Marine ecosystem management: Fish abundance and size under exploitation

This study is targeted on policy makers and environmental scientists to illustrate the typical historical scale of depletion of our fish stocks, and what current and emerging legislation might mean for fisheries management and the metrics of fish stocks. The population demography of the Bristol Channel sole is described since 1820. Their decline in abundance, and change in length compositions, are modelled. By 2000, the mature stock had been depleted to less than 5% of its original size, and larger sole were rarely caught. The implications of maximum sustainable yield targets, and of good environmental status, are examined.     

Sedimentary environments of the east-central Bering Sea continental shelf

A factor analysis of 180 bottom sediment samples from the east-central Bering Sea continental shelf identifies five factors that account for 95% of the variation in the 17 whole ø size classes that were used as variables. Factor I represents coarse sediments that have been bypassed in areas of active water circulation. Factors II and III represent fine and very fine sands that have been hydraulically sorted, reworked, and mixed. Factor IV represents coarse to medium silt that has been segregated from areas of relatively high energy. Factor V represents both the production of sediments finer than medium silt and deposition within the lowest-energy environment in this area.Modern and palimpsest sediments are areally prevalent over this section of the shelf. Relict sediments occur in only a few small areas. The dispersal of sediments is affected by surface and tidal currents as well as wave action. Ice rafting is not an important geological agent. Data from the eastcentral Bering Sea shelf indicate that sediments on subarctic continental shelves are not necessarily characterized by an abundance of rocky sediments or gravel.     

Book reviews

Kakadu: natural and cultural heritage and management

E.J. Press, D.A.M. Lea, A. Webb & A. Graham (eds), 1995

Australian Nature Conservation Agency & North Australia Research Unit, Australian National University, Darwin

17 × 25 cm, xviii + 318 pp., ISBN 0–7315–2171–4, A$29.95 (soft)

The landforms of England and Wales

A. Goudie, 1993 (first published in hardcover in 1990)

Blackwell, Oxford

23 × 15 cm, xi + 394 pp., ISBN 0–631–16367–0, A$45.00 (soft)

Technological change and the city

P.N. Troy (ed.), 1995

Federation Press, Sydney

14 × 21 cm, vi + 202 pp., ISBN 1–86287–184–1, A$35.00 (soft)

Oceanic islands

P.D. Nunn, 1994

Blackwell, Oxford

17 × 25 cm, xv + 413 pp., ISBN 0–631–17811–7, A$90.00 (soft)

Aotearoa and New Zealand: a historical geography

A. Grey, 1994

Canterbury University Press, Christchurch

14 × 21 cm, xix + 476 pp., ISBN 0–908812–34–5, NZ$29.95 (soft)     

Solar activity at λ = 15.8 mm (19 GHz) during the period 27 October–4 November 1968 and its relation to proton events

Four 15.8 mm solar bursts are discussed and the results are compared with data on recorded proton events, in relation to the prediction of such events from radio observations.Currently on leave of absence as 1969–70 NRC-OAR Senior Post-Doctoral Research Associate, AFCRL, Hanscom Field, Bedford, Mass. 01730.     

Hydrochemical variations during flood pulses in the south‐west China peak cluster karst: impacts of CaCO3–H2O–CO2 interactions

High‐resolution measurements of rainfall, water level, pH, conductivity, temperature and carbonate chemistry parameters of groundwater at two adjacent locations within the peak cluster karst of the Guilin Karst Experimental Site in Guangxi Province, China, were made with different types of multiparameter sonde. The data were stored using data loggers recording with 2 min or 15 min resolution. Waters from a large, perennial spring represent the exit for the aquifer's conduit flow, and a nearby well measures water in the conduit‐adjacent, fractured media. During flood pulses, the pH of the conduit flow water rises as the conductivity falls. In contrast, and at the same time, the pH of groundwater in the fractures drops, as conductivity rises. As Ca²⁺ and HCO₃^? were the dominant (>90%) ions, we developed linear relationships (both r² > 0·91) between conductivity and those ions, respectively, and in turn calculated variations in the calcite saturation index (SI_C) and CO₂ partial pressure (P) of water during flood pulses. Results indicate that the P of fracture water during flood periods is higher than that at lower flows, and its SI_C is lower. Simultaneously, P of conduit water during the flood period is lower than that at lower flows, and its SI_C also is lower. From these results we conclude that at least two key processes are controlling hydrochemical variations during flood periods: (i) dilution by precipitation and (ii) water–rock–gas interactions. To explain hydrochemical variations in the fracture water, the water–rock–gas interactions may be more important. For example, during flood periods, soil gas with high CO₂ concentrations dissolves in water and enters the fracture system, the water, which in turn has become more highly undersaturated, dissolves more limestone, and the conductivity increases. Dilution of rainfall is more important in controlling hydrochemical variations of conduit water, because rainfall with higher pH (in this area apparently owing to interaction with limestone dust in the lower atmosphere) and low conductivity travels through the conduit system rapidly. These results illustrate that to understand the hydrochemical variations in karst systems, considering only water–rock interactions is not sufficient, and the variable effects of CO₂ on the system should be evaluated. Consideration of water–rock–gas interactions is thus a must in understanding variations in karst hydrochemistry. Copyright © 2004 John Wiley & Sons, Ltd.     

Influence of the seasonal thermocline on the intrusion of Kuroshio across the continental shelf northeast of Taiwan

The hydrographic surveys in an area immediately northeast of Taiwan showed that the Kuroshio surface water intruded onto the shelf in the spring and there was a thick mixed layer and weak vertical stratification in the Kuroshio at the time. During the summer season, a strong thermocline was developed in the Kuroshio and the flow shifted offshore from Taiwan in front of the continental shelf break of the East China Sea. A numerical model is used to examine the effect of this seasonal thermocline on the flow pattern of the survey area. We find that the surface strength of the disturbance above the Su-Ao ridge is closely related to the occurrence of the on-shelf intrusion of Kuroshio. The presence of a seasonal thermocline in the Kuroshio can greatly diminish this disturbance in the surface level.     

Small-scale Spatial Distribution of a Pyramidellid Snail Ectoparasite, Boonea impressa, in Relation to its Host, Crassostrea virginica, on Oyster Reefs

Abstract. The pyramidellid snail, Boonea (= Odostomia) impressa (SAY), is an important ectoparasite of the oyster, Crassostrea virginica (G melin ). The small-scale spatial distribution of snails was examined in 11 oyster populations in the Copano Bay - Aransas Bay area of the Texas coast. Snails were contagiously distributed (s²/>1) and positively spatially autocorrelated. Patchiness was better developed and patches were larger on enclosed reefs than on more exposed open-bay reefs, probably because higher wave energy and increased distance between oyster clumps limited clump-to-clump movement by snails on open-bay reefs. Small snails were consistently distributed differently from large snails because small snails were less host specific. As the snails grow, a narrowing of host preference results in a substantial change in spatial distribution. Snails were consistently more contagiously distributed than their hosts; however, snail distribution was not host-density dependent. Factors determining the spatial distribution of the host had surprisingly little effect on the distribution of snails. Snail aggregates apparently form randomly by chance meetings of a few snails and then grow by attraction of additional snails. Consequently, snail behavior, which determines the size and length of time a patch retains its integrity, and the age distribution of the snail population may be the two most important factors, besides population size, determining the impact of B. impressa on oyster populations.     

Influence of parasitism in controlling the health, reproduction and PAH body burden of petroleum seep mussels

Petroleum seep mussels are often exposed to high hydrocarbon concentrations in their natural habitat and, thus, offer the opportunity to examine the relationship between parasitism, disease and contaminant exposure under natural conditions. This is the first report on the histopathology of cold-seep mussels. Seep mussels were collected by submersible from four primary sites in the Gulf of Mexico, lease blocks Green Canyon (GC) 184, GC-234, GC-233, and Garden Banks 425 in 550–650 m water depth. Five types of parasites were identified in section: (1) gill “rosettes” of unknown affinity associated with the gill bacteriocytes, (2) gill “inclusions” similar to chlamydia/rickettsia inclusions, (3) extracellular gill ciliates, (4) body “inclusions” that also resemble chlamydial/rickettsial inclusions, and (5) Bucephalus-like trematodes. Comparison to shallow-water mytilids demonstrates that: (1) both have similar parasite faunas; (2) seep mytilids are relatively heavily parasitized; and (3) infection intensities are extremely high in comparison to shallow-water mytilids for Bucephalus and chlamydia/rickettsia. In this study, the lowest prevalence for chlamydia/rickettsia was 67%. Prevalences of 100% were recorded from three populations. Bucephalus prevalence was 70% in three of 10 populations. The parasite fauna was highly variable between populations. Some important parasites were not observed in some primary sites. Even within primary sites, some important parasites were not observed in some populations. Bucephalus may exert a significant influence on seep mussel population dynamics. Forty percent of the populations in this study are severely reproductively compromised by Bucephalus infection. Only a fraction of petroleum seep mussel populations are maintaining the entire beta-level population structure of this species. Variation in two parasites, gill ciliates and Bucephalus, explained most of the variation in PAH body burden between mussel populations. PAHs are known to be sequestered preferentially in gametic tissue. Bucephalus would be expected to reduce overall body burden, at high infection intensities, by replacing gametic tissue. PAH concentrations exceeded 1 ppm in 4 of 9 populations, a ratio significantly higher than the 8 of 30 mussel locales in the NOAA Mussel Watch Program. Only five Mussel Watch locales exceeded the highest value for a petroleum seep population. Digestive gland and gill tissue atrophy were not significantly correlated with PAH body burden, even though some populations were characterized by body burdens exceeding 1 ppm, suggesting that seep mussels may not be as sensitive to PAH exposure as are some shallow-water mytilid populations.     

Short-Term and Small-Scale Variation in Food Availability to Natural Oyster Populations: Food, Flow and Flux

Abstract. Food resources and water flow speeds vary on short-temporal and small-spatial scales important to feeding in natural populations of oysters (Crassostrea virginica). To better describe food availability to the oyster population, micro-current meters were used to measure flow 1–3 cm above the bottom and frequent water sampling was used to determine the quantity and composition of the seston on Confederate Reef, Galveston Bay, Texas. The quantity and quality of food varied on time scales at least as short as 3 h and throughout the water column. Seston was high in detrital carbon, nitrogen poor, and had a high fraction of inorganic particles. The quantity of food at the bed was not predictable from samples taken near the surface. Water flow speeds were also highly variable. Whereas rapid flow speeds were measured, flow speed tended to cluster about a narrower range of slower speeds resulting in the mean speed being much higher than the median. Fluxes of food calculated from the quantity of food and water flow speeds were low and highly variable. Under most conditions, water flow speed was more important than food quantity in determining flux. Rapid flow rates could overcome low food quantity to result in relatively high fluxes, whereas slow flow resulted in small fluxes even if food was abundant. Changes in water flow rate associated with tidal changes and changes in speed are probably the primary agents controlling food availability on a daily basis. Decreased wind speed at night normally reduced flux in the early morning hours, for example. These results stress the importance of measuring food quantity and water flow speed on spatial and temporal scales important to oyster feeding. Model simulations of oyster populations at varying densities, flow rates and clump heights across the range of measured values show that flow rate is insufficient during most hours of the day and days of the year to prevent oysters from locally reducing food supply by their filtering efforts. Thus flow rate probably limits population density and adult size on Confederate Reef.