Understanding the sources and effects of abandoned,lost, and discarded fishing gear on marine turtles in northern Australia

Globally, 6.4 million tons of fishing gear are lost in the oceans annually. This gear (i.e., ghost nets), whether accidently lost, abandoned, or deliberately discarded, threatens marine wildlife as it drifts with prevailing currents and continues to entangle marine organisms indiscriminately. Northern Australia has some of the highest densities of ghost nets in the world, with up to 3 tons washing ashore per kilometer of shoreline annually. This region supports globally significant populations of internationally threatened marine fauna, including 6 of the 7 extant marine turtles. We examined the threat ghost nets pose to marine turtles and assessed whether nets associated with particular fisheries are linked with turtle entanglement by analyzing the capture rates of turtles and potential source fisheries from nearly 9000 nets found on Australia's northern coast. Nets with relatively larger mesh and smaller twine sizes (e.g., pelagic drift nets) had the highest probability of entanglement for marine turtles. Net size was important; larger nets appeared to attract turtles, which further increased their catch rates. Our results point to issues with trawl and drift‐net fisheries, the former due to the large number of nets and fragments found and the latter due to the very high catch rates resulting from the net design. Catch rates for fine‐mesh gill nets can reach as high as 4 turtles/100 m of net length. We estimated that the total number of turtles caught by the 8690 ghost nets we sampled was between 4866 and 14,600, assuming nets drift for 1 year. Ghost nets continue to accumulate on Australia's northern shore due to both legal and illegal fishing; over 13,000 nets have been removed since 2005. This is an important and ongoing transboundary threat to biodiversity in the region that requires attention from the countries surrounding the Arafura and Timor Seas. Entender las Fuentes y Efectos de Equipo de Pesca Abandonado, Perdido y Desechado sobre las Tortugas Marinas del Atlántico Norte     

Clogging and filtration coefficient in a high-speed plankton sampler

Data upon water exit-velocity in an encased high-speed plankton sampler were obtained during field use and analysed; the results show that reduction in size of mouth opening allows complete avoidance of clogging under normal working conditions in temperate waters. Reduction of mouth opening has the further effect of increasing the filtration coefficient in such a way that the sampler apparentlyfilters more than the theoretical quantity of water that would be expected from the mouth area. Unobstructed flow through the net probably reduces avoidance by fast-moving plankters.     

Ecological aspects of plankton production

The study was carried out in the neritic and estuarine waters of Porto Novo, Coromandel Coast, Bay of Bengal, India during the period January, 1960 to December, 1967. The average displacement volume of plankton usually varied between 2 and 4 c²/m². During summer, with a season of high plankton productivity, the average plankton displacement volume rose to 8 c²/m³. Generally speaking, the average zooplankton density (standing crop) was usually between 80,000 and 100,000 organisms/m³, of which copepods alone comprised usually between 70,000 and 90,000 organisms/m³. The average copepod density per sample varied from 30,000 to 50,000 organisms/m³. However, in the summer months, the copepod density was usually not less than 100,000 organisms/m³; in some years this was even higher (from 125,000 to 170,000 organisms/m³). Copepods comprised between 80 and 95% of the zooplankton population. The maximum non-copepod population in the zooplankton seldom reached 30%, was ofter below 25%, and usually less than 20%. During the period March to October (in some years as early as February, and in some years up to November), either an increasing or a steady trend of plankton production was evident. It would appear that salinity and rainfall determine the occurrence and distribution of plankton in Porto Novo.Contribution No. 189 from the Centre of Advanced Study in Marine Biology, Marine Biological Station of Annamalai University, Porto Novo, Tamilnadu, India.     

Effects of mesh size on estimation of population densities of tropical euthecosomatous pteropods

Euthecosomatous pteropods were collected in successive oblique plankton tows with 1 m diameter nets equipped with No. 0, No. 6, and No. 20 nylon mesh. Comparisons of samples revealed that the same species were collected by all 3 nets, but the No. 0 and No. 6 nets failed to collect veligers and juveniles of Spiratella inflata and Creseis virgula conica. Collections with nets of larger mesh size may erroneously suggest seasonal variations in population densities due, not to actual changes in total density, but to changes in the size of individuals and changes in the relative numbers of the different life stages. A large diameter net equipped with No. 20 mesh is recommended for work that includes the smallest life stages of tropical pteropods.     

Bioluminescence in the high Arctic during the polar night

This study examines the composition and activity of the planktonic community during the polar night in the high Arctic Kongsfjord, Svalbard. Our results are the first published evidence of bioluminescence among zooplankton during the Arctic polar night. The observations were collected by a bathyphotometer detecting bioluminescence, integrated into an autonomous underwater vehicle, to determine the concentration and intensity of bioluminescent flashes as a function of time of day and depth. To further understand community dynamics and composition, plankton nets were used to collect organisms passing through the bathyphotometer along with traditional vertical net tows. Additionally, using a moored bathyphotometer closed to the sampling site, the bioluminescence potential itself was shown not to have a diurnal or circadian rhythm. Rather, our results provide evidence for a diel vertical migration of bioluminescent zooplankton that does not correspond to any externally detectable changes in illumination.     

Sorted samples and quantitative counts in appendicularian catches

Sorting centers are important for processing material collected during extensive oceanic expeditions. Based on information obtained from samples of appendicularians, this paper demonstrates that usual procedures adopted in sorting centers produce samples which contain only a fraction of the organisms present in the original catch (or subsample). In addition, the proportion of individuals sorted varies considerably. In four Indian Ocean Standard Net catches collected in the Arabian Sea, subsamples (called sorted samples in this paper) are compared with careful microscopical counts (quantitative counts) performed by the author. The numbers of appendicularians found in quantitative counts were 2.4 to 19.2 times higher than those in the sorted samples. Sorting efficiency varies in different forms of appendicularians; it is particularly low in Fritillaria sp. Bad preservation and small body size further reduce the portion sorted out. Under certain conditions (e.g. net clogging), numerous small appendicularians are retained which normally would pass easily through the meshes of the plankton net.     

Size fractionation of natural aquatic populations associated with autotrophic and heterotrophic carbon uptake

Natural phytoplankton samples from the Gulf of California were incubated with radioactive bicarbonate or glucose, with or without the addition of gentamycin, a broad spectrum antibiotic which inhibits bacterial activity but has little effect on many algae. The relative distribution of chlorophyll and radioactivity in particles retained by filters of 3 and 0.45 μ mean pore size was measured. From 13 to 83% of the ¹⁴C-bicarbonate assimilated by natural populations appeared to be associated with organisms having a smallest dimension less than 3 μ. These were probably algae, since the distribution of chlorophyll on the filters was similar to that of the radioactivity. Furthermore, gentamycin treatment did not change the distribution of radioactivity from ¹⁴C-bicarbonate. In these waters, considerable photosynthesis may be carried out by ultra-nannoplankton (3 to 1 μ). Glucose was assimilated mainly by bacteria, of which more than 90% passed through 3 μ filters. Gentamycin significantly reduced glucose uptake in most cases. Although this method cannot be adopted uncritically for the estimation of carbon flux from photoautotrophs to heterotrophs, differential filtration can be used to elucidate the size distribution of various metabolic groups in the plankton.     

The feeding mechanisms in plankton eaters

The feeding mechanisms of 15 whitefish Coregonus lavaretus (Lin.) from the Gulf of Bothnia near Holmsund were studied. The fishes were very susceptible to infection. Different preparation and anaesthesia methods were tested. Careful dissections revealed a vestigial epibranchial organ extending from the dorsal part of the fifth gill slit. It is suggested that this little sac receives and accumulates plankton organisms which may then be carried to the oesophagus via a special pharynx groove. In this way the whitefish could effectively use small plankton as a complementary food source to larger food particles.     

Aspects of the community ecology of deep-sea,benthopelagic plankton,with special attention to gymnopleid copepods

Benthopelagic plankton was collected 10 to 100 m above the bottom of the deep sea (1100 to 3200 m) with an opening-closing net. Five samples were taken from the San Diego Trough, 2 from the eastern tropical Pacific and 2 from the northeast Atlantic. Roughly 80% of the individual organisms collected were copepods; the next most abundant groups were usually isopods, ostracods, and chaetognaths. Within the depth range studied, absolute abundances of most animal groups decreased with depth, but relative abundances (%) did not change. Of over 100 species of adult gymnopleid copepods collected, a few species were abundant, but most were rare. Copepod exoskeletons were more abundant than living copepods. For the gymnopleids, similarities among the 3 geographical regions were strong at the family and genus level but weak at the species level. Species that were widely distributed horizontally tended to occupy different positions (in terms of rank order of abundance) in each community. Gut content analysis indicated that the most abundant gymnopleids were generalized particle feeders. However, size-frequency distributions provided indirect evidence for niche separation. The deep-sea benthopelagic plankton was divisible into a truly planktonic component and a bottom-associated component. Abundances of some copepods changed relative to distance off the bottom. Several copepods and other animals, previously described as epibenthic or bottom-associated, were collected more that 10 m above the bottom and thus appeared to move freely between the bottom and lower water column.     

Pre-and Post-Liming Aquatic Communities and Trout Diets

In the period following liming and before the introduction of brown trout to Loch Fleet, there was a reduction in the numbers of invertebrates, including the Chironomidae, Oligochaeta and Hydracarina in the loch, but an overall increase in invertebrates in the inlet and outlet streams. This was followed in the loch by a recovery in the numbers of certain groups (particularly the Trichoptera and Ephemeroptera) to pre-liming levels in spite of the introduction of the trout.

The diets of the trout comprised a wide variety of aquatic and terrestrial organisms ranging in size from Fprotozoans to newts and are dominated by chironomids. Opportunistic type of feeding as found in the present study appears to be typical of the brown trout and is reported for many water bodies by other workers.

Significant differences (P < 0.05 and P < 0.001) were found in the numbers and weights of certain food organisms in the stomachs of brown trout caught by different fishing methods in the summers of 1992 and 1993. Fish caught by gill net contained fewer planktonic crustaceans and chironomids, but greater numbers of trichopterans and terrestrial organisms, than in those caught by fly and spinner. the differences are probably related to differences in the depth at which the fish, caught by the various methods, were feeding.     

Vertical distribution and abundance of Acantharia and their symbionts

Microzooplankton was sampled during two cruises (Galápagos Vents, March 1985; Tongue of the Ocean and western edge of the Sargasso Sea, October/November 1985) by various collection methods (Niskin bottles, plankton nets, divers) to determine the vertical distribution and abundance of Acantharia. The larger size classes of Protozoa are dominated by the sarcodines, and Acantharia are frequently the most abundant of these in mesotrophic and oligotrophic oceans. The absolute densities of Acantharia have been consistently underestimated in many previous studies for two reasons: their skeletons dissolve in preserved samples, and they are undersampled by fine-meshed plankton nets. The previously identified dissolution problem may be less severe for concentrated samples because the dissolution of a portion of the Acantharia will raise the dissolved strontium concentration in the sample. Twenty five and 160 m-mesh plankton nets consistently underestimate the abundance of net plankton by one to two orders of magnitude. Possible reasons for this significant error are discussed. In the Equatorial Pacific Ocean, Acantharia were found at densities as high as 30 liter^-1 and integrated abundances of 1.58 to 5.34x10⁵ Acantharia m^-2. Up to 90% were concentrated near the surface; their abundance declined sharply below 20 m. At two stations in the Atlantic, peak densities reached 6.4 liter^-1. Wind-mixing may spread individuals more evenly through the euphotic zone, but they reestablish their surface maximum during period of calm. Acantharia generally have relatively few, but large symbionts. Small individuals average about 6 symbionts per host, larger hosts average 40 symbionts, and some individuals may have thousands of algal cells. Acantharia symbionts made up less than 1% of the chlorophyll in the water column, even at their host's peak abundances of 30 liter^-1. However, production estimates, using published sarcodine-symbiont production-rates, suggest that Acantharia could occasionally account for up to 20% or more of the carbon fixation in the upper euphotic zone of oligotrophic oceans.     

Two-way coupling versus one-way forcing of plankton and fish models to predict ecosystem changes in the Benguela

‘End-to-end’ models have been adopted in an attempt to capture more of the processes that influence the ecology of marine ecosystems and to make system wide predictions of the effects of fishing and climate change. Here, we develop an end-to-end model by coupling existing models that describe the dynamics of low (ROMS–N₂P₂Z₂D₂) and high trophic levels (OSMOSE). ROMS–N₂P₂Z₂D₂ is a biogeochemical model representing phytoplankton and zooplankton seasonal dynamics forced by hydrodynamics in the Benguela upwelling ecosystem. OSMOSE is an individual-based model representing the dynamics of several species of fish, linked through opportunistic and size-based trophic interactions. The models are coupled through a two-way size-based predation process. Plankton provides prey for fish, and the effects of predation by fish on the plankton are described by a plankton mortality term that is variable in space and time. Using the end-to-end model, we compare the effects of two-way coupling versus one-way forcing of the fish model with the plankton biomass field. The fish-induced mortality on plankton is temporally variable, in part explained by seasonal changes in fish biomass. Inclusion of two-way feedback affects the seasonal dynamics of plankton groups and usually reduces the amplitude of variation in abundance (top-down effect). Forcing and coupling lead to different predicted food web structures owing to changes in the dominant food chain which is supported by plankton (bottom-up effect). Our comparisons of one-way forcing and two-way coupling show how feedbacks may affect abundance, food web structure and food web function and emphasise the need to critically examine the consequences of different model architectures when seeking to predict the effects of fishing and climate change.     

Characteristics of plankton Hg bioaccumulations based on a global data set and the implications for aquatic systems with aggravating nutrient imbalance

• Hg bioaccumulation by phytoplankton varies among aquatic ecosystems. • Active Hg uptake may exist for the phytoplankton in aquatic ecosystems. • Impacts of nutrient imbalance on food chain Hg transfer should be addressed. The bioaccumulation of mercury (Hg) in aquatic ecosystem poses a potential health risk to human being and aquatic organism. Bioaccumulations by plankton represent a crucial process of Hg transfer from water to aquatic food chain. However, the current understanding of major factors affecting Hg accumulation by plankton is inadequate. In this study, a data set of 89 aquatic ecosystems worldwide, including inland water, nearshore water and open sea, was established. Key factors influencing plankton Hg bioaccumulation (i.e., plankton species, cell sizes and biomasses) were discussed. The results indicated that total Hg (THg) and methylmercury (MeHg) concentrations in plankton in inland waters were significantly higher than those in nearshore waters and open seas. Bioaccumulation factors for the logarithm of THg and MeHg of phytoplankton were 2.4–6.0 and 2.6–6.7 L/kg, respectively, in all aquatic ecosystems. They could be further biomagnified by a factor of 2.1–15.1 and 5.3–28.2 from phytoplankton to zooplankton. Higher MeHg concentrations were observed with the increases of cell size for both phyto- and zooplankton. A contrasting trend was observed between the plankton biomasses and BAF_MeHg, with a positive relationship for zooplankton and a negative relationship for phytoplankton. Plankton physiologic traits impose constraints on the rates of nutrients and contaminants obtaining process from water. Nowadays, many aquatic ecosystems are facing rapid shifts in nutrient compositions. We suggested that these potential influences on the growth and composition of plankton should be incorporated in future aquatic Hg modeling and ecological risk assessments.     

A controlled-temperature plankton wheel

A controlled-temperature plankton wheel is described that is suitable for use on board a ship. The IMER plankton wheel system allows the use of various sizes of experimental bottles, up to 2.2 litres, the simulation of ambient light regimes and variable speed control for the rotation of the experimental bottles. The flexibility of the system was demonstrated by investigating the relationship between temperature and ingestion rate of an herbivorous copepod. Using four of the IMER plankton wheels simultaneously at four different temperatures (5°, 10°, 15° and 20° C), the ingestion rate of Calanus helgolandicus, feeding on Thalassiosira weissflogii, was shown to increase with increasing temperature; from a transformation of log_e (ingestion rate), this relationship was calculated as a Q₁₀ (10° to 20°C) for Copepodite Stage V (Q₁₀ 4.5) and adult female (Q₁₀ 2.7) C. helgolandicus. The possibility of damaging cells, by rotation at 2 rpm, was investigated using the spinose form of the diatom T. weissflogii. Such rotation did not cause any damage to the spines of T. weissflogii, but mixing this diatom with a magnetic stirrer bar did damage the spines to varying degrees, depending on the volume being mixed.     

Shift in nutrient and plankton community in eutrophic lake following introduction of a freshwater bivalve

The impact of the freshwater bivalve Corbicula leana on plankton community dynamics was examined during a cyanobacterial bloom period. Nutrient and chlorophyll concentrations, primary productivity, and phytoplankton and zooplankton communities in the experimental enclosures were measured at 2-3 day intervals. The introduction of mussels reduced net primary productivity and phytoplankton and chlorophyll. Chlorophyll decreased immediately following addition of 100 mussels and then increased over time. After 600 mussels were added, chlorophyll decreased continuously from 87to 25 microg l(-1), approaching that in the mussel-free enclosure. Simultaneously, water transparency increased and concentrations of suspended solids and total phosphorus decreased. Mussel addition caused short-term increases in nutrient concentrations, especially following high-density treatment: phytoplankton density decreased, while cell density in the mussel-free enclosure increased. Zooplankton densities in the two enclosures were similar; however, carbon biomass in the mussel enclosure increased, associated with an increase in large zooplankton. The trophic relationship between phytoplankton and zooplankton was positive in the mussel-free enclosure and negative in the mussel-treatment enclosure, possibly reflecting effects of mussels on both consumer and resource control in the plankton community. Thus, filter feeding by Corbicula affects nutrient recycling and plankton community structure in a freshwater ecosystem through direct feeding and competition for food resources.     

Ultrastructural and radiotracer studies of pore function in foraminifera

Clusters of mitochondria have been observed close to the inner end of the pores in Nonionella stella, Globobulimina pacifica, Buliminella tenuata, Bolivina argentea, Loxostomum pseudobeyrichi, Cassidulinoides cornuta and Bolivina cf. subexcavata, pointing to a pore function of oxygen-uptake. The relationship between mitochondria and pores is most distinct in Buliminella tenuata, Bolivina argentea and L. pseudobeyrichi collected from a deep-water, low-oxygen environment. Amphistegina lobifera Larsen, 1976 readily took up ¹⁴CO₂ through its pores, indicating a pore function related to the photosynthetic activities of its symbionts. ¹⁴C-labelled glucose did not pass through the pores in traceable amounts during a 2 h exposure time.     

Redistribution of benefits but not detection in a fisheries bycatch‐reduction management initiative

Reducing the capture of small fish, discarded fish, and bycatch is a primary concern of fisheries managers who propose to maintain high yields, species diversity, and ecosystem functions. Modified fishing gear is one of the primary ways to reduce by‐catch and capture of small fish. The outcomes of gear modification may depend on c ompetition among fishers using other similar resources and other gears in the same fishing grounds and the subsequent adoption or abandonment of modified gears by fishers. We evaluated adoption of modified gear, catch size, catch per unit effort (CPUE), yield, and fisher incomes in a coral reef fishery in which a 3‐cm escape gap was introduced into traditional traps. There were 26.1 (SD 4.9) fishers who used the experimental landing sites and 228(SD 15.7) fishers who used the control landing sites annually over 7 years. The size of fish increased by 10.6% in the modified traps, but the catch of smaller fish increased by 11.2% among the other gears. There was no change in the overall CPUE, yields, or per area incomes; rather, yield benefits were redistributed in favor of the unmodified gears. For example, estimated incomes of fishers who adopted the modified traps remained unchanged but increased for net and spear fishers. Fishers using escape‐gap traps had a high proportion of income from larger fish, which may have led to a perception of benefits, high status, and no abandonment of the modified traps. The commensal rather than competitive outcome may explain the continued use of escape‐gap traps 3 years after their introduction. Trap fishers showed an interest in negotiating other management improvements, such as increased mesh sizes for nets, which could ultimately catalyze community‐level decisions and restrictions that could increase their profits.     

Production and respiration of overall plankton and ultraplankton communities at the entrance to the Gulf of Finland in the Baltic Sea

Primary productivity and respiration of the overall plankton community and of ultraplankton (organisms passing through a 3-m Nuclepore filter) were studied at the entrance to the Gulf of Finland during the growth season in 1982. Data of the respiration measurements from previous years are also presented. During the development of a diatom spring bloom, the algal component could be successfully separated from the bacterial component by size fractionation with a 3-m Nuclepore filter and thus the algal respiration could be approximated, being on the order of 10 to 20% of the gross production. After the phytoplankton spring maximum, bacteria played an important role in mediating the energy flow from phytoplankton exudates to higher trophic levels. Maximum values of 1 230 and 740 mg O₂ m^-2 d^-1 were recorded for overall and for ultraplankton respiration, respectively, during late July. High productivity values coupled with low phytoplankton biomass and low inorganic nutrient values were also recorded in late July, indicating effective nutrient regeneration and rapid turnover of the plankton community. During late summer, a considerable fraction (over 30%) of phytoplankton production was released as exudates, suggesting that much of the energy is channeled to higher trophic levels via bacterial pathways rather than by direct herbivorous grazing during this season. The summer development of phytoplankton community structure and functioning is strongly controlled by hydrographic conditions, i.e. by nutrient inputs via upwelling and by water temperature. A carbon budget for late summer indicated that bacteria may contribute only up to 50% of the overall respiration of the plankton community, which suggests that heterotrophs other than bacteria play an important role in nutrient regeneration. The present study stresses the importance of energy flow via the phytoplankton exudatebacteria-micrograzer pathway in relatively oligotrophic, brackish water ecosystems.     

Development of north sea coastal plankton communities in separate plastic bags under identical conditions

In two experiments lasting 4 to 6 weeks, communities of North Sea coastal plankton kept in separate plastic bags (of about 1400 l) and exposed to the same environmental conditions showed very similar patterns of growth and decline. This result means that the method is suitable for the evaluation of toxic effects of environmental pollutants at low concentrations on complex plankton systems. The phytoplankton in the bags produced a succession of blooms, which were probably limited by shortage of nutrients. The dominant zooplankton organisms were various species of copepods which can develop in the bags from egg to adult. Strong indications were found that mineralization of organic matter occurs in the bags. Chemical parameters and phytoplankton biomass were found not to be stratified, indicating that the contents of the bags were well mixed.Work carried out under Contract No. 110-75-1 ENVN of the E.C. Environmental Research Programme.     

A simple plankton model for the Oregon upwelling ecosystem: Sensitivity and validation against time-series ocean data

Simple plankton models serve as useful platforms for testing our understanding of the mechanisms underlying ecosystem dynamics. A simple, one-dimensional plankton model was developed to describe the dynamics of nitrate, ammonium, two phytoplankton size-classes, meso-zooplankton, and detritus in the Oregon upwelling ecosystem. Computational simplicity was maintained by linking the biological model to a one-dimensional, cross-shelf physical model driven by the daily coastal upwelling index. The model sacrificed resolution of regional-scale and along-shore (north to south) processes and assumed that seasonal productivity is primarily driven by local cross-shelf Ekman transport of surface waters and upwelling of nutrient-rich water from depth.Our goals were to see how well a simple plankton model could capture the general temporal and spatial dynamics of the system, test system sensitivity to alternate parameter set values, and observe system response to the effective scale of potential retention mechanisms. Model performance across the central Oregon shelf was evaluated against two years (2000-2001) of chlorophyll and copepod time-series observations. While the modeled meso-zooplankton biomass was close in scale to the observed copepod biomass, phytoplankton was overestimated relative to that inferred from the observed surface chlorophyll concentration. Inshore, the system was most sensitive to the nutrient uptake kinetics of diatom-size phytoplankton and to the functional grazing response of meso-zooplankton. Meso-zooplankton was more sensitive to alternate parameter values than was phytoplankton. Reduction of meso-zooplankton cross-shelf advection rates (crudely representing behavioral retention mechanisms) reduced the scale of model error relative to the observed seasonal mean inshore copepod biomass but had little effect of the modeled meso-zooplankton biomass offshore nor upon phytoplankton biomass across the entire shelf.