scholarly journals Evaluation of sampling strategies for age determination of cod (Gadus morhua) sampled at the North Sea International Bottom Trawl Survey

2020 ◽  
Vol 77 (3) ◽  
pp. 859-869
Author(s):  
N O A S Jourdain ◽  
O Breivik ◽  
E Fuglebakk ◽  
S Aanes ◽  
J H Vølstad
Keyword(s):  
North Sea ◽  
Age Distribution ◽  
Age Determination ◽  
Length Distribution ◽  
Bottom Trawl ◽  
Trawl Survey ◽  
The North ◽  
The North Sea ◽  
Trawl Surveys

Abstract The North Sea cod stock assessment is based on indices of abundance-at-age from fishery-independent bottom trawl surveys. The age structure of the catch is estimated by sampling fish for otoliths collection in a length-stratified manner from trawl hauls. Since age determination of fish is costly and time consuming, only a fraction of fish is sampled for age from a larger sample of the length distribution and an age–length key (ALK) is then used to obtain the age distribution. In this study, we evaluate ALK estimators for calculating the indices of abundance-at-age, with and without the assumption of constant age–length structures over relatively large areas. We show that the ALK estimators give similar point estimates of abundance-at-age and yield similar performance with respect to precision. We also quantify the uncertainty of indices of abundance and examine the effect of reducing the number of fish sampled for age determination on precision. For various subsampling strategies of otoliths collection, we show that one fish per 5-cm-length group width per trawl haul is sufficient and the total number of fish subsampled for age from trawl surveys could be reduced by at least half (50%) without appreciable loss in precision.

scholarly journals Spatial interactions between saithe (Pollachius virens) and hake (Merluccius merluccius) in the North Sea

2014 ◽  
Vol 71 (6) ◽  
pp. 1342-1355 ◽  
Author(s):  
Xochitl Cormon ◽  
Christophe Loots ◽  
Sandrine Vaz ◽  
Youen Vermard ◽  
Paul Marchal
Keyword(s):  
North Sea ◽  
Linear Models ◽  
Spatial Dynamics ◽  
Distribution Patterns ◽  
Trawl Survey ◽  
Spatial Interactions ◽  
Merluccius Merluccius ◽  
The North ◽  
Sediment Types ◽  
The North Sea

Spatial interactions between saithe (Pollachius virens) and hake (Merluccius merluccius) were investigated in the North Sea. Saithe is a well-established species in the North Sea, while occurrence of the less common hake has recently increased in the area. Spatial dynamics of these two species and their potential spatial interactions were explored using binomial generalized linear models (GLM) applied to the International Bottom Trawl Survey (IBTS) data from 1991 to 2012. Models included different types of variables: (i) abiotic variables including sediment types, temperature, and bathymetry; (ii) biotic variables including potential competitors and potential preys presence; and (iii) spatial variables. The models were reduced and used to predict and map probable habitats of saithe, hake but also, for the first time in the North Sea, the distribution of the spatial overlap between these two species. Changes in distribution patterns of these two species and of their overlap were also investigated by comparing species’ presence and overlap probabilities predicted over an early (1991–1996) and a late period (2007–2012). The results show an increase in the probability over time of the overlap between saithe and hake along with an expansion towards the southwest and Scottish waters. These shifts follow trends observed in temperature data and might be indirectly induced by climate changes. Saithe, hake, and their overlap are positively influenced by potential preys and/or competitors, which confirms spatial co-occurrence of the species concerned and leads to the questions of predator–prey relationships and competition. Finally, the present study provides robust predictions concerning the spatial distribution of saithe, hake, and of their overlap in the North Sea, which may be of interest for fishery managers.

Storm surges in the North Sea during the winter 1953-4

1956 ◽  
Vol 235 (1201) ◽  
pp. 260-274 ◽  
Keyword(s):  
Sea Level ◽  
North Sea ◽  
Storm Surges ◽  
Rate Of Change ◽  
The North ◽  
Order Of Magnitude ◽  
The North Sea ◽  
Sea Ports ◽  
Phase Angles

Records of sea level for several North Sea ports for the winter of 1953-4 have been in vestigated. They were split into 14-day intervals, and each 14-day record was Fourieranalyzed to determine if any non-astronomical periods were present. There was evidence of some activity between 40 and 50 h period, and a determination of the phase angles at different ports showed that the activity could be due to a disturbance travelling southwards from the north of the North Sea. The disturbance was partly reflected somewhere near the line from Lowestoft to Flushing, so that one part returned past Flushing and Esbjerg towards Bergen while the other part travelled towards Dover, and there was evidence of its existence on the sea-current records taken near St Margaret's Bay. These results were confirmed by subtracting the predicted astronomical tidal levels from the observed values of sea level and cross-correlating the residuals so obtained for each port with those found at Lowestoft. The residuals at Lowestoft and Aberdeen were compared with the meteorological conditions, and it was found that, although they could be attributed to a large extent to conditions within the North Sea, there was an additional effect due to a travelling surge which was of the same order of magnitude at both Lowestoft and Aberdeen and which was closely related to the rate of change with time of the atmospheric pressure difference between Wick and Bergen.

scholarly journals Opportunistically recorded acoustic data support Northeast Atlantic mackerel expansion theory

2015 ◽  
Vol 73 (4) ◽  
pp. 1115-1126 ◽  
Author(s):  
Jeroen van der Kooij ◽  
Sascha M.M. Fässler ◽  
David Stephens ◽  
Lisa Readdy ◽  
Beth E. Scott ◽  
...  
Keyword(s):  
North Sea ◽  
Stock Assessment ◽  
Detection Algorithm ◽  
Trawl Survey ◽  
Northeast Atlantic ◽  
Acoustic Data ◽  
The North ◽  
The North Sea ◽  
Northern North Sea ◽  
Feeding Grounds

Abstract Fisheries independent monitoring of widely distributed pelagic fish species which conduct large seasonal migrations is logistically complex and expensive. One of the commercially most important examples of such a species in the Northeast Atlantic Ocean is mackerel for which up to recently only an international triennial egg survey contributed to the stock assessment. In this study, we explore whether fisheries acoustic data, recorded opportunistically during the English component of the North Sea International Bottom Trawl Survey, can contribute to an improved understanding of mackerel distribution and provide supplementary data to existing dedicated monitoring surveys. Using a previously published multifrequency acoustic mackerel detection algorithm, we extracted the distribution and abundance of schooling mackerel for the whole of the North Sea during August and September between 2007 and 2013. The spatio-temporal coverage of this unique dataset is of particular interest because it includes part of the unsurveyed summer mackerel feeding grounds in the northern North Sea. Recent increases in landings in Icelandic waters during this season suggested that changes have occurred in the mackerel feeding distribution. Thus far it is poorly understood whether these changes are due to a shift, i.e. mackerel moving away from their traditional feeding grounds in the northern North Sea and southern Norwegian Sea, or whether the species' distribution has expanded. We therefore explored whether acoustically derived biomass of schooling mackerel declined in the northern North Sea during the study period, which would suggest a shift in mackerel distribution rather than an expansion. The results of this study show that in the North Sea, schooling mackerel abundance has increased and that its distribution in this area has not changed over this period. Both of these findings provide, to our knowledge, the first evidence in support of the hypothesis that mackerel have expanded their distribution rather than moved away.

Spatial and temporal dynamics of European hake (Merluccius merluccius) in the North Sea

2018 ◽  
Vol 75 (6) ◽  
pp. 2033-2044 ◽  
Author(s):  
Arved Staby ◽  
Jon Egil Skjæraasen ◽  
Audrey J Geffen ◽  
Daniel Howell
Keyword(s):  
North Sea ◽  
Temporal Dynamics ◽  
Trawl Survey ◽  
Merluccius Merluccius ◽  
Frequency Distributions ◽  
The North ◽  
The North Sea ◽  
Northern North Sea ◽  
Central North Sea ◽  
European Hake

Abstract Catches of European hake (Merluccius merluccius) in the North Sea have increased substantially during the last decade, even though there is no directed commercial fishery of hake in this area. We analysed the spatial distributions of hake in the northern the parts of its range, (where it is less well-studied), using ICES international bottom trawl survey data from 1997 to 2015. We examine length-frequency distributions for (i) distinct modes enabling the assignment of fish into categories which likely corresponded to the ages 1, 2, and 3+ and (ii) patterns of seasonal spatial distribution for the different groups. Age categories 1 and 2 fish were most abundant in the northern North Sea, and appear to remain in the North Sea until 2 years of age, when they move into deeper waters. Their distribution has expanded into the western-central North Sea in the last decade. Age category 3+ fish were most abundant in the northern and central North Sea during summer, indicating a seasonal influx of large individuals into this area likely associated with spawning activity. The distribution of these older fish has gradually expanded westward in both seasons.

Loran-C Chains: Determination of Extent of Cover

1988 ◽  
Vol 41 (02) ◽  
pp. 149-173 ◽  
Author(s):  
K. F. Woodman ◽  
J. Juleff ◽  
R. A. Allen
Keyword(s):  
Field Strength ◽  
North Sea ◽  
Western Europe ◽  
Accurate Determination ◽  
Propagation Path ◽  
North West ◽  
The North ◽  
The North Sea ◽  
Geometric Effects

A mainly theoretical study has been undertaken to demonstrate how the extent of cover from a hyperbolic navigation system chain can be evaluated. The impetus for the study was the need to assess how Loran-C could be extended over Western Europe, particularly in the South western Approaches, North Sea, English Channel and Bay of Biscay sea areas.The technique described in this article leads to an accurate determination of the electric field strength at a distance from each transmitting site and takes into account the complexities of the ground-wave propagation path. This field-strength contour is combined with the geometric effects of station siting (expansion factors) to yield a constant S/N contour (–10 dB) which defines the ¼n.m. error and hence the limit of cover for the hyperbolic chain under study.In order to exercise the analytical methods a hypothetical Loran-C chain was studied comprising a master station at Lessay (France), with secondary stations at Soustons (also in France), at Sylt (dual rated; off the North Sea coast of Germany, near the Danish border) and at a fourth station located in north-west Britain on the Hebridean island of Barra. The study indicated that such a hypothetical chain would significantly improve Loran-C cover over much of western Europe.

scholarly journals Modelling potential impacts of bottom trawl fisheries on soft sediment biogeochemistry in the North Sea†

2001 ◽  
Vol 2 (1) ◽  
Author(s):  
Daniel E Duplisea ◽  
Simon Jennings ◽  
Stephen J Malcolm ◽  
Ruth Parker ◽  
David B Sivyer
Keyword(s):  
North Sea ◽  
Bottom Trawl ◽  
Soft Sediment ◽  
The North ◽  
Sediment Biogeochemistry ◽  
The North Sea ◽  
Trawl Fisheries
Sign in / Sign up

Export Citation Format

Share Document