Malthusian overfishing and efforts to overcome it on Kenyan coral reefs

This study examined trends along a gradient of fishing intensity in an artisanal coral reef fishery over a 10-year period along 75 km of Kenya's most populated coastline. As predicted by Malthusian scenarios, catch per unit effort (CPUE), mean trophic level, the functional diversity of fished taxa, and the diversity of gear declined, while total annual catch and catch variability increased along the fishing pressure gradient. The fishery was able to sustain high (approximately 16 Mg x km(-2) x yr(-1)) but variable yields at high fishing pressure due to the dominance of a few productive herbivorous fish species in the catch. The effect of two separate management strategies to overcome this Malthusian pattern was investigated: fisheries area closure and elimination of the dominant and most "competitive" gear. We found that sites within 5 km of the enforced closure showed significantly lower total catch and CPUE, but increased yield stability and trophic level of catch than predicted by regression models normalized for fishing effort. Sites that had excluded illegal beach seine use through active gear management exhibited increased total catch and CPUE. There was a strong interaction between closure and gear management, which indicates that, for closures to be effective at increasing catch, there must be simultaneous efforts at gear management around the periphery of the closures. We propose that Malthusian effects are responsible for the variation in gear and catch and that active management through reduced effort and reductions in the most competitive gear have the greatest potential to increase the functional and trophic diversity and per-person productivity.     

The relationships between mercury and selenium in plankton and fish from a tropical food web

Background, aim, and scope  Selenium (Se) has been shown to reduce mercury (Hg) bioavailability and trophic transfer in aquatic ecosystems. The study of methylmercury (MeHg) and Se bioaccumulation by plankton is therefore of great significance in order to obtain a better understanding of the estuarine processes concerning Hg and Se accumulation and biomagnification throughout the food web. In the western South Atlantic, few studies have documented trace element and MeHg in fish tissues. No previous study about trace elements and MeHg in plankton has been conducted concerning tropical marine food webs. Se, Hg, and MeHg were determined in two size classes of plankton, microplankton (70–290 μm) and mesoplankton (≥290 μm), and also in muscle tissues and livers of four fish species of different trophic levels (Mugil liza, a planktivorous fish; Bagre spp., an omnivorous fish; Micropogonias furnieri, a benthic carnivorous fish; and Centropomus undecimalis, a pelagic carnivorous fish) from a polluted estuary in the Brazilian Southeast coast, Guanabara Bay. Biological and ecological factors such as body length, feeding habits, and trophic transfer were considered in order to outline the relationships between these two elements. The differences in trace element levels among the different trophic levels were investigated. Materials and methods  Fish were collected from July 2004 to August 2005 at Guanabara Bay. Plankton was collected from six locations within the bay in August 2005. Total mercury (THg) was determined by cold vapor atomic absorption spectrometry (CV-AAS) with sodium borohydride as a reducing agent. MeHg analysis was conducted by digesting samples with an alcoholic potassium hydroxide solution followed by dithizone-toluene extraction. MeHg was then identified and quantified in the toluene layer by gas chromatography with an electron capture detector (GC-ECD). Se was determined by AAS using graphite tube with Pin platform and Zeeman background correction. Results and discussion  Total mercury, MeHg, and Se increased with plankton size class. THg and Se values were below 2.0 and 4.8 μg g⁻¹ dry wt in microplankton and mesoplankton, respectively. A large excess of molar concentrations of Se in relation to THg was observed in both plankton size class and both fish tissues. Plankton presented the lowest concentrations of this element. In fish, the liver showed the highest THg and Se concentrations. THg and Se in muscle were higher in Centropomus undecimalis (3.4 and 25.5 nmol g⁻¹) than in Micropogonias furnieri (2.9 and 15.3 nmol g⁻¹), Bagre spp (1.3 and 3.4 nmol g⁻¹) and Mugil liza (0.3 and 5.1 nmol g⁻¹), respectively. The trophic transfer of THg and Se was observed between trophic levels from prey (considering microplankton and mesoplankton) to top predator (fish). The top predators in this ecosystem, Centropomus undecimalis and Micropogonias furnieri, presented similar MeHg concentrations in muscles and liver. Microplankton presented lower ratios of methylmercury to total mercury concentration (MeHg/THg) (34%) than those found in mesoplankton (69%) and in the muscle of planktivorous fish, Mugil liza (56%). The other fish species presented similar MeHg/THg in muscle tissue (of around 100%). M. liza showed lower MeHg/THg in the liver than C. undecimalis (35%), M. furnieri (31%) and Bagre spp. (22%). Significant positive linear relationships were observed between the molar concentrations of THg and Se in the muscle tissue of M. furnieri and M. liza. These fish species also showed significant inverse linear relationships between hepatic MeHg and Se, suggesting a strong antagonistic effect of Se on MeHg assimilation and accumulation. Conclusions  Differences found among the concentrations THg, MeHg, and Se in microplankton, mesozooplankton, and fishes were probably related to the preferred prey and bioavailability of these elements in the marine environment. The increasing concentration of MeHg and Se at successively higher trophic levels of the food web of Guanabara Bay corresponds to a transfer between trophic levels from the lower trophic level to the top-level predator, suggesting that MeHg and Se were biomagnified throughout the food web. Hg and Se were positively correlated with the fish standard length, suggesting that larger and older fish bioaccumulated more of these trace elements. THg, MeHg, and Se were a function of the plankton size. Recommendations and perspectives  There is a need to assess the role of selenium in mercury accumulation in tropical ecosystems. Without further studies of the speciation of selenium in livers of fishes from this region, the precise role of this element, if any, cannot be verified in positively affecting mercury accumulation. Further studies of this element in the study of marine species should include liver samples containing relatively high concentrations of mercury. A basin-wide survey of selenium in fishes is also recommended.     

提供给热带森林管理的地方知识:植物次生化学节律和月亮周期的关系

本研究运用满月时收获非木材林产品的当地实践如识,来证明控制叶片分解速率的化学物质随月亮周期而波动,并且这种现象可能已发展为一种植物一食草动物之间的相互关系.当地知识认为,满月期收获的叶片更为耐用.满月期收获的棕榈叶片具有较高的总碳量、半纤维素和碳化合物、而钙的浓度则较低.这些化学上的变化使棕榈叶在满月收获时对食草动物不太敏感且更为耐用.本研究提出了热带植物减少食叶功物食用及影响衰老叶片的分解速度和耐用性(尤其在满月期)的作用机制.本研究认为森林经营中有必要利用天然的生命周期,并为当地人的收获实践提供了科学依据.     

Habitat associations and diet of young-of-the-year Pacific cod (<Emphasis Type="Italic">Gadus macrocephalus</Emphasis>) near Kodiak,Alaska

The influence of environmental variables and habitat on growth and survival of juvenile gadoid species in the Atlantic has been clearly demonstrated; conversely, in the North Pacific little is known about the habitat and ecology of juvenile Pacific cod (Gadus macrocephalus Tilesius). The hypothesis that density of young-of-the-year (YOY) Pacific cod in nearshore habitats is predicted by shallow depth and the presence of eelgrass and macroalgae was tested in a variety of nearshore habitats adjacent to commercial fishing grounds near Kodiak Island, AK. From 10 to 22 August 2002, a beach seine and small-meshed beam trawl were used to capture YOY Pacific cod (n = 254) ranging from 42 to 110 mm fork length. Depth, water temperature, salinity, sediment grain size, and percent cover by emergent structure (i.e. tube-dwelling polychaetes, sea cucumber mounds, macroalgae) were measured prior to fishing. Density of YOY Pacific cod was highest in areas of moderate depth (15–20 m) and positively and linearly related to percent cover by sea cucumber mounds and to salinity. No previous studies have documented fish utilizing sea cucumber mounds as habitat. Furthermore, eelgrass and macroalgae were inconsequential to cod distribution. Diets consisted mainly of small calanoid copepods, mysids, and gammarid amphipods and were significantly related to cod length and depth. This work provides important information on previously undocumented factors that affect distribution and feeding of YOY Pacific cod, which ultimately influence growth and survival in this species.     

Spatial distribution of polybrominated diphenyl ethers and polybrominated biphenyls in lake trout from the Laurentian Great Lakes

Concentrations of two types of brominated flame-retardants (BFRs); polybrominated diphenyl ethers (PBDEs) and polybrominated biphenyls (PBBs) were determined in a single age class of lake trout (Salvelinus namaycush) collected from the Laurentian Great Lakes in 1997. Mean concentrations of total PBDE were highest in samples from Lake Ontario at 95+/-22 ng/g wet weight (ww) or 434+/-100 ng/g lipid weight (lw) while the lowest concentrations were observed in Lake Erie lake trout (27+/-8.6 ng/g ww, 117+/-37 ng/g lw). In all samples, the predominant PBDE congeners were 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), 2,2',4,4',5-pentabromodiphenyl ether (BDE-99), and 2,2',4,4', 6-pentabromodiphenyl ether (BDE-100), which are the primary components of the commonly used penta-BDE formulation flame retardant. Lake trout collected from Lake Huron had the highest concentrations of PBBs (3.1+/-1.7 ng/g ww, 15+/-8.5 ng/g lw), while the lowest levels were detected in fish from Lake Superior (0.25+/-0.13 ng/g ww, 1.7+/-0.89 ng/g lw). In all lake trout samples, 2,2',4,4',5,5'-hexabromobiphenyl (BB-153), a major constituent of the flame-retardant FireMaster BP-6, was the predominant PBB congener.     

Organochlorine residues in harbour porpoises from Southwest Greenland

During the 1995 hunting season, 75 harbour porpoises (Phocoena phocoena) were sampled in three locations in West Greenland: Maniitsoq, Nuuk, and Paamiut. Sex, age, morphometrics, reproductive condition, and organochlorine compound (OC) levels in blubber were determined for each individual. OC levels were extremely low and, therefore considered unlikely to affect the population adversely: mean blubber concentrations, expressed on lipid weight basis were 1.98 (S.D.=1.1) mg/kg for PCBs, 2.76 (S.D.=1.66) mg/kg for tDDT and 0.21 (S.D.=0.11) mg/kg for HCB. No statistical differences were observed among individuals caught in the various locations. OC concentrations showed statistically significant positive associations with age in males but negative in females; consequently, mature females presented lower pollutant loads than their male counterparts. Juveniles did not show differences between sexes. A higher proportion of less chlorinated and more metabolizable polychlorinated biphenyls (PCBs) compared to tPCBs was found in calves (age< or =1) than in mature females, indicating that the feeding habits of these two groups differ and that a greater transfer of less chlorinated compounds is passed from females to their pups through lactation and parturition. Harbour porpoises significantly contribute to the dietary intake of OCs by local Inuit populations. This contribution could be reduced if mature males were selectively avoided; however, current hunting procedures make this selection impracticable.     

The imprecision in rates of down syndrome by 1-year maternal age intervals: A critical analysis of rates used in biochemical screening

Current algorithms to determine eligibility for prenatal cytogenetic diagnostic services depend critically on the accuracy and precision of the underlying rates of cytogenetic abnormality used in the calculations. We examine the maternal age-specific rates of Down syndrome livebirths in eight studies of European-origin populations, pooled rates from which are widely used for baseline calculations in biochemical screening. These studies vary significantly in such factors as methods of ascertainment of cases, likelihood of complete ascertainment, and methods of correction, if any, for underascertainment. Restriction of analysis to those two studies among the eight whose methods suggest the greatest likelihood of complete ascertainment for Down syndrome generates rates significantly higher than those in widespread use. Confidence intervals about previously reported and currently derived rates indicate that even with large-scale data, there is considerable residual uncertainty in derived rates.