Significant Diel Variation Research Articles

Diel variation in blue whale calls recorded in the eastern tropical Pacific

Blue whales, Balaenoptera musculus, in northeast Pacific waters often produce a two-part call. The first part, A, is amplitude-modulated, lasting about 17 s, and the second part, B, is frequency-modulated, lasting about 19 s. Type B calls are more easily detected in recordings and are often the signal tracked in long-term monitoring. Automatic detection methods applied to data from a hydrophone moored at 8°N, 95°W in the eastern tropical Pacific (ETP) from 15 May 1996 to 15 May 1997 showed seasonal variability in call detection. Calls were detected most often from February to June. Blue whales are believed to use the ETP region as a wintering ground but probably also feed there. Blue whales forage primarily on krill that undertake diel vertical migrations from several hundred metres depth during the day to near the surface at night. We tested the null hypothesis that blue whale calling behaviour does not show significant diel variation in order to determine whether variability in calling could provide an indication as to the behavioural function of B calls. The hourly rate of B calls showed a strong diel pattern. There were significantly more calls during dark and twilight periods than during light periods (P < 0.001). These diel patterns in call production correlate well with previously established diel patterns in prey distribution, with the highest calling rates occurring when prey have been reported to be closest to the surface.

Diel changes in endolymph aragonite saturation rate and mRNA expression of otolith matrix proteins in the trout otolith organ

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 294:249-256 (2005) - doi:10.3354/meps294249 Diel changes in endolymph aragonite saturation rate and mRNA expression of otolith matrix proteins in the trout otolith organ Yasuaki Takagi1,*, Hidekazu Tohse1,2, Emi Murayama2,3, Tsuyoshi Ohira2,4, Hiromichi Nagasawa2 1Graduate School of Fisheries Sciences, Hokkaido University, Minato, Hakodate, Hokkaido 041-8611, Japan2Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan3Present address: Unité Macrophages et Développement de l’Immunité, Département de Biologie du Développement, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris cedex 15, France4Present address: Japan International Research Center for Agricultural Sciences, Owashi, Tsukuba, Ibaraki 305-8686, Japan *Email: takagi@fish.hokudai.ac.jp ABSTRACT: In the teleost fish otolith, the alternate deposition of CaCO3-rich and protein-rich layers results in the formation of daily increments. In order to clarify the mechanism of daily increment formation, a precise understanding of the relationship between ionic and organic controls of otolith growth is essential. In the present study, we studied diel variations in the aragonite saturation rate (Sa) of the endolymph and the mRNA expression of 2 major otolith matrix proteins, OMP-1 and otolin-1, in the saccular tissue. A new technique for simultaneously quantifying endolymph Sa and mRNA expression in the saccular tissue from an individual rainbow trout was developed. Endolymph Ca2+ levels, CO2 partial pressure (PCO2) and pH were simultaneously measured using an automatic pH/gas/electrolyte analyzer, and the Sa was calculated. Total RNA was isolated from sacculi of individual fish after the endolymph was obtained. cDNAs were synthesized and used to quantify OMP-1 and otolin-1 mRNA expression levels by real-time PCR. Significant diel variations were observed in endolymph pH and PCO2 levels in an antiphasic manner. Endolymph Sa did not exhibit significant diel variations and was maintained at a value of more than 2.0, indicating that the endolymph was kept supersaturated with respect to aragonite during the day-night cycle. Expression of otolin-1 mRNA had apparent diel variations with high levels at night, whereas that of OMP-1 mRNA was almost constant. These data strongly suggest an organic control of daily increment formation in the otolith. The most probable candidate protein for daily increment formation is otolin-1. KEY WORDS: Calcification · mRNA expression · Endolymph · Daily increment · Aragonite · Otolith · Rainbow trout Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 294. Online publication date: June 09, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Swimming behaviour and photoresponses of the iridescent copepods, Sapphirina gastrica and Sapphirina opalina (Copepoda: Poecilostomatoida)

The swimming behaviour and photoresponses of the iridescent epipelagic copepods, Sapphirina gastrica and S. opalina were investigated in the laboratory. In continuous dark conditions, both species showed no significant diel variation in their swimming activities. When stimulated with light, they exhibited spiral-swimming in which the males showed a significantly higher speed and frequency of turning than the females. Both sexes of S. gastrica and S. opalina showed positive phototaxis at intensities higher than 0·05×1014 quanta cm−2 s−1 for light sources of 430 nm and 580 nm. Sapphirina gastrica showed increased locomotion over a broad range of wavelengths from 430 nm to 580 nm, while S. opalina showed a gradual increase of activity with decreasing wavelength, with the highest value at 430 nm. The photoresponses of these two species suggest that light conditions play an important role in their daytime ascent and in determining the depth distributions that were observed in our previous study. It is suggested that the iridescence and fast spiral-swimming of males, and the species-specific photoresponses of both sexes constitute a putative mate recognition system in the open ocean.

Variations in the feeding intensity and diet composition of red mullet (Mullus barbatus) during 24-h period in the summertime in Hisarönü Bay

Diel feeding pattern of red mullet in the Hisarönü Bay was studied from samples collected by bottom trawl during a 24-h period in the summer of 1997. Feeding activity of red mullet commenced at dawn and reached its maximum value at noon. During the period between noon to early evening, feeding intensity slowed down and then showed a small increase towards sunset. After sunset, the feeding activity nearly stopped and fish with empty stomachs increased. At night almost all stomachs were empty. Polychaetes, decapods, bivalves and amphipods were the major prey groups in the diet of red mullet and no significant diel variations were observed in the relative importance of these groups.

Studies of Peroxyacetyl nitrate (PAN) and its interaction with the snowpack at Summit, Greenland

Peroxyacetyl nitrate (PAN) was measured in ambient and snowpack interstitial air at Summit, Greenland, in June and July of 1998 and 1999 and at a rural/forest site in the Keewenaw Peninsula of Michigan in January of 1999. At Summit, we found that PAN typically represented between 30 and 60% of NOy. In the summer of 1999, a significant diel variation in both PAN/NOy and NOx/NOy was observed, but this was much less pronounced in 1998. Experiments during SNOW99 near Houghton, Michigan, indicated that PAN undergoes weak uptake onto snow grain surfaces. At Summit, we found that PAN concentrations in the snowpack interstitial air were significantly elevated (by as much as 2–5 times) relative to ambient levels, implying a flux of PAN out of the snowpack during the study period. We also observed evidence that PAN can be photochemically produced in snow that is exposed to polluted air. These observations indicate that interactions with the snowpack can have a significant impact on PAN concentrations in the boundary layer and point to potential difficulties associated with investigation of long‐term changes in PAN uptake into ice cores because of the impact of postdepositional processes.

Diel variation in spawning orange roughy (Hoplostethus atlanticus, Trachichthyidae) abundance over a seamount feature on the north‐west Chatham Rise

The diel variation in abundance of an orange roughy (Hoplostethus atlanticus Collett) spawning aggregation on a deepsea seamount on the Chatham Rise known as “Graveyard” was studied using acoustics. A 4‐day survey was carried out using a Latin cube statistical design. The acoustic data was echo‐integrated and analysis of variance was carried out to estimate the effects of day, time of day, and transect path. The estimated biomass of the spawning aggregation did not display any significant diel variation. We conclude that this result validates the current practice of acoustically surveying orange roughy on this seamount without considering diel effects. The aggregation fluctuated in abundance over the survey period, suggesting that turnover offish was occurring. Differences in roughy biomass with transect orientation indicate the importance of randomised radial transects for these surveys.

Summertime Diel Variations in the Diet Composition and Feeding Periodicity of Red Pandora (Pagellus erythrinus) in Hisarönü Bay

In summer 1997, the stomach contents of 235 red pandora collected by bottom trawl from Hisarönü Bay in the southern Aegean coast of Turkey, were analysed. It was observed that red pandora is predominantly a diurnal feeder. More intensive feeding started in the afternoon, reaching its maximum value in the evening and continued until dusk. The minimum feeding intensity occurred during the night. The diet was found to consist primarily of polychaetes, especially Glyceridae, and crustaceans, mainly Natantia. No significant diel variation was observed in the diet composition of red pandora.

The distribution of hydrogen peroxide in the southern and central Atlantic ocean

Abstract The near-surface distribution and processes controlling the distribution of hydrogen peroxide were examined in the South and central Atlantic Ocean during a transect from Uruguay to Barbados in May and June 1996. Four kinds of field experiments were conducted during the cruise including diel observations, dark decay experiments, photochemical production experiments, and hydrogen peroxide-enrichment experiments. Significant diel variations (∼25 nM) of hydrogen peroxide were observed, with surface-water concentrations increasing during the day and decreasing at night. With a dark decay half-life of 5.5 days and a net rate of photochemical production of 8.3 nM/h at local noon, it appears that both decay rate and photo-production rate of hydrogen peroxide are much smaller in oligotrophic seawater than in coastal seawater. The experimental results indicate that: (1) the decay reaction is a second-order reaction over all, first-order with both the concentration of hydrogen peroxide and the concentration of colloidal material; (2) seawater in the study area could restore its ambient levels of hydrogen peroxide in about 4 d after external perturbations. A total of 20 vertical profiles were obtained at 11 stations that can be classified as: surface maximum, surface mixed, and sub-surface maximum. Generally, the concentration of H 2 O 2 decreased with depth to less than 5 nM below 200 m. Hydrogen peroxide also was determined in some water samples from below 200 m, which revealed a slight increase of hydrogen peroxide with depth. In the surface waters of the open ocean, hydrogen peroxide increased with latitude from about 24 nM in the south (33.8°S) to about 80 nM in the north (8.9°N). This latitudinal variation of hydrogen peroxide correlated with model-calculated solar irradiance, satellite-measured wet deposition, depth of the mixed layer, and possibly total organic carbon. The water-column hydrogen peroxide inventory varied from 1.5 to 6.3×10 −3 mol/m 2 . Although the greatest shallow water concentrations were observed at stations in the Amazon plume, these stations showed a dramatic decrease in hydrogen peroxide with depth and integrated-water-column hydrogen peroxide was not significantly higher than at open ocean stations.

Carbon content and nutritional condition of sardine larvae ( Sardina pilchardus ) off the Atlantic coast of Spain

As part of a “European Sardine/Anchovy Recruitment Program” (SARP), sardine larvae (Sardina pilchardus) were sampled off the Atlantic coast of Spain through the spawning season from March to June. The larvae were analysed for carbon and nitrogen content as a measure of nutritional condition and survival potential. There was no significant diel variation in larval carbon content, but there was a small significant diel variation in nitrogen; the absence of a strong diel signal in elemental composition was ascribed to the overnight retention of the gut contents. There was an increase in carbon content with increase in body length which reached an asymptote at ∼40% carbon content at a larval length of 20 mm. It is argued that larvae with a carbon content of <25% of body weight were nutritionally stressed, with the smaller larvae (<10 mm in length) appearing to be more vulnerable to food limitation. Although larvae with the lowest age-specific carbon content (poorest condition) occurred on the cruise with the lowest food availability, there was no consistent relationship between carbon content and food availability. While the successive monthly estimates of carbon content revealed differences in potential recruitment between months, these were not related to the birth-date distribution of the surviving juveniles.

Diel variation in habitat use by planktivores in field enclosure experiments: the effect of submerged macrophytes and predation

The importance of submerged macrophytes and predation risk for habitat use by 0+ perch (Perca fluviatilis) and 0+ roach (Rutilus rutilus) was investigated in triplicate 78-m2 field enclosures with and without macrophytes in the middle. During three experimental runs, habitat use by fish were monitored every 6 h with Breder traps. Each period included 2 days of fish monitoring before stocking with piscivorous perch, and 2 days after. Predation risk significantly changed habitat use by 0+ perch in the morning, midday and evening, but not at night. By comparing with the unvegetated controls, we found a refuging effect of macrophytes in the morning. Under predation risk there was significant diel variation in habitat use by 0+ perch, suggesting a migration from the open water habitat at night into the macrophytes in the morning. Roach continued to use open water even after predators were stocked, but responded like perch by reducing overall activity.

Diel variation in habitat use by planktivores in field enclosure experiments: the effect of submerged macrophytes and predation

The importance of submerged macrophytes and predation risk for habitat use by 0+ perch (Perca fluviatilis) and 0+ roach (Rutilus rutilus) was investigated in triplicate 78‐m2 field enclosures with and without macrophytes in the middle. During three experimental runs, habitat use by fish were monitored every 6 h with Breder traps. Each period included 2 days of fish monitoring before stocking with piscivorous perch, and 2 days after. Predation risk significantly changed habitat use by 0+ perch in the morning, midday and evening, but not at night. By comparing with the unvegetated controls, we found a refuging effect of macrophytes in the morning. Under predation risk there was significant diel variation in habitat use by 0+ perch, suggesting a migration from the open water habitat at night into the macrophytes in the morning. Roach continued to use open water even after predators were stocked, but responded like perch by reducing overall activity.

Short-term variations in the abundance and cell volume of bacterioplankton in an artificial tropical lake

Circadian and spatial fluctuations in bacterioplankton abundance and cell volume were examined, for the first time, in the Municipal Lake located in the down town area of Yaounde (Capital of Cameroon, Central Africa, ca 3° 52′ N, 11° 31′ E). Bacterial cell volumes (range, 0.05 to 0.2 μm3) were consistent with those reported for other aquatic systems while bacterial densities (0.8 to 2 × 108 cells ml-1) were among the highest values reported in pelagic systems. These variables and chlorophyll a and dissolved oxygen concentrations within a single depth-zone varied from 13 to 61%, while water temperature fluctuated only from 2 to 6%. Spatial fluctuations of physical-chemical and biological variables were generally higher during the day-time than during the night-time. A significant diel variation was provided for bacterial cell volume in the surface waters where synchronized cell division was occurring during the night. The measured bacterial abundances in this study were 4 to 17 fold higher than values known from other lakes of similar trophic status, and both cell abundance and volume were not correlated with chlorophyll. We conclude that this was due to the dependence of bacterial populations to different sources of allochthonous substrates, including untreated sewage from the major influents of the lake, resuspension of benthic material, and substrate releasing from macrophytes which are prevalent in the littoral zone of the lake.

Net zooplankton and the biological pump: a comparison between the oligotrophic and mesotrophic equatorial Pacific

Structure and functioning of the zooplankton community and their consequences on the export fluxes of carbon, nitrogen and phosphorus were studied in two contrasting systems of the equatorial Pacific: the oligotrophic TTS (Typical Tropical Structure) to the west, and the mesotrophic HNLC (High Nutrient-Low Chlorophyll) in the central Pacific. Data were collected during the FLUPAC cruise equatorial transect (September-October 1994) of R.V. L'Atalante and four 6–8-day long time-series stations made between 165°E and 150°W. Along the equator, a sharp 2.5-fold increase in mesozooplankton biomass (200–2000 pm) was observed between 174° and 172°W, with a simultaneous change in surface salinity and chlorophyll concentration, corresponding to the shift between the TTS and HNLC systems. No significant zonal trend was observed within the two systems. Compared with TTS, HNLC presented a significantly greater contribution of the (500–2000 μm) size class to total mesozooplankton biomass, less diel variations, and a shallower vertical distribution. Lower metabolic rates in HNLC were accounted for by different taxonomic compositions in the two areas. Microzooplankton (35–200 gm) had a rather uniform biomass in TTS and HNLC, presented no significant diel variations in the 0–100 or 0–200 m layers, and displayed a shallower vertical distribution than the mesozooplankton. Consequences of such zooplanktonic features on the “biological pump” are assessed. Dissolved nitrogen (DN) and phosphorus (DP) active fluxes, resulting from excretion of interzonal mesozooplankton migrants were 2.1 times higher in the TTS than in the HNLC. However, dissolved inorganic carbon active fluxes were equal in the TTS and HNLC systems, due to differences in C, N, P metabolisms. Combined mesozooplankton and estimated micronekton nitrogen active fluxes represented 40% of the passive flux as measured by sediment traps in the TTS, and 9% in the HNLC. Estimates of mesozooplankton fecal production in the pbotic zone lead to a 2-fold increment between the oligotrophic and mesotrophic stations, and a larger contribution of the fecal production to the sinking flux. It is, therefore, concluded that the mesozooplankton role in the biological pump is mainly passive in the HNLC system, in contrast to the TTS site.

Effects of experimental conditions on the feeding rate of Mysis mixta (Crustacea, Mysidacea)

Predation by the Baltic Sea mysid Mysis mixta onnewly hatched Artemia sp. and culturedDaphnia magna was examined in the laboratory. Wedesigned experiments to study the effects onconsumption rate of different experimental conditions:light, container size, diel variation, duration ofadaptation period, starvation and the duration of afeeding experiment.Consumption differed significantly between mysids keptin bright light vs mysids in complete darkness, withabout twice the consumption in darkness. Acclimationto light conditions prior to the experiment did notinfluence this result. The duration of an experimenthas large effect on estimated consumption. Predationrates in 1-h experiments were up to 60% higher thanthose recorded in 4-, 12- and 24-h incubations.Consumption rates of starved mysids were significantlyhigher (27% on average) than that of unstarvedindividuals. No significant diel variation inconsumption was observed over an experimental periodat 36 hours. We found a tendency, however notstatistically significant, that the food consumptionincreased when the container size ws increased between1 and 8 l.

Development of Enzyme Immunoassays for 3,5,3′-Triiodo-l-thyronine andl-Thyroxine: Time-Course Studies on the Effect of Food Deprivation on Plasma Thyroid Hormones in Two Marine Teleosts, Sea Bass (Dicentrarchus labraxL.) and Sea Bream (Sparus aurataL.)

The effects of short-term food deprivation and photoperiod on plasma thyroid hormone levels of sea bass and sea bream were studied. Animals were acclimated under constant photoperiod regime (15L/9D) and feeding times (2 hr after light onset and 2 hr before light offset). Time-course studies involved monitoring plasma hormone levels every 4 hr throughout 1.5 24-hr cycles. Plasma 3,5,3′-Triiodo-l-thyronine (T3) andl-thyroxine (T4) were assayed using a newly developed competitive enzyme immunoassay, utilizing acetylcholinesterase as a label of enzymatic tracers. Enzyme immunoassays had sensitivities of 1.25–0.02 and 62.5–0.2 ng/ml for T3and T4, respectively, and reproducibilities of 3.7 and 5.6% intraassay variation for T3and T4, respectively; interassay variations for T3and T4assays respectively were 1.6%, 11% and 6.6%, 8% for sea bass and sea bream plasma similar to RIA. In sea bass, 3 days of food deprivation resulted in depressed plasma T3and T4, overriding significant diel variations seen during the second day of starvation. Sea bream displayed a slight decrease of T4plasma levels while T3levels remained constant for the whole sampling period. Both thyroidal systems responded to photoperiod with a significant increase in plasma T4level at the time of light onset. In addition, sea bass also displayed increased T3levels and decreases in both hormone levels coinciding with “lightoff.” Data show different responses of the sea bass and sea bream thyroidal systems to both nutritional state and photoperiod in that the latter state is influenced by the former. Data suggest plasma thyroid levels can be used as a rapid indicator of nutritional status.

2210 Location of the circadian pacemaker of the hagfish and the retinal information route to it

Two groups of goldfish were maintained on a 12L:12D photoperiod at 19°C and fed either at 0800 (light onset) or 1600 h daily. After 3 weeks, blood samples were taken at one of six times of day and the serum was analyzed for thyroxine, triiodothyronine, and cortisol by radioimmunoassay. Fish fed in the morning had significant diel variations in circulating titers of thyroxine and cortisol, those fed in the afternoon had significant variations of thyroxine and triiodothyronine. Meal feeding appeared to entrain, and phase shift, the cortisol rhythm but not the rhythms of thyroid hormones. These results are briefly discussed in light of the effects of timing the daily meal on weight gain in fish.

Relationship between the maximum quantum yield of carbon fixation and the minimum quantum yield of chlorophyll a in vivo fluorescence in the Gulf of St. Lawrence

Intensive monitoring of the maximum quantum yield of carbon fixation (&,,,) and of the minimum quantum yield of chlorophyll a in vivo fluorescence ($& was followed along the path of drifting buoys in the estuary and the Gulf of St. Lawrence, Canada. Samples were collected every third hour during two 24-h periods. Over the 170 measurements, &,,X varied by a factor of 20 [from 0.004 to 0.08 mol C (mol quanta))‘]. As predicted by a model of primary processes in photosystem 2, a significant and positive linear relationship was observed between $,,, and q5I;,. In the estuary, the covariability of &,,,, and $t;, seems to be driven by light, because it was observed only during daytime and was characterized by a parallel increase of &,,, and $r;, with depth. Examination of the spectral values of $FO suggests a contribution of photoprotectant carotenoids. In the gulf, because a parallel, although weaker, increase of &,, and rbr;, with depth also persisted during nighttime, nutrient deficiency is probably an additional cause of covariation between (6,,, and cbr;,. In both regions, an overall diel cycle was observed for $,,X with a maximum at around noon; $I;, did not show a clear diel periodicity. Despite the vertical covariability of &,,X and 4F0, the diel cycle of &,,X was uncoupled to $FO and thus to photosystem 2 activity. These results suggest a significant diel variation in the photosynthetic quotient of natural phytoplankton.

Activity and Thermal Biology of the Sand-Swimming Skink Neoseps reynoldsi: Diel and Seasonal Patterns

Body temperatures selected by skinks were measured using a laboratory thermal gradient; observations were made seven times during the diel cycle. Neoseps reynoldsi individuals exhibited significant diel variation in selected body temperature with highs of 31-32 C and lows of 28-29 C during both spring and winter. The length of time lizards selected relatively high temperatures was longer in spring than winter. In spring, lizards exhibited two periods of activity, in the morning and the evening. Lizards selected the highest temperatures when they were active in the evening and the lowest temperatures when they were active in the morning. In winter, lizards were active during the day. Because lizards did not alter their selected body temperature seasonally, activity during both spring and winter corresponded largely to daylight hours when subsurface temperatures were similar to selected body temperatures. Diel patterns of activity and selected body temperature of N. reynoldsi individuals were similar to those of an unrelated African sand-swimming skink, Sphenops sepsoides. These observations suggest that sand-swimming skinks may be convergent in activity patterns and thermal biology as well as in morphology.

Atmospheric sulfur chemistry and cloud condensation nuclei (CCN) concentrations over the northeastern Pacific Coast

Correlated measurements of dimethylsulfide (DMS), gas phase dimethylsulfoxide (DMSO), methane‐sulfonic acid (MSA)(g), sulfuric acid (H2SO4), and cloud condensation nuclei (CCN) were conducted in April 1991 at a Pacific coastal site in northern Washington. Measurements of SO2, aerosol methanesulfonate (MSA)(p), and non‐sea‐salt sulfate (nss‐SO4) concentrations were also included. Maximum DMS concentrations between 100 and 240 pptv were observed when the measurement site (480 m above sea level) was embedded in clouds and air from the marine boundary layer was flowing upslope to the site. DMS levels measured in continental air and/or above the mixed layer were typically less than 20 pptv. The sulfur gases DMSO, H2SO4, and MSA(g) were measured in real time on a continuous basis (once every 60–150 s) using selected ion chemical ionization mass spectrometry. Corresponding concentrations ranged between &lt;0.5–3.2 pptv, 0.001–1.19 pptv, and 0.002–0.19 pptv, respectively. All three sulfur gases showed significant diel variations mostly in phase with each other. Their corresponding lifetimes in the marine atmosphere are estimated to be of the order of a few hours. The results in connection with recent laboratory studies and model calculations suggest that dimethylsulfone (DMSO2) was the dominant end product of DMS oxidation under the present conditions. CCN concentrations measured in marine air ranged roughly between 10–200 cm−3 and 200–400 cm−3 at 0.3% and 0.9% supersaturation, respectively. A statistical analysis using only data obtained in predominantly marine air and during non‐fog/non‐precipitation periods showed significant correlations between individual sulfur species and between CCN (0.3% ss) and H2SO4, and CCN (0.3% ss) and nss‐SO4. The results indirectly support a relationship between DMS and CCN concentrations. However, other observations also suggest that at higher supersaturations (0.9%), compounds less soluble than sulfate may become important in marine CCN formation.

Diel variation in the catch of fishes and penaeid shrimps in a tropical estuary

In Laguna Joyuda, Puerto Rico, 24-h trawl surveys were conducted to test the hypothesis that there is no diel variation in the catch rate of fishes and penaeid shrimps in a turbid, microtidal estuary relatively free of visual predation. The null hypothesis was not rejected for the two most abundant fishes in the lagoon, Eucinostomus gula (Gerreidae) and Achirus lineatus (Bothidae). Conversely, the total number of fishes, Diapterus rhombeus (Gerreidae), and the engraulids Anchoa cubana and A. hepsetus were collected in significantly higher numbers during hours of darkness. Total penaeid catch was significantly higher in night than in day collections. The most abundant species were Penaeus notialis and P. subtilis, both of which appeared to be strong daytime burrowers. Small penaeids demonstrated greater diel variation in trawl capture rate than larger (subadult) shrimps. Diel variation in capture rate was related to habitat associations of individual species. Fauna resident in the very turbid lagoon demonstrated no diel variation in activity patterns and trawl susceptibility. Those species which spend only the early part of their lives in the lagoon, or live in both turbid and clear water, had significant diel variation in activity and trawl catchability.