denmark, fertilised and incubated in the laboratory. After hatching, the larvae were transferred to rearing tanks of 172 litres. The behaviour of larvae (6 to 7 mm long) was observed visually, and prey attacks, swimming activity and gut contents were registered across a range of 1 to 120 copepod nauplii l^-1. When prey density decreased, larvae increased their swimming activity, increased their responsiveness to prey (distance of reaction) and decreased their prey size selectivity. Behavioural response was to a large degree determined by the level of hunger, represented by the number of newly ingested prey in the gut. The findings show that cod larvae have a flexible response to changes in feeding conditions and imply that larvae can grow and survive even in the lower range of (mean) prey densities measured at sea.     

Fine-scale observations of the predatory behaviour of the carnivorous copepod <Emphasis Type="Italic">Paraeuchaeta norvegica</Emphasis> and the escape responses of their ichthyoplankton prey,Atlantic cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>)

Paraeuchaeta norvegica (8.5 mm total length) and yolk-sac stage Atlantic cod larvae (4 mm total length) (Gadus morhua) larvae were observed in aquaria (3 l of water) using silhouette video photography. This allowed direct observations (and quantitative measurement) of predator–prey interactions between these two species in 3-dimensions. Tail beats, used by cod larvae to propel themselves through the viscous fluid environment, also generate signals detectable by mechanoreceptive copepod predators. When the prey is close enough for detection and successful capture (approximately half a body-length), the copepod launches an extremely rapid high Reynolds number attack, grabbing the larva around its midsection. While capture itself takes place in milliseconds, minutes are required to subdue and completely ingest a cod larva. The behavioural observations are used to estimate the hydrodynamic signal strength of the cod larva’s tail beats and the copepod’s perceptive field for larval fish prey. Cod larvae are more sensitive to fluid velocity than P. norvegica and also appear capable of distinguishing between the signal generated by a swimming and an attacking copepod. However, the copepod can lunge at much faster velocities than a yolk-sac cod larva can escape, leading to the larva’s capture. These observations can serve as input to the predator–prey component of ecosystem models intended to assess the impact of P. norvegica on cod larvae.     

Electrophoretic survey of genetic variation in Pecten maximus,Chlamys opercularis,C. varia and C. distorta from the Irish Sea

Food selection by young larvae of the gulf menhaden (Brevoortia patronus) was studied in the laboratory at Beaufort, North Carolina (USA) in 1982 and 1983; this species is especially interesting, since the larvae began feeding on phytoplankton as well as microzooplankton. When dinoflagellates (Prorocentrum micans), tintinnids (Favella sp.), and N1 nauplii of a copepod (Acartia tonsa) were presented to laboratory-reared, larval menhaden (3.9 to 4.2 mm notochord length), the fish larvae ate dinoflagellates and tintinnids, but not copepod nauplii. Larvae showed significant (P<0.001) selection for the tintinnids. Given the same mixture of food items, larger larvae (6.4 mm notochord length) ate copepod nauplii as well as the other food organisms. These feeding responses are consistent with larval feeding in the northern Gulf of Mexico, where gulf menhaden larvae between 3 and 5 mm in notochord length frequently ate large numbers of dinoflagellates (mostly P. micans and P. compressum) and tintinnids (mostly Favella sp.), but did not eat copepod nauplii. As larvae grew, copepod nauplii and other food organisms became important, while dinoflagellates and tintinnids became relatively less important in the diet. Since the tintinnids and nauplii used in the laboratory feeding experiments were similar in size as well as carbon and nitrogen contents, the feeding selectivity and dietary ontogeny that we observed were likely due to a combination of prey capturability and larval fish maturation and learning.Contribution No. 5575 of the Woods Hole Oceanographic Institution     

Essential fatty acid (docosahexaenoic acid, DHA) availability affects growth of larval herring in the field

Larval fish growth and survival depends not only on prey quantity, but also on prey quality. To investigate effects of prey fatty acid concentration on larval herring growth, we collected different prey organisms and larval herring (Clupea harengus L.) in the Kiel Canal during the spring season of 2009. Along with biotic background data, we analysed fatty acids both in prey organisms and in the larvae and used biochemically derived growth rates of the larvae as the response variable. Larval herring reached their highest RNA/DNA derived growth rates only at high docosahexaenoic acid (DHA) concentration. When the ratio of copepodids to lesser quality cirriped nauplii was low, larval growth and larval DHA concentration were both significantly negatively affected. This was true even as prey abundance was increasing. This finding indicates that even in mixed, natural feeding conditions, growth variations are associated with DHA availability in larval fish.     

A laboratory study of predation by Aurelia aurita on larval herring (Clupea harengus): Experimental observations compared with model predictions

Predation by the medusa Aurelia aurita L. on early first-feeding stage larvae of the herring clupea harengus L. was studied in the laboratory. The medusae were captured in Loch Etive, Scotland. Herring larvae were reared from the extificially fertilized eggs of spawning Clyde herring caught in March, 1982. Swimming speeds, volume searched”, capture efficiency and predation rates increased as medusa size increased. Predation rates on fish larvae increased with prey density, but appeared to approach a maximum at high prey densities; in 1 h experiments, a maximum rate of predation of 6.64 larvae h^-1 was estimated by fitting an Ivlev function. A model to predict predation rates was constructed from swimming speeds, sizes and densities of medusae and larvae, and capture efficiency. The rates of predation predicted from the model fell within the range of experimental data, but tended to underestimate rates and did not account for saturation of medusae. Swimming patterns of medusae changed after prey capture: (a) before capture, encounter rates were low and medusae were relatively less active; (b) after capture of 1 larva, encounter rates doubled, with the stimulated medusae exhibiting increased activity and an aftered “searching” path; and (c) after capture of many larvae, swimming speeds and encounter rates of medusae decreased.     

Mechanics of prey selection by ephyrae of the scyphomedusa Aurelia aurita

In situ feeding patterns of ephyrae of the jellyfish Aurelia aurita (Linnaeus) revealed the importance of relatively large (>1 mm) prey in the diet of these scyphozoan predators. These studies were carried out in Narragansett Bay, Rhode Island, USA in March and April, from 1993 through 1996. Rotifers were the only small prey ingested in quantity, and then only when they were unusually abundant in the plankton. Copepod nauplii, similar in size to rotifers and equally abundant, were rarely consumed. Since copepods evince rapid escape responses, this observation suggested a role for prey escape in determining prey vulnerability, while the predominance of large prey in the diet suggested a role for prey size. Using two dimensional video observations of free-swimming ephyrae and their prey in the laboratory we tested hypotheses about the mechanisms underlying these dietary patterns, comparing mechanisms for capture of large versus small prey and for prey of equal size but differing escape behaviors. Capture efficiencies of ephyrae feeding on large prey were 4 to 12 times greater than for small prey taxa. Capture efficiencies for prey of equal size also differed significantly, indicating that other factors influence the outcome of predator–prey interactions. Most prey captures occurred while the ephyrae were swimming and creating fluid flows that entrained prey into the subumbrellar region. Even copepod nauplii were frequently drawn into the subumbrella of swimming ephyrae despite average potential escape velocities (25.7 mm s⁻¹) that exceeded mean maximum velocity of fluid flows around the ephyrae (13.1 mm s⁻¹). Large prey were more likely than small prey to contact nematocyst-bearing surfaces both before and after entrainment in flow fields. With regard to behavior, prey escape speeds were not the only predictor of prey vulnerability. Prey that continued swimming after entrainment (rotifers and brine shrimp) were captured more often than prey of equal size that ceased normal swimming (copepod nauplii and barnacle nauplii). Copepod nauplii were the prey least likely to be captured because they either “played dead” and were expelled from the subumbrella of the ephyrae before contacting a surface, or they eventually escaped at high velocity. These observations indicate that size-selective predation by ephyrae of A. aurita can be influenced by a variety of behavioral responses of the prey. Received: 9 April 1997 / Accepted: 5 September 1997     

How does cod (Gadus morhua) cope with variability in feeding conditions during early larval stages?

Response of mesocosm-reared cod (Gadus morhua L.) larvae to different feeding conditions was investigated in 1988 in two mesocosms: a large basin and a smaller bag enclosure within the basin. The basin was filled with seawater, and a community of naturally occurring plankton developed. Plankton concentrations were monitored, and cod larvae stocked in the enclosures were sampled for determination of growth, survival, and gut content. In the bag, insufficient amounts of energetically favourable prey, as copepod nauplii, led to non-selective ingestion of plankton from a broad range of sizes, including considerable amounts of protozoans (tintinnid and oligotrich ciliates). Growth of larvae from the bag was low, with daily specific growth rates (SGR) less than 2.8% the first 3 wk post-hatch. This was followd by rapid increase of SGR to 21.7%, which coincided with a large increase in availability of copepod nauplii. In the basin, high nauplii concentrations led to SGR of 13.7 to 21.7% from onset of feeding to 16 d post-hatch, respectively. Under such conditions, the larvae were highly selective feeders. At 3 wk post-hatch, survival was 36.7 and 38.3% in the basin and bag enclosure, respectively. To cope with variations in the feeding conditions, the cod larvae were shown to be opportunists when nauplii were scarce, and included plankton from several trophic levels in their diet. When nauplii were abundant, cod larvae realized their high potential for growth. Both opportunism and realization of a high growth potential may enhance survival of the larvae.     

Feeding opportunities of larval and juvenile cod (Gadus morhua) in a Greenlandic fjord: temporal and spatial linkages between cod and their preferred prey

Feeding of fish depends on a spatial and temporal match with prey, and since larval and juvenile feeding can be highly selective, their preferences for given prey sizes and taxa should be considered when quantifying the actual availability of potential prey. We investigated the diet and prey preferences of the early-life stages of Atlantic cod (Gadus morhua) to quantify the availability of prey during a spring-summer season in a West Greenlandic fjord. We hypothesized that abundances of larval and juvenile cod at size were synchronized to optimal availability of preferred prey in space and time. The present analysis is based on nine cruises each covering 5 stations visited between 24 May and 5 August 2010 comparing zooplankton abundance, cod gut content and distribution patterns. Cod 4–25 mm in length preferred prey of about 5 % of their own length. During ontogeny, their preferences changed from calanoid nauplii towards Pseudocalanus spp. and Calanus spp. copepodites. The larvae/juvenile had an exceptionally high dietary contribution from cladocerans, which were highly preferred by cod larger than 9 mm, while the abundant Metridia longa and the non-calanoid copepods contributed less. These findings stress the importance of focusing on abundance of preferred prey when assessing the actual prey availability to young fish. We found a spatio-temporal overlap between cod and their preferred prey, and observations suggest that advection of both zooplankton and cod contributed to this overlap. Hence, the larval feeding opportunities might be sensitive to climate-related changes affecting the circulation patterns in this fjord.     

Foraging behaviour of larval cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) at low light intensities

The ability to forage at low light intensities can be of great importance for the survival of fish larvae in a pelagic environment. Three-dimensional silhouette imaging was used to observe larval cod foraging and swimming behaviour at three light intensities (dusk ~1.36 × 10⁻³ W/m², night ~1.38 × 10⁻⁴ W/m² and darkness ~3.67 × 10⁻⁶ W/m²) at 4 different ages from 6 to 53 days post-hatch (dph). At 6 dph, active pursuit of prey was only observed under dusk conditions. Attacks, and frequent orientations, were observed from 26 dph under night conditions. This was consistent with swimming behaviour which suggested that turn angles were the same under dusk and night conditions, but lower in darkness. Cod at 53 dph attacked prey in darkness and turn angles were not different from those under other light conditions. This suggests that larvae are still able to feed at light intensities of 3.67 × 10⁻⁶ W/m². We conclude that larval cod can maintain foraging behaviour under light intensities that correspond to night-time at depths at which they are observed in the field, at least if they encounter high-density patches of prey such as those that they would encounter at thin layers or fronts.     

The effects of zooplankton swimming behavior on prey-capture kinematics of red drum larvae, <Emphasis Type="Italic">Sciaenops ocellatus</Emphasis>

Most marine fishes undergo a pelagic larval phase, the early life history stage that is often associated with a high rate of mortality due to starvation and predation. We present the first study that examines the effects of prey swimming behavior on prey-capture kinematics in marine fish larvae. Using a digital high-speed video camera, we recorded the swimming velocity of zooplankton prey (Artemia franciscana, Brachionus rotundiformis, a ciliate species, and two species of copepods) and the feeding behavior of red drum (Sciaenops ocellatus) larvae. From the video recordings we measured: (1) zooplankton swimming velocity in the absence of a red drum larva; (2) zooplankton swimming velocity in the presence of a red drum larva; and (3) the excursion and timing of key kinematic events during prey capture in red drum larvae. Two-way ANOVA revealed that: (1) swimming velocity varied among zooplankton prey; and (2) all zooplankton prey, except rotifers and ciliates, increased their swimming velocity in the presence of a red drum larva. The kinematics of prey capture differed between two developmental stages in S. ocellatus larvae. Hyoid-stage larvae (3–14 days old) fed on slow swimming B. rotundiformis (rotifers) while hyoid-opercular stage larvae (15 days and older) ate fast moving A. franciscana. Hyoid-opercular stage red drum larvae had a larger gape, hyoid depression and lower jaw angle, and a longer gape cycle duration relative to their hyoid-stage conspecifics. Interestingly, the feeding repertoire within either stage of red drum development was not affected by prey type. Knowledge of the direct relationship between fish larvae and their prey aids in our understanding of optimal foraging strategies and of the sources of mortality in marine fish larvae.     

Laboratory study of predation by Aurelia aurita on larvae of cod,flounder, plaice and herring: development and vulnerability to capture

Capture success of the medusa Aurelia aurita preying on various developmental stages of fish larvae was measured together with larval reactivity and escape speed after being stung. These experiments were conducted in the spring of 1983 with A. aurita medusae collected from Loch Etive, Scotland and laboratory-reared larvae of Gadus morhua L., Platichthys flesus L., Pleuronectes platessa L. and Clupea harengus L. Capture success of the medusae increased with medusa size, but decreased with advancing larval development. Smaller species of larvae were more vulnerable to capture. Larval reactivity to encounters with medusae increased with advancing development, and larger species of larvae were more reactive to encounters. Larval escape swimming speeds also increased with advancing larval development and size. These results indicate that earlier stages of larvae within a species and smaller species of larvae at a given stage are more vulnerable to predation by medusae since they are less reactive to encounters. Apparently they are more susceptible to the effects of neurotoxins. Predation rates on different developmental stages of herring larvae are documented and compared with rates predicted by a predation model. Predictions fell within the range of observed predation rates, but tended to overestimate rates by larger medusae feeding on larger herring larvae. This indicates the possibility of predator satiation and/or behavioural avoidance.     

Untersuchungen über das Beutefangverhalten bei larven des herings Clupea harengus

Herring larvae were obtained via artificial spawning (Baltic spring spawners, Downs herring). Eggs were immediately transported to the Marine Station (“Meeresstation”) of the Biologische Anstalt Helgoland, transferred into 140] tanks, and incubated at about 10°C. Sea water was circulated through an internal filter. Artificial illumination (neon tubes) was kept at about 1000 Lux (water surface) during 12 h per day; it was than decreased gradually to complete darkness within 30 min. Dawn was also simulated in order to avoid abrupt changes in light intensity. Food consisted of wild plankton (mainly crustacean nauplii) caught every day on Helgoland Roads, and of Artemia salina nauplii. The larvae were fed 1 to 3 times a day; they took the food always within the first half hour after it was offered. Over periods of 5 min each, the time spent for various activities (different modes of swimming, feeding) were recorded. The behavioural patterns of comparable larvae were filmed. The initial phase of prey catching consists of s-shaped body bending; usually the main bend of the body (upper arrows in Figs. 2 and 3) bears a typical directional relationship to the swimming path of the prey focussed (lower arrows). Such body bending is not always succeeded by subsequent steps of prey catching. In the normal prey catching process, aiming is followed by sudden stretching of the body and swallowing of the prey within 0.2 to 0.3 sec. Yolk sac larvae can use their pectoral fins, larvae of more then 15 mm total length also their tail- and dorsal-fins, for stabilization and correction of prey catching movements. In yolk sac larvae, complete prey catching lasts about 1 to 3 sec. Percentage successful prey catching manoeuvres increases with age and experience (Table 2). Initial success percentage was about 1% in Baltic Sea larvae (Kiel) and about 10% in Downs larvae; it rose within 30 to 35 days in Kiel larvae to nearly 60%, in Downs larvae to over 70%. The possible reasons for these differences are discussed; they may be related to body size and composition of planktonic food. Visual perception of food depends on optic capacities of larvae, size and distance of prey, visibility, and “duration of presentation” (time span during which the image of the prey is projected onto the retina). This, in turn, appears to be subject to frequency and amplitude of undulating movements of the head during swimming. The percentage of body positioning for prey catching attains maximum values at prey distances of 2 to 8 mm in yolk sac larvae (Downs), and of 3 to 40 mm in larvae of 15 to 20 mm body length; it decreases steadily with increasing prey distance. Larvae up to 15 mm total length take mainly copepod nauplii, larger larvae preferably copepodites. Distance of prey perception is wider in the horizontal than in the vertical plane; in fact, larvae do not perceive prey underneath the horizontal plane.     

Prey selection by larval weakfish (Cynoscion regalis): the effects of prey size,speed, and abundance

We examined feeding by larval weakfish, Cynoscion regalis (Bloch and Schneider), in laboratory experiments conducted during the 1991 spawning season. under natural conditions weakfish larval development is ca. 3 wk, and we ran separate experiments with larvae of five different ages (5, 8, 11, 14, and 17 d post-hatching). We used two different size classes of rotifers (Brachionus plicatilis) and brine shrimp nauplii (Artemia sp.) as prey organisms. Contrary to results of previous research, weakfish larvae did not select prey based on size alone. When prey abundance was above 100 itemsl^-1 weakfish, larvae always chose large rotifers (length = 216 m) over small rotifers (length = 160 m). At 11 d post-hatching, larvae switched their diet from large rotifers to small brine shrimp nauplii (length = 449 m); however, when fed small rotifers and small brine shrimp nauplii the change in diet occurred at 14 d post-hatching. This pattern of selectivity was maintained in each larval age class. Early-stage larvae (5 and 8 d post-hatching) did not feed selectively when prey abundance was less than 100 itemsl^-1. Late-stage larvae (17 d post-hatching) fed selectively at abundances ranging from 10 to 10000 items^-1. Lwimming speeds of prey items, which ranged from 1 to 6 mms^-1, had no consistent effect on prey selection. These results suggest that weakfish larvae are able to feed selectively, that selectivity changes as larvae age, and that selectivity is also influenced by prey abundance.     

Search tactics of a pause-travel predator: adaptive adjustments of perching times and move distances by hawk owls (Surnia ulula)

Summary After having erected artificial perches of different heights in a clear-cut, I studied the effect of perching height on the allocation of search and pursuit effort of 9 individuals of the hawk owl. Hawk owls are diurnal and locate their small mammal prey visually. Horizontal attack and move distances increased with perching height in most individual cases, as well as across cases. Prey detection and giving-up times increased with perching height in half of the individual cases, but not across cases. Attack and move distances increased with perching time in only a few individual cases, and not across cases. Perching height explained more of the variation in attack and move distances than did perching time. Move distance depended on both preceding and subsequent perching heights. When giving up a perch the owls on average moved slightly farther than the distance predicted if travel distance as well as overlap between successive search areas are to be minimized. When the ground was snow-free, distances of successful attacks tended to be shorter than those of unsuccessful attacks. Moreover, perches from which prey were captured, but not perches from which unsuccessful attacks were launched, tended to be lower than perches which were given up. This may suggest that the predator-prey distance is more critical to the probability of capture than to the probability of detection. Longer horizontal attack distances from higher perches suggest that the size of the area effectively searched around each perch increases with the height of the perch. Therefore, longer giving-up times on higher perches indicate allocation of more search time to larger search areas, and longer moves after giving up or before landing on higher perches indicate avoidance of overlap with the larger search areas of these perches. These adjustments of giving-up time and move distance allow hawk owls to forage efficiently.     

Development of the escape response in larval walleye pollock (Theragra chalcogramma)

The development of the escape response of walleye pollock (Theragra chalcogramma) larvae from attacks by macrozooplanktonic and small-fish predators was quantified in laboratory experiments. Behavior was recorded using video cameras with silhouette illumination from infrared-emitting diodes and by visual observation. Laboratory-reared larvae of 1, 3, 8, 10, 12, 18, 22, 27, 42 days post-hatching, ranging in size from 4 mm to 10 mm total length, were used in the experiments. Even the youngest larvae were observed to exhibit a fast startle response. The percentage of successful larval escapes from the different predators increased as the larvae developed. Euphausiids (Thysanoessa raschii) and amphipods (Calliopiella pratti) often touched larvae but the larvae were usually able to escape and no successful captures of larvae over 22 days old were observed. Although successful escape from initial attacks by three-spine sticklebacks (Gasterosteus aculeatus) increased ontogenetically, sticklebacks were able to consume most larvae, even of the oldest age group, by repeated attacks. Day-old larvae had the lowest percent of escapes after encounters with jellyfish (Sarsia sp.), but the percentage of escapes increased dramatically for 3-day-old larvae. Escape speeds after an attack also increased with age, and tended to be higher after stickleback attacks and lower after jellyfish attacks. This study revealed that the escape response of larval pollock to attack by predators improves rapidly with development during the early larval stage.     

Thermal effects on swimming activity and habitat choice in juvenile Pacific cod (Gadus macrocephalus)

The behavioral responses of fishes to temperature variation have received less attention than physiological responses, despite their direct implications for predator–prey dynamics in aquatic ecosystems. In this paper, we describe the temperature dependence of swimming performance and behavioral characteristics of juvenile Pacific cod (Gadus macrocephalus; 75–125 mm total length). Maximum swimming speeds increased with temperature and body size. Routine swimming speeds of Pacific cod in small groups of similarly sized fish (N = 6) increased with body size and were 34 % faster at 9 °C than at 2 °C. The response to temperature was opposite that previously described for juvenile walleye pollock (Theragra chalcogramma), reflecting species-specific differences in behavioral responses. In a separate experiment, we demonstrated the effect of temperature on habitat selection of juvenile Pacific cod: Use of an artificial eelgrass patch in a 5-m-long laboratory tank was significantly greater at 9 °C than at 2 °C. These results illustrate that temperature affects a range of behavioral traits that play important roles in determining the frequency and outcomes of predator–prey interactions.     

Diet of 0-group stages of capelin (<Emphasis Type="Italic">Mallotus villosus</Emphasis>), herring (<Emphasis Type="Italic">Clupea harengus</Emphasis>) and cod (<Emphasis Type="Italic">Gadus morhua</Emphasis>) during spring and summer in the Barents Sea

Recruitment of capelin in the Barents Sea fail when juvenile herring and cod are abundant and the potential for feeding competition of wild sympatric capelin and herring larvae and small cod juveniles were investigated. The frequency of gut evacuation after capture of capelin larvae were also studied in mesocosms. Small capelin larvae (<35 mm length) fed on small prey including phytoplankton, invertebrate eggs and nauplii, bivalves, other invertebrate larvae and small copepods. Calanus copepodites were only observed in large capelin larvae (>26 mm length). Calanus copepodites were the major food sources for contemporary herring larvae (25–35 mm length) and Calanus and euphausiids were the major prey for small juvenile herring (37–60 mm length) and cod (18–40 mm length). Capelin larvae reared in mesocosms evacuated the guts shortly after capture. Capelin larvae had a smaller mouth and fed on smaller prey than herring and cod of the same length. This implies that the small capelin larvae, in contrast to sympatric small herring and cod, are not tightly linked to the food chain involving Calanus and euphausiids. Thus, exploitative competition between capelin larvae and planktivorous fish that rely on Calanus and euphausiids in the Barents Sea may be relaxed.     

Swimming behavior of juvenile anchovies (<Emphasis Type="Italic">Anchoa</Emphasis> spp.) in an episodically hypoxic estuary: implications for individual energetics and trophic dynamics

Diel swimming behaviors of juvenile anchovies (Anchoa spp.) were observed using stationary hydroacoustics and synoptic physicochemical and zooplankton profiles during four unique water quality scenarios in the Neuse River Estuary, NC, USA. Vertical distribution of fish was restricted to waters with DO greater than 2.5 mg O₂ l⁻¹, except when greater than 70% of the water column was hypoxic and a subset of fish were occupying water with 1 mg O₂ l⁻¹. We made the prediction that an individual fish would select a swim speed that would maximize net energy gain given the abundance and availability of prey in the normoxic waters. During the day, fish adopted swim speeds between 7 and 8.8 bl s⁻¹ that were near the theoretical optimum speeds between 7.0 and 8.0 bl s⁻¹. An exception was found during severe hypoxia, when fish were swimming at 60% above the optimum speed (observed speed = 10.6 bl s⁻¹, expected = 6.4 bl s⁻¹). The anchovy is a visual planktivore; therefore, we expected a diel activity pattern characteristic of a diurnal species, with quiescence at night to minimize energetic costs. Under stratified and hypoxic conditions with high fish density coupled with limited prey availability, anchovies sustained high swimming speeds at night. The sustained nighttime activity resulted in estimated daily energy expenditure over 20% greater than fish that adopted a diurnal activity pattern. We provide evidence that the sustained nighttime activity patterns are a result of foraging at night due to a lower ration achieved during the day. During severe hypoxic events, we also observed individual fish making brief forays into the hypoxic hypolimnion. These bottom waters generally contained higher prey (copepod) concentrations than the surface waters. The bay anchovy, a facultative particle forager, adopts a range of behaviors to compensate for the effects of increased conspecific density and reduced prey availability in the presence of stratification-induced hypoxia.     

Experimental analysis of the contribution of swimming and drifting to the displacement of reef fish larvae

The extent to which behaviour affects the dispersal of pelagic larvae in reef fishes has been a topic of major discussion among marine ecologists. Here, we experimentally quantified the extent to which the displacement of late-stage larvae of Abudefduf saxatilis is due to active movement (i.e. swimming) and drifting. We consider drifting as the component of larval displacement accounted for by the current. Drifting was quantified by comparing larval displacement to the displacement of passive particles in an extended flow chamber that gave larvae the free choice of swimming (i.e. swim with or against the current or not swim at all). We also determine whether drifting results from currents exceeding larval swimming capabilities or from the behavioural choice of larvae of not to swim against adverse currents. To do this, we compare the speeds of larval swimming in the extended flow chamber to those obtained in a smaller chamber in which larvae are behaviourally forced to swim due to space constraints and a retaining fence (most available data on larval swimming is based on this sort of chamber). Within the extended chamber, larvae tended to face the current and swim slower than it. This resulted in a net displacement increasingly determined by drifting. We also found that in the extended chamber, larvae swam at speeds between one and six times slower than the speeds they achieved in the “behaviourally modifying” smaller chamber. This suggests that the net displacement in the extended chamber was in part due to the behavioural choice of the larvae of not to swim. The importance of this “behavioural drifting” is discussed in terms of energy savings required for successful completion of the larval period and post-settlement survival. The idea that larvae may modulate their swimming behaviour raises caution for the use of published data regarding swimming capabilities of reef fish larvae when assessing the extent to which these fish actively affect their dispersal.     

Experimental evidence of chemokinesis in newly hatched cod larvae (Gadus morhua L.)

The swimming behaviour of laboratory-reared newly hatched cod larvae (Gadus morhua L.) was observed in a control solution of artificial seawater and in seven solutions, each with a different concentration of arginine (10⁹ to 10^-3 M). The behaviour of 20 larvae was analysed in each of the eight solutions; the individual observation time was 1 min. Individual movements were recorded on video and analyzed using a computer-assisted program. The larvae swam in straight lines (a trajectory), rested, moved and started swimming again. For the parameters analyzed, i.e., number of movements, angle between successive trajectories and straightness index, there was no significant difference between the behaviour of the larvae in the different solutions. However, for the larvae in 10^-5, 10^-4 and 10^-3 M arginine solutions, the analyzed parameters, i.e., time active, frequency of trajectories (number of movements exceeding body length), distance swum min^-1, length of individual trajectories and trajectory velocity, were all significantly lower than for the larvae in the control solution of artificial seawater and for larvae in the solutions of 10^-9, 10^-8, 10^-7 and 10^-6 M arginine. The results show that the mean distance swum by cod larvae min^-1 was two to five times longer in artificial seawater without arginine and in the four lower concentrations of arginine than in the three higher concentrations. Scanning micrographs show that newly hatched (pre-feeding) cod larvae possess olfactory organs. It seems reasonalbe to assume that the observed changes in swimming behaviour are mediated by the olfactory sense and are important in the feeding strategy of cod larvae. We suggest that the observed behaviour increases the probability of the larvae localizing patches of prey organisms and remaining in the patch once they have found it. The results show that chemokinesis is a mechanism by which the spatial distribution of fish larvae will be correlated with their prey.