Exhibit Diel Vertical Migration Research Articles

Effects of particle buoyancy, release location, and diel vertical migration on exposure of marine organisms to microplastics in Delaware Bay

Plastics are ubiquitous in the global ocean and present a potential hazard to marine organisms. One challenge in assessing the impact of plastics on marine organisms is calculating their exposure to the background plastics field. Guided by observational data of marine organisms and microplastics in the Delaware Bay, this study applies a rational approach for determining exposure times based on a hydrodynamic model of the Delaware Bay paired with particle tracking. We consider two different marine organism behaviors, organisms that stay at the surface and those that exhibit diel vertical migration (DVM), as well as different release locations in the bay in order to understand the spatial variability in microplastics exposure. Microplastics are either assumed to be neutrally-buoyant or surface-trapped, so that they either move with the full three-dimensional estuarine circulation or only with the surface currents. Total exposure time is then determined from the modeled time-integrated background microplastics concentrations for each simulated marine organism path. Exposure was found to be greatest for surface-trapped organisms exposed to surface-trapped plastics as they both tend to aggregate in tidelines. DVM limits exposure to plastics by removing organisms from the surface-trapped plastics field. For neutrally-buoyant plastics, surface-trapped organismal exposure time greatly depends on the organism release location. Exposure is modified by two competing DVM influences: Lesser plastic concentrations at depth decrease the exposure, whereas limiting motion toward the mouth of the bay where the plastic concentrations are smaller increases the exposure. Marine organism exposure to microplastics in a dynamic estuarine environment greatly depends on plastic buoyancy and an organism's vertical position. Both of these factors need to be taken into account for comprehensive microplastic impact studies.

Integrating multiple chemical tracers to elucidate the diet and habitat of Cookiecutter Sharks

The Cookiecutter shark (Isistius brasiliensis) is an ectoparasitic, mesopelagic shark that is known for removing plugs of tissue from larger prey, including teleosts, chondrichthyans, cephalopods, and marine mammals. Although this species is widely distributed throughout the world’s tropical and subtropical oceanic waters, like many deep-water species, it remains very poorly understood due to its mesopelagic distribution. We used a suite of biochemical tracers, including stable isotope analysis (SIA), fatty acid analysis (FAA), and environmental DNA (eDNA), to investigate the trophic ecology of this species in the Central Pacific around Hawaii. We found that large epipelagic prey constituted a relatively minor part of the overall diet. Surprisingly, small micronektonic and forage species (meso- and epipelagic) are the most important prey group for Cookiecutter sharks across the studied size range (17–43 cm total length), with larger mesopelagic species or species that exhibit diel vertical migration also being important prey. These results were consistent across all the tracer techniques employed. Our results indicate that Cookiecutter sharks play a unique role in pelagic food webs, feeding on prey ranging from the largest apex predators to small, low trophic level species, in particular those that overlap with the depth distribution of the sharks throughout the diel cycle. We also found evidence of a potential shift in diet and/or habitat with size and season. Environmental DNA metabarcoding revealed new prey items for Cookiecutter sharks while also demonstrating that eDNA can be used to identify recent prey in stomachs frozen for extended periods. Integrating across chemical tracers is a powerful tool for investigating the ecology of elusive and difficult to study species, such as meso- and bathypelagic chondrichthyans, and can increase the amount of information gained from small sample sizes. Better resolving the foraging ecology of these mesopelagic predators is critical for effective conservation and management of these taxa and ecosystems, which are intrinsically vulnerable to overfishing and exploitation.

The freshwater mysid Mysis diluviana (Audzijonyte & Väinölä, 2005) (Mysida: Mysidae) consumes detritus in the presence of Daphnia (Cladocera: Daphniidae)

AbstractFreshwater mysids of the Mysis relicta group are omnivorous macroinvertebrates that form an important link between fishes and lower trophic levels in many north temperate to Arctic lakes, where they exhibit diel vertical migration (DVM) to exploit subsurface food-rich layers at night. Benthic food resources have been assumed to be less important for mysid diets than pelagic zooplankton. Studies have nevertheless indicated that mysids consume benthic sedimented detritus, calling this assumption into question. We conducted a food-choice experiment to evaluate the feeding preferences of Mysis diluviana (Audzijonyte & Väinölä, 2005) by presenting field-caught specimens in individual foraging arenas with multiple choices of food. Experimental food treatments included a preferred pelagic prey (Daphnia), a presumed less desirable benthic resource (detritus), and a combination of both. We hypothesized that M. diluviana 1) prefers Daphnia over detritus and consumes only Daphnia in combination treatments, and 2) would not consume detritus except when detritus was the only food source available. Contrary to our hypothesis, M. diluviana readily consumed detritus in the presence of Daphnia. Mysis diluviana unexpectedly consumed more individuals of Daphnia in the presence rather than in the absence of detritus. Our results demonstrate that mysids take advantage of benthic food resources even in the presence of a presumably preferred zooplankton prey, calling to question the long-held assumption that benthic resources are unimportant when considering the trophic role of freshwater mysids of the M. relicta group.

The effects of hypolimnetic anoxia on the diel vertical migration of freshwater crustacean zooplankton

AbstractLakes and reservoirs worldwide are increasingly experiencing depletion of dissolved oxygen (anoxia) in their bottom waters (the hypolimnion) because of climate change and eutrophication, which is altering the dynamics of many freshwater ecological communities. Hypolimnetic anoxia may substantially alter the daily migration and distribution of zooplankton, the dominant grazers of phytoplankton in aquatic food webs. In waterbodies with oxic hypolimnia, zooplankton exhibit diel vertical migration (DVM), in which they migrate to the dark hypolimnion during the day to escape fish predation or ultraviolet (UV) radiation damage in the well‐lit surface waters (the epilimnion). However, due to the physiologically stressful conditions of anoxic hypolimnia, we hypothesized that zooplankton may be forced to remain in the epilimnion during daylight, trading oxic stress for increased predation risk or UV radiation damage. To examine how anoxia impacts zooplankton vertical migration, distribution, biomass, and community composition over day–night periods, we conducted multiple diel sampling campaigns on reservoirs that spanned oxic, hypoxic, and anoxic hypolimnetic conditions. In addition, we sampled the same reservoirs fortnightly during the daytime to examine the vertical position of zooplankton throughout the summer stratified season. Under anoxic conditions, most zooplankton taxa were predominantly found in the epilimnion during the day and night, did not exhibit DVM, and had lower seasonal biomass than in reservoirs with oxic hypolimnia. Only the phantom midge larva, Chaoborus spp., was consistently anoxia‐tolerant. Consequently, our results suggest that hypolimnetic anoxia may alter zooplankton migration, biomass, and behavior, which may in turn exacerbate water quality degradation due to the critical role zooplankton play in freshwater ecosystems.

Evolution or Extinction: Might Zooplankton Adaptively Respond to the Novel Invasive Predator Bythotrephes longimanus?

Invasive predators are a large and growing threat to species diversity and human well-being. One of the reasons invasive predators have a negative impact is that native prey species do not possess appropriate anti-predator defenses. However, rapid evolution may allow prey species to respond adaptively to introduced predators. When this occurs the impacts of invasive predators are mitigated. An invasive predator that is of concern in North America is the spiny water flea, Bythotrephes longimanus. It disrupts freshwater ecosystems through voracious consumption of zooplankton. Declines in zooplankton abundance and richness reduce water quality and recreational fishing opportunities. However, a species of zooplankton, Daphnia mendotae has been found to adaptively respond to B. longimanus. This adaptation is diel vertical migration, the behavioral change of occupying a lower position in the water column during the day to reduce predation risk. Despite the ecological and economic implications of this behavior in response to B. longimanus, it has only been studied in a few lakes. This study investigated adaptive diel vertical migration in D. mendotae from multiple lakes. This was done by measuring the vertical position of D. mendotae in artificial water columns. It was hypothesized that D. mendotae from lakes that have been invaded by B. longimanus will exhibit diel vertical migration in the presence of B. longimanus. If this hypothesis is supported, rapid evolution of diel vertical migration can be established as a widespread response. This would strengthen understanding of rapid evolution and allow lakes more vulnerable to B.longimanus to be identified.

What goes up must come down: Diel vertical migration in the deep‐water sablefish (Anoplopoma fimbria) revealed by pop‐up satellite archival tags

AbstractThe sablefish (Anoplopoma fimbria) is a long‐lived species with wide distribution throughout the North Pacific Ocean. While adult sablefish are considered a deep‐water fish, diet analyses suggest that they undergo vertical migrations that could be related to prey movement and feeding. Pop‐up satellite archival tags (PSATs) were used to observe the fine‐scale depth selection behavior of adult sablefish tagged off the Washington coast during the summer from June to August. Tags were physically retrieved after they surfaced using direction‐finding equipment so that complete datasets over the entire deployment were obtained from 14 tags. PSATs that recorded depth and temperature every 4 min during the deployment confirm that sablefish inhabit depths of 750 m or greater. However, a majority of the tagged fish underwent extensive vertical migrations that averaged 254.4 m overall and occurred at a 24 hr periodicity. Variations were observed among individuals in the amount of the deployment during which vertical migrations occurred, ranging from 12.37% to 63.48% of the time. During the vertical migration, fish ascended towards the surface at night and descended prior to daylight (i.e., diel vertical migration). Sablefish generally inhabited temperatures of 5°C but during the vertical migrations were found at temperatures from 6 to 10°C. Sablefish are opportunistic feeders with a large proportion of their diet being fish, euphausiids and cephalopods. Because these prey items also exhibit diel vertical migrations, it is possible that the vertical migratory behavior displayed by the sablefish was in response to the movements or the location of their prey.

Sex and life stage dependent phototactic response of the marine copepod Calanus finmarchicus (Copepoda: Calanoida)

Irradiance thresholds for phototactic response were determined for the marine calanoid copepod Calanus finmarchicus (Gunnerus). C. finmarchicus is one of many zooplankton species that exhibit diel vertical migration. Light is considered the main proximate cause of diel vertical migration, but irradiance sensitivity is unknown for many ecologically important zooplankton taxa, including C. finmarchicus. Here we studied phototaxis in C. finmarchicus in response to low levels of irradiance using a custom-made experimental setup under controlled laboratory conditions. The setup consisted of an aquarium with a light stimulus in one end of a raceway. A video camera and near-infrared light for illumination were applied to monitor the response to light in the horizontal plane. Low levels of irradiance were achieved using a white LED and a combination of absorptive neutral density filters and diode pulsing.Stage V copepodites and adult females displayed negative phototaxis, and the threshold for phototactic response was 9.8×10−6μmolphotonsm−2s−1. Adult males displayed positive phototaxis and the corresponding threshold value was 9.9×10−7μmolphotonsm−2s−1. The results from the experiments were used to estimate the depths at which phototaxis is elicited in natural light conditions by conducting light simulations for an ocean scenario and a fjord scenario during peak spring bloom conditions. The estimated depths for irradiances that elicit a phototactic response corresponded to approximate depths of 158–186m in the ocean scenario and 48–57m in the fjord scenario. These depths are within the range of depth distributions of C. finmarchicus reported for ocean and fjord populations.

Vertical distributions of the early life stages of walleye pollock (Theragra chalcogramma) in the Southeastern Bering Sea

The present study examines vertical distributions of the early life stages of walleye pollock (Theragra chalcogramma) in the Southeastern Bering Sea to assess ontogenetic and diel vertical migration in relation to development and habitat. Walleye pollock demonstrated a decrease in the depth of occurrence following hatching, indicating an ontogenetic change in vertical distribution. Eggs occurred deepest in the water column and early juveniles occurred shallowest. Vertical distributions were related to the date of collection, water column depth, and thermocline depth. Non-feeding stages (eggs and yolksac larvae, <4.5mm standard length [SL]) did not exhibit diel vertical migration. Feeding larvae exhibited diel vertical migration, although patterns varied between two feeding stages. Preflexion stage larvae (4.5–9.9mm SL) were concentrated between 10 and 20m during the day and deeper at night. Postflexion stage larvae (flexion and postflexion, 10.0–24.5mm SL) underwent regular diel migrations (0–20m, night; 10–40m, day). Low sample sizes limited our ability to assess diel vertical migration in early juveniles, but this stage tends to occur in the upper 20m of the water column, regardless of time of day. These results suggest that vertical distributions and diel migration potentially are driven by prey availability at sufficient light levels for preflexion larvae to feed and a trade-off between prey access and predation risk for postflexion larvae. Vertical distributions of eggs and preflexion larvae varied with habitat examined (on the continental shelf versus over the continental slope). Vertical distributions of walleye pollock eggs, yolksac larvae, and preflexion larvae in the Bering Sea are different from distributions in other ecosystems, which can impact transport and modeling efforts.

Shark Predation on Migrating Adult American Eels (Anguilla rostrata) in the Gulf of St. Lawrence

In an attempt to document the migratory pathways and the environmental conditions encountered by American eels during their oceanic migration to the Sargasso Sea, we tagged eight silver eels with miniature satellite pop-up tags during their migration from the St. Lawrence River in Québec, Canada. Surprisingly, of the seven tags that successfully transmitted archived data, six were ingested by warm-gutted predators, as observed by a sudden increase in water temperature. Gut temperatures were in the range of 20 to 25°C—too cold for marine mammals but within the range of endothermic fish. In order to identify the eel predators, we compared their vertical migratory behavior with those of satellite-tagged porbeagle shark and bluefin tuna, the only endothermic fishes occurring non-marginally in the Gulf of St. Lawrence. We accurately distinguished between tuna and shark by using the behavioral criteria generated by comparing the diving behavior of these two species with those of our unknown predators. Depth profile characteristics of most eel predators more closely resembled those of sharks than those of tuna. During the first days following tagging, all eels remained in surface waters and did not exhibit diel vertical migrations. Three eels were eaten at this time. Two eels exhibited inverse diel vertical migrations (at surface during the day) during several days prior to predation. Four eels were eaten during daytime, whereas the two night-predation events occurred at full moon. Although tagging itself may contribute to increasing the eel's susceptibility to predation, we discuss evidence suggesting that predation of silver-stage American eels by porbeagle sharks may represent a significant source of mortality inside the Gulf of St. Lawrence and raises the possibility that eels may represent a reliable, predictable food resource for porbeagle sharks.

Distribution of myctophid resources in the Indian Ocean

Mesopelagics are one of the largest under-exploited marine resources with wide distribution in the world oceans. Lanternfishes are the key members of mesopelagic communities and the total resource in the world oceans is estimated at 600 million tons. Lanternfishes belong to the family Myctophidae which comprises of about 250 species in 35 genera. Myctophids account for about 75% of total global catch of small mesopelagic fishes. They are known to exhibit diel vertical migration, concentrating during the day time between 400 and 1,000 m, and between 5 and 100 m, during the night. In this paper, an attempt is made to review the existing information on the occurrence and distribution of myctophid resources in the Indian Ocean. 137 myctophid species have been reported from the entire Indian Ocean. Studies in the Arabian Sea have indicated that the area is rich in the midwater fish stocks dominated by myctophids with an estimated potential of 100 million tons.

Diel migration and swimbladder resonance of small fish: some implications for analyses of multifrequency echo data

Abstract Godø, O. R., Patel, R., and Pedersen, G. 2009. Diel migration and swimbladder resonance of small fish: some implications for analyses of multifrequency echo data. – ICES Journal of Marine Science, 66: 1143–1148. Many fish with swimbladders exhibit diel vertical migrations (DVM). Ascents and descents of hundreds of metres occur, and altered swimbladder volume and buoyancy can result from incomplete secretion and resorption of gas. When acoustic observations are made near the resonance frequency of the swimbladder, the estimated fish biomass can be positively biased. When multiple-frequency echosounders are used, the frequency response of the backscatter might vary temporally and spatially and compromise the effectiveness of conventional target-identification methods. In this paper, variations in backscatter from mesopelagic fish are studied using data collected west of the British Isles with a five-frequency echosounder (Simrad EK60). Two acoustic layers, one dominated by pearlsides (Maurolicus muelleri) and the other by myctophids (Myctophidae), were monitored during their DVM. The frequency responses of the layers changed systematically, mainly characterized by increases in the nautical-area-backscattering coefficient (sA) values at 18 kHz relative to those at 38 kHz. This could have been caused by changes in the resonance frequencies of fish swimbladders, as they expanded and contracted during ascent and descent. Two sA maxima in the myctophids layer suggest the presence of two types of target with different scattering characteristics. Models of sound scatter from myctophid swimbladders suggest that these peaks have resulted from resonance scattering. The sA at 18 kHz attributed to M. muelleri also peaked, but at the maximum depth of their distribution. Spatial and temporal changes in the frequency responses of fish should be taken into account when pelagic fish communities are surveyed with multiple-frequency echosounders.

The Role of Light and Oxygen in Chaoborus punctipennis (Insecta: Diptera) Diel Vertical Migration

ABSTRACT Aquatic phantom midge larvae (Chaoborus punctipennis) exhibit diel vertical migration (DVM) in response to fish visual predation, remaining in deeper waters during the day and ascending nocturnally to feed. Light penetration into the water column and dissolved oxygen concentrations are hypothesized to determine the depths where larvae are found during daylight hours. We collected samples in Yellow Poplar Tree Lake in Millington, Tennessee before and after sunset during a day in March, May, July, and September of 2005. Average daytime depth (Dc) and use of daytime refugia (dark or hypoxic waters) were significantly different (P<0.05) among larval instars (I/II<III<IV) in May, July, and September when the lake was highly stratified. Final instars utilized hypoxic waters (DO2 <3.5 mg/L) more than III instars, while I/II instars did not use this refuge. When the lake was highly stratified, the average depth of all instars during daylight hours was below the depth of maximum light penetration (1.0 μEinsteins/m2/s); IV instars remained below this level in May, July, and September, but III instars did so only in July and September, and I/II instars did so only in September. Further, IV instar Dc was significantly correlated with the critical oxygen threshold for fish (DO2=3.5 mg/L; R2=0.783, df=l1, P<0.01) but not with maximum light penetration (R2=0.183, df=11, P>0.05), indicating that oxygen was the ovemding stimulus governing C. punctipennis DVM seasonal changes and average daytime depth.

Individual variation in feeding habitat use by adult female green sea turtles (Chelonia mydas): are they obligately neritic herbivores?

Satellite telemetry and stable isotope analysis were used to confirm that oceanic areas (where water depths are >200 m) are alternative feeding habitats for adult female green sea turtles (Chelonia mydas), which have been thought to be obligate herbivores in neritic areas (where depths are <200 m). Four females were tagged with satellite transmitters and tracked during post-nesting periods from Ogasawara Islands, Japan. Three females migrated to neritic habitats, while transmissions from another female ceased in an oceanic habitat. The overall mean nighttime dive depths during oceanic swimming periods in two females were <20 m, implying that the main function of their nighttime dives were resting with neutral buoyancy, whereas the means in two other females were >20 m, implying that they not only rested, but also foraged on macroplankton that exhibit diel vertical migration. Comparisons of stable carbon and nitrogen isotope ratios between 89 females and the prey items in a three-source mixing model estimated that 69% of the females nesting on Ogasawara Islands mainly used neritic habitats and 31% mainly used oceanic habitats. Out of four females tracked by satellite, two females were inferred from isotope ratios to be neritic herbivores and the two others oceanic planktivores. Although post-nesting movements for four females were not completely consistent with the inferences from isotope ratios, possibly due to short tracking periods (28-42 days), their diving behaviors were consistent with the inferences. There were no relationships between body size and the two isotope ratios, indicating a lack of size-related differences in feeding habitat use by adult female green turtles, which was in contrast with loggerhead sea turtles (Caretta caretta). These results and previous findings suggest that ontogenetic habitat shifts by sea turtles are facultative, and consequently, their life histories are polymorphic.

MOCNESS estimates of the size and abundance of a pelagic gonostomatid fish Cyclothone pallida off the Bahamas

Depth patterns of size and abundance in a pelagic bristlemouth fish Cyclothone pallida (Stomiformes: Gonostomatidae) are reported. Sampling was conducted during the day and night at two sites in the Bahamas using a Multiple Opening and Closing Net and Electronic Sensing System (MOCNESS). All samples show peak abundance in the 500–700 m depth range irrespective of time or sampling location. The mean and variance of body length increased with depth, and size–depth patterns do not significantly differ among samples. Results suggest that these populations of C. pallida do not exhibit diel vertical migration.

Diel vertical migrations of zooplankton in a shallow, fishless pond: a possible avoidance‐response cascade induced by notonectids

1. Day (noon) and night (midnight) vertical distributions of zooplankton and phytoplankton in the water column (1.5 m) of a Vermont pond were determined on two consecutive days from 470 mL water samples taken at three depths (0.1, 0.5 and 1.0 m) at three sites. There was little variation across depths in temperature, dissolved oxygen concentration and phytoplankton. All individuals of each zooplankton species (a small copepod, Tropocyclops extensus and six rotifers) were counted.2. A three‐way ANOVA on the zooplankton data showed no effect of date or time of day on the abundance of any species. Significant diel shifts in vertical distribution (depth × time‐of‐day interactions) were found for T. extensus (nauplii, as well as copepodites and adults) and Polyarthra remata, but not for Hexarthra mira, Keratella cochlearis, Anuraeopsis fissa, Ascomorpha ovalis and Plationus patulus. Tropocyclops extensus showed a pronounced, typical diel vertical migration, avoiding the surface and occurring most abundantly near the bottom during the day. Polyarthra remata showed an equally pronounced, reverse diel vertical migration, avoiding the bottom and being most abundant near the surface during the day.3. The diurnal descent of Tropocyclops is interpreted as an avoidance response to Buenoa macrotibialis, a notonectid which feeds on this copepod at the surface during the day but not at night. The diurnal ascent of Polyarthra is thought to be an avoidance response to Tropocyclops, which strongly suppresses this rotifer in field enclosures and laboratory vessels. Thus, these out‐of‐phase migrations may be coupled and represent a behavioural cascade initiated by Buenoa.4. At night, Tropocyclops and Polyarthra both were uniformly distributed across depths. This is believed to reflect the absence of appreciable depth‐related variation in temperature, algal food resources (biovolume of cryptomonads and chrysophyte flagellates) and predation risk at this time.5. The five rotifer species that did not exhibit diel vertical migrations may be less susceptible to Tropocyclops predation than Polyarthra.

A Long-Term Study of Larval Chaoborus punctipennis (Diptera: Insecta) Vertical Migration and the Role of Critical Oxygen Concentration

ABSTRACT The aquatic insect larvae of Chaoborus punctipennis exhibit diel vertical migration, remaining in deeper waters during the day and ascending at night. This behavior reduces visual predation on the larvae, but incurs costs. We attempted to determine if patterns existed in the vertical migration of C. punctipennis larvae related to changes in water oxygen concentration. For 13 of the past 19 years, data were collected over a 24-hour period in the fall in Poplar Tree Lake, Shelby County, Tennessee. We determined the mean depth at which larvae were found during the nighttime and daytime. There was no consistent change in either mean nighttime or daytime depth over time (P<0.22). We assumed that an oxygen concentration below 2.0 mg/l could not be tolerated by fish. Comparison of the depths at which this critical oxygen concentration occurred with mean larval depth showed a significant correlation (P>0.05), supporting the hypothesis that diel vertical migration affords protection from fish predation.

Size‐dependent visual predation risk and the timing of vertical migration in zooplankton

Zooplankton commonly exhibit diel vertical migration (DVM), descending from food‐rich surface waters during the day. If DVM is a tradeoff between avoiding size‐selective visually hunting predators and maximizing energy gain, smaller bodied prey should enter surface waters earlier and leave later than larger, more visually conspicuous organisms. Conventional sampling technologies lack the temporal resolution to test this prediction. Here, we report on the first test of this prediction using a new submersible optical‐acoustic imaging system capable of resolving the timing of migration of the euphausiid crustacean Euphausia pacifica Hansen. Smaller bodied animals consistently ascended as much as 30 min earlier and descended up to 45 min later than adults. The timing of vertical migration reflects how the size‐dependent risk of attack by visual predators alters the tradeoff between feeding and predator avoidance, supporting the predator‐avoidance hypothesis for DVM.

Migration strategies of young fishes under temporal constraints: the effect of size-dependent overwinter mortality

Habitat selection by fishes involves balancing the benefits of foraging opportunities and the risk of predation. Size-dependent overwinter mortality imposes a time constraint on behavioral programs of juvenile fishes: individuals must achieve a sufficient size by the end of the growing season to survive. I developed a dynamic optimization model to evaluate how severity of size-dependent overwinter mortality alters habitat use by minnows. The littoral zone was the safest habitat with respect to predation. In lakes where zooplankton exhibit diel vertical migration, fish migrate to the pelagic at night and return to the littoral during the day to maximize foraging rates. Model results demonstrated that size-dependent overwinter mortality should lead to development of more pronounced predation risk-taking by fishes in lakes with severe winters. Individuals are predicted to initiate diel littoral-pelagic migrations at smaller sizes as winter severity increases or as hatch date within a season increases. This analysis suggests that the time constraint imposed by size-selective overwinter mortality has important consequences for life history strategies of juvenile fishes. The effects of time constraints on behavior are expected to be especially important near the high latitudinal limits of a species' geographic distribution.

Regulation of zooplankton composition and distribution by a zooplanktivorous fish in a shallow, eutrophic floodplain lake in south east Australia

The diet of Australian smelt (Retropinna semoni) and the patterns of vertical distribution and diel vertical migration of fish and zooplankton were studied over a single Austral spring/summer in a shallow, eutrophic, floodplain lake. The diet of smelt was dominated by cladocera despite the frequently low abundances of such prey in the water column. Calanoid copepods were abundant in the zooplankton community at all times yet formed only a minor component of smelt diet. Patterns of vertical distribution and diel vertical migration exhibited by the various zooplankton taxa appeared to be related to the intensity of predation by smelt. The most heavily predated cladoceran taxa tended to remain in deeper water, whilst lightly predated calanoid copepods were abundant throughout the water column at all times. Neither taxon exhibited marked diel vertical migration. In contrast, cladoceran taxa suffering intermediate levels of predation exhibited marked diel vertical migration tending to avoid surface waters during the day. Smelt also appeared to exhibit diel vertical migration with few fish being caught in the water column during the day. The patterns of predation on zooplankton and the distribution and abundance of zooplankton suggest that smelt may play a significant role in regulating zooplankton community structure in the lowland habitats of south east Australia.

Relationships between geotaxis/phototaxis and diel vertical migration in autotrophic dinoflagellates

Marine dinoflagellate diel vertical migrations are often conceptually explained by a species' geotactic and phototactic preferences, but actual simultaneous measurements are rare. Newly collected simultaneous measurements on Heterocapsa ( Cachonina ) illdefina (Herman and Sweeney) and Gymnodinium breve (Davis) are combined with similar literature information on Amphidinium carterae (Hulbert), Peridinium faeroense (Paulsen) and Prorocentrum micans (Ehrenberg) to explore several examples of the actual relationships between diel vertical migration and geotaxis/phototaxis. Amphidinium carterae does not migrate, but it exhibits a negative geotaxis that may counter a small sinking velocity. The four other species all exhibit diel vertical migrations that yield surface aggregations during daylight, but the associated combinations of geotaxis and phototaxis precision (which is strongest when every cell in a population exhibits the same response to a stimulus and weakest when the response is random) and sign [which is positive (negative) when motion is toward (away from) the stimulus] are different in each case. These different taxis combinations may be related to species-specific sensor structure and/or placement. Furthermore, variations in the different biochemical pools over a species' cell cycle may contribute to structural/mechanical changes that influence how a given sensory array functions at a given time. If so, this coupling may be an important link in the growth optimization mechanisms and occasional bloom successes of different autotrophic dinoflagellate species under varying environmental conditions.