Day–night and depth effects on catch rates during trawl surveys in the North Sea

Abstract Brown crab (Cancer pagurus) is a widely distributed crustacean that occurs around the British coastline supporting important commercial ﬁsheries. The habitat preferences of brown crab around Scotland are poorly documented and for the purposes of stock assessment, the species is considered data-poor. Based on an analysis of dredge and trawl surveys taking place in the North Sea (2008–2018), we describe the spatial distribution of brown crab and for the first time, develop abundance and recruitment indices for the species. We make use of geostatistical methods and apply generalized additive models to model catch rates in relation to a number of explanatory variables (depth, distance to the coast, sediment type and year). The dredge and trawl abundance indices were correlated showing a similar trend of increasing catch rates in the early years of the time series up to 2016 and a subsequent reduction. The recruitment index showed a gradual increase in captured juvenile crabs up to 2014 followed by a steep decrease with 2018 being the lowest value estimated. The derivation of robust indicators of stock abundance will contribute to the stock assessment of this species and enable the provision of improved fisheries management advice for brown crab around Scotland.

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Geographic variation in the abundance and morphology of the bryozoan Alcyonidium diaphanum (Ctenostomata: Alcyonidiidae) in UK coastal waters

Journal of the Marine Biological Association of the United Kingdom ◽

10.1017/s0025315402005842 ◽

2002 ◽

Vol 82 (4) ◽

pp. 529-535 ◽

Cited By ~ 5

Author(s):

J.S. Porter ◽

J.R. Ellis ◽

P.J. Hayward ◽

S.I. Rogers ◽

R. Callaway

Keyword(s):

North Sea ◽

Coastal Waters ◽

Additional Data ◽

Distribution Patterns ◽

Irish Sea ◽

Southern North Sea ◽

The North ◽

The North Sea ◽

Catch Rates ◽

Trawl Catches

The ctenostome bryozoan Alcyonidium diaphanum is widespread and abundant in the coastal waters of England and Wales. It was recorded in 77–90% of beam trawl catches in the eastern English Channel and southern North Sea, and in 53–73% of catches in the Bristol Channel and Irish Sea. The maximum catch rates in these two study areas were 1410 and 751 kg h−1 respectively. Alcyonidium diaphanum was most abundant in the greater Thames Estuary. Additional data from the North Sea indicated that A. diaphanum is widely distributed throughout the southern North Sea. Alcyonidium diaphanum was found intertidally in only seven of 99 locations sampled. Several different morphotypes were observed, ranging from cylindrical to lobate forms. The distribution patterns of various morphotypes are discussed.

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

ICES Journal of Marine Science ◽

10.1093/icesjms/fsaa013 ◽

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.

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Modelling fishing location choice within mixed fisheries: English North Sea beam trawlers in 2000 and 2001

ICES Journal of Marine Science ◽

10.1016/j.icesjms.2004.08.016 ◽

2004 ◽

Vol 61 (8) ◽

pp. 1443-1452 ◽

Cited By ~ 87

Author(s):

Trevor Hutton ◽

Simon Mardle ◽

Sean Pascoe ◽

Robin A. Clark

Keyword(s):

Simulation Model ◽

North Sea ◽

Location Choice ◽

Spatial Location ◽

Fishing Effort ◽

The North ◽

Sea Beam ◽

The North Sea ◽

Catch Rates ◽

Temporary Closure

Abstract Numerous studies have proposed methodologies to model fisher behaviour with the aim of predicting the outcomes of decision-making on board a fishing vessel. Both short- and long-term processes (e.g. investment) impact fleet dynamics. The proposed structure of the models has tended to depend upon the nature of the fishery and the control variables (technical restrictions, quotas, effort control, and/or closed areas). For example, within the context of multi-stock, multi-fleet fisheries (mixed fisheries), a skipper will allocate effort (as input to the production process) to harvest a range of species. Spatial complexity is normally excluded in models of behaviour. In this paper, two spatial analyses are presented for modelling location choice: an analysis based on a random utility model (RUM), and a simplified simulation model of individual vessels that depends on the results of the RUM. These models are applied to the English beam-trawl fleet operating in the North Sea in 2000. The results from the RUM indicate that the number of trips, the average trip length, and the average effort in each ICES rectangle are significant variables affecting location choice, in addition to catch rate for the previous year (1999), weighted by value. The last result is used as an assumption in a simulation model of fishing effort, i.e. fishers make decisions on spatial location of operation on the basis of past catch rates. The simulation model is used to predict the distribution of the same fleet for one month during the temporary closure in the North Sea in 2001. The predicted values for effort relate well to the fishing patterns observed.

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