Changes In Trophic Level Research Articles

Effects of the 2013-2016 warm anomalies on the California Current phytoplankton

Anomalies occurring in the California Current System during 2013–2016 were examined using remote sensing data and in-situ measurements. Positive anomalies in sea surface temperature (SST, up to 4°C) and absolute dynamic topography (ADT, up to 20cm), as well as negative anomalies in satellite chlorophyll concentration (CHL, up to -3.0mgm-3) were recorded. In-situ temperature anomalies in August 2014 increased equatorward, from 2°C along the northernmost line (CalCOFI hydrographic line 76.7) to 10°C (below ~80m depth) at the southernmost hydrographic line (off Baja California). In-situ negative Chlorophyll-a (Chl-a) surface anomalies were similar in most of the lines examined (~ -1.5mgm-3), but for the CalCOFI lines, larger anomalies (~ -3.5mgm-3) were recorded. As a result of increased stratification, mirrored in the increase in mean monthly ADT and the upper water-column temperature, the 2003–2016 CHL time series showed a declining trend in phytoplankton production and biomass, which was particularly pronounced during the last four years in the central and southern zones of the California Current System. The mechanisms responsible for this trend seem to be related to a climate shift from cold to warm in the northeastern Pacific Ocean, as evidenced by large correlations between the CHL anomalies and the Pacific Decadal Oscillation, Multivariate ENSO Index, and Oceanic El Niño indices. Generalized additive models were used to explore the relationship between stratification and the decline in phytoplankton production. Two ADT ranges—characteristic of either highly dynamic or stratified environments—were defined, both of which led to a drop in primary production. The effects of these environments have been associated with the photo acclimation of phytoplankton to irradiance and nutrient availability regimes, by which primary production and biomass decrease due to the limitation of either of these factors. This study reveals some of the effects of the warm anomalies occurring from 2013 to 2016 in the California Current ecosystem, which will help to understand changes in higher trophic levels in this region.

Influence of timing of fishing on trophic levels and diets of typical fish and invertebrate species in the Bohai Strait over a single year based on carbon and nitrogen isotope analysis

Fishing is the most widespread human exploitation of marine resources, which has an annual cyclical influence on aquatic species in Chinese offshore waters. This study used carbon and nitrogen isotopic ratios as tracers to reveal the changes in trophic level and dietary composition of offshore organisms during four cruises in March, June, August and November 2014. The results indicated that the trophic levels of fishes declined during two fishing periods, from March (average trophic level=3.36) to June (3.01), and from August (2.99) to November (2.57), while most invertebrates did not show this trend. The self-restoring ability of this ecosystem was reflected in the trophic level changes after the closed fishing season (from June 1 to September 1). The trophic levels of fishes remained stable, and some species even recovered such as Enchelyopus elongates (trophic level increased from 2.84 in June to 2.86 in August), Cryptocentrus filifer (from 3.10 to 3.12), and Ernogrammus hexagrammus (from 2.91 to 2.96). According to the trophic results, we selected the invertebrates Octopus minor and Asterias amurensis from the top trophic levels for dietary composition analysis. The composition of their diets changed significantly after fishing periods, and the proportions of some smaller and “non-commercial” species increased, such as Notoacmea schrenckii and Chlorostoma rustica. After the closed fishing season, the larger and “commercial” species contributed a greater proportion to their diet composition. These results indicated that the closed fishing season should be prolonged to give the ecosystem enough time to restore itself and further halt the trend of this fishery towards environment deterioration.

Onset and demise of Cretaceous oceanic anoxic events: The coupling of surface and bottom oceanic processes in two pelagic basins of the western Tethys

AbstractThe upper Albian–lower Turonian pelagic successions of the Tethys record processes acting during the onset, core, and recovery from perturbed conditions across oceanic anoxic event (OAE) 1d, OAE 2, and the mid‐Cenomanian event I (MCE I) relative to intervening intervals. Five sections from Umbria‐Marche and Belluno Basins (Italy) were analyzed at high resolution to assess processes in surface and deep waters. Recurrent facies stacking patterns (SP) and their associations record periods of bottom current activity coupled with surface changes in trophic level. Climate changes appear to have been influential on deep circulation dynamics. Under greenhouse conditions, vigorous bottom currents were arguably induced by warm and dense saline deep waters originated on tropical shelves in the Tethys and/or proto‐Atlantic Ocean. Tractive facies postdating intermittent anoxia during OAE 1d and in the interval bracketed by MCE I and OAE 2 are indicative of feeble bottom currents, though capable of disrupting stratification and replenish deep water with oxygen. The major warming at the onset of OAE 2 might have enhanced the formation of warm salty waters, possibly producing local hiatuses at the base of the Bonarelli Level and winnowing at the seafloor. Hiatuses detected at the top of the Bonarelli Level possibly resulted from most effective bottom currents during the early Turonian thermal maximum. Times of minimal sediment displacement correlate with cooler climatic conditions and testify a different mechanism of deep water formation, as further suggested by a color change to reddish lithologies of the post‐OAE 1d and post‐OAE 2 intervals.

Response of organic carbon burial to trophic level changes in a shallow eutrophic lake in SE China

Lakes are an important component of terrestrial carbon cycling. As the trend of eutrophication in many lakes continues, the mechanisms of organic carbon (OC) burial remain unclear. This paper aims to understand the distribution of OC and the effect of trophic level changes on OC burial in Chaohu Lake, a shallow eutrophic lake located in the lower reaches of the Yangtze River, SE China. Two hundred and one surface sediment samples (0–20cm) and 53 subsurface samples (150–200cm) from the lake were collected. The OC accumulation rates (OCARs) are relatively low, with an average of 10.01g/m2/year in the surface sediments. The spatial distribution of the OCARs is similar to that of allochthonous OC. The difference in total phosphate (TP) content between the surface and subsurface sediments (ΔTP) is significantly correlated with the autochthonous OC, suggesting that TP loading is a critical limiting nutrient for the lake's primary productivity. It is concluded that allochthonous OC is the dominant source of total OC in surface sediments compared to autochthonous OC. The primary productivity of Lake Chaohu increased due to increasing nutrient loading. However, the autochthonous OC contributed 11% of the total OC in the surface sediments. This could be ascribed to strong mineralization in the water column or surface sediments.

Trophic Magnification of Organic Chemicals: A Global Synthesis.

Production of organic chemicals (OCs) is increasing exponentially, and some OCs biomagnify through food webs to potentially toxic levels. Biomagnification under field conditions is best described by trophic magnification factors (TMFs; per trophic level change in log-concentration of a chemical) which have been measured for more than two decades. Syntheses of TMF behavior relative to chemical traits and ecosystem properties are lacking. We analyzed >1500 TMFs to identify OCs predisposed to biomagnify and to assess ecosystem vulnerability. The highest TMFs were for OCs that are slowly metabolized by animals (metabolic rate kM < 0.01 day(-1)) and are moderately hydrophobic (log KOW 6-8). TMFs were more variable in marine than freshwaters, unrelated to latitude, and highest in food webs containing endotherms. We modeled the probability that any OC would biomagnify as a combined function of KOW and kM. Probability is greatest (∼100%) for slowly metabolized compounds, regardless of KOW, and lowest for chemicals with rapid transformation rates (kM > 0.2 day(-1)). This probabilistic model provides a new global tool for screening existing and new OCs for their biomagnification potential.

Misapplied survey data and model uncertainty result in incorrect conclusions about the role of predation on alewife population dynamics in Lake Huron: a comment on

Drastic recent and ongoing changes to fish populations and food webs in the Great Lakes have been well-described (Riley et al. 2008; Barbiero et al. 2009; Nalepa et al. 2009; Fahnenstiel et al. 2010; Evans et al. 2011; Gobin et al. 2015), and uncertainty regarding their potential effects on fisheries has caused concern among scientists and fishery managers (e.g., Dettmers et al. 2012). In particular, the relative importance of “bottom-up” (e.g., lower trophic level changes) versus “top-down” (e.g., predation) factors to fish community changes in the Great Lakes have been widely debated (e.g., Barbiero et al. 2011; Eshenroder and Lantry 2012; Bunnell et al. 2014). In Lake Huron, recent ecosystem changes have been particularly profound, and populations of alewife (Alosa pseudoharengus), an offshore pelagic prey fish, collapsed in 2003 and have yet to recover (Riley et al. 2008, 2014). He et al. (2015) recently used a series of linked ecologicalmodels to assess the role of predation in the dynamics of the offshore prey fish community in Lake Huron. While we believe that they provide a novel method for combining bioenergetics and stock assessment modeling, we question the validity of their conclusions because of the misapplication of survey data and the lack of critical interpretation of theirmodeling efforts. Here we describe howHe et al. (2015) have misapplied bottom trawl data from Lake Huron, and we provide examples of how this has resulted in erroneous conclusions regarding the importance of predation to the population dynamics and collapse of alewife in Lake Huron.

Isotopic variation in five species of stream fishes under the influence of different land uses.

The aim of this study was to test if changes in land use alter the isotopic signature of fish species, promoting changes in the trophic position and food resource partitioning between these consumers. Three different systems were investigated: pasture streams (n = 3), streams in sugar cane plantations (n = 3) and reference streams (n = 3). Fish species Aspidoras fuscoguttatus, Astyanax altiparanae, Characidium zebra, Hisonotus piracanjuba and Knodus moenkhausii were selected, and their nitrogen and carbon isotopic compositions were estimated to assess changes in the trophic level and partitioning of food items consumed. The composition of δ(13) C (‰) only differed among the land use categories for A. altiparanae, H. piracanjuba and K. moenkhausii. Resource partitioning was different for all species, with changes in the sources or proportions they consumed in each land use category, but only A. altiparanae introduced new food sources in large quantity in altered land uses. It is important to note, however, that the results from the resource partitioning analysis are limited due to large overlapping of isotopic signatures between the analysed food resources. All fish species exhibited variation in δ(15) N (‰), with the highest values found in streams under sugar cane or pasture influence. Despite the variation in nitrogen isotopic values, only C. zebra and H. piracanjuba displayed changes in trophic level. Therefore, it is believed that the increase in the δ(15) N (‰) value of the individuals collected in streams under the influence of sugar cane or pasture was due to the greater influence of livestock dung and chemical and organic fertilizers. The results also highlight the importance of studying consumer species along with all forms of resources available at each location separately, because the signatures of these resources also vary within different land uses.

Phytoplankton response to the environmental and climatic variability in a temperate lake over the last 14,500 years in eastern Latvia

Phytoplankton species are the primary producers in lakes and play important roles in food-web structures. Any shift in their diversity and productivity has an impact on other aquatic life forms. We use a range of environmental variables to explore the possible drivers influencing phytoplankton composition over the last 14,500 years in a temperate lake Lielais Svētiņu, eastern Latvia. Using pollen, non-pollen palynomorphs, temperature reconstructions and lithological information as proxies of environmental factors, we statistically test their associations with the fossil phytoplankton community composition. Our results reveal that during the Late Glacial, the climate warming, the decrease in landscape openness, and increase in organic matter were significant environmental variables affecting dynamics of phytoplankton communities , especially in the prevalence of Botryococcus, Tetraedron, Scenedesmus and Pediastrum. According to the Redundancy Analysis and Generalized Least Squares models, Pediastrum, Scenedesmus and Tetraedron were positively associated with waterlogging tolerance that indicates moist soils in surroundings of the lake, during the Early Holocene. The 8.2 ka cold event with a 2–3 °C cooling led to a strong environmental disturbance for nearly 700 years, indicated by an increased chlorophyta accumulation rates and a decrease in the organic matter. Our results indicate that Coelastrum reticulatum and C. polychordum are characteristic for the 8.2 ka cold event. Positive association between cyanobacteria and mean air summer temperature suggests that a warming favoured cyanobacteria over other phytoplankton taxa between 8000 and 4000 cal yr BP. High nutrient loads and water turbidity were more important for the dynamics of cyanobacteria from 4000 to 2000 cal yr BP. Human-driven trophic level change was recorded in the last 2000 years by abundances of fungi Sporormiella and Sordaria, and by the peaks of Gloeotrichia pisum, C. reticulatum and C. polychordum indicating eutrophication.

Telomere length is a strong predictor of foraging behavior in a long‐lived seabird

Telomeres are an increasingly studied component of physiological ecology. However, in long‐lived birds a large telomere loss with chronological age is not the norm. Telomeres are now regarded less as a chronological aging tool and more as an indicator of individual quality, residual lifespan, or biological age. If telomeres indicate biological aging processes, then they should also be associated with other variables that change with age, especially foraging and reproductive behaviors. This study compared telomere length to a suite of foraging parameters in Thick‐billed Murres breeding on three colonies in the Bering Sea. Telomere length, environmental conditions at colonies, and sex played pivotal roles in determining foraging habitat selection. Spatial habitat use, foraging efficiency, and prey selection variables all changed with telomere length. The behavioral evidence indicates that despite losing telomeres, birds with short telomere length retain their ability to use the environment efficiently. This indicates that aging birds remain behaviorally flexible, despite paying physiological costs. Changes in spatial use were largely sex‐dependent: females and males differed in their use of the environment as telomere lengths declined. Prey selection was related to telomere length and colony; changes in murre trophic level depended on telomere length, but their direction also depended on habitat quality. We found much support for the continued able functioning of birds with shorter telomeres, indicating that physiological aging does not carry only costs. Murres appear to modify their behavior depending on environmental conditions as their physiological reserves decline.

SYMPTOMS OF WATER EUTROPHICATION IN THE BACHMATY RESERVOIR

When analyzing the symptoms of eutrophication and water quality in a small retention reservoir Bachmaty, an attempt to assess changes in its trophic condition before and after modernization, was undertaken. The study upon changes in water trophic level was carried out on a small retention reservoir Bachmaty located in Podlasie region on the Orlanka river in the municipality of Dubicze Cerkiewne. Tests of water samples collected from the surface layer of the coastal zone were performed once a month, and the time of the research was divided into two periods. The first period was conducted since April 2007 to March 2008 and since April 2009 to March 2010. The second research period included the time immediately after the reservoir reclamation, which ended in May 2011. The study was conducted since May 2011 to March 2012 and since April 2013 to March 2014. The trophic level of Bachmaty reservoir was also assessed according to the concentration criteria and based on the trophic status indices (TSI) calculated after Carlson’s as well as Kratzer and Brezonik’s. Reclamation of the reservoir, which consisted mainly in removing the sediments, caused a reduction in all TSI values. Analysing the overall trophic level index, i.e. the average of TSI (TP), TSI (TN), and TSI (Chl), water of the Bachmaty reservoir may be classified as eutrophic in individual years of the research, although a gradual decline in the TSI value could be observed directly after the reservoir modernizing. After testing it was found that the formation of the trophic level of Bachmaty reservoir was largely affected by the amount of total phosphorus supplied to the water. Water quality in the reservoirs, and also the content of phosphorus compounds is largely determined by external supply.

Shifts in microphytoplankton species and cell size at Admiralty Bay, Antarctica

AbstractPhytoplankton (&gt;15 µm) was investigated in three shallow coastal areas at Admiralty Bay (AB) between the summers of 2002–03 and 2008–09. Phytoplankton abundance was low (103cells l-1) and, over time, the prevailing cell size decreased due to a shift in phytoplankton dominant species from diatoms to dinoflagellates.In situand remote sensing data showed that oscillations in sea surface temperature, precipitation, ice formation/melting, irradiance (cloud cover) and bottom circulation (indexed by the Antarctic Oscillation Index; AAO) were shown to govern the structure of the phytoplankton. Under negative AAO, diatoms prevailed, with the dominance of large (&gt;80 µm) benthic diatoms (e.g.Corethron pennatumandNavicula directa) in periods of low production (102cells l-1in 2002–03), and medium-sized (31–80 µm) centrics (e.g.Thalassiosiraspp. andStellarima microtrias) when the abundance was higher (104cells l-1in 2003–04). Conversely, positive AAO led to the co-dominance of dinoflagellates and planktonic diatoms (e.g.Pseudo-nitzschiaspp.) in the summers of 2007–08 and 2008–09. These results suggest that the AAO can be a good predictor of phytoplankton in coastal areas around the western Antarctic Peninsula, and may help our understanding of changes in other trophic levels of the food web.

Region-based MTI: resolving geographic expansion in the Marine Trophic Index

The Marine Trophic Index (MTI), which tracks the mean trophic level of fishery catches from an ecosystem, generally, but not always, tracks changes in mean trophic level of an ensemble of exploited species in response to fishing pressure. However, one of the disadvantages of this indicator is that declines in trophic level can be masked by geographic expansion and/or the development of offshore fisheries, where higher trophic levels of newly accessed resources can overwhelm fishing-down effects closer inshore. Here, we show that the MTI should not be used without accounting for changes in the spatial and bathymetric reach of the fishing fleet, and we develop a new index that accounts for the potential geographic expansion of fisheries, called the region-based MTI (RMTI). To calculate the RMTI, the potential catch that can be obtained given the observed trophic structure of the actual catch is used to assess the fisheries in an initial (usu- ally coastal) region. When the actual catch exceeds the potential catch, this is indicative of a new fishing region being exploited. The MTI of the new region can then be calculated, and subsequent regions are determined in a sequential manner. This method improves upon the use of the Fish- ing-in-Balance (FiB) index in conjunction with the original MTI calculated over the whole time series because assumptions of fleet and stock stationarity over the entire time series and geo- graphic area are removed. We illustrate the utility of the RMTI with simulated examples and actual data, and generalize these results.

Assessing the vulnerability of the marine bird community in the western North Sea to climate change and other anthropogenic impacts

Ocean warming and anthropogenic activities such as fishing, shipping and marine renewable developments are affecting marine top predators. Research has focussed on the impacts of single stressors on single species, yet understanding cumulative effects of multiple stressors on communities is vital for effective conservation management. We studied a marine bird community (45 species; 11 families) that used the Forth and Tay region of the North Sea for breeding, overwintering or migration between 1980 and 2011. Local sea surface temperature (SST) increased significantly over this period, with concomitant changes in lower trophic levels. Simultaneously, the region has been subject to fishing pressure and shipping disturbance and is a priority area for renewable energy developments. We used colony-based and at-sea data to quantitatively assess relationships between SST and counts, productivity and survival of 25 species for which sufficient data were available for analysis. For the remaining species, we applied a qualitative approach using published population trends, published climate relationships and foraging sensitivity. In total, 53% of species showed negative relationships with SST. Trends in counts and demography were combined with climate vulnerability to give an index of population concern to future climate warming, and 44% of species were classified as high or very high concern, notably cormorants, grebes, skuas, shearwaters, terns and auks, as well as species breeding in the region. Qualitative assessments of vulnerability to fisheries, pollutants, disturbance (including introduced predators), marine renewables and climate found that 93% of species were vulnerable to ≥2 threats, and 58% to ≥4. Our results indicate that the majority of birds in this region of the North Sea face an uncertain future, potentially threatening the resilience of this important marine bird community.

Spatial, ontogenetic and interspecific variability in stable isotope ratios of nitrogen and carbon of Merluccius capensis and Merluccius paradoxus off South Africa.

General linear models (GLMs) were used to determine the relative importance of interspecific, ontogenetic and spatial effects in explaining variability in stable isotope ratios of nitrogen (δ(15) N) and carbon (δ(13) C) of the co-occurring Cape hakes Merluccius capensis and Merluccius paradoxus off South Africa. Significant GLMs were derived for both isotopes, explaining 74 and 56% of observed variance in Merluccius spp. δ(15) N and δ(13) C, respectively. Spatial effects (west or south coast) contributed most towards explaining variability in the δ(15) N model, with Merluccius spp. off the west coast having higher (by c. 1.4‰) δ(15) N levels than Merluccius spp. off the south coast. Fish size and species were also significant in explaining variability in δ(15) N, with both species showing significant linear increases in δ(15) N with size and M. capensis having higher (by c. 0.7‰) δ(15) N values than M. paradoxus. Species and coast explained most and similar amounts of variability in the δ(13) C model, with M. capensis having higher (by c. 0.8‰) δ(13) C values than M. paradoxus, and values being lower (by c. 0.7‰) for fishes off the west coast compared with the south coast. These results not only corroborate the knowledge of Merluccius spp. feeding ecology gained from dietary studies, in particular the ontogenetic change in trophic level corresponding to a changing diet, but also that M. capensis feeds at a slightly higher trophic level than M. paradoxus. The spatial difference in Merluccius spp. δ(15) N appears due to a difference in isotopic baseline, and not as a result of Merluccius spp. feeding higher in the food web off the west than the south coast, and provides new evidence that corroborates previous observations of biogeographic differences in isotopic baselines around the South African coast. This study also provides quantitative data on the relative trophic level and trophic width of Cape hakes over a large size range that can be used in ecosystem models of the southern Benguela.

Using stable isotope analysis to validate effective trophic levels from Ecopath models of areas closed and open to shrimp trawling in Core Sound, NC, USA

We used stable isotope ratios of nitrogen (15N/14N) as a validation tool for determining the accuracy of the effective trophic levels computed in network analysis models for Core Sound, North Carolina, USA. Ecological network models (using Ecopath) were assembled for two trawling management areas, closed areas where shrimp trawling has been restricted for 30 years, and open trawling areas within the estuary. Effective trophic levels (ETLs) were computed from the two Ecopath models and compared with δ15N values measured from the same species at the same time in the two areas. Comparisons of ETLs between species compartments of the two models were very similar, and ETLs were positively related to δ15N values. Linear regression of ETL as a predictor of δ15N value was highly significant (R2=0.50, p>0.00001), suggesting that δ15N values increase by 2.52‰ per trophic level. This increase in δ15N value agrees well with a recent global meta-analysis of nitrogen fractionation estimates. There was no significant difference in δ15N values between trawling zones over all species examined, although there were significantly higher δ15N values in the areas open to trawling for certain species of fish (pinfish, spot) and invertebrate (blue crabs). Atlantic menhaden, which feed on detritus and zooplankton, had much higher δ15N values than the Ecopath ETL and δ15N regression model predicted (large studentized residuals), suggesting that Ecopath incorrectly computes the trophic level of animals that consume dead material (in our model, detritus and shrimp trawl bycatch species). Shrimp trawl bycatch species are mostly pinfish, spot and blue crabs, with ETLs of 2.5 or higher, but are inappropriately assumed to be at trophic level 1 when dead in Ecopath. Despite this discrepancy for some species of detritivores and scavengers, we conclude that this Ecopath network model of Core Sound is reliable and predicts with good accuracy the shifts and changes in trophic level and diet as measured by stable isotopes and observed in diet studies. The Core Sound network model will be a useful approach when predicting impacts in the ecosystem due to changes in trawling or other fisheries management decisions.

Farming-up coastal fish assemblages through a massive aquaculture escape event

We investigated the changes on the mean trophic level of fish assemblages across different spatiotemporal scales, before and after a massive escape event occurred off La Palma (Canary Islands), which resulted in the release of 1.5 million fish (mostly Dicentrarchus labrax) into the wild. The presence of escaped fish altered significantly the mean trophic level of fish assemblages in shallow coastal waters. This alteration was exacerbated by the massive escape. A nearby marine protected area buffered the changes in mean trophic level but exhibited the same temporal patterns as highly fished areas. Moreover, escaped fish exploited natural resources according to their total length and possibly, time since escapement. New concerns arise as a “farming up” process is detected in shallow coastal fish assemblages where marine aquaculture is established.

Regime shifts of Cruces River wetland ecosystem: current conditions, future uncertainties

During April-May 2004 the Cruces River wetland ecosystem, located in Valdivia (40oS), southern Chile, was described as shifting from a clear water regime, dominated by the exotic macrophyte Egeria densa, to turbid waters and absence of submerged macrophytes. We analyzed the trophic status and ecological regime of the wetland from November 2011 through January 2013. The trophic status was determined comparing values of selected variables (nutrients, chlorophyll-a and transparency) with OECD criteria. The ecological regime was determined comparing the same variables with the criteria proposed by Ibelings et al. (2007). We further compared the concentration of nutrients and suspended solids with previous measurements. Current trophic status of the wetland is between eutrophic and hypereutrophic, as shown by results, and its ecological regime intermediate, between clear and turbid waters, with a considerable risk of returning to turbid waters. In this article we discuss the potential relationship between the watershed land use (agriculture, cattle feeding and forestry), the change in wetland's trophic level and future conditions.

Historical changes in mean trophic level of southern Australian fisheries

Decreases in the mean trophic level (MTL) of fishery catches have been used to infer reductions in the abundance of high trophic level species caused by fishing pressure. Previous assessments of southern Australian fisheries have been inconclusive. The objectives of the present study were to provide more accurate estimates of MTL using disaggregated taxonomic and spatial data. We applied the model of MTL to fisheries catch statistics for the state of South Australia from 1951 to 2010 and a novel set of historical market data from 1936 to 1946. Results show that from 1951 to 2010, MTL declined by 0.16 of a trophic level per decade; a rate greater than the global average of 0.10 but equivalent to similar regional investigations in other areas. This change is mainly attributable to large increases in catches of sardine, rather than reductions in the catches of high trophic level species. The pattern is maintained when the historical data is included, providing a time line from 1936 to 2010. Our results show a broadening of the catch of lower trophic levels and suggest care in interpretation of MTL of catches because reductions do not necessarily reflect change in high trophic level species by fishing pressure.

Variation in feeding ecology within the fish community in the north Yellow Sea

Our objective was to evaluate ecosystem status before and after fishery enhancement.We analyzed the feeding habits,food consumption,and trophic level of the species constituting 90% of total biomass in bottom trawl surveys conducted in May and August of 2010.We compared the traits of the five dominant species(by total biomass) with those collected in 1985 and 2000.In addition,we evaluated decadal-scale variation in the mean TL of the fish community in the north Yellow Sea. The fish community in the north Yellow Sea was dominated by nine species,though these differed between May and August.The most dominant species in May included small yellow croaker Pseudosciaena polyactis,elongate eelpout Enchelyopus elongates,half-fin anchovy Setipinna taty,angler Lophius litulon,pointhead plaice Cleisthenes herzensteini,Pacific cod Gadus macrocephalus,and sea perch Lateolabrax japonicus.Conversely,in August the fishery was dominated by anchovy Engraulis japonicus,sea snail Liparis tanakae,small yellow croaker,elongate eelpout,and Pacific cod.Thus,the assemblage in May was characterized by a lack of nektivores,a decrease in the proportion of generalist predators,and an increase in the proportion of zooplanktivores relative to August.The mean trophic level of the fish community was 3.75 in May and 3.57 in August. The primary prey groups included euphausiids,copepods,hyperiid amphipods,crustacea larva,shrimps,ophiuroidea,bivalvia,polychaeta,and other fish species.We estimated the fish community consumed 0.5 million tons and 19.6 million tons of these prey items in May and August,respectively.Fish and shrimp were the dominant prey groups in May,accounting for 46.9% and 39.2% of the total food consumption,respectively.Similarly,fish,shrimp,and zooplankton were the dominant prey groups in August,accounting for 33.9%,31.4%,and 30.8% of the total food consumed,respectively.The primary prey species were Crangon affinis,Eualus sinensis,Euphausia pacifica,small yellow croaker,Johnius belengerii,and Ophiura sarsii in May.Consumption of these species equaled 0.3 million tons,accounting for 65% of the total food consumption.The primary prey species in August were C.affinis,E.sinensis,Themisto gracilipes,Calanus sinicus,and anchovy.Consumption of these species equaled 12.4 million tons,accounting for 62.9% of the total food consumption.Thus,as in the central and southern Yellow Sea,T.gracilipes,C.sinicus,C.affinis,E.sinensis,small yellow croaker,and anchovy were the most im-portant prey species in the north Yellow Sea. The mean trophic level of fish community declined from 4.26 in 1985 to 3.66 in 2010 in the north Yellow Sea,or 0.24 decade?1.This decrease was more rapid than in the Bohai Sea and central and southern Yellow Sea and was caused by a shift in the dominant species in the fish community.The percentage of nektivores decreased concomi-tantly with an increase in generalist predators between 1985 to 2010.The rate of change in trophic level has de-creased from 0.28 decade?1 between 1985 and 2000 to 0.18 decade-1 between 2000 and 2010,but has remained higher than the global rate.Given this,we recommend continuing efforts to enhance the fishery.

Interpreting variation in fish-based food web indicators: the importance of “bottom-up limitation” and “top-down control” processes

AbstractReilly, T. O. M., Fraser, H. M., Fryer, R. J., Clarke, J., and Greenstreet, S. P. R. 2014. Interpreting variation in fish-based food web indicators: the importance of “bottom-up limitation” and “top-down control” processes. – ICES Journal of Marine Science, 71: 406–416. Proposed indicators for the Marine Strategy Framework Directive (MSFD) food webs Descriptor focus on structural elements of food webs, and in particular on the abundance and productivity of top predators. However, the inferences that can be drawn from such indicators depend on whether or not the predators are “bottom-up limited” by the availability of their prey. Many seabird populations appear to be “bottom-up limited” so that variation in their reproductive success and/or abundance reflects changes in lower trophic levels. Here we find that gadoid fish predators off the Firth of Forth, southeast Scotland, do not appear to be “bottom-up limited” by the biomass of their main prey, 0-group sandeels; gadoid biomass and feeding performance was independent of sandeel biomass. Variability in food web indicators based on these gadoid predators seems to impart little insight into underlying processes occurring at lower trophic levels in the local food web. The implications of this in terms of how the currently proposed MSFD food web indicators should be used and interpreted are considered, and the ramifications in terms of setting targets representing good environmental status for both fish and seabird communities are discussed.