Industry participation in stock assessment: the New Zealand SNA1 snapper (Pagrus auratus) fishery

We describe industry funded contributions to the assessment of the SNA1 snapper (Pagrus auratus) fishery during the 1990s and discuss the advantages and disadvantages of industry funded research. We also provide background on the history of fisheries management in New Zealand, on the current assessment and management processes, and on the SNA1 snapper fishery. In the SNA1 fishery, the contributions of industry and the cooperation with Government scientists has resulted in high quality assessments. In our opinion, the advantages of industry funded research out weigh the disadvantages and suggest that industry funded assessments are highly desirable and should be an integral part of any management system.     

Measurement and modeling of bed shear stress under solitary waves

Direct measurements of bed shear stresses (using a shear cell apparatus) generated by non-breaking solitary waves are presented. The measurements were carried out over a smooth bed in laminar and transitional flow regimes (~ 10⁴ < R_e < ~ 10⁵). Measurements were carried out where the wave height to water depth (h/d) ratio varied between 0.12 and 0.68; maximum near bed velocity varied between 0.16 m/s and 0.51 m/s and the maximum total shear stress (sum of skin shear stress and Froude–Krylov force) varied between 0.386 Pa and 2.06 Pa. The total stress is important in determining the stability of submarine sediment and in sheet flow regimes. Analytical modeling was carried out to predict total and skin shear stresses using convolution integration methods forced with the free stream velocity and incorporating a range of eddy viscosity models. Wave friction factors were estimated from skin shear stress at different instances over the wave (viz., time of maximum positive total shear stress, maximum skin shear stress and at the time of maximum velocity) using both the maximum velocity and the instantaneous velocity at that phase of the wave cycle. Similarly, force coefficients obtained from total stress were estimated at time of maximum positive and negative total stress and at maximum velocity. Maximum positive total shear stress was approximately 1.5 times larger than minimum negative total stress. Modeled and measured positive bed shear stresses are well correlated using the best convolution model, but the model underestimates the data by about 4%. Friction factors are dependent on the choice of normalizing using the maximum velocity, as is conventional, or the instantaneous velocity. These differ because the stress is not in phase with the velocity in general. Friction factors are consistent with previous data for monochromatic waves, and vary inversely with the square-root of the Reynolds number. The total shear stress leads the free stream fluid velocity by approximately 50°, whereas the skin friction shear stress leads by about 30°, which is similar to that reported by earlier researchers.     

Dissolved carbon and 33-133-133-1anomalies in the water column caused by hydrocarbon seeps on the northwestern Gulf of Mexico slope

Alkalinity, dissolved inorganic carbon (DIC), and ¹³C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative ¹³C values of up to –4.5 PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the¹³C-depletions are attributed to the release of CO₂ by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The ¹³C composition of the migrating CO₂ is estimated to be –26.0 on the basis of dissolved carbon inventory. Manifestation of DIC and ¹³C anomalies in the water column caused by hydrocarbon seepage holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory.     

Geochemistry of suspended matter from the Baltic Sea 2. Results of bulk trace metal analysis by AAS

In 1984, on a transect covering the whole Baltic Sea and parts of the adjacent North Sea, 160 water samples were taken and analysed for their concentrations of particulate and dissolved metals. In addition, the suspended materials were investigated for their elemental bulk composition.The particulate fractions represented from about 5% (Cd, Cu and Ni) to 50% (Fe and Pb) of the total (particulate plus dissolved) concentrations. For some elements (Ba, Cd, Cu, Pb and Zn), the particulate matter from the surface microlayer was enriched with respect to those suspended materials taken from 0.2 m depth. This could reflect the atmospheric input of metal-rich aerosols. In anoxic deep waters, maximum contents of Zn (6400 μg g⁻¹), Cu (1330 μg g⁻¹) and Cd (12 μg g⁻¹) were observed in the particulate matter, indicating sulphidic forms. On the other hand, under oxic conditions the distribution coefficients (K_d) decreased with the water depth (Cd, Fe and Pb).Relative to global background levels, the particulate matter contained metal “excesses” amounting to more than 90% of the total contents (Cd, Mn, Pb and Zn). Automated electron probe X-ray microanalysis (EPXMA) revealed that the elemental composition of sediments is mainly governed by post-depositional processes of early diagenesis and is only weakly related to the composition of suspended matter in the overlying water body. For instance, in relation to surface mud sediments of the central Baltic net-sedimentation basins, Zn, Cd, Cu and Mn had 30–100% higher levels in the suspended materials. The general pattern of metal contents of particulate matter taken from 10 m depth on a transect between the Bothnian Bay and the North Sea were—possibly as a result of anthropogenic inputs—rather similar for Pb, Zn and Cu. For Fe and Mn, the distribution patterns along the transect were probably governed by the natural loading characteristics and by the biogeochemistry of those elements.     

Carbon balance in a salt marsh: Interactions of diffusive export, tidal deposition and rainfall-caused erosion

Studies of the concentrations of particulate and dissolved organic carbon in the Duplin River, of the tidal exchange of POC and DOC in the marsh, of the standing stock and movement of Spartina alterniflora wrack in the Duplin, and of the removal of carbon from the surface of the marsh by rain were conducted at Sapelo Island, Georgia in order to test three hypotheses about export of carbon from the Duplin River watershed. We found that the gradients in POC and DOC concentrations are such that carbon is being transported down the Duplin River throughout the year, although in smaller quantities than previously believed. In contrast, almost all tidal exchanges within the marsh result in deposition of carbon. Most of this deposited carbon is subsequently eroded as a result of rain falling on the exposed marsh surface, and is washed back into the tidal creeks. This cycle of deposition and erosion is a possible mechanism keeping POC in the thin aerobic surface layer of the marsh, thus increasing its availability to detritivores and aerobic microbes. The standing stock of wrack is only a fraction of the S. alterniflora produced each year, and its export is a negligible term in the carbon balance equation.     

Was a carbon balance measured in the equatorial Pacific during JGOFS?

The likelihood that the carbon fluxes measured as part of the US-JGOFS field program in the equatorial Pacific ocean (EgPac) during 1992 yielded a balanced carbon budget for the surface ocean was determined. The major carbon fluxes incorporated into a surface carbon budget were: new production, particulate organic carbon (POC) and dissolved organic carbon (DOC) export, CaC0₃ export, C0₂ gas evasion, dissolved inorganic carbon (DIC) supply, and the time rate of charge. The ratio of the measured concentration gradients of DOC and DIC provided a constraint on the ratio of POC/DOC export. Uncertainties of ±30–50% for individual carbon flux measurements reduce the likelihood that a carbon balance can be measured during a JGOFS process-type study. As a benchmark, carbon fluxes were prescribed to yield a hypothetical surface carbon budget that was, on average, balanced. Given the typical errors in the individual carbon fluxes, however, there was only about a 30% chance that this hypothetical budget could be measured to be balanced to ±50%. Using this benchmark, it was determined that there was a 95 % chance that the carbon flux measurements yielded a surface DIC budget balanced (to ±50%) during El Nino conditions in boreal spring 1992, when the total organic carbon export rate was - 5 mmol C m-2 day- 1 and the POC export was 3 mmol C m⁻² day⁻¹. In boreal fall 1992, during cold period conditions, there was a 70% chance that the surface carbon DIC budget was balanced when the total organic carbon export rate was 20 mmol C m⁻² day⁻¹ and export was -13 mmol C m-2 day-'. The DOC to DIC concentration gradient ratio of - -0.15, measured in depth profiles down to 100m and in surface waters, was used as an important constraint that most (> 70%) of the organic carbon exported from the euphotic zone was POC rather than DOC. If a balanced surface DIC budget was used to test the compatibility of individual carbon fluxes measured during EgPac, then a three- to four-fold increase in total and particulate organic carbon export between spring and fall is indicated. This increase was not reflected in the POC loss rates measured by drifting sediment trap collections or estimated by²³⁴Th deficiencies coupled with the C/Th measured on suspended particles.     

Short-term and interannual variability of redox-sensitive chemical parameters in hypoxic/anoxic bottom waters of the Chesapeake Bay

A combination of CTD casts, discrete bottle sampling and in situ voltammetric microelectrode profiling was used to examine changing redox conditions in the water column at a single station south of the Bay Bridge in the upper Chesapeake Bay in late July/early August, 2002–2005. Short-term (2–4 h) fluctuations in the oxic/suboxic/anoxic interface were documented using in situ voltammetric solid-state electrodes. Profiles of dissolved oxygen and sulfide revealed tidally-driven vertical fluctuations of several meters in the depth and thickness of the suboxic zone. Bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ also varied over the tidal cycle by approximately an order of magnitude. These data indicate that redox species concentrations at this site varied more due to physical processes than biogeochemical processes. Based on analysis of ADCP data, tidal currents at this station were strongly polarized, with the principal axis of tidal currents aligned with the mainstem channel. Together with the chemical data, the ADCP analysis suggests tidal flushing of anoxic bottom waters with suboxic water from north of the site. The present study is thus unique because while most previous studies have focused on processes across relatively stable redox interfaces, our data clearly demonstrate the influence of rapidly changing physical mixing processes on water column redox chemistry.Also noted during the study were interannual differences in maximum bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺. In 2003, for example, heavy spring rains resulted in severe hypoxia/anoxia in June and early July. While reported storm-induced mixing in late July/early August 2003 partially alleviated the low-oxygen conditions, bottom water concentrations of sulfide, Mn²⁺ and Fe²⁺ were still much higher than in the previous year. The latter implies that the response time of the microbial community inhabiting the suboxic/anoxic bottom waters to changing redox conditions is slow compared to the time scale of episodic mixing events. Bottom water concentrations of the redox-sensitive chemical species should thus be useful as a tracer to infer prior hypoxic/anoxic conditions not apparent from ambient oxygen levels at the time of sampling.