Recent trends in the abundance of plaice Pleuronectes platessa and cod Gadus morhua in shallow coastal waters of the Northeastern Atlantic continental shelf – a review

Shallow, near-shore water habitats on the continental shelf of the Northeast Atlantic have been productive fishing areas in the past. Here, we review the present knowledge about (i) recent trends in the abundance of plaice and cod in these habitats and (ii) hypotheses regarding the factors responsible for any trends. At present, only a few studies exist on the trends of abundance of plaice or cod, namely from the Bay of Biscay, the North Sea and the Skagerrak/Kattegat. They suggest a declining abundance in coastal, shallow areas and – at least for plaice – a latitudinal gradient with an erosion of the southern distribution boundary in the Bay of Biscay and deepening of stocks in the North Sea. In contrast, no trend in shallow water abundance of plaice similar to a decline in deep-water stocks during the 1970s and their slow recovery during the 2000s is apparent in the Skagerrak/Kattegat. Although shallow habitats fundamentally differ from deeper areas by the prevalence of juvenile stages, the declining trends coincide with decreasing abundance/landings and spatial stock relocations in the deeper areas. Whether this indicates a common trend pointing at connectivity between shallow and deep water remains open. Fundamental differences exist in the suggested causes of the trends in different geographical areas. High fishing pressure together with low local recruitment apparently prevents the recovery of overexploited plaice and cod stocks in the Skagerrak/Kattegat. In contrast, the responses of juveniles and adult fish to increasing seawater temperature are the main hypotheses for changes in distribution and abundance of both fish species in the North Sea/Bay of Biscay. However, temperature alone cannot explain the observed decline of fish in coastal areas, and the causes may be more complex, involving nutrient loading, primary productivity or food availability, although at present, knowledge of these factors is insufficient.     

Diet composition and biomass consumption of harbour seals in Telemark and Aust-Agder,Norwegian Skagerrak

ABSTRACT

To explore ecosystem dynamics and functions it is vital to obtain knowledge on predator–prey relations. Harbour seals are piscivorous predators that can come into conflict with fisheries. Recently, as the Skagerrak and Kattegat population of harbour seals has increased, claims have emerged that seals are depleting coastal cod populations. The diet of harbour seals in Norwegian Skagerrak was investigated based on otolith identification from scats. The overall seal diet included 20 different fish species/groups. The most important prey (combined index Q _i ) were haddock/pollack/saithe (32.7%), genus Trisopterus (Norway pout/poor cod/bib, 12.5%), plaice (12.4%) and herring (10.0%). Plaice also had the largest biomass (24.1%). Gadoids and pleuronectids comprised 88.6% of the diet (combined index Q _i ) and 87.1% in terms of biomass. Cod constituted 0.7% (combined index Q _i ) of the overall diet and 2.3% in terms of biomass. Fish length estimates showed that seals generally prefer small fish below minimum allowed landing size. Estimated total amount of fish consumed was 315 tons per year and was dominated by non-commercial species. Annual cod consumption was an estimated 7.1 tons, representing 5% of annual cod landings, suggesting that competition between local fisheries and harbour seals is limited.     

Effect of spatial differences in growth on distribution of seasonally co‐occurring herring Clupea harengus stocks

The mechanisms most likely to determine the distribution of the two major herring Clupea harengus stocks in their common early summer feeding ground in the eastern North Sea, Skagerrak and Kattegat were investigated through analysis of acoustic survey data from six consecutive years. No change was detected in biomass of North Sea autumn spawning C. harengus (NSAS) over time, whereas the biomass of western Baltic spring spawning C. harengus (WBSS) declined severely. Analyses of centre of abundance by stock showed no change in NSAS distribution, whereas the WBSS changed to a more western distribution over time. Contrary to previous perception of the juvenile migration, NSAS were found to leave the study area at the age between 1 and 2 years and WBSS 1 year olds were encountered in the Skagerrak. The estimated parameters of von Bertalanffy growth equations showed marked differences between areas with fish in the eastern part of the area having the lowest size at age at all ages. Further, their growth conditions appeared to deteriorate progressively over the period studied. Both NSAS and WBSS showed the highest condition in the North Sea and Skagerrak while condition was substantially lower in age Kattegat. The westward movement of spring spawners over time suggests that growth rate and possibly density of conspecifics influence the migration pattern and distribution of C. harengus in the area. In contrast, there was no evidence to suggest that distribution was constant over time within stocks or that distribution reflected size‐dependent limitations on migration distance.     

TaqMan DNA technology confirms likely overestimation of cod (Gadus morhua L.) egg abundance in the Irish Sea: implications for the assessment of the cod stock and mapping of spawning areas using egg-based methods

Recent substantial declines in northeastern Atlantic cod stocks necessitate improved biological knowledge and the development of techniques to complement standard stock assessment methods (which largely depend on accurate commercial catch data). In 2003, an ichthyoplankton survey was undertaken in the Irish Sea and subsamples of 'cod-like' eggs were analysed using a TaqMan multiplex, PCR (polymerase chain reaction) assay (with specific probes for cod, haddock and whiting). The TaqMan method was readily applied to the large number of samples (n = 2770) generated during the survey and when combined with a manual DNA extraction protocol had a low failure rate of 6%. Of the early stage 'cod-like' eggs (1.2-1.75 mm diameter) positively identified: 34% were cod, 8% haddock and 58% whiting. As previous stock estimates based on egg surveys for Irish Sea cod assumed that the majority of 'cod-like' eggs were from cod, the TaqMan results confirm that there was probably substantial contamination by eggs of whiting and haddock that would have inflated estimates of the stock biomass.     

Predation on northern shrimp (Pandalus borealis) by three gadoid species

Northern shrimp (Pandalus borealis) is targeted by commercial fisheries, but is also an important prey in the marine food web. In this study, stomach content data were used to study predation on shrimp by three gadoid species, cod (Gadus morhua), haddock (Melanogrammus aeglefinus) and whiting (Merlangius merlangus), in six inshore areas around Iceland. The results showed that shrimp was more important in the diet of cod compared with the other two predators. However, the overall predation pressure was similar for cod and haddock due to the high abundance of haddock. Therefore, even though shrimp is not the primary source of food for haddock, this species can have a substantial effect on shrimp stocks when haddock abundance is high. In addition, while cod and haddock did not select for any particular size of shrimp, whiting seemed to preferentially target juvenile shrimp. The results suggest that due to the overall effects of these three predators on shrimp stocks, gadoids need to be included in the management of shrimp stocks as predation is one of the major drivers in the development of this important prey stock.     

Synchronous patterns of fluctuations in two stocks of anchovy Engraulis ringens Jenyns, 1842 in the Humboldt Current System

Most of the studies investigating synchrony in fluctuations of abundance of small pelagic fish have been based on catch data only, which do not describe the dynamics of populations as a relative abundance index. In this paper, catch, biomass, recruitment and recruitment rate, were used to compare synchronous changes for two stocks of anchovy (Engraulis ringens) from 1982 to 2004. One is the North Central Peru stock (NCP) and the other is the shared South Peru‐Northern Chile stock (SPNC). Correlation analysis demonstrated a significant association between population time series, particularly during the growing phase of the stocks. Thus, the synchronous fluctuation pattern of the two stocks is due to the recovery phase and probably driven by density‐independent effects of simultaneous favorable environmental conditions occurring in the two regions. The conclusions were: (i) both NCP and SPNC anchovy stocks are in an overall positive phase of synchrony, (ii) higher correlations in the synchronous pattern of fluctuations occurred during simultaneous increase of biomass, and (iii) short‐term fluctuations were negative when the abundance of the stock was lower and/or impacted by El Niño events.     

Overfishing and the Replacement of Demersal Finfish by Shellfish: An Example from the English Channel

The worldwide depletion of major fish stocks through intensive industrial fishing is thought to have profoundly altered the trophic structure of marine ecosystems. Here we assess changes in the trophic structure of the English Channel marine ecosystem using a 90-year time-series (1920–2010) of commercial fishery landings. Our analysis was based on estimates of the mean trophic level (mTL) of annual landings and the Fishing-in-Balance index (FiB). Food webs of the Channel ecosystem have been altered, as shown by a significant decline in the mTL of fishery landings whilst increases in the FiB index suggest increased fishing effort and fishery expansion. Large, high trophic level species (e.g. spurdog, cod, ling) have been increasingly replaced by smaller, low trophic level fish (e.g. small spotted catsharks) and invertebrates (e.g. scallops, crabs and lobster). Declining trophic levels in fisheries catches have occurred worldwide, with fish catches progressively being replaced by invertebrates. We argue that a network of fisheries closures would help rebalance the trophic status of the Channel and allow regeneration of marine ecosystems.     

Patterns of macroalgal diversity, community composition and long-term changes along the Swedish west coast

This study analyses the complicated patterns of vertical distribution of the macroalgal vegetation in an area where brackish and marine waters meet and mix. Variables used to record vegetation characteristics are algal cover, species composition and diversity. The data set includes 64 diving profiles, all from sites exposed to wave action, along a ca. 260 km long coastline. The profiles belong to four categories: coastal sites in the Skagerrak (more marine), coastal sites in the Kattegat (more brackish), coastal sites in the Kattegat after a toxic phytoplankton bloom, and submerged offshore stone reefs in the Kattegat. The highest species diversity was found at the reefs, which are not affected directly by land runoff. At the reefs the 18 most common perennial species penetrate 2–11 m (on average 5.5 m) deeper than at the coastal sites. The virtual absence of sedimentation, and thus the availability of substratum, at the reefs may explain the differences so that the lower limit for the algae is determined by light penetration or by recruitment problems caused by strong currents at the reefs, whereas sedimentation limits the settlement of algae in coastal sites. Ordination analysis based on species composition reveals that the major environmental gradients structuring the algal vegetation in the Kattegat and the Skagerrak are salinity and water depth. The large data set of this study made it possible to quantify the downward dislocation of Atlantic intertidal species to the sublittoral along the Swedish west coast. For example, the mean upper limit of Corallina officinalis is 2 m in the Skagerrak but 12.5 m in the Kattegat and the mean occurrence interval of Fucus serratus is 0.9–2.7 m in the Skagerrak, but 1.1–6.3 m in the Kattegat. This downward dislocation is suggested to be the result of decreased competition when species successively disappear with lower salinity. Comparisons of the present study's results with those of previous investigations show that eight common red algal species have moved upwards compared to the situation before the large-scale eutrophication started in the 1960s, e.g. Cystoclonium purpureum and Polysiphonia elongata by ca. 8 m, Phycodrys rubens and Delesseria sanguinea by ca. 5 m. A toxic phytoplankton bloom affected macroalgal community composition on the whole only slightly, but it had a negative effect on algal cover and species richness below a water depth of ca. 5 m, the algae were visibly damaged and the lower vegetation limit temporarily moved upwards.     

Stock structure analysis of ‘Bombay duck’ (Harpadon nehereus Hamilton, 1822) along the Indian coast using truss network morphometrics

Harpadon nehereus, commonly known as ‘Bombay duck’, is a fish with a discontinuous distribution along the Indian peninsula. The fisheries are dominant on the north‐east and north‐west coast but are absent in commercial landings below 15° north latitude. Heretofore stock assessment studies had not considered the various spawning stock components that replenish this fishery, therefore the present study. Fish samples were collected from four locations: two each from the northeast and the northwest coasts. Twenty‐four morphometric variables were measured using a box‐truss network method. Factor analysis of these variables differentiated the east and the west coast fish populations. Multiple comparisons on the factor scores indicated two independent stocks on the east coast, whereas the fishery on the west coast is replenished by a single stock. The important morphometric traits that accounted for most of the stock variations were related to swimming adaptations of the fish. Future stock assessments can consider the population on the west coast as a single stock when formulating management plans. To harvest the resource in a sustainable manner, the maritime states on the west coast should adopt collaborative efforts towards managing this fishery.     

Fire effects and ecological recovery pathways of tropical montane cloud forests along a time chronosequence

Tropical montane cloud forests (TMCFs) harbour high levels of biodiversity and large carbon stocks. Their location at high elevations make them especially sensitive to climate change, because a warming climate is enhancing upslope species migration, but human disturbance (especially fire) may in many cases be pushing the treeline downslope. TMCFs are increasingly being affected by fire, and the long‐term effects of fire are still unknown. Here, we present a 28‐year chronosequence to assess the effects of fire and recovery pathways of burned TMCFs, with a detailed analysis of carbon stocks, forest structure and diversity. We assessed rates of change of carbon (C) stock pools, forest structure and tree‐size distribution pathways and tested several hypotheses regarding metabolic scaling theory (MST), C recovery and biodiversity. We found four different C stock recovery pathways depending on the selected C pool and time since last fire, with a recovery of total C stocks but not of aboveground C stocks. In terms of forest structure, there was an increase in the number of small stems in the burned forests up to 5–9 years after fire because of regeneration patterns, but no differences on larger trees between burned and unburned plots in the long term. In support of MST, after fire, forest structure appears to approximate steady‐state size distribution in less than 30 years. However, our results also provide new evidence that the species recovery of TMCF after fire is idiosyncratic and follows multiple pathways. While fire increased species richness, it also enhanced species dissimilarity with geographical distance. This is the first study to report a long‐term chronosequence of recovery pathways to fire suggesting faster recovery rates than previously reported, but at the expense of biodiversity and aboveground C stocks.     

Effect of land‐use and land‐cover change on mangrove blue carbon: A systematic review

Mangroves shift from carbon sinks to sources when affected by anthropogenic land‐use and land‐cover change (LULCC). Yet, the magnitude and temporal scale of these impacts are largely unknown. We undertook a systematic review to examine the influence of LULCC on mangrove carbon stocks and soil greenhouse gas (GHG) effluxes. A search of 478 data points from the peer‐reviewed literature revealed a substantial reduction of biomass (82% ± 35%) and soil (54% ± 13%) carbon stocks due to LULCC. The relative loss depended on LULCC type, time since LULCC and geographical and climatic conditions of sites. We also observed that the loss of soil carbon stocks was linked to the decreased soil carbon content and increased soil bulk density over the first 100 cm depth. We found no significant effect of LULCC on soil GHG effluxes. Regeneration efforts (i.e. restoration, rehabilitation and afforestation) led to biomass recovery after ~40 years. However, we found no clear patterns of mangrove soil carbon stock re‐establishment following biomass recovery. Our findings suggest that regeneration may help restore carbon stocks back to pre‐disturbed levels over decadal to century time scales only, with a faster rate for biomass recovery than for soil carbon stocks. Therefore, improved mangrove ecosystem management by preventing further LULCC and promoting rehabilitation is fundamental for effective climate change mitigation policy.     

Associations between wintering avian predators and schooling fish in the Skagerrak-Kattegat suggest reliance on predictable aggregations of herring Clupeaharengus

We investigated the distributional dynamics of piscivorous seabirds in relation to the distribution of young fish outside the breeding season in the Skagerrak and Kattegat, using synoptic bottom trawl and seabird surveys. Kittiwake Rissa tridactyla , Common Guillemot Uria aalge and Razorbill Alca torda were significantly correlated with herring Clupea harengus abundance at coarse spatial scales (>20 km). The association between the three seabird species and other abundant fish species like sprat Sprattus sprattus , whiting Merlangius merlangus and Norway pout Trisopterus esmarki was considerably weaker. In the Skagerrak and Kattegat, the location of aggregations of immature herring during winter is relatively stable and predictable. The correlation with herring and the lack of correlation with the abundant, but less stable occurrence of sprat of similar caloric value suggest that the tendency among piscivorous seabirds to depend on predictable concentrations of prey during the breeding season is a common feature during the non-breeding season as well. The results are discussed in relation to ecological and winter habitat characteristics of wintering piscivorous seabirds in the eastern North Atlantic, and in relation to the management and conservation of nursering herring in the Skagerrak and Kattegat.     

Long‐term recovery of the functional community assembly and carbon pools in an African tropical forest succession

On the African continent, the population is expected to expand fourfold in the next century, which will increasingly impact the global carbon cycle and biodiversity conservation. Therefore, it is of vital importance to understand how carbon stocks and community assembly recover after slash‐and‐burn events in tropical second growth forests. We inventoried a chronosequence of 15 1‐ha plots in lowland tropical forest of the central Congo Basin and evaluated changes in aboveground and soil organic carbon stocks and in tree species diversity, functional composition, and community‐weighted functional traits with succession. We aimed to track long‐term recovery trajectories of species and carbon stocks in secondary forests, comparing 5 to 200 + year old secondary forest with reference primary forest. Along the successional gradient, the functional composition followed a trajectory from resource acquisition to resource conservation, except for nitrogen‐related leaf traits. Despite a fast, initial recovery of species diversity and functional composition, there were still important structural and carbon stock differences between old growth secondary and pristine forest, which suggests that a full recovery of secondary forests might take much longer than currently shown. As such, the aboveground carbon stocks of 200 + year old forest were only 57% of those in the pristine reference forest, which suggests a slow recovery of aboveground carbon stocks, although more research is needed to confirm this observation. The results of this study highlight the need for more in‐depth studies on forest recovery in Central Africa, to gain insight into the processes that control biodiversity and carbon stock recovery.     

Have jellyfish in the Irish Sea benefited from climate change and overfishing?

Climate change and overfishing may lead to ecosystem instability and may benefit nonexploited organisms such as jellyfish. In the Irish Sea, an increase in jellyfish abundance was evident (r²=0.29, P=0.03) in a 16‐year time‐series (1994–2009) collected during juvenile fish surveys. Jellyfish abundance correlated positively with sea surface temperature (SST) over the preceding 18 months (r=0.65, p_ACF<0.001) and copepod biomass in the previous year (r=0.56, p_ACF=0.03) and negatively with spring (February–May) precipitation (r=?0.57, p_ACF=0.02). Principal components regression indicated that climatic indices explained 68% of the interannual variability in jellyfish abundance (P=0.003), where the components were based on the North Atlantic Oscillation Index, SST and precipitation. The frequency of cnidarian material present in Continuous Plankton Recorder (CPR) samples has also increased since 1970, with a period of frequent outbreaks between 1982 and 1991. Before this period, the herring stock in the northern Irish Sea declined rapidly to a low level, potentially stimulating structural change in the ecosystem. In 1985, there was a step decrease in CPR copepod biomass and in 1989, a step increase in the phytoplankton colour index, suggesting a cascading regime shift during the 1980s. Subsequent overexploitation of gadids, coupled with warm temperatures and the poor recruitment of cod, led to the rapid decline in cod biomass from 1990. While the biomass of sprat has decreased in the last decade, the herring stock has recovered partially. Reductions in demersal fishing pressure since 2000, intended to stimulate cod recovery, appear to have facilitated further rises in haddock biomass. Since the 1980s regime shift, sea temperatures have increased, the fish community has altered and jellyfish abundance has risen such that jellyfish and haddock may now play an increasingly important role in the ecosystem.     

Diatoms from the surface sediments of the Skagerrak and the Kattegat and their relationship to the spatial changes of environmental variables

This paper deals with the distribution of diatoms in the surface sediments of the Skagerrak and the Kattegat and its correlation with the spatial changes of eight environmental variables. Eigenvalues and the values of the species-environmental correlation show that the measured environment variables account for the major gradients of diatom composition. The results of forward selection of the environmental variables and Monte Carlo permutation tests of statistical significance, together with canonical coefficients and intra-set correlations between environmental variables and the axes, show that summer and winter surface salinities are the most important environmental factors affecting diatom distribution in this area. Five diatom assemblages are distinguished in the Skagerrak and the Kattegat. The distribution of these assemblages is clearly correlated with the strong variations in salinity, and in particular with surface salinities during summer and winter.     

The influence of maternal length and age on the size and weight of the eggs and the relative fecundity of the haddock, Melanogrammus aeglefinus, in British waters

The diameter and dry weight of the eggs of haddock, Melanogrammus aeglefinus , are positively correlated with fish length. The correlations are largely due to the fact that many of the smaller fish that were sampled were 2 years old, and the eggs of these young haddock, which can be regarded as precocious spawners, are significantly smaller and lighter than those of older fish. The relative fecundity of 2–year–old haddock (274 eggs g⁻¹) is also lower than that of the other age classes (493 eggs g⁻¹); this has important implications for the estimation of egg production from female spawning stock biomass. It is pointed out that in some years precocious spawners represent a large component of the North Sea haddock spawning stock. When the annual egg production of this stock is calculated, it may be appropriate to apply a weighting factor to the number of eggs contributed by the 2–year–olds, on the assumption that the small eggs of these fish produce larvae that are less viable than those of older haddock.     

Population structure of flounder (Platichthys flesus) in the Baltic Sea: differences among demersal and pelagic spawners

We found significant population structure and isolation by distance among samples of flounder (Platichthys flesus) in the Baltic, Kattegat and Skagerrak seas using microsatellite genetic markers. This pattern was almost entirely due to a difference between flounder that have demersal spawning in the northern Baltic, as compared to pelagic spawners in the southern Baltic and on the west coast of Sweden. Among demersal spawners we found neither genetic differentiation nor any isolation by distance among sampling sites. We speculate that demersal flounder are descendants of a population that colonized the Baltic previous to pelagic spawners. The demersal flounder may thus have had longer time to adapt to the low salinity in the Baltic, and accordingly display egg characteristics that make it possible to reproduce at the low salinity levels in the northern Baltic. Among pelagic spawners significant isolation by distance was detected. Pelagic spawners have previously been shown to display clinal variation in egg size, which allows them to float also at the moderate salinity levels up to the region north of the island Bornholm. Management units for harvesting should ideally be based on true biological populations, and for the commercially important flounder up to 15 different management stocks in the Baltic have been suggested. We could not find a population genetic foundation for such a high number of management units, and our data suggest three management units: the northern Baltic (demersal populations), southern Baltic with the Oresund straits and the most northwestern sampling sites (Skagerrak, Kattegat and North Sea).     

Influence of oceanic factors on Anguilla anguilla (L.) over the twentieth century in coastal habitats of the Skagerrak,southern Norway

The European eel (Anguilla anguilla L.) is distributed in coastal and inland habitats all over Europe, but spawns in the Sargasso Sea and is thus affected by both continental and oceanic factors. Since the 1980s a steady decline has been observed in the recruitment of glass eels to freshwater and in total eel landings. The eel is considered as critically endangered on the International Union for the Conservation of Nature and Natural Resources Red List of species. The Skagerrak beach seine survey from Norway constitutes the longest fishery-independent dataset on yellow/silver eels (starting in 1904). The Skagerrak coastal region receives larvae born in the Sargasso Sea spawning areas that have followed the Gulf Stream/North Atlantic Drift before they penetrate far into the North Sea. The Skagerrak coastal time series is therefore particularly valuable for exploring the impacts of oceanic factors on fluctuations in eel recruitment abundance. Analyses showed that Sargasso Sea surface temperature was negatively correlated with eel abundance, with a lag of 12 years revealing a cyclic and detrimental effect of high temperatures on the newly hatched larvae. The North Atlantic Oscillation index and inflow of North Atlantic water into the North Sea were negatively correlated with eel abundance, with a lag of 11 years. Increased currents towards the North Atlantic during high North Atlantic Oscillation years may send larvae into the subpolar gyre before they are ready to metamorphose and settle, resulting in low recruitment in the northern part of the distribution area for these years. The Skagerrak time series was compared with glass eel recruitment to freshwater in the Netherlands (Den Oever glass eel time series), and similar patterns were found revealing a cycle linked to changes in oceanic factors affecting glass eel recruitment. The recent decline of eels in the Skagerrak also coincided with previously documented shifts in environmental conditions of the North Sea ecosystem.     

Ecological Meltdown in the Firth of Clyde,Scotland: Two Centuries of Change in a Coastal Marine Ecosystem

Background

The Firth of Clyde is a large inlet of the sea that extends over 100 km into Scotland''s west coast.

Methods

We compiled detailed fisheries landings data for this area and combined them with historical accounts to build a picture of change due to fishing activity over the last 200 years.

Findings

In the early 19^th century, prior to the onset of industrial fishing, the Firth of Clyde supported diverse and productive fisheries for species such as herring (Clupea harengus, Clupeidae), cod (Gadus morhua, Gadidae), haddock (Melanogrammus aeglefinus, Gadidae), turbot (Psetta maxima, Scophthalmidae) and flounder (Platichthys flesus, Pleuronectidae). The 19^th century saw increased demand for fish, which encouraged more indiscriminate methods of fishing such as bottom trawling. During the 1880s, fish landings began to decline, and upon the recommendation of local fishers and scientists, the Firth of Clyde was closed to large trawling vessels in 1889. This closure remained in place until 1962 when bottom trawling for Norway lobster (Nephrops norvegicus, Nephropidae) was approved in areas more than three nautical miles from the coast. During the 1960s and 1970s, landings of bottomfish increased as trawling intensified. The trawl closure within three nautical miles of the coast was repealed in 1984 under pressure from the industry. Thereafter, bottomfish landings went into terminal decline, with all species collapsing to zero or near zero landings by the early 21^st century. Herring fisheries collapsed in the 1970s as more efficient mid-water trawls and fish finders were introduced, while a fishery for mid-water saithe (Pollachius virens, Gadidae) underwent a boom and bust shortly after discovery in the late 1960s. The only commercial fisheries that remain today are for Nephrops and scallops (Pecten maximus, Pectinidae).

Significance

The Firth of Clyde is a marine ecosystem nearing the endpoint of overfishing, a time when no species remain that are capable of sustaining commercial catches. The evidence suggests that trawl closures helped maintain productive fisheries through the mid-20^th century, and their reopening precipitated collapse of bottomfish stocks. We argue that continued intensive bottom trawling for Nephrops with fine mesh nets will prevent the recovery of other species. This once diverse and highly productive environment will only be restored if trawl closures or other protected areas are re-introduced. The Firth of Clyde represents at a small scale a process that is occurring ocean-wide today, and its experience serves as a warning to others.     

Effects of long‐term straw return on soil organic carbon storage and sequestration rate in North China upland crops: A meta‐analysis

Soil organic carbon (SOC) is essential for soil fertility and climate change mitigation, and carbon can be sequestered in soil through proper soil management, including straw return. However, results of studies of long‐term straw return on SOC are contradictory and increasing SOC stocks in upland soils is challenging. This study of North China upland agricultural fields quantified the effects of several fertilizer and straw return treatments on SOC storage changes and crop yields, considering different cropping duration periods, soil types, and cropping systems to establish the relationships of SOC sequestration rates with initial SOC stocks and annual straw C inputs. Our meta‐analysis using long‐term field experiments showed that SOC stock responses to straw return were greater than that of mineral fertilizers alone. Black soils with higher initial SOC stocks also had lower SOC stock increases than did soils with lower initial SOC stocks (fluvo‐aquic and loessial soils) following applications of nitrogen‐phosphorous‐potassium (NPK) fertilizer and NPK+S (straw). Soil C stocks under the NPK and NPK+S treatments increased in the more‐than‐20‐year duration period, while significant SOC stock increases in the NP and NP+S treatment groups were limited to the 11‐ to 20‐year period. Annual crop productivity was higher in double‐cropped wheat and maize under all fertilization treatments, including control (no fertilization), than in the single‐crop systems (wheat or maize). Also, the annual soil sequestration rates and annual straw C inputs of the treatments with straw return (NP+S and NPK+S) were significantly positively related. Moreover, initial SOC stocks and SOC sequestration rates of those treatments were highly negatively correlated. Thus, long‐term straw return integrated with mineral fertilization in upland wheat and maize croplands leads to increased crop yields and SOC stocks. However, those effects of straw return are highly dependent on fertilizer management, cropping system, soil type, duration period, and the initial SOC content.