The scope of this study is to investigate the extent to which area-based management may have influenced the fishing efficiency of the Danish and Dutch demersal fleets harvesting cod, plaice and sole in the North Sea. Special consideration is given to the ‘plaice box’, a restricted area where fishing is prohibited to towed-gear fleets of horsepower exceeding 300 hp. An index of fishing power is calculated as the log-ratio between the catch per unit effort (CPUE) of any vessel and some survey abundance index. Annual trends in fishing are calculated as the year-effect derived from a general linear model (GLM) analysis of the index of fishing power. The fishing efficiency of Danish gill-netters and, to some extent, Danish seiners, has overall increased inside the ‘plaice box’, whilst remaining relatively stable outside. However, the fishing efficiency of the other exemption fleets has apparently been subject to little change. Overall, it is not possible to determine the extent to which the ‘plaice box’ has contributed to the fishing efficiency dynamics of the exemption fleets. However, some of the results of this study bear out the conclusions from recent investigations.

The recruitment of Strangomera bentincki (common sardine) and Engraulis ringens (anchovy) and the relationships with oceanographic conditions in the upwelling ecosystem of central-south Chile were investigated from 1990 to 1998, with emphasis on the 1997–1998 El Niño. Time series of recruitment, biomass, local sea surface temperature, and a coastal upwelling index were used to explore relationships during the spawning (July–August) and pre-recruitment (August–December) periods. The 1997–1998 El Niño caused physical changes in the small pelagic fish habitat off central-south Chile. Anomalous sea surface temperatures (SST) and upwelling indexes began to be detected from May 1997 and persisted into 1998. Recruitment of common sardine showed significantly negative relationship with SST anomalies during the pre-recruitment period, as well as with the upwelling index during the peak of spawning. However, the recruitment of anchovy did not seem to be affected by the environmental changes observed in the 1990s. Instead, the recruitment rate of anchovy showed negative relationship with the recruitment rate of common sardine. We conclude that the conditions of the 1997–1998 El Niño off central-south Chile affected the survival of common sardine offspring, and that the recruitment success of anchovy could be determined by less-abundant cohorts of common sardine through a biological mechanism of interaction.

Few studies have compared the suitability of different artificial reef construction materials in terms of their efficacy in acquiring diverse faunal assemblages. We compared the fishes associated with 12 co-located reefs constructed of limestone quarry boulders, concrete-gravel aggregate, or concrete-tire aggregate (four of each substrate) in 7 m of water, 200 m offshore Miami Beach, Florida, USA. All 12 reefs were deployed 100 m apart the same day in two lines of six. The four quarry stone reefs consist of a pile of 50 boulders each. The remaining eight reefs, of concrete-gravel aggregate and concrete-tire aggregate, were each constructed with 25 1.5 m edge and 25 1.2 m edge tetrahedron modules. Every two months from October 1998 to February 2001, SCUBA divers recorded fish species, abundance, and length, as well as spiny lobster, Panulirus argus, abundance. One hundred and forty-six species of fishes were recorded during the study period. The abundance and species richness of fish on each treatment exhibited a significant (p<0.05) seasonal variation with summer months having the greatest numbers and winter the lowest. There was no significant difference in total fish or spiny lobster abundance or fish biomass amongst the three reef types (p>0.05). Likewise, multi-dimensional scaling of Bray-Curtis dissimilarity indices did not indicate clustering of fish assemblages by reef type. Comparison of pre-deployment fish counts from the reef sites and neighboring hard bottom and jetty with counts from the same sites two years post-deployment indicate the artificial reefs increased both fish abundance and richness in the local area.

The impetus for this study was the mass bleaching event of giant clams in 1997–1998 at several reefs in the Great Barrier Reef (GBR, Australia). From September until December 1999, the study investigated the effects of high light intensities and increased temperature accompanied by a nutrient limitation on the metabolism of Tridacna gigas, to test, if these environmental changes can induce bleaching. In a 50-day trial similar conditions of the mass bleaching event were imitated stressing clams, collected from Orpheus Island and Nelly Bay, by a two- to threefold light intensity and an increased temperature of 4–6 °C. The objectives of the experiments were to determine whether high light intensities can induce changes in chlorophyll content or alter zooxanthella cell sizes and populations in the tissue. After 50 days of exposure to high light intensity the mantle tissue of stressed clams exhibited a decreased number of zooxanthellae per unit area from 19.8 ± 0.8 (× 107·cm–2) to 0.2 ± 0.2 (× 107·cm–2) (mean ± CL). Additionally, the average cell size of zooxanthellae were downsized from 7.4 ± 0.1 μm to 5.3 ± 0.1 μm (mean ± CL). Subsequently, the chlorophyll content of both, chl a and chl c1, declined as well, chl a from 192 ± 4 to 0.1 ± 0.1 μg·ml–1 and chl c1 from 145 ± 6 to 0 μg·ml–1 (mean ± CL). This study shows that increased light intensity and temperature are the main causes for bleaching in giant clams. Thus, the study confirmed the four major aspects involved in bleaching: (1) loss of symbiotic algae, (2) decrease of chl a/c1 in the remaining symbiotic algae, (3) retention of small zooxanthellae in the tissue and (4) release of ammonium (NH4+) into the water column while nutrient uptake of ammonium was largely blocked.

We studied the distribution patterns of the native European white-clawed crayfish (Austropotamobius pallipes) and the introduced signal crayfish (Pacifastacus leniusculus), and looked at the water chemistry in several streams in the north of the Iberian Peninsula (Cantabrian watershed). Fifty fluvial reaches which were currently or previously inhabited by crayfish and had physical attributes similar to the known habitat requirements of crayfish were sampled. P. leniusculus was the most common species encountered (54% of the samples), A. pallipes was found in 19% and neither species was recorded in the remaining reaches. In this paper we determine the relationship between crayfish presence and water chemistry. As neither species inhabits waters with the highest NO2– concentrations found, it seems both species are sensitive to NO2–. Mg+2 and SO4–2 concentrations are the factors separating sites with and without crayfish (crayfish inhabit water with higher concentrations of both cations). The main differences between reaches inhabited by native and signal crayfish lies in SO4–2 concentration: higher concentrations favour signal crayfish. Applications to management based on the river water chemistry are proposed.

The abundance of prey ingested by brown trout (Salmo trutta) were compared with the abundance of drifting invertebrates in a mountain stream managed by hydropeaking, upstream (site A) and downstream (site B) of a hydroelectric power plant. During power generation, flow and temperature were the two main environmental factors modified. The natural flow in the river below the outlet may be enhanced several times a day from 1 to 11 m3.s–1 in summer and winter, and from 5 to 15 m3.s–1 during spring spates. During hydropeaking, the water was cooled by an average of 6 °C in summer and warmed by an average of 2 °C in winter. Overall drift density was higher at site B than at site A. There was a clear diel pattern of drift at site A, with low drift density during the day and high drift density at night, whereas no clear diel pattern was observed at site B. Below the plant, at site B, drift pattern appeared to be influenced most by hydropeaking. The flushing action of peaking flows caused a catastrophic drift, which was highest in autumn when the difference between natural and peak flows was greatest. Juvenile trout were adversely affected by hydropeaking conditions and subsequently their density and biomass were reduced by 30% from site A to site B, whereas no significant difference was noticed for adults. Gut contents analysis showed that brown trout chiefly fed on the most available prey items at both sites. Fish did not seem to feed in response to diel drift patterns above the plant, whereas they chiefly used drift pulses generated by peaking flows below the outlet. Under natural conditions, fullness indices increased from autumn to summer, suggesting they may be related to prey availability and changes in water temperature. In the regulated section, fullness indices were the lowest in spring, i.e. the season when peak flows added to snowmelt floods, suggesting a prominent role of high current velocities through habitat suitability, position maintenance, and ability to capture preys. Although hydropeaking is known to disturb trout population dynamics in this and other rivers, this kind of river regulation (natural discharge except during periods of power generation, and intermittent hydropeaking from a separate reservoir) allowed the maintain of brown trout below the outlet, probably because the river returned to natural conditions when the plant was inoperative, and because daily artificial fluctuations in flow and temperature remained within the limits of natural seasonal variations.

The European catfish Silurus glanis has been cultivated in extensive ponds in central and Eastern Europe in association with carps for more than 100 years. The total production of S. glanis in aquaculture from ten European countries (Austria, Bulgaria, Croatia, Germany, France, Hungary, Greece, Macedonia, Poland, Czech Republic and Romania) was 602 t in 1993 and presently is about 2000 t. Production is increasingly from intensive pond culture or in heated/geothermal waters. The flesh is white, boneless, easy to prepare, and low in fat (6 to 8%). The taste is mild but is subject to off-flavour. The fish are filleted (yields about 40% without skin) and the flesh is cut into steaks or smoked). During the last 10 years, research in Czech Republic, France and other European countries have developed technologies for artificial reproduction, evaluated population genetics and addressed problems of conservation. Growth is higher in males and research is in progress to produce triploids fish. While propagation techniques have progressed, production remains low and the cost of production is still high. Thus availability is low and the product remains poorly known.