Model-supported estimation of mortality rates in Baltic cod (Gadus morhua callarias L.) larvae: the varying impact of 'critical periods'.
(17/591)
BACKGROUND: Changes in the survival-rate during the larval phase may strongly influence the recruitment level in marine fish species. During the larval phase different 'critical periods' are discussed, e.g. the hatching period and the first-feeding period. No such information was available for the Baltic cod stock, a commercially important stock showing reproduction failure during the last years. We calculated field-based mortality rates for larval Baltic cod during these phases using basin-wide abundance estimates from two consecutive surveys. Survey information was corrected by three dimensional hydrodynamic model runs. RESULTS: The corrections applied for transport were of variable impact, depending on the prevailing circulation patterns. Especially at high wind forcing scenarios, abundance estimates have the potential to be biased without accounting for transport processes. In May 1988 mortality between hatch and first feeding amounted to approximately 20% per day. Mortality rates during the onset of feeding were considerably lower with only 7% per day. In August 1991 the situation was vice versa: Extremely low mortality rates of 0.08% per day were calculated between hatch and first feeding, while the period between the onset of feeding to the state of an established feeder was more critical with mortality rates of 22% per day. CONCLUSIONS: Mortality rates during the different proposed 'critical periods' were found to be highly variable. Survival rates of Baltic cod are not only influenced by a single 'critical period', but can be limited at different points during the larval phase, depending on several biotic and abiotic factors. (+info)
Morphological plasticity in the tropical sponge Anthosigmella varians: responses to predators and wave energy.
(18/591)
The goal of the research presented here was to examine phenotypic plasticity exhibited by three morphotypes of the common Caribbean sponge Anthosigmella varians (Duchassaing & Michelotti). We were interested in examining the biotic (and, to a lesser extent, abiotic) factors responsible for branch production in this species. We also tested the hypothesis that the skeleton may serve an antipredator function in this sponge, focusing on vertebrate fish predators (i.e., angelfish) in this work. In transplant and caging experiments, unprotected forma varians replicates were immediately consumed by angelfish, while caged replicates persisted on the reef for several months. These findings support the hypothesis that predators (and not wave energy) restrict forma varians to lagoonal habitats. Branch production was not observed in A. varians forma incrustans when sponges were protected from predators or placed in predator-free, low-wave-energy environments. It is not clear from our work whether forma incrustans is capable of producing branches (i.e., whether branch production is a plastic trait in this morph). Additional field experiments demonstrated that A. varians forma varians increased spicule concentrations, compared to uninjured sponges, in response to artificial predation events, and A. varians forma rigida reduced spicule concentrations, compared to uncaged controls, when protected from predators. These findings indicate that spicule concentration is a plastic morphological trait that can be induced by damage, and that A. varians may be able to reduce spicule concentrations when environmental conditions change (e.g., in the absence of predators). The potential significance of inducible defenses and structural anti-predator defenses in sponges is discussed in relation to recent work on sponge chemical defenses. (+info)
Shell clamping behaviour in the limpet Cellana tramoserica.
(19/591)
The behaviour of clamping the shell against the substratum may play an important role in the limpet adhesion mechanism because friction generated by this behaviour resists dislodgement by shear forces. This paper describes the development of an apparatus to analyse limpet clamping activity in relation to known forces, including simulated wave activity and predator attack. The results show that Cellana tramoserica clamps its shell in a closely regulated manner consistent with an active role in the limpet adhesion mechanism. Limpets clamped sharply for several seconds in response to single disturbances such as tapping the shell. In response to more continuous disturbance simulating a concerted predator attack, limpets clamped tightly for several minutes. In response to lifting forces applied to the shell, limpets clamped at a set proportion of the lifting force, even if the lift force was a highly dynamic wave profile. This behaviour has implications for numerical models that attempt to describe limpet adhesion because it shows that limpets cannot be represented by a simple mechanical analogue and that the clamping behaviour must be accounted for if useful predictions are to be drawn. (+info)
The potential impact of flooding on confined animal feeding operations in eastern North Carolina.
(20/591)
Thousands of confined animal feeding operations (CAFOs) have been constructed in eastern North Carolina. The fecal waste pit and spray field waste management systems used by these operations are susceptible to flooding in this low-lying region. To investigate the potential that flood events can lead to environmental dispersion of animal wastes containing numerous biologic and chemical hazards, we compared the geographic coordinates of 2,287 CAFOs permitted by the North Carolina Division of Water Quality (DWQ) with estimates of flooding derived from digital satellite images of eastern North Carolina taken approximately 1 week after Hurricane Floyd dropped as much as 15-20 inches of rain in September 1999. Three cattle, one poultry, and 237 swine operations had geographic coordinates within the satellite-based flooded area. DWQ confirmed 46 operations with breached or flooded fecal waste pits in the same area. Only 20 of these 46 CAFOs were within the satellite-based estimate of the inundated area. CAFOs within the satellite-based flood area were located in 132 census block groups with a population of 171,498 persons in the 2000 census. African Americans were more likely than whites to live in areas with flooded CAFOs according to satellite estimates, but not according to DWQ reports. These areas have high poverty rates and dependence on wells for drinking water. Our analysis suggests that flood events have a significant potential to degrade environmental health because of dispersion of wastes from industrial animal operations in areas with vulnerable populations. (+info)
A BETR way to track toxic pollutants.
(21/591)
Why are chemicals that have been banned for over a decade still being detected in the environment? How is it that chemicals and other pollutants such as particulate matter turn up hundreds or even thousands of miles from their sources? These are just two of the questions that a team of American and Canadian scientists hope will be answered using a model they have developed to track pollutants across North America. In the short term, the model's developers are concentrating on using it to assess rates of exposure from food sources, but they hope to expand the model to include global data, a step they hope will help policy makers better understand the consequences of pollution. (+info)
Sand bar opening in a coastal lagoon (Iquipari) in the northern region of Rio De Janeiro State: hydrological and hydrochemical changes.
(22/591)
The aim of this paper was to verify hydrological and hydrochemical changes in the Iquipari Lagoon water column, during three months, as related to an artificial sand bar opening. A drastic reduction in water volume occurred 28 hours after the sand bar opening, exposing the bottom sediment. This was densely colonized by submerged aquatic plants, which were dominant in the ecosystem metabolism, specially near the sand bar. An increase in salinity values, and dissolved and total nutrient concentrations, was observed during the sand bar opening period. In contrast, chlorophyll a concentration decreased. These changes were associated with: 1) mix of remained brackish and marine waters; 2) nutrient release from sediments by physical processes; and 3) submerged plant death. After the sand bar closure, lagoon metabolism was completely regulated by a dense phytoplankton community. We observed that the sand bar opening resulted in an extensile replacement of the lagoon's water and a change in the community mainly responsible for the ecosystem metabolism. (+info)
Hatching controlled by the circatidal clock, and the role of the medulla terminalis in the optic peduncle of the eyestalk, in an estuarine crab Sesarma haematocheir.
(23/591)
Embryos attached to the female crab Sesarma haematocheir hatch synchronously within 1 h. Hatching is also synchronized near the time of the expected nocturnal high tide. These events are governed by a single circatidal clock (or pacemaker) in the female crab. The present study examined the role of the optic peduncle of the eyestalk on hatching and hatching synchrony. Surgery was performed either from the tip of the eyestalk [to remove the region of the optic peduncle from the compound eye-retina complex to the medulla interna (MI)] or from a small triangle 'window' opened on the eyestalk exoskeleton [to create lesions on the medulla terminalis (MT) of the optic peduncle]. Neither hatching nor hatching synchrony was affected by removal of the region of the optic peduncle from the compound eye-retina complex to the MI: the circatidal rhythm also remained. Removal of the MI probably caused damage to the sinus gland and the bundle of axons running from the sinus gland to the X organ. Nevertheless, maintenance of highly synchronized hatching indicates that the X organ-sinus gland system is not related to hatching. Hatching and hatching synchrony were not affected by dorsal-half cuts of the MT: the timing of hatching was not affected either. By contrast, transverse and ventral-half cuts of the MT caused severe damage to most females; hatching of many females was suppressed, while hatching of some females was either periodic, at intervals of approximately 24 h, or arrhythmic for a few days. The bundle of neuronal axons is tangled in the MT, and the axons inducing hatching pass through the ventral half of the MT. Complete incision of these axon bundles may have suppressed hatching. Incomplete incision of the axon bundle or partial damage to the neurons may have caused periodic or arrhythmic patterns of hatching. There are two possible roles for MT in hatching. One possibility is that neurons in the MT only induce hatching under the control of the circatidal pacemaker located in a site somewhere other than the optic peduncle. Another possibility is that the circatidal pacemaker is actually present in the MT. The second possibility seems more plausible. Each embryo has a special 48-49.5 h developmental program for hatching. This program could be initiated by the circatidal pacemaker in the female, and hatching synchrony may also be enhanced by the same pacemaker. (+info)
Climate change and latitudinal patterns of intertidal thermal stress.
(24/591)
The interaction of climate and the timing of low tides along the West Coast of the United States creates a complex mosaic of thermal environments, in which northern sites can be more thermally stressful than southern sites. Thus, climate change may not lead to a poleward shift in the distribution of intertidal organisms, as has been proposed, but instead will likely cause localized extinctions at a series of "hot spots." Patterns of exposure to extreme climatic conditions are temporally variable, and tidal predictions suggest that in the next 3 to 5 years "hot spots" are likely to appear at several northern sites. (+info)