The Precision Of International Market Sampling For North Sea Plaice (Pleuronectes Platessa L.) And Its Influence On Stock Assessment

No abstracts are to be cited without prior reference to the author.Market sampling is an essential source of data for age-based finfish stock assessment. Previously, little has been documented about the influence of potential error in these data on the precision of stock assessments and the management information they produce. This paper presents the results of an EU-funded study (CFP Study Project 98/075) of the precision of North Sea plaice (Pleuronectes platessa L.) fish market sampling carried out by the United Kingdom (England & Wales), Denmark and the Netherlands. Data from eight years of market sampling conducted for the period 1991-1998 were analysed to obtain the precision of estimated numbers-at-age in the landed catch. The annual market sample data were then used to generate 1000 realisations of the international numbers-at-age and mean weights-at-age in the landed catch for the eight-year period selected. Matrices of the catch numbers-at-age were computed for the generated realisations and these were used to produce 1000 stock assessments conditional on the XSA (Extended Survivors Analysis) model as implemented in 1999 at the ICES Working Group on the Assessment of Demersal Stocks in the North Sea. From the outcome of these assessments the influence of the market sampling programmes on the perception of the dynamics of the stock are graphically presented as simulation envelopes on the main management parameters

The Precision Of International Market Sampling For North Sea Cod (Gadus Morhua L.) And Its Influence On Stock Assessment

No abstracts are to be cited without prior reference to the author.Market sampling is an essential source of data for age-based finfish stock assessment. Previously, little has been documented about the influence of potential error in these data on the precision of stock assessments and the management information they produce. This paper presents the results of an EU-funded study (CFP Study Project 98/075) of the precision of North Sea cod (Gadus morhua L.) fish market sampling carried out by the United Kingdom (England & Wales, Scotland), Denmark and the Netherlands. Data from eight years of market sampling conducted for the period 1991-1998 were analysed to obtain the precision of estimated numbers-at-age in the landed catch. The annual market sample data were then used to generate 1000 realisations of the international numbers-at-age and mean weights-at-age in the landed catch for the eight-year period selected. Matrices of the catch numbers-at-age were computed for the generated realisations and these were used to produce 1000 stock assessments conditional on the XSA (Extended Survivors Analysis) model as implemented in 1999 at the ICES Working Group on the Assessment of Demersal Stocks in the North Sea. From the outcome of these assessments the influence of the market sampling programmes on the perception of the dynamics of the stock are graphically presented as simulation envelopes on the main management parameters

Impact of a large-scale area closure on patterns of fishing disturbance and the consequences for benthic communities

Seasonal area closures of fisheries are primarily used to reduce fishing mortality on target species. In the absence of effort controls, fishing vessels displaced from a closed area will impact fish populations and the environment elsewhere. Based on the observed response of the North Sea beam trawl fleet to the closure of the "cod box" and an existing size-based model of the impacts of beam trawling, we predict the effects of seasonal area closures on benthic communities in the central North Sea. We suggest that repeated seasonal area closures would lead to a slightly more homogeneous distribution of annual trawling activity, although the distribution would remain patchy rather than random. The increased homogeneity, coupled with the displacement of trawling activity to previously unfished areas, is predicted to have slightly greater cumulative impacts on total benthic invertebrate production and lead to localized reductions in benthic biomass for several years. To ensure the effective integration of fisheries and environmental management, the wider consequences of fishery management actions should be considered a priori. Thus, when seasonal closures increase the homogeneity of overall disturbance or lead to the redistribution of trawling activity to environmentally sensitive or previously unfished areas, then effort reductions or permanent area closures should be considered as a management option. The latter would lead to a single but permanent redistribution of fishing disturbance, with lower cumulative impacts on benthic communities in the long run

Evidence for possible climatic forcing of late-Holocene vegetation changes in Norfolk broadland floodplain mires, UK.

Plant macrofossil analyses of five peat cores from undisturbed fens in the flood-plain of the Ant Valley of the Norfolk Broadland show the sequence of vegetation development during the last two millennia. Macrofossil assemblages have been grouped into five regional phases and are interpreted largely in terms of the response of the vegetation to changes in sea level, climate and management. Phase 1 represents pre-Roman fen woodland communities (>2000 cal. BP); phase 2 represents salt-marsh communities formed during an estuarine phase in Romano-British times (c. 2000–1600 cal. BP); phase 3 represents ‘tussock-fen’ and carr communities suggestive of drier conditions in the post-Roman to early Medieval period (c. 1600–800 cal. BP); phase 4 represents aquatic communities indicative of wetter conditions from the late Medieval period to c. 300 cal. BP; phase 5 represents communities comparable with present-day vegetation. The biostratigraphic development of the Ant Valley floodplain mires has analogues in climatically induced humification changes of some British ombrotrophic mires, suggesting a response to similar climatic controls. Widespread human interference and control of the fen vegetation may be a relatively recent phenomenon (beginning possibly,400 cal. BP). Peat-accumulation rates in the undisturbed mire sites suggest that the original Medieval turbaries which later flooded to form the Norfolk Broads may have been at least 0.5 m shallower when dug than their present depth. The wide range of environmental conditions experienced by the mires during the last two millennia is of relevance to the development of strategies for their conservation