49 research outputs found
Catchability of pelagic trawls for sampling deep-living nekton in the mid-North Atlantic
Material collected in summer 2004 from the Mid-Atlantic Ridge between Iceland and the Azores with three pelagic trawls was used to estimate relative catchabilities of common fish, cephalopod, decapod, and jellyfish species. Catchability is defined as the ratio of numbers caught between two trawls, standardized for towed distance. Taxon-specific catchability coefficients were estimated for two large pelagic trawls with graded meshes, using a smaller pelagic trawl with a uniform mesh size as the reference trawl. Two of the trawls were equipped with multiple openingâclosing codends that allowed sampling of different depth layers. Generalized linear and mixed models suggest that most of the taxa have catchabilities much lower than expected from the area of opening alone, indicating that only a few species are herded by the large mesh at the mouth of larger trawls. Catchability coefficients across taxa show a very large spread, indicating that the sampled volume for the larger trawls with graded meshes was highly
taxon-specific. Part of this variability can be explained by body size and taxonomic group, the latter probably reflecting differences in body form and behaviour. The catchability estimates presented here form the basis for combining data for quantitative analyses of community structure
Choosing best practices for managing impacts of trawl fishing on seabed habitats and biota
Bottom trawling accounts for almost one quarter of global fish landings but may also have significant and unwanted impacts on seabed habitats and biota. Management measures and voluntary industry actions can reduce these impacts, helping to meet sustainability objectives for fisheries, conservation and environmental management. These include changes in gear design and operation of trawls, spatial controls, impact quotas and effort controls. We review nine different measures and actions and use published studies anda simple conceptual model to evaluate and compare their performance. The risks and benefits of these management measures depend on the extent to which the fishery is already achieving management objectives for target stocks and the characteristics of the management system that is already in place. We offer guidance on identifying best practices for trawl-fisheries management and show that best practices and their likelihood of reducing trawling impacts depend on local, national and regional management objectives and priorities, societal values and resources for implementation. There is no universalbest practice, and multiple management measures and industry actions are required to meet sustainability objectives and improve trade-offs between food production and environmental protection
Short-term fate of seagrass and macroalgal detritus in Arenicola marina bioturbated sediments
© The authors 2020. Seagrass meadows are globally important ecosystems for carbon (C) sequestration. However, bioturbation by benthic fauna can alter the distribution, degradation and overall preservation of C in the sediment. We performed a 4 wk laboratory experiment to investigate the short-term degradation and burial of 2 major C sources in bare sediments associated with seagrass ecosystems. Eelgrass Zostera marina and macroalgal (Fucus vesiculosus) detritus were amended in sediment with and without bioturbation by the common polychaete Arenicola marina. Bioturbation did not significantly affect the loss of eelgrass detritus (>0.5 mm), but caused a rapid burial of this material as a discrete layer (55% recovery) at sediment depths ranging from 8 to 14 cm. A. marina effects on macroalgal detritus were more pronounced, resulting, in total, in an 80% loss of macroalgal detritus by microbial degradation and worm ingestion. We conclude that A. marina bioturbation effectively buries eelgrass detritus into deep anoxic sediments, but we cannot confirm that this leads to enhanced C preservation in coastal ecosystems. In contrast, A. marina bioturbation significantly increases the degradation of macroalgal tissue, and it is unlikely that this detritus is a major source for permanent C burial
Carbon degradation in agricultural soils flooded with seawater after managed coastal realignment
Permanent flooding of low-lying coastal areas is a growing threat
due to climate change and related sea-level rise. An increasingly common
solution to protect coastal areas lying below sea level is intentional
flooding by "managed coastal realignment". However, the biogeochemical
implications of flooding agricultural soils with seawater are still not well
understood. We conducted a 1-year mesocosm experiment to investigate
microbial carbon degradation processes in soils flooded with seawater.
Agricultural soils were sampled on the northern coast of the island Fyn
(Denmark) at Gyldensteen Strand, an area that was subsequently flooded in a
coastal realignment project. We found rapid carbon degradation to
TCO2 1Â day after experimental flooding and onwards and microbial
sulfate reduction established quickly as an important mineralization pathway.
Nevertheless, no free sulfide was observed as it precipitated as FeâS
compounds with Fe acting as a natural buffer, preventing toxic effects of
free sulfide in soils flooded with seawater. Organic carbon degradation
decreased significantly after 6Â months, indicating that most of the soil
organic carbon was refractory towards microbial degradation under the anoxic
conditions created in the soil after flooding. During the experiment only
6â7âŻ% of the initial soil organic carbon pools were degraded. On this
basis we suggest that most of the organic carbon present in coastal soils
exposed to flooding through sea-level rise or managed coastal realignment
will be permanently preserved
Organic N and P in eutrophic fjord sediments â rates of mineralization and consequences for internal nutrient loading
Nutrient release from the sediments in shallow eutrophic estuaries may
counteract reductions of the external nutrient load and prevent or prolong
ecosystem recovery. The magnitude and temporal dynamics of this potential
source, termed internal nutrient loading, is poorly under\-stood. We
quantified the internal nutrient loading driven by microbial mineralization
of accumulated organic N (ON) and P (OP) in sediments from a shallow
eutrophic estuary (Odense Fjord, Denmark). Sediments were collected from
eight stations within the system and nutrient production and effluxes were
measured over a period of ~ 2 years. Dissolved inorganic nitrogen
(DIN) effluxes were high
initially but quickly faded to low and stable levels after 50â200 days, whereas
PO<sub>4</sub><sup>3â</sup> effluxes were highly variable in the different sediments.
Mineralization patterns suggested that internal N loading would quickly
(< 200 days) fade to insignificant levels, whereas internal
PO<sub>4</sub><sup>3â</sup> loading could be sustained for extended time (years). When
results from all stations were combined, internal N loading and P loading
from the fjord bottom was up to 121 Ă 10<sup>3</sup> kg N yr<sup>â1</sup> (20 kg N ha<sup>â1</sup> yr<sup>â1</sup>)
and 22 Ă 10<sup>3</sup> kg P yr<sup>â1</sup> (3.6 kg P ha<sup>â1</sup> yr<sup>â1</sup>) corresponding to 6 (N) and 36% (P) of the external
nutrient loading to the system. We conclude that the internal N loading
resulting from degradation of accumulated ON is low in shallow eutrophic
estuaries, whereas microbial mineralization of accumulated OP is a potential
source of P. Overall it appears that, in N-limited eutrophic systems,
internal nutrient resulting from mineralization of ON and OP in sediments is
of minor importance