The fishery for jack mackerel (Trachurus symmetricus) off central Chile competes for the resource with southern sea lions (Otaria flavescens), and during purse-seining makes the fish more accessible to the pinnipeds. Interactions with sea lions were recorded during 31 purse-seine sets off central Chile during October 1999. The sea lion behaviour associated with the fishing...
Diving activity, foraging locations and pup provisioning behaviour of 10 female Antarctic fur seals (Arctocephalus gazella) were examined with respect to a range of oceanographic parameters (sea surface temperature, chlorophyll distribution and bathymetry) at the Kerguelen Archipelago in the southern Indian Ocean in February 2000. A multivariate analysis of the environmental...
The distribution and abundance of scavenging seabirds and their utilization of discards and offal between June and December 1998 were studied in the western Baltic Sea. Herring gulls were clearly the most numerous scavenging species in all areas and all seasons, followed by great black-backed gulls, lesser black-backed gulls and mew gulls. High percentages of discarded gadoids...
For species like cod and haddock, commonly distributed from bottom to far above bottom, swept area (bottom trawl) indices and acoustic indices of abundance cover different fractions of the stock. It has been argued that a combination of the two density estimates, into one estimate of absolute abundance, will improve the reliability of the survey results. The effective fishing...
The bycatch fish community was compared between areas open and closed to prawn trawling in Australia's Northern Prawn Fishery to investigate the impacts of the fishery. Two regions of a large (∼6648 km2) closure were compared, with three areas in each region, one closed to trawling (Closed) and two open to trawling, one near the closure (Near) and one farther from the closure...
Because fisheries operate within a complex array of species interactions, scientists increasingly recommend multispecies approaches to fisheries management. We created a food web model for the Baltic Sea proper, using the Ecopath with Ecosim software, to evaluate interactions between fisheries and the food web from 1974 to 2000. The model was based largely on values generated by...
The frequency and geographic distribution of trawlnet hangs from a fishery-independent survey are evaluated. The hangs data were plotted on a substratum map to confirm that many, but not all, were naturally occurring, high relief substrata. The data were also coupled with the occurrence of juvenile cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) to assess the degree of...
Primary determinants of the productivity of most fisheries are the level of fishing mortality and the age structure of the catch. Constraints on nominal effort or catch and technical measures such as mesh sizes are typically the most effective methods of ensuring productivity and sustainability. Stock assessment models are generally well adapted to assess the biological...
A total of 871 females (76–122 cm) and 86 males (68–100 cm) of Centroscymnus coelolepis caught in Portuguese mainland waters were examined for reproductive characteristics. One hermaphroditic individual of 95 cm was found. Males were few in numbers and dominated by immature specimens. Female length at first sexual maturity was 98.5 cm. Mean ovarian and uterine fecundity were 13.2...
The effects of tagging with acoustic transmitters on the growth of juvenile seabass, Dicentrarchus labrax (L.) (initial mean mass±SD, 173 g ± 23.4) in a 47 d tank experiment, and sole, Solea solea (L.) (103.2 g ± 14.8) in a 72 d tank experiment and (104.0 g ± 18.4) in a 58 d salt marsh mesocosm experiment were examined. Twenty externally tagged seabass grew more slowly than the...
This paper describes a new, enhanced version of the macrosounding method. It was first introduced over 10 years ago by the author. The purpose was to integrate acoustic data with environmental and biological parameters in a time and space scale, for use in marine-ecosystem analyses. Previous visualizations were replaced in 2002 by a vari-coloured topographical space- or time...
Inaccuracy in herring target strength can be an important source of bias in the acoustic assessment of several important herring stocks. New acoustic data on herring target strength (Ona et al., 2001, submitted for publication; Ona, 2003) confirm previous suggestions and evidence on a possible reduction of the size of the herring swimbladder as a result of its compression with...
The resolution, temporal variability and survey vessel speed dependence of the acoustic ground discrimination system RoxAnn™ was assessed over a 1 km2 area in Loch Linnhe on the west coast of Scotland. The resolution of the system was relatively poor and of the sediment parameters quantified (stone cover and sediment texture), only stone cover was consistently and significantly...
This paper describes the main characteristics of sardine schools detected in the Spanish-Atlantic surveys carried out from 1992 to 1997 (except 1994). A series of parameters were obtained for each school (morphological, positional and energetic) as well as environmental factors (temperature and salinity). The relationships between the school parameters were analyzed by a PCA and...
During the past decade, regional changes in the dynamics of the Atlanto-Iberian stock of sardine, and its exploitation by Portuguese and Spanish purse-seine fisheries, have increased the uncertainties in estimated trends of spawning biomass, stock abundance, and fishing mortality. Together with recent evidence for lack of discontinuities in the distribution of sardine eggs at the...
Vertical distribution patterns of larval and juvenile sandeels were investigated at four locations in the North Sea. Sandeels between 6 and 65 mm were found to depths of 80 m, with vertical distributions dependent on both length and environmental factors. At one location with a stratified water column, the highest densities were found during the day in midwater where food...
European hake (Merluccius merluccius) were caught alive in the northern Bay of Biscay in June/July 2002, using a codend specially designed to avoid crushing fish and also to retain water while hauling the trawl. In all, 1307 fish were tagged with anchor T-bar tags, injected with tetracycline, then released. The length range of the tagged and marked fish varied from 13 to 58 cm...
Diel variation in the catchability of 51 species of fish and 13 invertebrate taxa from an annual bottom-trawl survey of the southern Gulf of St. Lawrence was examined using generalized linear (Poisson) models. Two approaches were used: comparative fishing based on spatially paired day and night tows, and statistical control of spatio-temporal effects in regular survey data. The...
A 150 mile2 (388 km2) area in the South Minch on the Scottish west coast was surveyed acoustically using the seabed discrimination system RoxAnn™. This site was chosen from BGS seabed sediment maps because of the wide variety of substratum types present within a relatively small area. The work presented here investigates different combinations of survey track spacing in relation...
Quantifying the degree of density-dependence in stock–recruit relationships is critical to understanding fish population dynamics. The Shepherd and Cushing (1980) model couples a simple model of density-dependent larval growth with a constant rate of mortality to predict numbers surviving to recruitment. The model has not been evaluated using field data, nor have its predictions...
Grey seal pup production on Sable Island, Nova Scotia, has been monitored since the early 1960s. We estimated pup production on Sable Island in 1997 using aerial photography with a correction for detection of pups on the imagery and a statistical model to account for the proportion of pups not born at the time of the survey. A total of 20 863 pups were counted on the colour...
Barents Sea capelin (Mallotus villosus), Norwegian spring-spawning herring (Cluea harengus), and Northeast Arctic cod (Gadus morhua) have been associated with large fluctuations of biomass growth. The cause of these large fluctuations has been poorly understood and led to problems in biomass management. The identification of a deterministic cause would provide the possibility of...
Geostrophic transport and hydrographic measurements derived from a historical database (1900–1998) were used to study the spring–summer mean circulation in the upper layer south and west of Cape St. Vincent, Southwest (SW) Portugal. The larger-scale circulation scheme is forced by equatorward winds from May to September, when the Iberian coastal transition zone (CTZ) is dominated...
Relative indices of abundance by age class for bluefin tuna from 1975 to 2000 were estimated using generalized linear mixed models. Age class was included as a fixed factor within the model specifications, with the Year×Age interaction as a fixed factor component in order to obtain annual indices by age. Catch and effort data on bluefin tuna were available from two sources...